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000
FXUS02 KWBC 230643
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
243 AM EDT Tue Apr 23 2024

Valid 12Z Fri Apr 26 2024 - 12Z Tue Apr 30 2024

...Severe thunderstorms and heavy rain possible for the Plains and
Mississippi Valley late this week through this weekend...


...Overview...

The upper level pattern through this weekend will feature mean
troughing over the western U.S. while a ridge builds into the East
Coast. This pattern will result in unsettled weather in the West
and fairly benign weather for the East Coast. A couple of potent
shortwaves/embedded upper lows will travel through the base of the
western trough and eject northeastwards across the Central U.S.
towards the Upper Midwest/Great Lakes, which will trigger rounds of
active, and potentially hazardous, weather across portions of the
Plains and Mississippi Valley. Early next week, a deep upper low
will slide south along the western coast of Canada towards the
Pacific Northwest, and the ridge that was situated over the East
Coast will shift offshore into the Atlantic.


...Guidance/Predictability Assessment...

Model guidance continues to show good agreement on the overall
upper level pattern, but there is still some disagreement on finer
scale details, which is to be expected in the medium range time
period. The main features of interest will be the two upper level
shortwaves that are forecast to bring impactful weather to the
Central U.S. through the weekend. Agreement is high on the timing
and location of the first shortwave Friday into Saturday, but model
solutions show a bit more divergence with the second shortwave
Sunday into Monday. The 12Z ECMWF is the quickest with the second
wave, the 12Z UKMET is the slowest, and the 18Z GFS and 12Z CMC
fall in between, closer to the ensemble means. A blend of the
deterministic GFS/ECMWF/CMC/UKMET was used for the WPC forecast for
Friday through Sunday to get a middle of the road solution close
to the GFS and ensemble mean solutions. The 00Z GFS and ECMWF were
not available at the time that the forecast was made; however,
they remain consistent with the trends shown in their previous
runs.

After Sunday, there are some more significant differences in how
the models handle the upper low moving down the West Coast, which
has an impact on the evolution of downstream features as well. One
way to mitigate these differences was to add ensemble means to the
WPC blend to smooth out some of the differences. The CMC seemed to
be an outlier compared to the other available guidance, so it was
removed from the WPC model blend for days 6 and 7 (Monday and
Tuesday).


...Weather/Hazards Highlights...

Late this week, an upper level shortwave will move northeast
across the Central U.S. and will be accompanied by a surface low
pressure system. The surface low will move across the Central
Plains and Upper Midwest on Friday, lifting a warm front north
across the South-Central U.S. and pushing a cold front across the
Central/Southern Plains and Middle/Upper Mississippi Valley. This
system is forecast to produce showers and thunderstorms over
portions of the Plains and Mississippi Valley, and the ingredients
(instability and moisture) will be in place for locally heavy
rainfall totals. A broad Marginal Risk area remains in place for
portions of the Plains and Middle/Upper Mississippi Valley in the
Day 5 (Friday) ERO. The southern tail of the trailing cold front is
forecast to stall across the Southern Plains on Saturday while
another surface low develops over the Southern/Central Plains,
which will focus another round of heavy precipitation across the
Southern Plains and Mid-Mississippi Valley. A Marginal Risk area
was introduced for these regions in the Day 5 (Saturday) ERO.

In addition to heavy rainfall, severe thunderstorms will be
possible ahead of the dryline in the warm sector of this system,
and the Storm Prediction Center has highlighted portions of the
Plains and Mississippi Valley in the Severe Weather Outlook for
Friday and Saturday. Isolated to scattered severe thunderstorms
will be possible and may bring a threat of damaging winds, large
hail, and tornadoes. On the western side of the dryline, strong
winds and low humidity will create and increased wildfire risk for
portions of New Mexico, west Texas, the Texas and Oklahoma
panhandles, and far southeastern Colorado.

Over the weekend, a second upper level shortwave will develop over
the Southwest then push across the Central U.S. towards the Upper
Midwest early next week. This system will bring another round of
showers and potentially severe thunderstorms to portions of the
Plains and much of the Mississippi Valley. Precipitation chances
will also spread into the Ohio Valley and Northeast over the
weekend as the leading system moves over the top of the East Coast
ridge. By Monday-Tuesday, the East Coast ridge will move offshore
into the Atlantic, and the trailing cold front associated with the
second low pressure system will bring precipitation chances to the
Gulf and East Coasts.

In the West, mean upper troughing will allow precipitation chances
to persist across much of the region through the end of this week,
but drier conditions will likely develop over the Southwest early
next week. Precipitation will fall as rain at lower elevations and
as snow at higher elevations. Daily high temperatures will likely
remain slightly below normal across the West, but highs may return
to near normal in the Southwest Sunday through Tuesday after
precipitation comes to an end.

Temperatures will trend above normal across much of the south-
central and eastern U.S. as a warm front lifts north later this
week, and high temperatures will likely reach the 70s and 80s for
much of these regions. Highs will likely reach the 90s and
potentially over 100 degrees for some locations in south Texas by
Saturday.


Dolan


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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