684
FXUS02 KWNH 120711
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
310 AM EDT Wed Mar 12 2025

Valid 12Z Sat Mar 15 2025 - 12Z Wed Mar 19 2025


...Heavy rain/flooding and severe weather possible for the
Ohio/Tennessee Valleys to East Coast this weekend...

...Widespread high winds cause fire danger in the southern Plains
and localized blizzard conditions in the north-central U.S. into
Saturday...

...Multiple rounds of precipitation likely in the West...


...Overview...

The first weather feature of concern this weekend will be an upper
low supporting a strong surface low in the Upper Midwest with
potent cold fronts to its south as the period begins Saturday.
Hazards associated with the surface low pressure system will
include heavy rain and severe thunderstorms in the Southeast and
Tennessee/Ohio Valleys Saturday and moving into the Eastern
Seaboard Sunday. Snow is possible on the backside of the low in the
northern Plains into the Upper Midwest Saturday. Meanwhile areas
of strong winds are likely, which could cause localized blizzard
conditions where it snows, as well as fire danger particularly in
the southern Plains. Warmer than normal temperatures will be common
ahead of the low and its cold fronts, with cooler temperatures
behind, but rebounding into next week. Behind that low, another
upper trough is forecast to move into the West this weekend and
early next week and spread additional precipitation there, with
possibly heavy rain and snow.


...Guidance/Predictability Assessment...

Model guidance continues to show reasonably good agreement for the
large scale upper pattern for the early part of the period. The
upper and surface lows atop the Midwest Saturday are examples where
a multi-model deterministic blend worked well. The 12Z UKMET did
become aggressive by Sunday with a secondary low atop Michigan`s
Lower Peninsula, which was not favored. Farther west, there is
general agreement in an upper trough in the eastern Pacific
pivoting east toward the West Coast. Surface lows near the Pacific
Northwest to Vancouver Island show more spread Sunday morning for
example. This affects the timing and orientation of a potential AR;
the 12Z ECMWF was farther north and slower with the AR coming into
the Pacific Northwest this weekend, but the 00Z ECMWF has adjusted
to be more like the bulk of guidance. A model and ensemble blend
worked so that no individual model was too dominant given the
uncertainty.

Into the workweek, more model spread arises with the evolution of
these features. The initially central U.S. trough axis moves into
the East, with spread on its timing. The 12Z ECMWF was among the
slowest solutions, even closing off an upper low centered over the
western Atlantic by Tuesday. Even the EC-based AI models were not
as slow as the EC, and the newer 00Z ECMWF has adjusted a bit
eastward. Meanwhile GFS runs may be a tad fast. The ensemble means
and the CMC along with the AI models appeared to be a good middle
ground. Meanwhile the next trough coming into the West Coast and
Interior West shows some timing variability as well. The 12Z and
new 00Z EC are slower than the GFS and CMC runs. This also affects
the potential for another (weaker) surface low to consolidate in
the central Plains or so. Did think that the 18Z and new 00Z GFS
runs were quite fast to eject this next low quickly eastward, but
the 12Z GFS seemed reasonable. With the EC and GFS showing some
issues that were not the preferred solutions, the WPC forecast used
GEFS and EC ensemble means in the forecast blend to half by Day 6
and a bit more Day 7 amid increasing spread.


...Weather/Hazards Highlights...

The strong low pressure system in the Midwest Saturday along with
its trailing cold fronts could cause several weather hazards. Ahead
of the cold fronts, ample moisture and instability will be in
place along with good dynamics for lift, causing widespread
thunderstorms in the east-central U.S. Saturday. Severe weather is
likely per SPC. Rainfall totals could be substantial as high rain
rates likely at least 1-2 inches per hour could be common in the
Tennessee Valley. A Slight Risk remains in place for Day 4/Saturday
in the ERO, with a larger Marginal from the Ohio Valley to central
Gulf Coast. This is considered a higher end Slight Risk, and a
Moderate Risk may be needed in future issuances especially once the
hi-res models cover the system and give a better idea of rainfall
rates and placement of heaviest amounts. Meanwhile on Saturday,
snow may wrap around the surface low focused in western Minnesota
and perhaps the eastern Dakotas. Widespread high winds in the
central U.S. are another potential hazard with the low. In northern
areas, locations that receive snow could experience blizzard
conditions into Saturday given the strong winds. Meanwhile farther
south, high winds in the southern Rockies and southern Plains will
cause fire weather concerns.

As the primary cold front progresses eastward on Sunday,
widespread convection is possible across the East. Severe
thunderstorms are possible from the Mid-Atlantic to Carolinas and
Southeast. Locally heavy rainfall is also possible. The first take
at the Day 5/Sunday ERO shows a Marginal Risk stretching across
much of the Eastern Seaboard. There is some uncertainty in how
quickly the cold front moves eastward into the Atlantic off the
Southeast coast, but heavy rain with high rates is likely ahead of
it. Meanwhile farther north in the northern Mid-Atlantic and
Northeast, moisture anomalies are generally over the 95th
percentile and some modest instability is in place, so rain rates
for the region could be locally heavy. All this seemed to warrant a
Marginal Risk but future adjustments certainly could be necessary.
Rain and storms may continue into Monday for coastal areas before
moving away.

Rounds of precipitation are likely in the West by the weekend with
frontal systems channeling Pacific moisture inland. An atmospheric
river of moderate strength is forecast to reach the West Coast
possibly late Saturday but especially by Sunday, causing chances
for heavy precipitation (lower elevation rain and higher elevation
snow) to increase for Oregon to northern California. With the
moderate AR, some localized rain rates of over an inch per hour are
likely, and have a Marginal Risk delineated for the Day 5/Sunday
ERO. Will have to monitor the timing of the AR`s arrival to see if
sufficient heavy rain enters the coast before 12Z Sunday, so during
the Day 4 ERO period, to show a Day 4 Marginal as well. The AR
currently looks to weaken as it pushes into central and southern
California Sunday-Monday, hopefully precluding too many flooding
issues over recent burn scars. Heavy snow may pile up over higher
elevations of the Cascades, Sierra Nevada, and the Interior West
and Rockies this weekend into early next week as precipitation
chances gradually shift east.

Above normal temperatures are likely across the eastern third of
the U.S. on Saturday, with highest anomalies of 20-30 degrees above
normal likely in the Ohio Valley/Great Lakes as some locations
there rise well into the 70s. The strong cold front gradually
pushing through that area and warm inflow ahead of it should push
these warm temperatures into the Mid-Atlantic and Northeast Sunday.
Warm morning lows will be common in the eastern U.S., which could
set records for warm mins--if the locations do not have those
temperatures fall lower before the calendar day after the cold
frontal passage. Milder than average temperatures are forecast to
rebuild in the central U.S. and spreading into the East for the
first half of next week underneath upper ridging. Meanwhile,
periods of troughing across the West will produce below normal
temperatures on average, with highs generally a bit more below
normal given clouds and precipitation compared to low temperatures.


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



$$