449
FXUS02 KWBC 230702
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
302 AM EDT Sun Jun 23 2024

Valid 12Z Wed Jun 26 2024 - 12Z Sun Jun 30 2024

...Hazardous heat possible for southern/central portions of the
Plains, Mississippi Valley, and into the Southeast for parts of
next week eventually building back into the Midwest next weekend...


...Overview...

An upper level ridge will remain anchored over southwestern to
south-central parts of the nation through the week, leading to
multiple days of above normal to possibly hazardous heat. To the
north, one shortwave through the northern tier will amplify
somewhat as it crosses the Midwest/East mid- to late week. The next
system reaching the West Coast around Thursday should push a
defined surface low pressure system into the northern Plains by
late week. A third shortwave looks to enter the West again next
weekend.


...Guidance/Predictability Assessment...

The latest suite of guidance continues to offer overall good
agreement on the large scale, and typical uncertainty in the
details that could have impacts on sensible weather/QPF. Models
continue to offer better agreement on the first shortwave into the
Northeast late week. 12z/18z guidance runs showed some greater
timing differences with the next shortwave through the northern
U.S. Friday-Saturday, but new 00z runs have trended closer
together. Details on energy through south- central Canada
reinforcing this trough as it moves into the Great Lakes and
Northeast next weekend remain uncertain and would impact overall
timing. With the next system into the West on Sunday, the CMC was a
little flatter than the consensus, which resulted in less
ridging/heat downstream into the central U.S. and Midwest.

The WPC forecast for tonight used a blend of the GFS, ECMWF, CMC,
and UKMET for the first half of the period amidst sufficient model
agreement. Transitioned to 50 percent of the ensemble means in the
blend (with the ECMWF and GFS) late period to mitigate some of the
model differences which will take additional time to resolve.
Overall, this maintained good agreement with the previous WPC
forecast.


...Weather/Hazards Highlights...

A cold front moving into the East on Wednesday and copious
moisture and instability in a typical summertime convective pattern
will support widespread showers and thunderstorms from parts of
the Lower Mississippi Valley into the Northeast. There remains
enough model QPF discrepancies to preclude any sort of slight
risks, and the front should be fairly progressive as well, limiting
the overall threat. Still, given moisture and instability, a very
broad marginal risk seemed reasonable for the Day 4 Excessive
Rainfall Outlook.

Another defined system will reach the northern Plains by Thursday
bringing a renewed threat for heavy rainfall across the north-
central Plains to Upper Midwest, and a broad marginal risk was
included on the Day 5/Thursday ERO period to cover this. There was
enough agreement in the guidance to place a slight risk from far
eastern Nebraska, into Iowa, and far southern Minnesota - an area
with very wet antecedent conditions and with additional rainfall
expected in the short range period as well. A heavy rainfall threat
may emerge with this system farther east into the Great Lakes and
Northeast Friday and Saturday as well.

Anomalous moisture and sufficient instability should persist over
the Four Corners states, eventually expanding into the southern
Rockies and parts of the High Plains much of the period and
potentially lead to some locally heavy rainfall, which could cause
flooding impacts particularly over burn scars. Marginal Risks are
in the outlook for Day 4/Wednesday and Day 5/Thursday for this
region. Though perhaps not everywhere in the quite broad Marginal
risk area will see heavy rain, slow- moving convection is a concern
for these areas and warrants the risk depiction in the ERO.
Elsewhere, parts of the Southeast/Florida may also see episodes of
diurnally enhanced convection, but overall dry conditions/high FFGs
precluded any excessive rainfall risk areas at this time.

The focus for hazardous heat by mid-week will be across the south-
central Plains/Mississippi Valley and parts of the Southeast where
a long duration heat wave will be ongoing and heat indices near
110F for some leading to widespread major to extreme HeatRisk. The
heat across the Southeast should briefly moderate on Thursday with
troughing moving through, but the upper ridge will build back into
the region and northward into the Midwest bringing several days
again of above normal temperatures (both daytime highs and
overnight lows) from the southern Plains into the Midwest and parts
of the Southeast. Above normal heat is also possible across the
Southwest where temperatures 105-115F are likely through much 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








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