311
FXUS02 KWNH 200652
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
251 AM EDT Mon May 20 2024

Valid 12Z Thu May 23 2024 - 12Z Mon May 27 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 Thursday, a potent shortwave and deep surface low will be
exiting the Upper Great Lakes 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 Tennessee
Valley and parts of the Mid-Atlantic as the boundary stalls and
lifts back north as a warm front. Additional energy/closed lows
look to traverse the northern tier late week and next weekend, but
with more uncertainty as strong upper ridging builds over Mexico
into South Texas with a continued threat for hazardous heat.


...Guidance/Predictability Assessment...

The greatest uncertainty during the medium range period lies with
a pair of upper lows through the northern tier. The first will be
in place over the Northwest by the start of the period on Thursday
but by Friday begins to show some timing uncertainties. The past
few runs of the GFS have been faster with this system while the
remainder of the deterministic solutions and ensembles are slower
through the Northern Plains and Upper Midwest. What is interesting
is that while most of the deterministic and ensemble guidance shows
good agreement on the progression of this feature, did notice the
ECMWF-initialized ML models keep this as a deep closed low farther
south across the Upper Midwest/Great Lakes rather than lift it into
south-central Canada around Saturday. Prefer a blend towards the
deterministic/ensemble solutions for now.

The next upper low drops into the Northwest next weekend and with
a lot of spread in the timing and progression of this system both
with the deterministic, ensembles, and ML/AI models. The 12z 5/19
run of the CMC was a much weaker outlier but the new 00z run did
trend closer to the GFS and ECMWF which show some deepening closed
low development over the Northern Plains Sunday-Monday. Significant
timing uncertainties and how far south some of the energy may
reach and also, whether some sort of southern stream energy out of
the Southwest may try to combine or stay separate with the northern
feature.

Given enough agreement, was able to use a blend of the
deterministic solutions (anchored by the ECMWF) for the first half
of the period. After this, increased the ensemble means to try and
cut down on the noise, while also maintaining some of the
deterministic GFS and ECMWF for added system definition. Overall,
good continuity with the previous WPC forecast.


...Weather/Hazards Highlights...

Main focus for late week convection will be from parts of the
southern Plains into the Lower Ohio Valley as the western part of a
frontal boundary stalls/lifts north ahead of the next upper low
into the region. The Day 4/Thursday Excessive Rainfall Outlook
continues to depict a fairly large slight risk area from northeast
Texas through Arkansas where ample moisture/instability should be
present within a very recently wet region. Heavy rainfall potential
across this region is also possible at the end of the short range
period/Wednesday. By Friday, some heavy rain/flood potential will
be possible farther north and east across the Lower Ohio
Valley/Appalachians/southern Mid-Atlantic where a marginal risk is
in place for the Day 5 ERO.

The Upper Low over the Northwest/northern Rockies on Thursday may
also bring some heavier rain to parts of the northern High Plains,
but with an overall lack of instability and anomalous moisture,
combined with dryer antecedent conditions, not anticipating a flash
flood risk at this time, despite some 1-2 inch QPF totals for
central-eastern Montana. Showers and storms should progress with
the accompanying cold front late week into the weekend through the
Midwest. An additional heavy rainfall threat may develop farther
south across the Middle Mississippi Valley region late this weekend
or early next week depending on still very uncertain upper level
dynamics and stream energies.

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 later in the period as an upper high begins to build more
into the region. Some daily records for highs/warm lows will be
possible. Above normal highs will also track east into the
Northeast/Mid- Atlantic on 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 but relatively normal temperatures
everywhere else.

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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