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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1856Z May 19, 2024)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Sun May 19 2024

Valid 12Z Wed May 22 2024 - 12Z Sun May 26 2024


...Overview...

Multiple progressive upper shortwaves/possible closed lows and
surface low pressure/frontal systems will traverse the West to
northern/central Plains and Midwest during the medium range period.
By Wednesday, a potent shortwave and deep surface low will be
moving through the Upper Midwest while its attendant cold front
will push across the East and partway into the South beyond
midweek, with rain and storms along and ahead of it and cooler
temperatures behind. There is some recurring threat for heavy
rainfall along the western side of this front from parts of Texas
into the Lower Mississippi Valley as the boundary stalls and lifts
back north as a warm front. Farther south, heat will continue to be
hazardous for at least southern Texas. Another couple rounds of
energy may push through the Northwest and eastward late week and
next weekend, but with more uncertainty.


...Guidance/Predictability Assessment...

Models show reasonable agreement with the first shortwave and
surface low in the Upper Midwest at the start of the period on
Wednesday, although the past few runs of the GFS have been slightly
faster with the system as it moves into southeast Canada. Still, a
general model blend seems to serve as a good starting point.

The next upper low should arrive into the Northwest by Wednesday
as well, but models have struggled with its ejection eastward into
the north-central U.S. late week. WPC's preference has been for an
upper low or at least strong shortwave to persist in the Dakotas
into Friday, consistent with the majority of the
deterministic/ensemble/AI guidance. For the 00Z/06Z cycle, that
eliminated the GFS runs as they sheared out the shortwave and had
the associated surface low farther northeast of other guidance,
even though the GFS runs from a day ago showed the feature. The new
12Z GFS seems in slightly better alignment though may be a little
fast still. Given the model waffling, there is fairly low
confidence in the details, especially in precipitation forced by
the features. Then upstream, guidance generally has shown an upper
low dropping south into the Northwest late week into the weekend,
other than a few rogue runs with weaker energy (like the 06Z GFS).
A model (excluding the GFS)/ensemble blend worked to handle this
feature.

Thus the WPC forecast used a blend of the deterministic guidance
to start the period, but removed the GFS by Friday due to its
north-central U.S. and then northwestern U.S. issues, in favor of
the GEFS and EC ensemble means. Increased the proportion of the
ensemble means to just over half by the end of the period.


...Weather/Hazards Highlights...

The cold front associated with a deep surface low in place across
the Upper Midwest on Wednesday will be the main focus for showers
and thunderstorms from the southern Plains into the Ohio Valley on
Wednesday. The Day 4/Wednesday ERO shows a broad and elongated
Marginal Risk stretching from the southern Plains to the Ohio
Valley. There is an embedded Slight from northeast Texas into
southern Illinois and vicinity as the western portion of the
boundary stalls and eventually begins to lift north as a warm front
in a moist and unstable environment. A similar setup is expected
on Thursday, so a Marginal to embedded Slight Risk remains in place
for Day 5/Thursday. Rainfall totals may decrease a bit by Thursday
but still could cause flash flooding issues after a wet Wednesday.
Some areas (with the exception of the southern half of Arkansas)
have wet antecedent conditions because of multiple heavy rainfall
episodes in the previous few weeks, so these may be particularly
vulnerable.

Higher elevations of the northern Rockies can expect several
inches of accumulating snow Wednesday-Thursday underneath an upper
low, with snow levels depending on the low's depth. Rain in lower
elevations of Montana may be locally heavy. Then rain will push
into the Plains and Mississippi Valley for late week into the
weekend. Amounts and positioning of heaviest rain are quite
uncertain but there may be a general focus in parts of the
Upper/Middle Mississippi Valley, with scattered convection
elsewhere.

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
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 into the
Northeast/Mid-Atlantic Wednesday-Thursday but should moderate
thereafter 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.


Tate/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