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FXUS02 KWBC 280700

PMDEPD



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





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