Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
302 AM EDT Sat May 18 2024

Valid 12Z Tue May 21 2024 - 12Z Sat May 25 2024


...Overview...

A fairly progressive pattern looks to evolve across the CONUS
during the medium range period, with much of the activity focused
across the West to north-central Plains and Upper Midwest. Early
next week, a potent shortwave will send a modestly deep surface low
into the Upper Midwest and potential for at least locally heavy
convection across the region. It's attendant cold front will push
across the Eastern U.S. midweek as the next system (possible closed
upper low) reloads into the Northwest and tracks eastward (with
much more uncertainty) into the northern Plains/Upper Midwest.
Another shortwave/possible closed low may drop into the Northwest
again late period.


...Guidance/Predictability Assessment...

Models show reasonable agreement with the first shortwave that
kicks out into the Upper Midwest Tuesday-Wednesday, though with
some uncertainty in the timing and depth of the surface low. The
first upper low into the Northwest shows a lot more uncertainty
even as early as Day 5/Thursday. The GFS is the deepest/slowest
with it as it progresses eastward through the north-central U.S.,
owing in part to weaker shortwave energy reloading the Western
trough later in the week. The ECMWF and CMC suggest two separate
upper lows into the Northwest (the first Wednesday/Thursday, the
next Friday/Saturday) and has some support but there is a lot of
spread in the details and timing of this per the latest ensembles
and ECMWF- initialized Machine Learning models.

The WPC forecast for tonight used a blend of the deterministic
models for the first few days of the period, incorporating the
ensemble means for the latter half of the period amidst plenty of
spread. Used some ECMWF contributions late period for some added
system definition. Overall, this maintained reasonable agreement
with the previous WPC forecast.


...Weather/Hazards Highlights...

An upper shortwave and leading cold front, with deepening surface
low, will push into the Upper Midwest Tuesday/Wednesday, helping to
fuel showers and thunderstorms across this region, with potential
for at least locally heavy rainfall given ample anomalous moisture
and instability present. The Day 4 Excessive Rainfall Outlook
continues to depict a broad marginal risk along the surface low
track across the Upper Mississippi Valley/Great Lakes and attendant
cold front southward into the middle Mississippi Valley. Models
show fairly good agreement for an area of higher QPF near the
surface low, but given somewhat drier antecedent conditions,
uncertainties in moisture/instability on the backside of the low,
and collaboration with the affected WFOs, opted to hold off on any
sort of upgrade to a slight risk at this time. There is also
potential for severe weather along the cold front. After Tuesday,
expect northeastward progression of the surface low to push the
trailing cold front and accompanying rain/storms farther east and
south with some more uncertain potential for heavy rainfall in the
eastern and southern U.S. for Wednesday-Thursday. There should be
ample moisture and instability present along the cold front so did
go ahead and introduce a marginal risk along the boundary for the
Day 5 ERO period from northeast Texas into the Ohio Valley.
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
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 into 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