145
FXUS02 KWNH 250658
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 AM EDT Wed Jun 25 2025

Valid 12Z Sat Jun 28 2025 - 12Z Wed Jul 02 2025

...Overview...

This weekend, quasi-zonal flow is likely across the lower 48 while
the main flow pattern sets up along the northern tier. These
northern shortwaves will support fronts and rounds of possibly
heavy rain across the Northeast Saturday and in the north-central
U.S. Saturday-Sunday. Meanwhile continued above normal 500mb
heights, though much weaker than in the near term, could cause
lingering above average temperatures in the Midwest to Carolinas
and possibly compound heat stress for areas that have been seeing
extreme heat. Into next week, upper troughing may deepen a bit as
it digs into the Great Lakes/Ohio Valley/Northeast, pushing a cold
front gradually southeast. Hot temperatures and scattered to
widespread showers and thunderstorms are likely ahead of the front
across the central U.S. to Eastern Seaboard. Meanwhile, monsoonal
moisture is forecast to increase in the southern/central Rockies
and High Plains, while the West should see increasing heat
underneath an upper ridge.


...Guidance/Predictability Assessment...

Model guidance is pretty agreeable with the overall pattern, with
wide southern tier ridging and a progressive northern stream. What
becomes the primary upper trough during the period will be shallow
as it moves into the northern Plains by Sunday, but deepen a little
as it pushes into the Midwest and Great Lakes Monday-Tuesday and
into the Northeast Wednesday. 12/18Z model guidance was all pretty
similar with this trough and a multi-model deterministic blend was
able to be used. Minor differences will affect the details like
exact frontal position and QPF though. Models agree well with a
Four Corners upper ridge forming behind the trough. Meanwhile a
southern stream trough gradually approaches California while a
northeastern Pacific upper low may send (uncertain) shortwaves
toward the Northwest next week. The main outlier seen with this was
the 12Z CMC, which pushed the northeastern Pacific low
progressively into Canada and ended up out of phase with the
broader model consensus. Thus by the later period, used some GEFS
and EC ensemble means in place of the CMC and UKMET, but maintained
a majority deterministic blend through Day 7 given the reasonably
good large scale agreement. One change seen in the new 00Z incoming
model suite is the EC/CMC indicating a digging shortwave on the
western side of the trough approaching the Upper Midwest next
Wednesday unlike the older guidance. This will affect frontal
positions and temperatures, so this will be monitored in future
forecast cycles.


...Weather/Hazards Highlights...

An initial shortwave and weak low pressure system along a frontal
boundary could bring locally heavy rain in the Northeast lingering
into Saturday. Ample moisture (PWs likely over the 90th percentile)
and some instability could allow for heavy rain rates and possible
flash flood concerns. Continue to depict a Marginal Risk in the
Day 4/Saturday ERO for this activity. Farther west, thunderstorms
are likely to form ahead of the broader upper trough and the
surface frontal boundary in the northern Plains to Upper Midwest
Saturday. Global models have already been showing high instability
with MUCAPE 4000-6000 J/kg for this region, so severe storms are
likely, and this will also support heavy rain rates that could
cause flash flooding. A similar setup is likely on Sunday, just
with the cold front a bit more progressive southeastward. This
allows for a broad Marginal Risk in the Upper/Middle Mississippi
Valley back into the central Plains for the Day 5/Sunday ERO.
Another concern is the wet antecedent conditions that may be in
place by then, after heavy rain in the short range period.
Refinements may be needed in future cycles since there is
uncertainty in the placement of heaviest rain.

Over the weekend into early next week, scattered thunderstorms are
likely farther south in the broad warm sector as well. There will
be less forcing for organization and sustaining of storms across
the central and southern tier away from the upper jet, but
instability could allow for heavy rain rates that may cause nonzero
chances of localized flash flooding, but that are likely dependent
on smaller scale boundaries and are less predictable at this
point. One area of focus that the models show for some heavier rain
is across the central to eastern Gulf Coast region. Will monitor
if there will be flash flooding concerns there but it would have to
battle with very high flash flood guidance. Into the workweek as
the main cold front continues to press south and east, heavier rain
and thunderstorms could stretch across the south-central Plains
into the Ohio/Tennessee Valleys and into the East.

Monsoonal type moisture is likely to increase over the
southern/central Rockies and Plains where there could be a weakness
in the upper ridge. On Saturday main flash flooding concerns will
be across the Sacramento Mountains, where the steep terrain and
burn scars cause the area to be particularly sensitive to rain, so
WPC is covering this with a small Marginal Risk in the Day 4 ERO.
But moisture levels and thus coverage and rain amounts of storms
should increase into early next week. A Marginal Risk is in place
across eastern New Mexico and far West Texas in the Day 5/Sunday
ERO with this activity. These areas are seeing heavy rain in the
near term, and the wetter antecedent conditions enhance flash
flooding concerns.

By the weekend, heat will be much less extreme than currently, but
above average temperatures by 5-12 degrees are likely across the
north-central U.S., Mississippi Valley, and into southern parts of
the Mid-Atlantic. HeatRisk shows some Major (level 3 of 4) areas in
these regions, indicating heat levels that affect anyone without
effective cooling and/or adequate hydration. Meanwhile the
Northeast will be much cooler behind a backdoor cold front. Into
next week, slightly above average temperatures will shift east
ahead of a cold front and moderate. Farther west, building heat is
expected next week as an upper ridge takes hold. Temperatures in
the Northwest are likely to be 10-15 (locally 20) degrees above
normal, for highs nearing 100F. In the Desert Southwest,
temperatures a few degrees above already high averages will equate
to 100s and 110s. Moderate to Major HeatRisk is shown for much of
the Interior West.


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


$$