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FXUS02 KWBC 150659

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Mon Apr 15 2024

Valid 12Z Thu Apr 18 2024 - 12Z Mon Apr 22 2024


A low pressure system exiting the Great Lakes on Thursday will
continue to bring some showers and storms across parts of the East.
Behind this, another upper low will skirt the northern tier states
eventually dragging some troughing through the Ohio Valley and
Northeast this weekend/next week. This may allow a secondary cold
front to briefly stall across the Central Plains-MS Valley region,
and aided by weak shortwave energy through the Southwest/Southern
Plains, may help to enhance precipitation across the ArkLaTex
region on Saturday. The next system should move into the Northwest
late weekend, with a very uncertain evolution still. This should
push upper ridging from the West this weekend into the Central U.S.
early next week.

...Guidance/Predictability Assessment...

Model guidance remains reasonably agreeable on the large scale at
least the first half of the period, though some differences in the
smaller scale details. A second broader upper low across south-
central to southeastern Canada continues to show some variation in
timing of shortwaves rounding its base into Ohio Valley/Northeast,
but models have overall trended toward less amplification of
troughing across the East late period, and stronger ridging across
the Southeast U.S.. Later in the period, yesterdays 12z/18z
guidance showed some significant variation in energy entering the
Northwest late weekend, and eventual tracking eastward. The ECMWF
was notably weaker/faster than other guidance with this energy,
while the GFS/CMC suggested a closed low would hang back over the
Northwest early next week. There wasn't as much support for the
GFS/CMC closed low from the ECMWF-initialized AI/ML models, and the
ensembles means were extremely washed out suggested a lot of
uncertainty. The new 00z runs tonight (available after forecaster
generation time) of the GFS and CMC have trended much faster with 
the system and timing wise, are in better agreement with the ECMWF
(just much stronger). 

The WPC forecast tonight utilized a blend of the deterministic
guidance for days 3-4 (Thursday-Friday), but trended more towards
the ensemble means later in the period (with some ECMWF) to account
for significant uncertainty in the western U.S. system.

...Weather/Hazards Highlights... 

Some generally light precipitation is expected as the period begins
on Thursday associated with an initial cold front. The next system
though right on its heels will renew rain and convection across the
East late week, while lingering across the south-central U.S. as 
the frontal system stalls briefly. Given anomalous moisture and
instability present, felt a low-end marginal risk was warranted 
from eastern Oklahoma into southern Missouri where storm training 
seemed most likely with some higher rain rate potential. This area 
however has been rather dry as of late, which should inhibit the 
overall flash flood threat. Weak shortwave energy into the Southern
Plains should help to further enhance precipitation across Texas on
Saturday with increasing moderate to heavy rain potential. After
this, some precipitation should move into the Southeast as the
Northeast dries out, and depending on upper pattern evolution,
precipitation should return to the Northwest late week/early next

Expect a broad area of above normal temperatures by 5-15 degrees 
over the southern half of the country into Thursday, while the 
northeastern quadrant of the U.S. gradually cools behind a cold 
front. Meanwhile the upper trough across the northern tier states 
will bring below normal temperatures (with greater anomalies for 
highs than lows) starting in the northern Rockies/Plains midweek. 
Cooler temperatures gradually expand farther south and east behind 
the secondary cold front late week into the weekend, with near to 
below average temperatures expected to reach everywhere but Florida
by next weekend. Meanwhile, above normal temperatures across the 
Southwest should gradually expand northward underneath a building 
Western U.S. ridge and progress eastward into the Rockies early
next week.


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:

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: