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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0646Z Apr 22, 2024)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
246 AM EDT Mon Apr 22 2024

Valid 12Z Thu Apr 25 2024 - 12Z Mon Apr 29 2024

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


...Overview...

The upper level pattern will be fairly consistent through the
medium range period, featuring mean troughing over the western U.S.
and a ridge shifting over the East Coast. A couple shortwaves will
eject from the base of the trough in the West across the Central
U.S., which will trigger rounds of active, and potentially
hazardous, weather across portions of the Plains and Mississippi
Valley. Persistent troughing over the western U.S. will result in a
prolonged period of unsettled weather for the region with rain at
low elevations and snow at high elevations. Ridging over the East
Coast will keep the weather calm through most of the period, but a
cold front will move towards the Northeast over the weekend,
bringing precipitation back to the region.


...Guidance/Predictability Assessment...

The main features of interest will be the upper level shortwaves
that could potentially produce severe thunderstorms and heavy rain
in the Central U.S. late this week and again over the weekend.
Models are in good agreement on the timing and location of the
first shortwave, bringing it across the Southwest on Thursday and
across the Plains to the Upper Midwest on Friday. Model agreement
decreases further out in time, so there is more spread surrounding
solutions for the second shortwave over the weekend, though the
amount of spread is fairly typical for this time range. Most of the
spread derives from how the models are handling upstream energy
moving southeast from the Gulf of Alaska. The ECMWF is more
progressive with this feature while the GFS is a bit slower.

A pure deterministic blend was used for the first half of the WPC
forecast with a nearly even blend of the 18Z GFS and 12Z
ECMWF/CMC/UKMET. For the second half of the period, ensemble means
were introduced to the blend to help smooth out some of the model
differences, but the majority of the blend was still composed of
deterministic models to help keep some definition of the secondary
shortwave feature.


...Weather/Hazards Highlights...

Thursday and Friday, an upper level shortwave will move northeast
across the Central U.S. and will be accompanied by a surface low
pressure system. Low pressure will strengthen over the Central
Plains on Thursday, then the system will move towards the Upper
Midwest on Friday, pulling a warm front north across the south-
central and eastern U.S. and a trailing cold front from the Mid-
Mississippi Valley to the Southern Plains. 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
Marginal Risk area remains in place over the Central Plains and
Mid-Mississippi Valley in the vicinity of the developing surface
low in the Day 4 (Thursday) ERO. Precipitation will become more
widespread on Friday, and another Marginal Risk area was
introduced for much of the Mississippi Valley and eastern portions
of the Plains in the Day 5 (Friday) ERO.

In addition to heavy rainfall, severe thunderstorms will be
possible ahead of a 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 Thursday
and Friday. Isolated to scattered severe thunderstorms will be
possible, and severe storm threats may include 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, the second upper level shortwave will move
northeast across the Central U.S. and create a precipitation
pattern that will look very similar to the first system. A low will
strengthen over the Central/Southern Plains Saturday and move
towards the Upper Midwest Sunday into Monday, bringing another
round of showers and storms 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.

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 on
Sunday and Monday. 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
and Monday 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 could reach the 90s and potentially
over 100 degrees for portions of 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