Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
413 AM EDT Sat Apr 27 2024

Valid 12Z Tue Apr 30 2024 - 12Z Sat May 04 2024

...Rounds of convection are likely in the central U.S. for much of
next week...


...Overview...

As the medium range period begins Tuesday, quasi-zonal upper flow
will be in place over much of the lower 48, but embedded shortwaves
could push frontal systems through the central and eastern U.S. and
cause some showers and thunderstorms. Mean troughing is forecast to
amplify in the Intermountain West Wednesday-Thursday and provide
support for increasing convection over much of the central U.S.,
with heavy rain and flash flooding possible hazards. A gradual
shift east with the convection is likely into late week.
Precipitation including some higher elevation snow is possible
across the Northwest into the northern/central Rockies, but with
rather low predictability.


...Guidance/Predictability Assessment...

Recent model guidance is fortunately in better agreement than a
day ago regarding energy across the northwestern U.S./British
Columbia on Tuesday. There is a general theme of some energy
developing a closed low and pressing east near the U.S./Canadian
border Wednesday-Thursday while secondary energy causes a trough to
dig into the Intermountain West. Operational models and AI/machine
learning models show some variability in how separate or phased
these features may be, but generally within typical spread for the
medium range. There is also some model spread in how deep the
trough may dig in the Intermountain West and pushing toward the
Plains by Friday. The 18Z GFS was quite aggressive with a
deeper/neutrally (rather than positively) tilted trough and a
resultant surface low track farther west compared to other
guidance. Thus for these features, preferred a model blend of the
12Z ECMWF/GFS/CMC. The newer 00Z GFS looked more in line with other
guidance.

Upstream, there are even larger model differences across the
northwestern U.S. by Thursday that lead to dynamical
models/ensembles/ML models showing out of phase solutions. Early
Thursday, most 12/18Z deterministic models indicated a ridge over
the West Coast while they held an upper low farther north into
western Canada compared to previous runs. Most of the 12Z ML models
also supported this ridge solution. On the other hand, the
ensemble means persisted in indicating troughing atop the Northwest
more similar to the previous runs. Individual ensemble members
show ample spread. Having to choose a camp, preferred the
deterministic and ML type solutions with ridging over the
Northwest, thinking that the ensemble means were lagging the trend.
The WPC model blend maintained more deterministic components than
normal for the latter part of the period, especially the ECMWF.
However, this may have been the wrong choice given the incoming 00Z
models generally show troughing (though think the 00Z CMC may be
too aggressive with its compact closed low). There may need to be
some changes made with frontal and precipitation timing in future
forecasts because of this. By Friday-Saturday, there are additional
uncertainties with northern Pacific and Canadian flow affecting
the West. Thus predictability and confidence are somewhat low for
the pattern in the western and central U.S. after midweek.


...Weather/Hazards Highlights...

On Tuesday, lessening upper-level support should lead to lower
rainfall amounts and rainfall rates in the central U.S. compared
earlier and later in the week. However, some convection is still
likely to occur in a moist and unstable environment. Currently the
best chance for rain with isolated flooding potential is across the
south-central Plains to Mid-Mississippi Valley as a front stalls.
Within that region there is some spread in location of locally
heavy convective potential--GFS runs have been persistent in
showing convection near the Red River while other models indicate
local maxima farther north. A Day 4 Marginal Risk is in place in
the ERO to cover these areas and will be refined with time.
However, rainfall amounts have trended down across the board
farther south in Texas and Louisiana, so a Marginal Risk that was
in place there on what is now Day 4 was removed. By Wednesday, the
upper flow (with troughing to the West and a southern stream
shortwave) will become more supportive of more widespread heavy
rain and thunderstorms for the central/southern Plains and
Mid/Lower Mississippi Valley as moisture surges ahead of a wavy
frontal system. Once again there is spread in the exact position of
the heaviest totals, but rain amounts of 3-5 inches are likely,
and some areas of heavy rain are likely to overlap with
current/short range heavy rainfall locations. A large Slight Risk
serves as a starting point for the Day 5/Wednesday ERO especially
given the likely sensitivity of these areas due to wet antecedent
conditions. Expect a slow trend of the rain/thunderstorms eastward
Thursday and Friday as the front presses east.

Elsewhere, one or more shortwaves and upper lows will lead to
rounds of precipitation across the northern tier. On Tuesday, ample
moisture with precipitable water values over the 90th or even 95th
percentile will be in place across the northeastern U.S., with
rain and thunderstorms likely. A shortwave aloft moving through and
some instability in place could lead to heavy rainfall causing
isolated flooding concerns, so added a Marginal Risk centered in
upstate New York with its footprint stretching a bit into the
northern Mid-Atlantic and western New England on Day 4/Tuesday.
Meanwhile the Northwest and into the northern and central Rockies
could get rounds of precipitation through the week, including
higher elevation snow. Some guidance suggests the potential for a
period of enhanced northern Rockies/Plains precipitation that will
be worth monitoring. Timing and amounts of precipitation are
dependent on the eventual uncertain evolution of the upper and
surface pattern.


Temperatures across the south-central U.S. all the way to the
Eastern Seaboard will be warmer than average by generally 10-15F on
Tuesday and Wednesday, while cooler than normal temperatures are
likely for the northwestern quadrant of the lower 48. Then upper
troughing and a cold front pushing from the Northwest should cool
temperatures to near or below normal across the central U.S. by
Thursday and reaching the east-central U.S. Friday and Saturday.
There is a possibility of a warmup across the West late week, but
the guidance spread is still yielding lower than average confidence
in the temperature forecast there.


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