284
FXUS02 KWNH 190706
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
306 AM 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. 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 though. 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 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 with plenty of lingering uncertainty late week as it moves
into the Upper Midwest. Models continue to struggle with its
evolution and possible combination of energy with another shortwave
across central Canada at the same time. The GFS and ECMWF (and now
the new 00z CMC) show consolidation of this energy into one big
low over south-central Canada next Saturday. However, the ECMWF-
initialized ML models show more support for stream separation. So
lots of questions on the evolution and the ensemble means are very
washed out with this energy.

The WPC forecast for tonight used a blend of the deterministic
solutions the first half of the period amidst better agreement. By
late week and especially next weekend, continued to maintain more
of the deterministic solutions than usual along with the ensemble
means to help smooth out the detail differences but still maintain
system definition. This late period blend also serves well for yet
another upper low that drops into the Northwest next Saturday.


...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 risk from northeast Texas into
the lower Ohio Valley as the western portion of the boundary stalls
and eventually begins to lift north as a warm front. This will
continue the heavy rainfall threat into Thursday as well across
parts of the southern Plains to lower Mississippi Valley - a region
that has been very late as of late. Given the overlap too with Day
4, a slight risk was introduced from northeast Texas into
southeast Oklahoma and central Arkansas with a broader marginal
risk surrounding. Approach/arrival of another couple of systems as
currently advertised into the Northwest by midweek and next weekend
would produce somewhat more organized precipitation there, and
potential for heavy snow in the mountains, with snow levels
dependent on the depth of the upper low(s).

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.

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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