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000
FXUS02 KWBC 241858
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Wed Apr 24 2024

Valid 12Z Sat Apr 27 2024 - 12Z Wed May 01 2024

***Severe thunderstorms and heavy rain expected for the Plains and
 Midwest over the weekend***


...General Overview...

An amplified upper level flow pattern will to be in place across
the Continental U.S. this weekend, with a building ridge across the
East and a trough across the West/High Plains. Behind a leading
Upper Midwest shortwave, another vigorous system will eject into
the Plains and continue the threat for heavy rain and strong to
severe thunderstorms, especially from central Texas to
Iowa/Missouri. A pronounced warming trend is likely for the
eastern half of the U.S. going into the beginning of next week, and
colder temperatures out West will support late season snowfall for
the higher terrain of the Central and Northern Rockies. The
unseasonable cold should extend into the Northern Plains for a day
or so as well. Expect a second upper trough to settle near the West
Coast by the middle of next week as an upper low now over the
Bering Sea approaches Vancouver Island or the Pacific Northwest.


...Guidance/Predictability Assessment...

Most dynamical models/ensemble means and ECMWF-initialized machine
learning models show better than average agreement with most
aspects of the forecast through the period. With timing
discrepancies for the Sunday Plains storm significantly reduced
from 24 hours ago, primary differences that remain are later in the
period and involve details of the upper trough nearing the West
Coast, leading energy, and some details over the Northeast. Latest
CMC runs produce most of these differences--being more closed with
upstream energy feeding into the East Pacific/West Coast upper
trough, more amplified with a leading shortwave tracking near the
Canadian border by midweek, and the 00Z run in particular showing a
suppressed surface pattern over the Northeast. The midweek West
Coast upper trough has trended a little deeper in consensus,
pulling back the western U.S. portion of the front trailing from
the system traversing the northern tier by Wednesday. Meanwhile,
individual ensemble members develop quite a bit of spread for the
character of the modest upper trough nearing the East Coast by the
middle of next week but averages of the dynamical/machine learning
models and the ensemble means have been similar and steady with the
depiction of this trough so far. The current array of guidance
supported an operational model composite (with greater GFS/ECMWF
emphasis) for the first half of the period and then a transition to
a model/mean blend along with eventual elimination of CMC input.


...Weather/Hazards Highlights...

The overall synoptic environment will be increasingly conducive for
episodes of excessive rainfall across portions of the
Central/Southern Plains and into portions of the Midwest states
over the weekend. There will likely be multiple mesoscale
convective systems that develop from central Texas to Missouri, and
there is a good chance of training convection given the slow
overall progression of the frontal boundary and nearly
unidirectional flow through the lower to middle troposphere. For
the Day 4 period Saturday, the Slight Risk area extending from
north-central Texas to around the Iowa-Missouri border has been
maintained from previous issuance. Potential continues for multiple
cases of MCS development within this area and rainfall rates may
reach two inches per hour with some storms. The core of the
heaviest rainfall should slowly progress eastward for the Day 5
period Sunday across the ArkLaTex region and into southern
Missouri, where a Slight Risk area persists given continued multi-
model signal for some 2-4 inch totals where storms train over the
same areas. There is a good chance that portions of both outlook
areas may eventually need a Moderate Risk in later forecast updates
if the QPF signal continues to remain robust, so this will
continue to be closely monitored. This potential may be enhanced by
wet conditions from rainfall forecast over portions of Plains and
vicinity prior to the Days 4-5 period. Meanwhile there is some
uncertainty over potentially greater northeastward extent of the
Day 4 Slight Risk area, as some solutions could argue for, but with
too much spread for specifics to make a confident adjustment at
this time. Beyond early Monday, deceleration of the front reaching
the Lower Mississippi Valley could continue the threat for locally
heavy rainfall over that region for another day before the front
weakens and eventually dissipates. The more progressive northern
part of the front should bring lesser rainfall to the Great Lakes
and Northeast during the first half of next week.

Elsewhere across the U.S., anticipate widespread moderate to
locally heavy rain from Minnesota to northern Michigan on Sunday
as the surface low lifts northeastward, and a Marginal Risk area
reflects this potential. Another area of enhanced rainfall is
probable over northeastern Colorado and portions of adjacent
states, where a Marginal Risk remains valid for Saturday. Colder
temperatures moving in across the Intermountain West and the
Rockies behind this storm system will likely result in several
inches of snow for the higher mountain ranges. Flow ahead of the
developing East Pacific/West Coast upper trough should support
multiple days of mostly light to moderate rain and mountain snow
over the Pacific Northwest from this weekend through the middle of
next week.

Temperatures will also be making weather headlines as most of the
country east of the Rockies will likely trend well above normal by
the weekend into early next week. Expect highs to rise well into
the 80s from the Central/Southern Plains (90s to around 100 on
Saturday over the typically hottest parts of far southern Texas) to
the Ohio Valley and then reaching the East Coast by Monday,
perhaps reaching 90 degrees in some cases. These readings are on
the order of 10-20 degrees above normal for late April, and there
may be some daily record highs set. Morning lows could reach 20-25
degrees above normal over parts of the Great Lakes with potential
for record warm lows. On the other hand, locations from the
Intermountain West into Northern Plains should see a day or so with
highs 5-15 degrees below average during the weekend. Next week`s
pattern will favor some moderation over the East (but keeping
temperatures somewhat above normal) while areas from the central
U.S. through Intermountain West trend 5-15 degrees above normal by
Tuesday-Wednesday. At that time, modestly below normal highs
should be confined to the Pacific Northwest due to the nearby upper
trough.

Rausch/Hamrick

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









$$