Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
423 PM EDT Tue Oct 15 2024

Valid 00Z Wed Oct 16 2024 - 00Z Fri Oct 18 2024

...Unseasonably cool temperatures continue for much of the East with frost
and freeze concerns from the Central U.S. to parts of the Mid-Atlantic and
Northeast...

...Above normal temperatures build across the central and northern High
Plains with the risk of fire weather increasing through Thursday...

...Significant pattern change begins Wednesday into Thursday across the
Pacific Northwest and Intermountain West with much colder temperatures and
the threat of widespread mountain snow...

A large area of Canadian high pressure, building southward into the
Central U.S. in the wake of yesterday's passing cold front, will continue
to keep unseasonably cool temperatures in place across much of the eastern
half of the United States over the next few days and nights. Starting
tonight, low temperatures from the nation's midsection to the Eastern
Seaboard will bottom out roughly 10 to 20 degrees below average for this
time of year. Low temperatures stretching from parts of Kansas all the way
to Pennsylvania are forecast to dip near or even below freezing, leading
to fairly widespread frost and freeze concerns. Those with sensitive
vegetation or other agricultural interests in these regions will want to
heed the Freeze Warnings and Frost Advisories that are currently in
effect. Despite increasing sunshine Wednesday into Thursday underneath the
bubble of high pressure, high temperatures will still struggle to climb
out of the 50s and 60s... equating to 10 to 20 degrees below average for
the middle of October.

As the large and sprawling area of high pressure moves into the Eastern
U.S. mid to late week, developing southwest flow on the backside of the
high will allow anomalously warm air to surge northward across the central
and northern High Plains. High temperatures will soar well into the 70s
and 80s on Wednesday with some places in Wyoming, Montana, and South
Dakota even having a shot at 90 degrees. This unseasonable warmth then
spreads eastward on Thursday, with high temperatures climbing 20 to 30
degrees above normal. The combination of near record warmth, low relative
humidity, gusty winds, and dry fuels will be conducive to critical fire
weather conditions across parts of the central and northern Plains on
Wednesday which may linger into Thursday.

The very warm conditions across much of the West as of late won't last too
much longer as a significant pattern change is on the way. A strong
Pacific storm system will slam into the Pacific Northwest beginning later
Wednesday into Thursday, bringing much colder and much more seasonable
temperatures into the region behind a powerful cold front. In addition to
the colder temperatures, the storm will also bring widespread
precipitation for much of the region. While valley locations will mostly
just see rain showers, dropping snow levels will result in the first snow
of the season for many of the higher mountain elevations. Winter Storm
Watches and Warnings have already been issued for portions of the central
and northern Rockies, which go into effect late Wednesday night into
Thursday morning. If you'll be traveling through the mountains later this
week, be sure to keep checking the forecast and be prepared for
potentially hazardous winter driving conditions.

Miller


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
333 PM EDT Tue Oct 15 2024

Valid 12Z Fri Oct 18 2024 - 12Z Tue Oct 22 2024


...Overview...

Deep upper troughing with a developing closed low in the Interior
West will lead to a cool and wet pattern there from late this week
into the weekend, including higher elevation snow and frost/freeze
concerns in some of the valleys. Heavy rain chances could increase
in the southern/central Plains by the weekend. Upper ridging ahead
of this trough/low will promote warmer than average temperatures
in the northern/central Plains to Midwest. The ridge should
gradually warm the East as well and promote dry conditions for the
most part, except for South Florida. Additional rounds of
precipitation may come into the Pacific Northwest late week into
the weekend.


...Guidance/Predictability Assessment...

Most guidance still agrees with the large scale pattern. This
evolution consists of eventual ejection of the Southwest/Four
Corners closed low into the Plains/Midwest as the leading edge of
an elongated mean trough over the northeastern Pacific/western
Canada pushes southeastward, while weak shortwave energy over the
southern Appalachians/Southeast may separate a couple ridges that
will comprise a larger scale overall mean ridge covering the East.

The primary model discrepancies involve the eventual progression of
the upper low and details of influencing upstream shortwave energy.
Most machine learning (ML) models offer the best support for the
intermediate timing of recent ECMWF runs. Latest GFS runs shift the
upper low a little north of consensus by around Sunday or so and
then stray faster, though incoming northern tier energy ultimately
yields a mean trough not too far off the guidance average for
position. By Day 7/Tuesday the GFS details aloft result in a
deeper southern Canadian surface low compared to other dynamical or
ML models. In contrast, per the ML models the 00Z ECMWF could be
underdone with surface low pressure due to slow timing of upstream
shortwave energy. CMC runs represent the slower extreme for the
ejecting upper low, though the new 12Z version has adjusted closer
to the favored intermediate timing. Also of note by the end of the
period, dynamical/ML guidance develops significant divergence for
how energy will be distributed within the western Canada into
eastern Pacific mean trough aloft. This spread begins to influence
sensible weather over the Northwest by next Tuesday but has the
potential to produce more significant spread and run-to-run
variability over more of the lower 48 thereafter.

Guidance comparisons led to starting the updated forecast with a
composite of 00Z/06Z operational models, replacing the GFS with the
GEFS mean around mid-period and then splitting ECMWF input between
the 00Z/15 and 12Z/14 runs. The 00Z ECens run was not available
locally for blending purposes, so the last part of the forecast
consisted of those ECMWF runs, the 06Z GEFS mean, and WPC
continuity.


...Weather/Hazards Highlights...

The upper low closing off across the Southwest late week into the
weekend will push precipitation into the Four Corners states,
Rockies, and High Plains. Expect enhanced precipitation totals in
terrain with higher elevation snow, which will be the first snow
of the season in some locations given the warm Fall in the West
thus far. The Day 4/Friday Excessive Rainfall Outlook has
introduced a Marginal Risk area centered over the central portion
of northern New Mexico into far southwestern Colorado in light of
guidance signals for potentially high localized rain rates over
terrain that may be more sensitive after rainfall during the Day 3
period, with the ECMWF also indicating some pockets of instability.
Some precipitation could continue across the central/southern
Rockies into the weekend. By later Friday into the weekend, Gulf
moisture may stream in just ahead of the upper low for increasing
rain chances in the central/southern High Plains. While forecast
details vary, models have been fairly persistent in showing some
moderate to heavy rain potential focused in northeastern New Mexico
and nearby, with some modest instability and slow movement of the
upper low around Saturday in particular. Thus the Day 5/Saturday
ERO continues to depict a Marginal Risk in that region and heavy
rain would be most concerning atop sensitive burn scar areas.
Latest guidance still suggests that the south- central Plains will
continue to be a focus for enhanced precipitation on Sunday before
rainfall finally moves eastward Monday, though with decreasing
intensity.

Meanwhile, the Pacific Northwest will likely see more
precipitation during the period. Initial focus for enhanced totals
will be over the Olympics and vicinity Friday into the weekend, on
the southern periphery of a strong jet pushing into British
Columbia. With time, expect upstream shortwave energy to expand the
precipitation shield southward and eastward but with gradually
lesser max amounts. Uncertainty about specifics of eastern
Pacific/West Coast flow aloft decreases confidence in precipitation
details over the Northwest by next Tuesday.

Meanwhile, the eastern U.S. is forecast to be generally dry late
week into early next week, with the exception of parts of the
Florida Peninsula. There, return flow of moisture near a meandering
front could increase rain and thunderstorm chances especially for
South Florida by Friday and into the weekend. There could be a
nonzero chance for flash flooding with this activity especially if
heavy rain rates occur in the sensitive urban areas. However, as
models continue to vary with how much rain may fall onshore versus
offshore, the Days 4-5/Friday-Saturday EROs continue to depict no
risk areas. Will continue to monitor this region for any trends in
model clustering.

An upper ridge aligned over the east-central U.S. Friday will
promote above normal temperatures in its vicinity. The biggest
anomalies of plus 10-20 degrees are forecast across the Midwest.
Meanwhile another round of chilly morning lows may cause
frost/freeze concerns for the Interior Northeast and Appalachians.
Northeastern temperatures will rebound during the weekend when
upper ridging moves overhead, bringing anomalies up to 5-10
degrees above normal. Then the ridge rebuilding over the central
into eastern U.S. early next week will lead to warmer than average
temperatures in the northern Plains to Upper Midwest once again,
with expanding coverage of plus 5-15 degree anomalies over the
East. On the other hand, the upper trough and frontal surges in the
West will cause below normal temperatures (by up to 10-20 degrees
for highs) to spread across the Great Basin/Intermountain West and
Southwest on Friday with slow moderation into the weekend. These
colder temperatures may also lead to additional frost/freeze
concerns for any remaining susceptible growing areas/valley
locations.


Rausch/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
356 AM EDT Tue Oct 15 2024

Valid 00Z Wed Oct 16 2024 - 00Z Wed Oct 23 2024

Some enhanced showers are forecast to wind down Tuesday as a weak
low level trough moves west of Hawaii. Meanwhile trades will
strengthen to breezy levels and likely continue into Wednesday or
Thursday. By Thursday, a weak upper low and perhaps a surface
trough approach the state and cause another surge of above average
moisture that could enhance rainfall amounts especially in favored
windward locations for Thursday-Friday. Meanwhile trades should
decrease to more moderate levels, which should continue into the
weekend though with drier conditions overall. The strength of the
trades may vary a bit from day to day into early next week, while
generally dry conditions persist.

Tate

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Tue Oct 15 2024
Day 1
Valid 16Z Tue Oct 15 2024 - 12Z Wed Oct 16 2024

...16Z Update...

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

Wegman


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Tue Oct 15 2024
Day 2
Valid 12Z Wed Oct 16 2024 - 12Z Thu Oct 17 2024

...2030Z Update...

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

Wegman


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Tue Oct 15 2024
Day 3
Valid 12Z Thu Oct 17 2024 - 12Z Fri Oct 18 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH-CENTRAL NEW MEXICO...

...2030Z Update...

A deepening trough over the Intermountain West will focus some of
the moisture across New Mexico into a corridor that upslopes along
the Sangre de Cristo Mountains on Thursday. While conditions in
this region have been dry, the hydrophobic dry soils in the area
as well as local burn scars such as Hermit's Peak of 2022 will
locally raise the flash flooding potential that even rainfall rates
to 1/4 inch per hour in these flood sensitive areas may result in
localized flash flooding should stationary storms form. In
coordination with ABQ/Albuquerque, NM forecast office, a Marginal
Risk was introduced for north central New Mexico with this update.
A few of the mountains into extreme south central Colorado may also
focus enough rainfall to result in an isolated flash flooding risk.
For the taller mountains of southern Colorado, the precipitation
should mostly fall as snow and thus negate the local flash flooding
risk, but future updates may require an expansion for the valley
areas of southwest Colorado. More rainfall with a developing
leeside low is expected in this area on Day 4/Friday.

Wegman

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
333 PM EDT Tue Oct 15 2024


The upper low closing off across the Southwest late week into the
weekend will push precipitation into the Four Corners states,
Rockies, and High Plains. Expect enhanced precipitation totals in
terrain with higher elevation snow, which will be the first snow
of the season in some locations given the warm Fall in the West
thus far. The Day 4/Friday Excessive Rainfall Outlook has
introduced a Marginal Risk area centered over the central portion
of northern New Mexico into far southwestern Colorado in light of
guidance signals for potentially high localized rain rates over
terrain that may be more sensitive after rainfall during the Day 3
period, with the ECMWF also indicating some pockets of instability.
Some precipitation could continue across the central/southern
Rockies into the weekend. By later Friday into the weekend, Gulf
moisture may stream in just ahead of the upper low for increasing
rain chances in the central/southern High Plains. While forecast
details vary, models have been fairly persistent in showing some
moderate to heavy rain potential focused in northeastern New Mexico
and nearby, with some modest instability and slow movement of the
upper low around Saturday in particular. Thus the Day 5/Saturday
ERO continues to depict a Marginal Risk in that region and heavy
rain would be most concerning atop sensitive burn scar areas.
Latest guidance still suggests that the south- central Plains will
continue to be a focus for enhanced precipitation on Sunday before
rainfall finally moves eastward Monday, though with decreasing
intensity.

Meanwhile, the Pacific Northwest will likely see more
precipitation during the period. Initial focus for enhanced totals
will be over the Olympics and vicinity Friday into the weekend, on
the southern periphery of a strong jet pushing into British
Columbia. With time, expect upstream shortwave energy to expand the
precipitation shield southward and eastward but with gradually
lesser max amounts. Uncertainty about specifics of eastern
Pacific/West Coast flow aloft decreases confidence in precipitation
details over the Northwest by next Tuesday.

Meanwhile, the eastern U.S. is forecast to be generally dry late
week into early next week, with the exception of parts of the
Florida Peninsula. There, return flow of moisture near a meandering
front could increase rain and thunderstorm chances especially for
South Florida by Friday and into the weekend. There could be a
nonzero chance for flash flooding with this activity especially if
heavy rain rates occur in the sensitive urban areas. However, as
models continue to vary with how much rain may fall onshore versus
offshore, the Days 4-5/Friday-Saturday EROs continue to depict no
risk areas. Will continue to monitor this region for any trends in
model clustering.

An upper ridge aligned over the east-central U.S. Friday will
promote above normal temperatures in its vicinity. The biggest
anomalies of plus 10-20 degrees are forecast across the Midwest.
Meanwhile another round of chilly morning lows may cause
frost/freeze concerns for the Interior Northeast and Appalachians.
Northeastern temperatures will rebound during the weekend when
upper ridging moves overhead, bringing anomalies up to 5-10
degrees above normal. Then the ridge rebuilding over the central
into eastern U.S. early next week will lead to warmer than average
temperatures in the northern Plains to Upper Midwest once again,
with expanding coverage of plus 5-15 degree anomalies over the
East. On the other hand, the upper trough and frontal surges in the
West will cause below normal temperatures (by up to 10-20 degrees
for highs) to spread across the Great Basin/Intermountain West and
Southwest on Friday with slow moderation into the weekend. These
colder temperatures may also lead to additional frost/freeze
concerns for any remaining susceptible growing areas/valley
locations.


Rausch/Tate

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
333 PM EDT Tue Oct 15 2024


The upper low closing off across the Southwest late week into the
weekend will push precipitation into the Four Corners states,
Rockies, and High Plains. Expect enhanced precipitation totals in
terrain with higher elevation snow, which will be the first snow
of the season in some locations given the warm Fall in the West
thus far. The Day 4/Friday Excessive Rainfall Outlook has
introduced a Marginal Risk area centered over the central portion
of northern New Mexico into far southwestern Colorado in light of
guidance signals for potentially high localized rain rates over
terrain that may be more sensitive after rainfall during the Day 3
period, with the ECMWF also indicating some pockets of instability.
Some precipitation could continue across the central/southern
Rockies into the weekend. By later Friday into the weekend, Gulf
moisture may stream in just ahead of the upper low for increasing
rain chances in the central/southern High Plains. While forecast
details vary, models have been fairly persistent in showing some
moderate to heavy rain potential focused in northeastern New Mexico
and nearby, with some modest instability and slow movement of the
upper low around Saturday in particular. Thus the Day 5/Saturday
ERO continues to depict a Marginal Risk in that region and heavy
rain would be most concerning atop sensitive burn scar areas.
Latest guidance still suggests that the south- central Plains will
continue to be a focus for enhanced precipitation on Sunday before
rainfall finally moves eastward Monday, though with decreasing
intensity.

Meanwhile, the Pacific Northwest will likely see more
precipitation during the period. Initial focus for enhanced totals
will be over the Olympics and vicinity Friday into the weekend, on
the southern periphery of a strong jet pushing into British
Columbia. With time, expect upstream shortwave energy to expand the
precipitation shield southward and eastward but with gradually
lesser max amounts. Uncertainty about specifics of eastern
Pacific/West Coast flow aloft decreases confidence in precipitation
details over the Northwest by next Tuesday.

Meanwhile, the eastern U.S. is forecast to be generally dry late
week into early next week, with the exception of parts of the
Florida Peninsula. There, return flow of moisture near a meandering
front could increase rain and thunderstorm chances especially for
South Florida by Friday and into the weekend. There could be a
nonzero chance for flash flooding with this activity especially if
heavy rain rates occur in the sensitive urban areas. However, as
models continue to vary with how much rain may fall onshore versus
offshore, the Days 4-5/Friday-Saturday EROs continue to depict no
risk areas. Will continue to monitor this region for any trends in
model clustering.

An upper ridge aligned over the east-central U.S. Friday will
promote above normal temperatures in its vicinity. The biggest
anomalies of plus 10-20 degrees are forecast across the Midwest.
Meanwhile another round of chilly morning lows may cause
frost/freeze concerns for the Interior Northeast and Appalachians.
Northeastern temperatures will rebound during the weekend when
upper ridging moves overhead, bringing anomalies up to 5-10
degrees above normal. Then the ridge rebuilding over the central
into eastern U.S. early next week will lead to warmer than average
temperatures in the northern Plains to Upper Midwest once again,
with expanding coverage of plus 5-15 degree anomalies over the
East. On the other hand, the upper trough and frontal surges in the
West will cause below normal temperatures (by up to 10-20 degrees
for highs) to spread across the Great Basin/Intermountain West and
Southwest on Friday with slow moderation into the weekend. These
colder temperatures may also lead to additional frost/freeze
concerns for any remaining susceptible growing areas/valley
locations.


Rausch/Tate

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue Oct 15 2024

Valid 00Z Wed Oct 16 2024 - 00Z Sat Oct 19 2024

...Northeast...
Day 1...

An elongated and anomalous 500mb trough extending from a stacked
upper low will amplify and pivot southeast across New England. A
surface trough beneath this feature will additional move southeast,
providing a period of enhanced ascent through convergence and
height falls to wring out modest, and lessening, available
moisture. Forecast soundings suggest the DGZ will dry rapidly
tonight into early Wednesday, but forced upslope ascent should
offset this drying at least subtly, resulting in a period of light
to moderate snow in higher elevations above 1500-2000 ft,
especially the first half of D1. Snow should not be too
significant, but the nighttime event and periods of heavier snow
will cause some accumulation, and WPC probabilities for more then 4
inches reach 10-30% in the highest elevations of VT and NH, with up
to 6" possible at the top of Mt. Washington.


...Pacific Northwest...
Days 1-2...

Two distinct shortwaves embedded within an amplifying trough across
the West will spread ascent and moisture into the Pacific Northwest
beginning later on Wednesday. The first of these shortwaves will
feature minimal amplitude as it digs across WA state Wednesday
evening, but will be accompanied by a surge in 700mb RH and modest
IVT lifting into the interior Pacific NW. A surface cold front
pushing east beneath this trough will help cool snow levels from
around 8000 ft to as low as 4500 ft by the end of D1, which when
combined with the enhanced ascent and moisture will result in
moderate snow which has a 30-50% chance of exceeding 4 inches in
some of the highest terrain of the Cascades.

Immediately in the wake of this first shortwave, a more pronounced
trough will dig along the Pacific Coast and amplify into the Great
Basin by Friday. A secondary surge in 7090-500mb moisture will
accompany this feature, but in general moisture will be waning as
IVT continues to push east. However, lower initial snow levels will
continue to fall, dropping to as low as 3000-4000 ft, resulting in
more widespread light to moderate snow, including some light
accumulations of 1-3 inches at Washington and Stevens Passes.
Otherwise, WPC probabilities D2 reach above 70% along the spine of
the Cascades as far south as central OR, with locally as much as
10" of snow in the higher terrain possible.


...Northern & Central Rockies...
Days 2-3...

The first potentially significant winter storm of the season, at
least for the terrain, looks likey to develop late-week across the
Intermountain West.

A shortwave trough digging out of the Pacific Northwest Thursday
aftn will rapidly amplify into a closed low centered over the
southern Great Basin by Friday aftn before shifting into the Four
Corners by the end of the forecast period. Ascent associated with
this feature will expand and intensify downstream in response to
pronounced height falls, potent mid-level divergence, and an
increasingly coupled upper level jet structure to drive diffluence
atop the other ascent. This will result in widespread impressive
lift across the Central Rockies, especially D3 (and beyond). This
ascent will be aided additionally by a potent cold front surging
southeast across the area, leading to enhanced baroclinicity upon
which a wave of low pressure will develop, and subsequent upslope
ascent within the post-frontal flow.

This forcing will encounter increasing moisture across the Central
Rockies where modest IVT will surge PWs to as much as +2 sigma
according to NAEFS, resulting in an environment favorable for heavy
precipitation, including snow in the higher elevations.

The models have come into better agreement in the timing and
placement of the upper low, reflected by shrinking IQR in the 500mb
height probabilities across the Central Rockies according to DESI,
which leads to inreasing confidence in the overall evolution of
this event. However, the timing of the front is still uncertain,
which by D3 leads to challenges in timing of precipitation changing
from rain to snow due to lowering snow levels and cooling
temperaturres. At this time, the GEFS cluster appears to be the
fastest/coldest solution, while the ECENS/GEPS is more similar.
Regardless of timing, as snow levels fall from 8000-10000 ft early
Friday to as low as 5000-6000 ft Friday night, snow will overspread
the higher elevations, and likely become heavy at times as some
instability develops in conjunction with sharpening fgen to drive
rates potentially above 1"/hr.

On D2, heavy snow is focused across the Absarokas, and WPC
probabilities for 6+ inches are 30-50%. The more significant and
heavy snow is likely D3, with accumulating snow spreading across
much of the Central Rockies and eastern Great Basin ranges. WPC
probabilities D3 for more than 6 inches of snow are as high as
70-90% in the Uintas, Wind Rivers, and Big Horns, with small
corridors above 70% also extending into the Abasarokas and portions
of the Wasatch. At this time impacts appear confined to higher
terrain, but at least short-duration heavy snow rates combined with
the early-season event will cause hazardous conditions on Friday.


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


Weiss

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
333 PM EDT Tue Oct 15 2024

Valid 12Z Fri Oct 18 2024 - 12Z Tue Oct 22 2024


...Overview...

Deep upper troughing with a developing closed low in the Interior
West will lead to a cool and wet pattern there from late this week
into the weekend, including higher elevation snow and frost/freeze
concerns in some of the valleys. Heavy rain chances could increase
in the southern/central Plains by the weekend. Upper ridging ahead
of this trough/low will promote warmer than average temperatures
in the northern/central Plains to Midwest. The ridge should
gradually warm the East as well and promote dry conditions for the
most part, except for South Florida. Additional rounds of
precipitation may come into the Pacific Northwest late week into
the weekend.


...Guidance/Predictability Assessment...

Most guidance still agrees with the large scale pattern. This
evolution consists of eventual ejection of the Southwest/Four
Corners closed low into the Plains/Midwest as the leading edge of
an elongated mean trough over the northeastern Pacific/western
Canada pushes southeastward, while weak shortwave energy over the
southern Appalachians/Southeast may separate a couple ridges that
will comprise a larger scale overall mean ridge covering the East.

The primary model discrepancies involve the eventual progression of
the upper low and details of influencing upstream shortwave energy.
Most machine learning (ML) models offer the best support for the
intermediate timing of recent ECMWF runs. Latest GFS runs shift the
upper low a little north of consensus by around Sunday or so and
then stray faster, though incoming northern tier energy ultimately
yields a mean trough not too far off the guidance average for
position. By Day 7/Tuesday the GFS details aloft result in a
deeper southern Canadian surface low compared to other dynamical or
ML models. In contrast, per the ML models the 00Z ECMWF could be
underdone with surface low pressure due to slow timing of upstream
shortwave energy. CMC runs represent the slower extreme for the
ejecting upper low, though the new 12Z version has adjusted closer
to the favored intermediate timing. Also of note by the end of the
period, dynamical/ML guidance develops significant divergence for
how energy will be distributed within the western Canada into
eastern Pacific mean trough aloft. This spread begins to influence
sensible weather over the Northwest by next Tuesday but has the
potential to produce more significant spread and run-to-run
variability over more of the lower 48 thereafter.

Guidance comparisons led to starting the updated forecast with a
composite of 00Z/06Z operational models, replacing the GFS with the
GEFS mean around mid-period and then splitting ECMWF input between
the 00Z/15 and 12Z/14 runs. The 00Z ECens run was not available
locally for blending purposes, so the last part of the forecast
consisted of those ECMWF runs, the 06Z GEFS mean, and WPC
continuity.


...Weather/Hazards Highlights...

The upper low closing off across the Southwest late week into the
weekend will push precipitation into the Four Corners states,
Rockies, and High Plains. Expect enhanced precipitation totals in
terrain with higher elevation snow, which will be the first snow
of the season in some locations given the warm Fall in the West
thus far. The Day 4/Friday Excessive Rainfall Outlook has
introduced a Marginal Risk area centered over the central portion
of northern New Mexico into far southwestern Colorado in light of
guidance signals for potentially high localized rain rates over
terrain that may be more sensitive after rainfall during the Day 3
period, with the ECMWF also indicating some pockets of instability.
Some precipitation could continue across the central/southern
Rockies into the weekend. By later Friday into the weekend, Gulf
moisture may stream in just ahead of the upper low for increasing
rain chances in the central/southern High Plains. While forecast
details vary, models have been fairly persistent in showing some
moderate to heavy rain potential focused in northeastern New Mexico
and nearby, with some modest instability and slow movement of the
upper low around Saturday in particular. Thus the Day 5/Saturday
ERO continues to depict a Marginal Risk in that region and heavy
rain would be most concerning atop sensitive burn scar areas.
Latest guidance still suggests that the south- central Plains will
continue to be a focus for enhanced precipitation on Sunday before
rainfall finally moves eastward Monday, though with decreasing
intensity.

Meanwhile, the Pacific Northwest will likely see more
precipitation during the period. Initial focus for enhanced totals
will be over the Olympics and vicinity Friday into the weekend, on
the southern periphery of a strong jet pushing into British
Columbia. With time, expect upstream shortwave energy to expand the
precipitation shield southward and eastward but with gradually
lesser max amounts. Uncertainty about specifics of eastern
Pacific/West Coast flow aloft decreases confidence in precipitation
details over the Northwest by next Tuesday.

Meanwhile, the eastern U.S. is forecast to be generally dry late
week into early next week, with the exception of parts of the
Florida Peninsula. There, return flow of moisture near a meandering
front could increase rain and thunderstorm chances especially for
South Florida by Friday and into the weekend. There could be a
nonzero chance for flash flooding with this activity especially if
heavy rain rates occur in the sensitive urban areas. However, as
models continue to vary with how much rain may fall onshore versus
offshore, the Days 4-5/Friday-Saturday EROs continue to depict no
risk areas. Will continue to monitor this region for any trends in
model clustering.

An upper ridge aligned over the east-central U.S. Friday will
promote above normal temperatures in its vicinity. The biggest
anomalies of plus 10-20 degrees are forecast across the Midwest.
Meanwhile another round of chilly morning lows may cause
frost/freeze concerns for the Interior Northeast and Appalachians.
Northeastern temperatures will rebound during the weekend when
upper ridging moves overhead, bringing anomalies up to 5-10
degrees above normal. Then the ridge rebuilding over the central
into eastern U.S. early next week will lead to warmer than average
temperatures in the northern Plains to Upper Midwest once again,
with expanding coverage of plus 5-15 degree anomalies over the
East. On the other hand, the upper trough and frontal surges in the
West will cause below normal temperatures (by up to 10-20 degrees
for highs) to spread across the Great Basin/Intermountain West and
Southwest on Friday with slow moderation into the weekend. These
colder temperatures may also lead to additional frost/freeze
concerns for any remaining susceptible growing areas/valley
locations.


Rausch/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