248
FXUS02 KWBC 240703
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
303 AM EDT Tue Jun 24 2025

Valid 12Z Fri Jun 27 2025 - 12Z Tue Jul 01 2025

...Record heat is less likely by late week, but dangerous heat
continues for the Ohio Valley, Mid-South, and Mid-Atlantic/Carolinas...

...Overview...

Into late week, the peak of the significant and extremely
dangerous heat wave will have waned. However, temperatures will
remain quite hot across the Ohio Valley back toward the Mid-South
and east toward the Mid-Atlantic and Carolinas as some weaker upper
ridging persists. The long duration of the heat wave will cause
exacerbated impacts, maintaining Major to Extreme HeatRisk in some
of these areas into late week and the weekend. Meanwhile, moisture
will advect north and then east around the broad southeastern U.S.
ridge in a "ring of fire" setup. Better chances for flash flooding
are now into the short range period, but a wavy frontal boundary
interacting with the remaining above average moisture could allow
for localized heavy rain and flash flooding across the central
Appalachians to Northeast Friday-Saturday. Additional rounds of
scattered thunderstorms are possible across much of the central and
eastern U.S. into the weekend and early next week, some of which
could have low-end heavy rain and flooding potential.


...Guidance/Predictability Assessment...

Model guidance continues to be agreeable in showing mean ridging
across the southeastern U.S. late week (though weaker compared the
near term) and expanding west over the weekend and early next week.
This creates a more zonal pattern across the country with the jet
stream staying in the northern tier. This puts much more emphasis
on the speed, depth, and evolution of individual shortwaves, which
can be more difficult to nail down at extended time ranges. The
first shortwave and weak surface low atop the Great Lakes seems to
show reasonable consensus but any seemingly minor waffling of the
features would lead to sensible weather differences, like the bulk
of QPF moving across the northern U.S. or southern Canada. Then the
next broader, but still shallow, trough should be coming across
the Northwest Friday and reach the north-central U.S. by Sunday.
For this feature, the main outlier was the 18Z GFS, which was
slower to bring the trough east while western Canadian energy gets
involved. The other operational models, including the 12Z GFS and
the new 00Z GFS, were more agreeable. Various AI/ML models show
some spread though, so adjustments may need to be made in the
future. The flow pattern may amplify a bit into next week as that
trough shows a little digging. Meanwhile ridging is forecast to
build with its axis along the Rockies or so, while a southern
stream trough gradually approaches California while a northeastern
Pacific upper low may send (uncertain) shortwaves toward the
Northwest.

The WPC forecast was initially based on a composite blend of the
12Z deterministic guidance. By Day 5-7, introduced and gradually
increased the proportion of the EC and GEFS ensemble means to
around half to lessen the influence of individual model
variability.


...Weather/Hazards Highlights...

Moisture advection around the southeastern U.S. upper ridge
combined with shortwaves aloft and a wavy frontal boundary will
provide a favorable environment for convection for portions of the
northern tier into late week. An initial wave moving across the
Lower Great Lakes and Northeast Friday-Saturday will allow for some
possibly heavy rain in the Interior Northeast on the cusp of the
instability gradient and where a weak surface low tracks along the
front. Have Marginal Risks delineated in the Day 4/Friday and Day
5/Saturday EROs for localized flash flooding with this activity.
Farther south, showers and thunderstorms will become scattered to
widespread through much of the period across the eastern third of
the lower 48, mostly diurnally driven in the warm and unstable
airmass. There looks to be some thunderstorm focus stretching
across the central Appalachians especially Friday, so will
encompass that area within the Marginal Risk Day 4 ERO as well.
Will continue to monitor any other areas that show enhanced rain
totals that could trigger flash flood concerns. The central Gulf
Coast may see a few days of more focused convection, with heavier
amounts possible early next week (after the ERO period, and would
have to battle against high flash flood guidance to see flooding
concerns there).

Another shallow upper trough and frontal system will push through
the north-central U.S. late week into the weekend, providing
forcing for thunderstorms. Instability is likely to be high (MUCAPE
of 3000-5000 J/kg already shown by the global models...even higher
farther south, but where there is a drier atmosphere that could be
capped). Have Marginal Risks delineated in parts of the Dakotas
into the Upper Midwest on Friday and Saturday since locally heavy
rain rates are likely in this unstable atmosphere. Convection is
forecast to shift southeast across the Midwest and central
Plains/Middle Mississippi Valley early next week ahead of the
trough and front.

After a very wet short range period, support for heavy rain in New
Mexico will be waning by late week. However, any showers and
thunderstorms that form could be problematic for sensitive areas
like burn scars and terrain, especially considering the wet
antecedent conditions by then. Thus show a Marginal Risk for the
Sacramento Mountains and vicinity into Friday for isolated flash
flooding. Rain will continue to decrease in coverage and amounts on
Saturday. But into early next week, monsoonal moisture looks to
increase, bringing rain and thunderstorm chances to the southern
and central Rockies and High Plains.

The strong upper high/ridge stretching across the south-central to
eastern U.S. will be weaker by late week but remain generally in
place, maintaining hot temperatures albeit slightly moderated from
the short range period and less likely to break records.
Nevertheless, temperatures will be well into the 90s in many
central and eastern areas, while dewpoints in the 70s will raise
heat indices into the 100s. Overnight lows will only drop into the
low/mid 70s for many areas, bringing little relief from the heat
and exacerbating potential impacts. Especially after the extreme
heat in the short range, through late week HeatRisk values will be
Extreme for portions of the Ohio Valley, and Major from the Mid-
South into the southern Mid-Atlantic -- levels 3 and 4 on a scale
from 1 to 4 (4 being Extreme). This indicates an intensity and
duration of heat that is extremely dangerous to anyone without
adequate cooling or hydration. Extreme heat is the number 1
weather-related killer -- please take precautions if you are
outside during the hottest part of the day and seek cooling if you
are without adequate means. Into the weekend and early next week,
temperatures should slowly moderate in the east-central U.S. in
particular for closer to normal summer heat. Areas like the Mid-
Atlantic are expected to stay hot most days with generally Major
HeatRisk. Meanwhile the Northeast should see relief from the heat
by the medium range period behind a backdoor front, but could
gradually warm early next week. The north-central U.S. can expect
warmer than average temperatures by 5 to 10 degrees around Friday-
Saturday and then cool behind a cold front. Temperatures in the
Northwest will warm by early next week under an upper ridge. The
Desert Southwest can expect typical summer heat with temperatures
in the 100s and 110s.


Tate


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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