779
FXUS02 KWNH 280701
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Sun Apr 28 2024

Valid 12Z Wed May 01 2024 - 12Z Sun May 05 2024

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


...Overview...

Into midweek, mean troughing will amplify over the Intermountain
West and provide support for widespread convection over much of the
central U.S., with heavy rain and flash flooding likely Wednesday
and Thursday across much of the Plains and Mississippi Valley.
Upper troughing and a surface cold front pressing east into late
week should push rain chances into the east-central and eastern
U.S., though some may remain over the southern Plains near a
lingering frontal boundary. Additionally, precipitation including
some higher elevation snow is possible at times across the
Northwest into the northern/central Rockies, but confidence in the
details is still lower than desired.


...Guidance/Predictability Assessment...

Model guidance begins the period Wednesday in fairly good
agreement of the larger scale features. A somewhat elongated west-
east upper low should be centered in southern Saskatchewan and
Manitoba while additional troughing to its west should be
amplifying across the Intermountain West/Rockies. Meanwhile ridging
will be present for the eastern U.S. with the exception of a
shortwave moving out of the Mid-Atlantic Wednesday. With the
general agreement on this, a multi-model deterministic blend
favoring the GFS and ECMWF was used for the early part of the
medium range period.

Upstream, recent models are in somewhat better agreement on timing
with a shortwave or compact upper low reaching western North
America by Thursday, with just some minor positional differences
with its center being over Washington or British Columbia. This
small feature moves east with still a bit of spread into Friday and
possibly impacts the downstream flow. By Friday the larger trough
farther east should be pushing into the Plains, with model
variability mainly in its timing but some in its depth as well. The
12Z ECMWF was on the slower side with the trough axis moving east
but it did have support from some of the AI/machine learning (ML)
models. Now the incoming 00Z ECMWF is among the faster solutions
though. This will impact surface low and frontal timing as well.

Then farther west, a large upper low or trough is forecast to push
across the Pacific late week into this weekend, but significant
differences remain with its timing and strength. GFS runs have been
persistent in showing a potent closed low reaching the West Coast,
while the 12Z ECMWF was on the slower and weaker side just sending
a tongue of energy into the Northwest. The 12Z CMC and many of the
ML models were in between with the timing and depth. The 00Z CMC
and ECMWF happen to agree better, but the 00Z GFS is now west and
south of the previous GFS position. As expected, GEFS/EC/CMC
ensemble members show considerable spread for this feature.
Overall, with increasing spread among the model guidance,
transitioned the forecast blend to contain over half GEFS and EC
ensemble means by Days 6-7 to minimize the individual model
differences.


...Weather/Hazards Highlights...

On Wednesday, ingredients for widespread rain and thunderstorms
potentially causing flash flooding will be in place across central
parts of the country. Lowering heights as upper troughing digs in
the Intermountain West will provide lift and combine with ample
moisture and instability. On Wednesday model guidance already shows
heavy rain totals of 3-5 inches in some areas and high rain rates
are likely as well. A large Slight Risk remains in place for the
Day 4/Wednesday ERO for the central/southern Plains and stretching
into the Mid-Mississippi Valley. Within this risk area, there may
be somewhat greater potential from northeast Kansas/southeast
Nebraska into northwest Missouri and much of Iowa given proximity
to the stalling surface front. There remains a little more spread
farther south. Then for Thursday, another large Slight Risk serves
as a starting point for the Day 5 ERO just shifted a bit eastward
from Wednesday with a little progression of the surface front. Many
locations within these areas will be sensitive to additional rain
due to wet antecedent conditions, and potential for embedded
upgrades is possible depending on details of prior convection and
how guidance consolidates for amounts/location of heaviest
rainfall. By Friday, the cold front progressing east in the
northern two-thirds of the country or so will transition rain
chances more into the Great Lakes and Ohio/Tennessee Valleys, and
toward the Appalachians to Eastern Seaboard on Saturday. However,
the frontal boundary looks to linger longer across the the southern
Plains and produce additional rounds of rain and storms.

Meanwhile, areas from the Northwest into the northern and central
Rockies may see rounds of precipitation including snow in higher
elevations through the latter half of the week into the weekend. On
Wednesday-Thursday, the cooling associated with the upper trough
could bring some early May snow into the northern and central
Rockies. There is some chance that snow may make it into the Front
Range of Colorado on Thursday. There is still quite a bit of model
spread into late week, which affects the precipitation forecast.
But
the general consensus is for another round of precipitation in the
northern Rockies Friday, while at some point late Friday or into
Saturday precipitation is renewed for the Northwest.

Temperatures across the south-central U.S. all the way to the
Eastern Seaboard will be warmer than average by generally 10-15F
on 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. on
Thursday or Friday and reaching the east-central U.S. Friday and
Saturday. Temperatures over the West may rebound to near normal or
moderately above normal levels by late week into next weekend, but
confidence remains lower than average given continued guidance
spread for upper flow details.


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


$$