495
FXUS02 KWNH 210701
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Tue May 21 2024

Valid 12Z Fri May 24 2024 - 12Z Tue May 28 2024


...Heavy Rainfall/Flooding Threats through the Mid-South and
vicinity through the weekend...

...South Texas to Central Gulf Coast and South Florida Heat Risk...


...Overview...

Multiple and generally progressive upper shortwaves/closed lows
and surface low pressure/frontal systems will traverse the West to
the Plains and Midwest during the medium range period. The first
will be moving through the Northern Plains by Friday, eventually
lifting into south-central Canada next weekend. Meanwhile, the next
upper low will drop into the Northwest next weekend, moving in an
eastward direction into early next week. A building ridge behind
this over the Western U.S. early next week will allow this system
to dig more into the Midwest than previous systems. Multiple
surface lows/fronts will track with these systems across the CONUS
with some recurring threat for heavy rainfall mainly across the
mid-South/Tennessee Valley to parts of the Mid-Atlantic as fronts
tend to stall in that region. Strong upper ridging will also build
into South Texas and the central Gulf Coast to spread a continued
threat for hazardous heat as indicated in the WPC and CPC Hazards
Outlooks.


...Guidance/Predictability Assessment...

The latest models and ensembles continue to show very good
agreement on the overall pattern across the CONUS during the medium
range period, but with plenty of uncertainty in the details. There
was good enough agreement the first half of the period to use a
blend of the deterministic models as a starting point for WPCs
forecast. After the weekend, there is some growing spread in the
exact track and speed of the second upper low through the Northern
Plains/Midwest although the ensemble means are quite a bit weaker
with the system. Given there seems to be good agreement for a
deeper surface low (including in the various ML/AI models),
maintained more deterministic models in the blend than usual late
period, ending up with a 60/40 split deterministic/ensemble means.
Also some uncertainty early next week on various shortwaves
rotating through a deep upper low parked off British Columbia which
may skirt the Pacific Northwest, but very good consensus for an
amplifying upper ridge over the interior West and a strong upper
high reaching into south Texas and the central Gulf Coast.


...Weather/Hazards Highlights...

A slow moving/stalled front across the Mid-South/Tennessee Valley
into the southern Mid-Atlantic will be the main focus for
convection late week. A marginal risk is in place across this
region for the Day 4/Friday Excessive Rainfall Outlook. A southern
stream shortwave into the Southern Plains next weekend should
ignite more heavy rainfall potential into the Mid-Mississippi
Valley where a marginal risk is highlighted for the Day 5 ERO. This
looks to continue and possibly get more widespread for Sunday into
the Tennessee Valley.

Northern stream energy will keep conditions showery and generally
unsettled across the northern tier into the Great Lakes during the
period. A more amplified pattern early next week should support a
strong cold front into the East with showers and thunderstorms
ahead of it across the East and into parts of the South as well.

Expect a hazardous heat threat to expand from South Texas through
the Central Gulf Coast and also South Florida 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. 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. Above normal
temperatures will build again across the West early next week
underneath of amplified upper ridging.

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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