378
FXUS02 KWBC 310736
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
336 AM EDT Sun Aug 31 2025

Valid 12Z Wed Sep 03 2025 - 12Z Sun Sep 07 2025


...Overview...

Upper troughing across the central and eastern U.S. will be
reinforced as the medium range period begins midweek, with strong
energy forming an upper low near the Upper Great Lakes Thursday.
This will bring in well below temperatures to the Plains, Midwest,
and Great Lakes/Ohio Valley. Rain is likely ahead of the trough
axis, including some moderate to heavy rain totals possible in the
Northeast later week. Meanwhile in the West, upper ridging is
likely to continue and bring above normal temperatures to the
Northwest before a likely cooling trend by next weekend. Some
monsoonal moisture may increase late week into next weekend across
the West.


...Guidance/Predictability Assessment...

Model guidance remains in good agreement with the amplified
synoptic scale pattern anchored by the ridge in the West and trough
in the East, with relatively more spread in the details. These
details include the exact position of the upper low (CMC runs
somewhat southeast of the better consensus by Friday) and possible
shortwaves rounding the upper low. Most models (with the exception
of the CMC) have now followed EC-AIFS runs to show a decent amount
of energy rounding the western/southern side of the trough Thursday
into Friday. This could enhance wraparound moisture for additional
cloudiness and showers in the north-central U.S., and the
overnight WPC forecast trended this direction. Models show the
upper low gradually lifting northeast into the weekend and
deamplifying the trough across the central and eastern CONUS.

Models indicate a larger amount of spread late week into next
weekend as Pacific troughing could move toward or into the West and
suppress and shift the Western ridge. There is not a lot of
consensus yet in the deterministic models with the resulting
pattern, so leaned more toward the ensemble means.

The WPC forecast was based on a multi-model deterministic blend
early in the forecast period. As the period progressed, lessened
the proportion of deterministic guidance in favor of the ensemble
means, with the means reaching half the model blend by Day 6 and
more Day 7 amid increasing spread.


...Weather/Hazards Highlights...

The cold front dropping into the north-central U.S. ahead of the
strong reinforcing upper trough could see some convection ahead of
it. However, instability and moisture are not too high on Wednesday
and the front will be moving progressively through the Upper Great
Lakes region, and since QPF came down there as well, will consider
this a less than 5 percent risk for flash flooding for no ERO area
there. By Thursday, the front should be slowing up while the upper
low does, while it should also tap into some Atlantic moisture,
possibly influenced by a coastal system. Thus for the Day
5/Thursday ERO, a Marginal Risk is delineated across the central to
northern Appalachians and surrounding areas for isolated flooding
concerns. Orographic enhancement may play a role in producing
heavier rain totals. One downside for flooding will be antecedent
dry conditions overall, though south-central New York has been
wetter the past couple of weeks. Moderate to heavy rain is forecast
across the Interior Northeast on Friday, pulling away Saturday.
Some rain may wrap around the Midwest/Upper Great Lakes late week
as upper energy moves through, and the cool westerly flow atop the
Great Lakes could lead to lake effect rain showers into next
weekend.

Monsoonal moisture is forecast to flow into the southwestern U.S.
underneath the mean upper ridge for scattered showers and
thunderstorms. Moisture anomalies are highest into parts of
California and Nevada, so have Marginal Risks over those states
despite low deterministic QPF and stretching into southwest Utah
and Arizona into New Mexico on Day 4/Wednesday and Day 5/Thursday.
The track of an Eastern Pacific tropical system that is likely to
form could also affect the moisture levels in the Southwest. Rain
chances are currently forecast to expand farther north into the
Intermountain West by next weekend. Convection could make its way
into the southern Plains at times.

Meanwhile, frontal boundaries are forecast to meander over Florida
in an unstable environment with above normal moisture into
midweek, allowing for localized flash flooding potential and a
Marginal Risk in the Day 4/Wednesday ERO. Most model guidance then
limits potential for heavy rainfall to southern parts of the
Florida Peninsula into Day 5/Thursday, allowing for a smaller
Marginal by that point. South Florida in particular could see some
lingering moisture and frontal boundaries for additional diurnal
convection into late week and next weekend.

The large trough covering the central and eastern U.S. for the
latter half of this week will promote well below average
temperatures. Reinforcing energy coming into the trough will push a
strong cold front into the north-central U.S. Wednesday and
spreading into much of the Plains and Midwest Thursday. Lows are
forecast to be generally around 10-15 degrees below average, while
highs may be around 15-20 degrees below normal in those areas.
These cool highs could set daily records for low maximum
temperatures, as highs only reach the 50s in some locations of the
Upper Midwest and Great Lakes, with even some possible highs in the
40s in the U.P. of Michigan. While record lows should be less
expansive, there may be a concern for frost around Lake Superior
particularly Thursday morning, depending on wind and cloud cover.
Temperatures are forecast to gradually warm closer to normal into
next weekend. On the other hand, the West can expect warm to hot
conditions under the upper ridge. Temperatures will be most
anomalous in the Northwest, with upper 90s and possibly reaching
100F east of the Cascades. Record high minimum and maximum
temperatures are possible, and the HeatRisk shows Major to locally
Extreme conditions in the northern Great Basin on Wednesday and
Thursday, with temperatures decreasing after that.


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


$$