FXUS02 KWBC 180702
PMDEPD

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














$$