Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
341 PM EDT Thu Oct 17 2024

Valid 00Z Fri Oct 18 2024 - 00Z Sun Oct 20 2024

...Powerful Pacific storm to bring much colder temperatures and widespread
mountain snow throughout the Intermountain West and Rockies over the next
few days...

...Heavy rain and scattered flash flooding possible for portions of the
central/southern High Plains on Saturday...

...Cooler temperatures continue to bring areas of Frost/Freeze potential
over the eastern half of the country Friday morning...

...Above average temperatures and gusty winds forecast across the central
and northern Plains today, expanding across the northern tier of the
country into the weekend...

A powerful Pacific storm system continues to make its way through the
Intermountain West today bringing showers and thunderstorms, heavy
mountain snow, gusty winds, and much cooler temperatures. Heavy snowfall
totals of 4-8", locally 12"+, are expected over the next couple of days
for higher elevations of areal mountain ranges in the northern Rockies
south through the Great Basin and into the Four Corners region. Strong,
gusty winds on the order of 30-40 mph, locally 50 mph, can also be
expected particularly along a cold front dropping southward through
southern California and into portions of the Southwest. Temperatures are
well below average, especially across the Great Basin and into the
Southwest. Forecast highs on Friday are mainly in the 40s and 50s across
the Intermountain West, with 60s to low 80s for Desert locations in the
Southwest. Morning lows will drop into the 20s and 30s for much of the
region outside of the Desert Southwest, especially across the Great Basin
and into lower elevations of the northern Rockies. Many valley locations
are covered with either Freeze or Hard Freeze Warnings as these frigid
temperatures could kill/damage crops and may freeze external pipes.
Further east, deep southerly flow on the east of the system will bring
very anomalous moisture northward over portions of the central and
southern High Plains, leading to widespread storms producing efficient
heavy rainfall on Saturday. A Slight Risk of Excessive Rainfall (level
2/4) is in effect as totals of 2-4"+ could lead to some flash flooding
despite dry antecedent conditions. Elsewhere, another system approaching
the Pacific Northwest on Friday will begin to bring moist, Pacific flow
inland along upslope portions of the Olympics and northern Cascades,
leading to periods of heavier rainfall that may lead to some isolated
flooding concerns.

Temperature-wise, highs remain below average across much of the eastern
U.S. in the wake of a strong cold front passage earlier this week, only
rising into the 50s to the north and 60s to the south. Morning lows on
Friday will once again drop into the 30s across the interior Northeast
south through the Appalachians/Ohio Valley and even as far south as the
Tennessee Valley. Frost Advisories are in place across much of the region,
with some Freeze Warnings into the central Appalachians and Upper Ohio
Valley. Conditions have moderated to the northwest over the Plains as
southerly flow increases ahead of the Pacific system. This has resulted in
widespread much above average temperatures into the 70s and low 80s,
especially for the northern Plains into the Upper Midwest, accompanied by
some gusty winds. Widespread Red Flag Warnings as well as a Critical Risk
of Fire Weather from the Storm Prediction Center (level 2/3) remain in
effect Thursday afternoon as the combination of these warmer temperatures
and winds along with dry conditions will bring a threat of wildfires.
Temperatures will remain well above average and in the upper 60s to low
70s for the Upper Midwest southwest through the central Plains Friday
while an incoming cold front will bring much cooler Fall temperatures to
the northern Plains, as highs drop back into the 40s and 50s. Temperatures
overall will moderate throughout the eastern U.S. and especially along the
northern tier of the country on Saturday making for a pleasant start to
the weekend. Forecast highs in the 60s and low 70s will stretch from New
England and the Northeast west through the Midwest, northern
Plains/Rockies, and into the Pacific Northwest. Temperatures will return
into the 70s for the South as well.

Putnam


Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Thu Oct 17 2024

Valid 12Z Sun Oct 20 2024 - 12Z Thu Oct 24 2024

...Overview...

A closed upper low atop the Four Corners into Sunday could lead to
lingering snow in the south-central Rockies and cool temperatures,
while moderate to locally heavy rain and some isolated flooding
concerns spread into the south-central Plains. Expect the upper
low to track northeast and weaken with time, producing some
lighter rain. Meanwhile, precipitation is likely in the Pacific
Northwest early next week with the tail end of an atmospheric
river. By midweek and beyond, broadly cyclonic flow should set up
across the eastern two-thirds of the CONUS while upper ridging may
form over or near the West as splitting trough energy yields a
trough/closed low over the eastern Pacific. Additionally, ridging
that retrogrades from the Gulf of Mexico through southern Texas
and Mexico will spread warmer than average temperatures to the
southern states, while cold fronts will gradually cool the
northern tier after that region sees very warm temperatures early
in the week.


...Guidance/Predictability Assessment...

Primary guidance issues continue to revolve around persistent minor
timing and detail differences for the upper low ejecting from the
Four Corners region and then pattern implications of the forecast
separation of flow within an elongated Canada into eastern Pacific
upper trough as of early next week.

For the Four Corners upper low, GFS trends over the past day have
brought its previous fast solution more into line with recent
consensus but there is still a little waffling among other
solutions. Of particular note, the 12Z CMC is a bit on the slower
side of the spread (with that model tending to be slow in recent
days) while the new 12Z ECMWF has also trended slower over the Four
Corners and then weaker/southward over the Plains/Midwest (but
closer to the guidance average at that point). Then this feature
should open up/shear out as stronger northern stream energy
approaches.

Guidance continues to show splitting of early-week elongated
Canada/eastern Pacific trough energy, with such flow separation
cases frequently lowering predictability as evidenced by sometimes
significant multi-run guidance spread and variability for the
downstream pattern. The most agreeable aspect of this evolution is
Tuesday-Wednesday closed low formation over the Pacific around
135-145W longitude and progressive Canada/northern tier U.S.
shortwave energy reaching eastern North America by Thursday
(supporing a leading cold front). The more contentious part of the
forecast involves location/strength of upper ridging that may set
up over the western U.S. or just offshore the West Coast, and
details of smaller scale energy that could be left behind
over/offshore of the Pacific Northwest and southwestern Canada.
Latest dynamical model runs have been leaning toward a stronger
upper ridge over the West, in contrast to some ECMWF/CMC ensemble
members that actually depict some trough/upper low potential. Those
members lead to flatter flow over the West in the resulting
ECens/CMCens means. Meanwhile the 00Z/06Z machine learning models
and 00Z AIFS mean depict upper ridging but on average with an axis
farther west than the dynamical model cluster. Compared to the full
guidance envelope, the new 12Z GFS is fairly extreme with the
amplitude and eastward position of its upper ridge, as well as the
eastward position of the Pacific upper low/trough. The 06Z GFS
compared better to other guidance.

Based on comparisons of 00Z/06Z models and means, the updated
forecast incorporated an even compromise of operational runs early
in the period and transitioned to an even blend of 06Z GFS/00Z
ECMWF and 00Z GEFS/ECens means to reflect the most common ideas of
guidance.


...Weather/Hazards Highlights...

The Four Corners upper low will provide forcing for precipitation
across south-central parts of the Rockies and Plains into Sunday.
Snow could linger in the higher elevations of Colorado in
particular, though likely decreasing in magnitude compared to late
this week. Ahead of the upper low, moisture streaming in from the
Pacific and the Gulf will provide an environment with PWs above the
90th if not 95th percentile for potentially heavy rain and storms
across the south-central Plains. The Day 4 Excessive Rainfall
Outlook maintains a Marginal Risk area centered over the Texas and
Oklahoma Panhandles into the western half of Kansas, plus parts of
northeastern New Mexico/far southeastern Colorado as heavy rain
may cause isolated flooding threats especially if areas of high
rain rates train in the same locations. Some showers will shift
into the central Plains/Middle Mississippi Valley by Monday and
perhaps the Ohio Valley Tuesday, though with decreasing intensity.

Meanwhile, the Pacific Northwest will likely see precipitation on
Sunday as an atmospheric river continues to spread moisture into
the region, with generally light to moderate rain and snow
amounts (lesser totals than over the Olympics in the short range).
Some precipitation may linger into Monday and spread farther
inland but with a continued trend toward lighter amounts. Gusty
winds are possible in the Rockies as fronts push through. There is
still uncertainty about specifics of eastern Pacific/western U.S.
flow aloft by Tuesday-Thursday, but most guidance currently
suggests that most of the West should be dry with upper ridging
setting up. The fast side of the spread for an eastern Pacific
upper trough/low would lead to some moisture reaching the West
Coast by next Thursday or Thursday night but that scenario is
somewhat in the minority at the moment.

Most of the eastern U.S. should remain dry through much of the
period. Easterly flow across the Florida Peninsula could bring
some light showers onshore. By Tuesday-Wednesday, light rain may
push across the north-central to northeastern U.S. with a front.

Cooler than average temperatures by 5-15 degrees should continue
into Sunday in the Four Corners states underneath the upper low,
but gradually trend warmer thereafter. Overall a majority of the
West should trend toward near/above normal temperatures by midweek,
though with some uncertainty in light of guidance differences for
the strength and location of upper ridging. Meanwhile, another
upper ridge initially over the east-central U.S. and then drifting
eastward will promote well above normal temperatures across the
northern Plains to Upper Midwest/Great Lakes and Northeast. The
Midwest can expect the greatest anomalies of 15-25 degrees above
average on Sunday and Monday as highs reach the 70s. Forecast
transition of the upper pattern toward broadly cyclonic or nearly
zonal mean flow along with a surface cold front would moderate
temperatures over the northern Plains and vicinity by midweek and
into the Great Lakes/Ohio Valley/Northeast by next Thursday. A
subtropical upper high remaining across the Gulf/northern
Mexico/Texas into Wednesday will allow for warmer temperatures to
expand across the the southern Plains into Southeast.


Rausch/Campbell/Tate


Additional 3-7 Day Hazard information can be found on the WPC
medium
range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
320 AM EDT Thu Oct 17 2024

Valid 00Z Fri Oct 18 2024 - 00Z Fri Oct 25 2024

Trade winds are forecast to weaken Thursday to more moderate
levels, while a mid-upper trough approaching from the east will
cause some above average moisture to come into Hawaii, enhancing
rainfall amounts especially in favored windward locations for
Thursday-Friday. Moisture levels should return to normal by the
weekend into early next week as an upper high comes overhead,
leading to fewer showers. Light to moderate winds may increase to
solid moderate levels around Sunday-Monday or so as a surface high
strengthens well north of the state. Into the middle of next week,
upper and surface lows are forecast to get established northwest
and northeast of Hawaii, but may send fronts as far south as the
state during the latter half of next week. That aspect of the
forecast is uncertain at this point and will continue to be
monitored.

Tate

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
844 PM EDT Thu Oct 17 2024
Day 1
Valid 01Z Fri Oct 18 2024 - 12Z Fri Oct 18 2024

The probability of rainfall exceeding flash flood guidance is less than
5 percent.

Hurley


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
844 PM EDT Thu Oct 17 2024
Day 2
Valid 12Z Fri Oct 18 2024 - 12Z Sat Oct 19 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE FOUR CORNERS REGION THROUGH NORTHERN NEW MEXICO...

...2030Z Update...

No changes were made to the Marginal Risk area in the forecast.

With the synoptic setup described below largely unchanged, the only
detail to add and one of the larger points of uncertainty will be
snow levels...as lower snow levels would mean a greater amount of
the total precipitation falls as snow and thus does not contribute
to flooding. Overall this will be the case further north for much
of UT and CO. Meanwhile for southern AZ and NM lesser amounts of
total rainfall are expected...thus the narrow area of northern NM
and the 4 Corners remains the corridor where there is both adequate
forcing and temperatures are warm enough that when added to the
preponderance of flash flood prone areas such as slot canyons, burn
scars, and any more developed and low-lying areas...isolated flash
flooding remains possible.

Wegman

...Previous Discussion...

Longwave pattern will continue evolving into a fairly prolific
upper trough/closed-low over the Great Basin, slowly migrating
eastward with sights on the Four Corners. Large scale ascent will
become maximized downstream of the mean trough with a pronounced
diffluent pattern in-of the Southern Rockies, including the Sangre
de Cristo and San Juan mountain ranges where heavier precip is
forecast. A cold front will slowly propagate eastward from the
primary disturbance with a return flow setup pressing westward
across the Eastern NM plains on the western flank of a sprawling
surface high east of the Mississippi. A secondary cold front to the
north will plunge southward thanks to a strong ridge of high
pressure developing upstream in wake of the primary western low,
along with a lee side trough developing over the Front Range. The
tandem of boundaries and backed flow will converge over Northern NM
with a strong isentropic ascent pattern initiating later Friday
evening through Saturday AM and beyond.

There has been very little deviation in the guidance to amass much
of a change compared to the previous forecast, so a general
continuity was maintained from the inherited MRGL risk across the
Four Corners into Northern NM. Ensemble guidance is in relative
agreement on the placement of two maxima during the forecast
period; one located Northeast AZ into Southeast UT with the initial
diffluent signature ahead of the evolving upper low over the Great
Basin, and the secondary maxima across Northeastern NM where the
convergence pattern aided by increasing large scale ascent will
generate a period of heavier precip Friday evening into Saturday
morning and beyond. Each areal maxima is similar with regards to
precip magnitude with a general neighborhood 1-2" max with a small
chance at upwards of 2.5" across the western maxima within the best
upper ascent within the nose of the 100kt 250mb jet streak. The
secondary maxima is generally within the last 12-hr window
encompassing 00-12z Saturday with the heaviest QPF footprint
actually highest in D3 (More on that in the D3 period below). In
any case, the very dry antecedent conditions, slot canyons, and
flashier river basins within the Four Corners area will offer a
greater risk of flash flooding with a secondary threat over the
remnant burn scars in Northern NM. This allowed for a maintenance
of the previous MRGL risk.

Kleebauer

Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
844 PM EDT Thu Oct 17 2024
Day 3
Valid 12Z Sat Oct 19 2024 - 12Z Sun Oct 20 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS
OF THE SOUTHERN HIGH PLAINS INTO SOUTHEASTERN COLORADO...

...2030Z Update...

...ERO Changes...

The Slight Risk area was nominally expanded east to include the
westernmost counties in the Texas Panhandle, the far western
Oklahoma Panhandle and a few miles into extreme southwestern Kansas
with this update. The surrounding Marginal was expanded west to
include the mountains north of Santa Fe, NM and much of the Texas
and Oklahoma Panhandles, as well as far western Kansas.
A new Marginal Risk area was introduced for the Olympic Peninsula
as well as the far northern Cascades of northwestern Washington
State.

...Southern Plains...

Forecast rainfall continues to slowly increase across the southern
Plains this afternoon. A nearly stationary upper low will begin to
tap into increasing Gulf moisture associated with a well
established LLJ across far western TX/OK/ and north up much of the
Plains overnight tonight. By Day 2/Friday the moisture plume will
be well established. The upper level low ever-so-slowly moving east
will gradually tap into more and more of that moisture. Given the
steering flow changes will be slow, the excellent forcing east of
the upper level low and increasing instability will all favor the
development of strong and few severe thunderstorms within the
moisture and instability axis.

These storms will be capable of very heavy rainfall over this
mostly parched/arid area. Due to the LLJ the storms will be pretty
fast-moving towards the NNE. However, the slow moving upper low and
constant resupply of Gulf moisture will strongly favor training
cells across the Slight Risk area, particularly NM. Thus, it's
likely that training storms will develop and move over the same
hard-hit areas causing widely scattered flash flooding.

Finally, the 12Z HRRR suggests a line of storms will follow these
individual training cells overnight Friday night. The more
widespread nature of the heavy rain associated with the line is
what's likely to push the more flood prone areas to begin to flood.
The latest guidance suggests the area near the NM/OK border at
greatest risk for the most rain, and therefore the highest threat
for flooding.

...Northwestern Washington State...

A cold front/atmospheric river impacting Vancouver Island and
portions of British Columbia just north of Washington on Friday
will push south into northwestern Washington on Saturday. 1-3
inches of rain are possible in the Olympics and in the Cascades
east of Bellingham Friday, so that by the Day 3/Saturday period,
the full force of the A.R. will move into WA, resulting in local
rainfall totals of 3-5 inches for the day. Integrated Vapor
Transport values at the peak could exceed 900 kg/m/s, which is a
bit stronger than the typical A.R., but nothing major. When added
to Friday's rain totals, this will likely be enough rain to result
in isolated flash flooding. Even if the highest elevations see
snow, the large majority of the rainfall into the mountains will be
in the form of rain. The usual rain shadowing from westerly or WSW
flow should keep much of Seattle and its surrounding suburbs with
significantly less (generally under an inch) of rain, so it was
excluded from the Marginal Risk area. Further evaluation of the
urban areas may be needed with future forecast updates.

Wegman

...Previous Discussion...

Closed upper circulation will slowly migrate eastward with a
persistent diffluent channel focused in-of the Eastern NM High
Plains up into the Colorado Front Range for Saturday. A uni-
directional flow located within the lowest 600mb will persistent
through much, if not all the forecast period allowing for moist
unstable air to be pulled poleward into the confines of the Caprock
and points north. ECENS PWAT anomalies are forecast to run upwards
of 3-3.5 deviations above normal with an M-Climate Percentile
(Forecasts relative to the last 20 years during time of year) that
is peeking into the seasonal max, a testament to the anomalous
nature of the moisture advection regime. With the poleward
advancement of the airmass, regional theta-E's spike with a theta-E
ridge positioning located right over the NM/TX border running clear
into Front Range of CO and nearby KS. This signal is consistent
with a persistent band of rainfall likely to be situated within the
theta-E ridge mentioned above and just out ahead of the slow-moving
quasi-stationary front trailing the very slow moving to perhaps stationary
low pressure across Southern CO.

Precip totals of 2-4" with locally higher amounts are increasingly
likely within that corridor between Clovis up through Southeastern
CO with the max likely driven by repeating convection as small
mid-level perturbations rotate around the general upper circulation
and move overhead of the areas above. The only saving grace for a
more significant flash flood potential is the antecedent conditions
leading in are very dry with the 0-10cm relative soil moisture
percentiles running between 5-20% across the area anticipated to
see the most rainfall. Initial rains will be beneficial, but will
add through the course of the period with waves of heavier rain
likely exacerbating areas by later in the period into early Sunday.
The previous SLGT risk was generally maintained with a MRGL
surrounding. Both risks were expanded somewhat on the southern and
northern peripheries to account for a trend in an expansion of
precip latitudinally in either direction. The heaviest will likely
encompass Eastern NM into Southeast CO with these areas likely
trending towards the higher end of SLGT risk if the signal remains
and/or amplifies further.

Kleebauer

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Thu Oct 17 2024


The Four Corners upper low will provide forcing for precipitation
across south-central parts of the Rockies and Plains into Sunday.
Snow could linger in the higher elevations of Colorado in
particular, though likely decreasing in magnitude compared to late
this week. Ahead of the upper low, moisture streaming in from the
Pacific and the Gulf will provide an environment with PWs above the
90th if not 95th percentile for potentially heavy rain and storms
across the south-central Plains. The Day 4 Excessive Rainfall
Outlook maintains a Marginal Risk area centered over the Texas and
Oklahoma Panhandles into the western half of Kansas, plus parts of
northeastern New Mexico/far southeastern Colorado as heavy rain
may cause isolated flooding threats especially if areas of high
rain rates train in the same locations. Some showers will shift
into the central Plains/Middle Mississippi Valley by Monday and
perhaps the Ohio Valley Tuesday, though with decreasing intensity.

Meanwhile, the Pacific Northwest will likely see precipitation on
Sunday as an atmospheric river continues to spread moisture into
the region, with generally light to moderate rain and snow
amounts (lesser totals than over the Olympics in the short range).
Some precipitation may linger into Monday and spread farther
inland but with a continued trend toward lighter amounts. Gusty
winds are possible in the Rockies as fronts push through. There is
still uncertainty about specifics of eastern Pacific/western U.S.
flow aloft by Tuesday-Thursday, but most guidance currently
suggests that most of the West should be dry with upper ridging
setting up. The fast side of the spread for an eastern Pacific
upper trough/low would lead to some moisture reaching the West
Coast by next Thursday or Thursday night but that scenario is
somewhat in the minority at the moment.

Most of the eastern U.S. should remain dry through much of the
period. Easterly flow across the Florida Peninsula could bring
some light showers onshore. By Tuesday-Wednesday, light rain may
push across the north-central to northeastern U.S. with a front.

Cooler than average temperatures by 5-15 degrees should continue
into Sunday in the Four Corners states underneath the upper low,
but gradually trend warmer thereafter. Overall a majority of the
West should trend toward near/above normal temperatures by midweek,
though with some uncertainty in light of guidance differences for
the strength and location of upper ridging. Meanwhile, another
upper ridge initially over the east-central U.S. and then drifting
eastward will promote well above normal temperatures across the
northern Plains to Upper Midwest/Great Lakes and Northeast. The
Midwest can expect the greatest anomalies of 15-25 degrees above
average on Sunday and Monday as highs reach the 70s. Forecast
transition of the upper pattern toward broadly cyclonic or nearly
zonal mean flow along with a surface cold front would moderate
temperatures over the northern Plains and vicinity by midweek and
into the Great Lakes/Ohio Valley/Northeast by next Thursday. A
subtropical upper high remaining across the Gulf/northern
Mexico/Texas into Wednesday will allow for warmer temperatures to
expand across the the southern Plains into Southeast.


Rausch/Campbell/Tate

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Thu Oct 17 2024


The Four Corners upper low will provide forcing for precipitation
across south-central parts of the Rockies and Plains into Sunday.
Snow could linger in the higher elevations of Colorado in
particular, though likely decreasing in magnitude compared to late
this week. Ahead of the upper low, moisture streaming in from the
Pacific and the Gulf will provide an environment with PWs above the
90th if not 95th percentile for potentially heavy rain and storms
across the south-central Plains. The Day 4 Excessive Rainfall
Outlook maintains a Marginal Risk area centered over the Texas and
Oklahoma Panhandles into the western half of Kansas, plus parts of
northeastern New Mexico/far southeastern Colorado as heavy rain
may cause isolated flooding threats especially if areas of high
rain rates train in the same locations. Some showers will shift
into the central Plains/Middle Mississippi Valley by Monday and
perhaps the Ohio Valley Tuesday, though with decreasing intensity.

Meanwhile, the Pacific Northwest will likely see precipitation on
Sunday as an atmospheric river continues to spread moisture into
the region, with generally light to moderate rain and snow
amounts (lesser totals than over the Olympics in the short range).
Some precipitation may linger into Monday and spread farther
inland but with a continued trend toward lighter amounts. Gusty
winds are possible in the Rockies as fronts push through. There is
still uncertainty about specifics of eastern Pacific/western U.S.
flow aloft by Tuesday-Thursday, but most guidance currently
suggests that most of the West should be dry with upper ridging
setting up. The fast side of the spread for an eastern Pacific
upper trough/low would lead to some moisture reaching the West
Coast by next Thursday or Thursday night but that scenario is
somewhat in the minority at the moment.

Most of the eastern U.S. should remain dry through much of the
period. Easterly flow across the Florida Peninsula could bring
some light showers onshore. By Tuesday-Wednesday, light rain may
push across the north-central to northeastern U.S. with a front.

Cooler than average temperatures by 5-15 degrees should continue
into Sunday in the Four Corners states underneath the upper low,
but gradually trend warmer thereafter. Overall a majority of the
West should trend toward near/above normal temperatures by midweek,
though with some uncertainty in light of guidance differences for
the strength and location of upper ridging. Meanwhile, another
upper ridge initially over the east-central U.S. and then drifting
eastward will promote well above normal temperatures across the
northern Plains to Upper Midwest/Great Lakes and Northeast. The
Midwest can expect the greatest anomalies of 15-25 degrees above
average on Sunday and Monday as highs reach the 70s. Forecast
transition of the upper pattern toward broadly cyclonic or nearly
zonal mean flow along with a surface cold front would moderate
temperatures over the northern Plains and vicinity by midweek and
into the Great Lakes/Ohio Valley/Northeast by next Thursday. A
subtropical upper high remaining across the Gulf/northern
Mexico/Texas into Wednesday will allow for warmer temperatures to
expand across the the southern Plains into Southeast.


Rausch/Campbell/Tate

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 PM EDT Thu Oct 17 2024

Valid 00Z Fri Oct 18 2024 - 00Z Mon Oct 21 2024


...Central Rockies...
Days 1-3...

**Significant early season winter to bring periods of heavy snow
beginning tonight***

Mid-level shortwave trough digging south from Oregon will amplify
into a potent closed low across the Great Basin early Friday, and
then continue to deepen as it ejects into the Four Corners Friday
night. NAEFS ensemble tables suggest mid-level heights will fall to
-3 sigma, reflecting the impressive intensity of this feature.

As is typical with lows of this magnitude, it will be very slow to
move and begin to fill, not exiting from Colorado until beyond
this forecast period. This will result in a prolonged period of
intense synoptic ascent focused across the Four Corners and Central
Rockies, where height falls, downstream divergence, and upper level
diffluence in response to modestly coupled jet streaks overlap. At
the same time, a surface cold front will traverse west to east,
moving across the Central Rockies Thursday night through Saturday
morning. This will have a two-pronged effect of enhancing ascent,
as a wave of low pressure develops along it, while impressive
baroclinicity results in sharpening fgen to drive locally more
intense lift. Where this most intense ascent occurs, especially in
areas of upslope flow, precipitation rates could become
intense.

Downstream of this upper low, moisture advection will ramp up on an
impressive surge of 700-500mb RH leading to PWs that are progged to
reach as high as +2 to +3 sigma, additionally supporting an
environment favorable for widespread heavy precipitation. Snow
levels initially will be quite high, varying between 8000-11000 ft,
but will fall quickly behind the front and in response to the
rapidly lowering heights beneath the anomalous low. Snow levels may
fall as low as 4000-5000 ft in the Great Basin and into WY, but
will struggle to only around 6000-8000 ft downstream of the low,
and even higher out into the High Plains. This will limit
significant snow accumulations to the higher terrain, although
convective snow rates could drag these snow levels at least subtly
further down during the event.

Heavy snow is likely D1 in the higher terrain spreading from the
Absarokas southward through the Big Horns, Wind Rivers, Uintas,
Wasatch, and into the San Juans. Snow exceeding 12 inches is
possible in the Uintas and San Juans, but otherwise WPC
probabilities for exceeding 8 inches are generally 50-70% across
these ranges. For D2, The heavy snow shifts primarily into the San
Juans, where WPC probabilities for more than 8 inches are again
above 70%, and locally an additional 12+ inches of snow is likely
in the San Juans, however some moderate snow will also expand into
the Sangre de Cristos. By D3 things start to ramp down quickly, but
an additional few inches of snow is possible in the San Juans, with
event total snow exceeding 3 feet in some places likely.

The probability of significant ice across the CONUS is less than
10 percent.

Weiss

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Thu Oct 17 2024

Valid 12Z Sun Oct 20 2024 - 12Z Thu Oct 24 2024

...Overview...

A closed upper low atop the Four Corners into Sunday could lead to
lingering snow in the south-central Rockies and cool temperatures,
while moderate to locally heavy rain and some isolated flooding
concerns spread into the south-central Plains. Expect the upper
low to track northeast and weaken with time, producing some
lighter rain. Meanwhile, precipitation is likely in the Pacific
Northwest early next week with the tail end of an atmospheric
river. By midweek and beyond, broadly cyclonic flow should set up
across the eastern two-thirds of the CONUS while upper ridging may
form over or near the West as splitting trough energy yields a
trough/closed low over the eastern Pacific. Additionally, ridging
that retrogrades from the Gulf of Mexico through southern Texas
and Mexico will spread warmer than average temperatures to the
southern states, while cold fronts will gradually cool the
northern tier after that region sees very warm temperatures early
in the week.


...Guidance/Predictability Assessment...

Primary guidance issues continue to revolve around persistent minor
timing and detail differences for the upper low ejecting from the
Four Corners region and then pattern implications of the forecast
separation of flow within an elongated Canada into eastern Pacific
upper trough as of early next week.

For the Four Corners upper low, GFS trends over the past day have
brought its previous fast solution more into line with recent
consensus but there is still a little waffling among other
solutions. Of particular note, the 12Z CMC is a bit on the slower
side of the spread (with that model tending to be slow in recent
days) while the new 12Z ECMWF has also trended slower over the Four
Corners and then weaker/southward over the Plains/Midwest (but
closer to the guidance average at that point). Then this feature
should open up/shear out as stronger northern stream energy
approaches.

Guidance continues to show splitting of early-week elongated
Canada/eastern Pacific trough energy, with such flow separation
cases frequently lowering predictability as evidenced by sometimes
significant multi-run guidance spread and variability for the
downstream pattern. The most agreeable aspect of this evolution is
Tuesday-Wednesday closed low formation over the Pacific around
135-145W longitude and progressive Canada/northern tier U.S.
shortwave energy reaching eastern North America by Thursday
(supporing a leading cold front). The more contentious part of the
forecast involves location/strength of upper ridging that may set
up over the western U.S. or just offshore the West Coast, and
details of smaller scale energy that could be left behind
over/offshore of the Pacific Northwest and southwestern Canada.
Latest dynamical model runs have been leaning toward a stronger
upper ridge over the West, in contrast to some ECMWF/CMC ensemble
members that actually depict some trough/upper low potential. Those
members lead to flatter flow over the West in the resulting
ECens/CMCens means. Meanwhile the 00Z/06Z machine learning models
and 00Z AIFS mean depict upper ridging but on average with an axis
farther west than the dynamical model cluster. Compared to the full
guidance envelope, the new 12Z GFS is fairly extreme with the
amplitude and eastward position of its upper ridge, as well as the
eastward position of the Pacific upper low/trough. The 06Z GFS
compared better to other guidance.

Based on comparisons of 00Z/06Z models and means, the updated
forecast incorporated an even compromise of operational runs early
in the period and transitioned to an even blend of 06Z GFS/00Z
ECMWF and 00Z GEFS/ECens means to reflect the most common ideas of
guidance.


...Weather/Hazards Highlights...

The Four Corners upper low will provide forcing for precipitation
across south-central parts of the Rockies and Plains into Sunday.
Snow could linger in the higher elevations of Colorado in
particular, though likely decreasing in magnitude compared to late
this week. Ahead of the upper low, moisture streaming in from the
Pacific and the Gulf will provide an environment with PWs above the
90th if not 95th percentile for potentially heavy rain and storms
across the south-central Plains. The Day 4 Excessive Rainfall
Outlook maintains a Marginal Risk area centered over the Texas and
Oklahoma Panhandles into the western half of Kansas, plus parts of
northeastern New Mexico/far southeastern Colorado as heavy rain
may cause isolated flooding threats especially if areas of high
rain rates train in the same locations. Some showers will shift
into the central Plains/Middle Mississippi Valley by Monday and
perhaps the Ohio Valley Tuesday, though with decreasing intensity.

Meanwhile, the Pacific Northwest will likely see precipitation on
Sunday as an atmospheric river continues to spread moisture into
the region, with generally light to moderate rain and snow
amounts (lesser totals than over the Olympics in the short range).
Some precipitation may linger into Monday and spread farther
inland but with a continued trend toward lighter amounts. Gusty
winds are possible in the Rockies as fronts push through. There is
still uncertainty about specifics of eastern Pacific/western U.S.
flow aloft by Tuesday-Thursday, but most guidance currently
suggests that most of the West should be dry with upper ridging
setting up. The fast side of the spread for an eastern Pacific
upper trough/low would lead to some moisture reaching the West
Coast by next Thursday or Thursday night but that scenario is
somewhat in the minority at the moment.

Most of the eastern U.S. should remain dry through much of the
period. Easterly flow across the Florida Peninsula could bring
some light showers onshore. By Tuesday-Wednesday, light rain may
push across the north-central to northeastern U.S. with a front.

Cooler than average temperatures by 5-15 degrees should continue
into Sunday in the Four Corners states underneath the upper low,
but gradually trend warmer thereafter. Overall a majority of the
West should trend toward near/above normal temperatures by midweek,
though with some uncertainty in light of guidance differences for
the strength and location of upper ridging. Meanwhile, another
upper ridge initially over the east-central U.S. and then drifting
eastward will promote well above normal temperatures across the
northern Plains to Upper Midwest/Great Lakes and Northeast. The
Midwest can expect the greatest anomalies of 15-25 degrees above
average on Sunday and Monday as highs reach the 70s. Forecast
transition of the upper pattern toward broadly cyclonic or nearly
zonal mean flow along with a surface cold front would moderate
temperatures over the northern Plains and vicinity by midweek and
into the Great Lakes/Ohio Valley/Northeast by next Thursday. A
subtropical upper high remaining across the Gulf/northern
Mexico/Texas into Wednesday will allow for warmer temperatures to
expand across the the southern Plains into Southeast.


Rausch/Campbell/Tate


Additional 3-7 Day Hazard information can be found on the WPC
medium
range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw