457
FXUS02 KWBC 170625
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
225 AM EDT Fri May 17 2024

Valid 12Z Mon May 20 2024 - 12Z Fri May 24 2024


...Overview...

An upper trough and surface low will be exiting the East Coast by
the start of the period, but may linger in the western Atlantic for
a day or two before getting pushed out to sea by the next system
behind it. Meanwhile, a shortwave will push into the northern
Plains/Upper Midwest early in the week along with a modest surface
reflection and potential for at least locally heavy convection
across the north- central U.S.. An upper low is forecast to drop
into the Northwest behind this system around midweek bringing
another cold front and unsettled weather to the West.


...Guidance/Predictability Assessment...

Recent model guidance is generally agreeable with the overall
pattern, but there are some notable differences in the details and
timing of individual systems. The latest CMC run continues to be an
outlier suggesting more energy hanging back over the Rockies
starting Tuesday- Wednesday as the first shortwave moves into the
Upper Midwest. The CMC is also much different late in the week
showing weak troughing, if anything, across the Northwest, while
the other operational models/ensembles show fairly good consistency
for a fairly deep upper low over the Northwest by Wednesday and
slowing edging eastward with time. Given this, the WPC forecast
blend for tonight used a non-CMC consensus. Increased weighting of
the ensemble means later in the period as timing and detail
uncertainties increased for both systems, but did maintain up to 50
percent of the deterministic GFS/ECMWF even late period just for
some added system definition. Overall, this maintained good
agreement with the previous WPC forecast as well.


...Weather/Hazards Highlights...

An upper shortwave and leading cold front pushing slowly through
the Plains/Upper Midwest early in the week will help fuel showers
and thunderstorms from the north-central Plains into the Great
Lakes, with potential for at least locally heavy rainfall given
ample anomalous moisture and instability present. Both the Days 4
and 5 (Monday-Tuesday) Excessive Rainfall Outlooks show just broad
marginal risks across this area, and in some places may be
dependent on activity towards the end of the short range period as
well. Farther West into the Rockies, some moderate snowfall may
fall in the higher elevations. After Tuesday, expect northeastward
progression of the surface low to push the trailing cold front and
accompany rain/storms farther east and south with some more
uncertain potential for heavy rainfall in the eastern and southern
U.S. for Wednesday-Thursday. Approach/arrival of another upper low
as currently advertised by midweek or so would produce somewhat
more organized precipitation across the Northwest, and potential
for heavy snow in the mountain, and snow levels dependent on the
depth of the upper low.

Expect South Texas to see multiple days of hazardous heat during
the period with highs persistently running 10-15F above normal with
max heat index values possibly reaching at least 110F. Highs near
or over 100 degrees could stretch farther north across the southern
High Plains at times as well. Some daily records for highs/warm
lows will be possible. Above normal highs will also track East
across the Midwest and Northeast Monday-Wednesday but should
moderate by Thursday as the cold front pushes through the region. The
forecast pattern will favor below average highs over the Northwest
to northern Plains for most of next week.


Santorelli


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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