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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1858Z May 21, 2024)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Tue May 21 2024

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


...Heavy Rainfall/Flooding Threats for the Mid-Mississippi,
Tennessee, and Ohio Valleys through the weekend...

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


...Overview...

Multiple upper level lows/shortwaves will parade across the
continental United Stated during the medium range period. The first
closed low will swing across the Upper Midwest on Friday and
deepen north of the Great Lakes over the weekend. The next low will
drop into the Pacific Northwest over the weekend and dig east
towards the Midwest early next week while a ridge builds over the
West. At the surface, multiple frontal systems will accompany the
upper level lows, creating an unsettled pattern for much of the
central and eastern United States. A couple of additional southern
stream shortwaves will move from the southern/central Plains to
the Mid- Atlantic ahead of the upper lows, providing support for
multiple rounds of heavy rainfall/flooding for portions of the Mid-
Mississippi, Tennessee, and Ohio Valleys. A strong upper ridge
will remain in place through the period across Mexico, the Gulf,
South Texas, and Florida, which will result in dangerously high
heat index values each day and anomalously warm nighttime lows.


...Guidance/Predictability Assessment...

Models remain in good agreement on the broad upper level pattern,
but there are some differences in how the models handle
interactions between upper lows/shortwaves, especially during the
second half of the period. The 00Z guidance suite was showing a
trend towards a faster moving southern stream shortwave ahead of
the second closed low, which would limit the interaction between
the two features and may result in a weaker upper level low. A
general blend of the deterministic GFS/ECMWF/CMC/UKMET seemed fine
for the first half of the forecast period, but ensemble means from
the GEFS and ECENS were added to the blend for the second half of
the period to mitigate some of the model differences. Even with the
ensemble means added to the blend, deterministic models still made
up 50-70% of the blend in order to retain some of the smaller
scale details.


...Weather/Hazards Highlights...

A slow moving front across the Middle/Lower Mississippi and
Tennessee Valleys and southern Mid-Atlantic will become a focus for
convection late this week. A Marginal Risk of Excessive Rainfall
(level 1/4) is in place across this region for the Day 4/Friday
Excessive Rainfall Outlook. A separate low pressure system will
also be moving towards the Upper Midwest on Friday, and there will
likely be enough moisture and instability to support isolated areas
of heavy rainfall in this region. For this reason, a second
Marginal Risk area was introduces for portions of the Upper Midwest
on Friday. A southern stream shortwave will likely ignite more
heavy rainfall potential over the weekend in the Mid-Mississippi
and Tennessee Valleys, and a Marginal Risk is in place for this
region in the Day 5/Saturday ERO. Heavy rain looks to continue and
possibly become more widespread and expand further in the Tennessee
and Ohio Valleys on Sunday.

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 hazardous heat to expand from South Texas through the
Central Gulf Coast and South Florida during the period with highs
persistently running 10-15 degrees above normal with max heat index
values possibly reaching 115 degrees. 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.


Dolan/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