853
FXUS02 KWBC 150658
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Thu May 15 2025

Valid 12Z Sun May 18 2025 - 12Z Thu May 22 2025

***Heat builds across the southern Plains while late season snow is
 expected for the Rockies***


...General Overview...

A transition to a more amplified pattern is expected as a leading
upper low over the northeastern U.S. briefly deepens and then moves
out, a large-scale trough digs southward over the West, and a
ridge builds northward between those troughs over the
Plains and Midwest states. The most significant weather during the
period will likely be associated with the western upper trough and
its reflection at the surface as a frontal system moves through
the Intermountain West. This upper trough will induce lee
cyclogenesis over the central/southern Plains during the Sunday-
Monday timeframe as the system ejects eastward over the Plains.
The amplified, blocky pattern will also result in the slower
progression of the individual systems leading to multiple days of
precipitation over the western and then central U.S., as well as
lingering precipitation over the Northeast this weekend.

...Model Guidance/Predictability Assessment...

The 00Z model guidance suite today continues to offer very good
synoptic scale agreement for the upcoming weekend, with a multi-
deterministic model blend sufficing as a starting point in the
forecast process for Sunday into Monday, and slightly more
weighting applied to the ECMWF/GFS solutions. In general, the CMC
solution is a little less amplified than the other guidance with
the storm system ejecting east across the Plains, and this remains
the case going into next Thursday as the low pressure system
reaches the Ohio Valley. More in the way of model differences exist
with the next shortwave reaching the Pacific Northwest by Thursday,
with the ensemble means serving as a good starting point there.
The WPC forecast trended towards half ensemble means by the middle
to end of next week which showed better consistency and smoothed
out the smaller scale differences.

...Weather/Hazards Highlights...

The active weather pattern with the strong surface low developing
over the western High Plains will lead to increasing showers and
thunderstorms for a wide expanse from the Dakotas to the Midwest,
and extending across portions of Texas and Oklahoma. Prolonged
advection of Gulf moisture combined with an increasingly dynamic
late spring storm system will fuel the development of multiple
mesoscale convective complexes, and the greatest concentration of
these will likely be over portions of Missouri and Arkansas. A
Slight Risk area will be valid for the Day 4 period Sunday where
the highest confidence exists for excessive rainfall. Even though a
broad Marginal Risk area will be in place for Day 5/Monday, it is
very likely that portions of that outlook area will eventually need
a Slight Risk as the event gets closer. Severe thunderstorms are
also likely to develop across portions of the central Plains.

Elsewhere across the Continental U.S., expect additional late
season snow for the central and northern Rockies by Sunday and into
Monday, with the potential for over a foot of accumulation for
some of the highest mountain ranges of Wyoming and Colorado. For
the lower elevations, a Marginal Risk area for excessive rainfall
will be valid for both Sunday and Monday across portions of the
Four Corners region where slow moving convection associated with
the upper low may result in some localized flooding across the high
desert. Across the Northeast U.S., anomalous moisture combined
with low flash flood guidance values across parts of Maine allowed
for the continuation of a Marginal Risk area for the Day
4/Sunday ERO for this region owing to re-development of low
pressure offshore.

There will be a temperature dichotomy across the country Sunday
into the early part of next week with the amplified upper level
pattern in place. It will be quite chilly for this time of year
across much of the Intermountain West and the northern Rockies,
with highs 10-20 degrees below mid-May climatology. This also holds
true across much of Arizona as well, followed by a moderation trend
by Tuesday as the upper low moves out. In contrast, it will
continue to be quite hot across southern Texas with Heat Risk in
the major category most days, and it will also be hot and humid for
much of the Deep South and into Florida with highs reaching 90-95
degrees and plenty of humidity.

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













































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