Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Tue Apr 8 2025

Valid 12Z Fri Apr 11 2025 - 12Z Tue Apr 15 2025


...General Overview...

The forecast shows moderate progression with two significant
weather systems of note. The leading one involves digging
Canada/Great Lakes energy within an amplified trough, forming a
closed low over the central Appalachians and Mid-Atlantic by late
this week into the weekend. Associated surface low pressure will
likely track just off the East Coast during the weekend, with this
system producing organized rainfall and perhaps localized high
elevation snow. A trailing upper ridge should progress from the
Intermountain West/Rockies to the East Coast during the period.
Then an upper trough reaching the Pacific Northwest by Saturday
will support the second system that should develop over the Plains
during the weekend and track through the upper Great Lakes into
east-central Canada early next week. Most of this system's rain and
higher elevation snow will likely extend over and east-northeast
from the northern Rockies. Most rain with the trailing cold front
should be fairly light. Another upper trough with more uncertain
specifics may reach the West Coast around next Tuesday.

...Guidance/Predictability Assessment...

Models and ensembles from the 00Z/06Z cycles continued to agree
fairly well for the general pattern through the period. However
meaningful embedded differences persist for individual systems.
These include the leading one affecting the East Coast, the upper
trough and associated surface system progressing from the Northeast
into the Great Lakes/Canada Saturday-Tuesday, and the upper trough
reaching near the West Coast by next Tuesday. Guidance comparisons
and continuity favored an early-mid period blend consisting of 40
percent 00Z ECMWF and a remaining even split among the 06Z GFS/00Z
UKMET/00Z CMC. Clustering ultimately favored a shift toward 60
percent total 06Z GFS and 12Z/07 ECMWF relative to 40 percent 06Z
GEFS/00Z ECens means by next Tuesday.

For the East Coast system, individual dynamical and machine
learning (ML) models show varying detail/position differences
without pronounced clustering. At least there is a general theme
toward the system having less northward elongation/precipitation
extent than depicted by latest GFS runs. The new 12Z UKMET strays
northeast with the upper low versus most other solutions by Sunday
while the 12Z ECMWF/CMC and GEFS mean maintain an upper low path
across the Mid-Atlantic.

Meanwhile guidance still shows important detail differences for
the evolution of shortwave energy reaching the Northwest by
Saturday. GFS/CMC runs have been forming an embedded upper low over
or near the northern Rockies by Sunday, with the 12Z ICON also
showing this (and with the farthest south track later on). UKMET
runs (and the 12Z GEFS mean) have had an intermediate upper low
track along the Canadian border while faster ECMWF runs wait until
farther eastward near the Canadian border to form the low by
Monday. Most ML models favor waiting until after Sunday to form an
upper low, and with a track near the Canadian border. The preferred
solution leans toward an intermediate evolution--an upper low near
the Canadian border, Plains low pressure between the weaker ECMWF
and deeper GFS/CMC, and eventually slower timing than the 00Z and
new 12Z ECMWF (12Z/07 ECMWF closer to the majority cluster by next
Tuesday). Northern tier QPF also reflects an intermediate idea
between the GFS/CMC versus ECMWF extremes.

By late in the period, individual dynamical/ML models and
ensembles develop a lot of spread for the character of troughing
that may approach the West Coast. This is also the case for leading
energy that may pull off from the southern part of the weekend
shortwave. At this point there is no meaningful consensus or
continuity regarding how much stream separation may occur within
the trough and where this separation may occur. The updated blend
valid next Tuesday shows some mid-latitude separation based on
preferences to the east but confidence along the West Coast is very
low and future cycles are likely to reflect different ideas for
specifics.

...Weather/Hazards Highlights...

The low pressure system exiting the Ohio Valley and reaching the
East Coast to close out the week will be the next system to monitor
for meaningful sensible weather. A deepening coastal low near the
Mid-Atlantic Coast on Friday and into early Saturday should bring
higher rain chances and breezy conditions from the Carolinas to New
England. A Marginal Risk area continues for Day 4/Friday from
Maryland northward to southern New England with more of an enhanced
QPF signal in the deterministic guidance, with perhaps 1-2 inches
of rainfall for some areas. New guidance continues to show some
spread for specifics but the best clustering and ensemble
probabilities recommend no change for the current Marginal Risk
area at this time. Higher elevation snowfall is also likely across
northern New England Friday night into Saturday. The rainfall
becomes lighter in general by Saturday (with additional spread for
details) across the Northeast, and the Day 5 ERO still reflects no
risk areas.

The forecast over and east of the northern Rockies from Saturday
onward depends on the specifics of potential upper low formation
within the trough supporting Plains low pressure development by
Sunday. Current preferences lie between the extremes, represented
by the GFS/CMC that produce some areas of heavy rain higher
elevation snow--due to earlier and farther south upper low
formation--versus the much lighter ECMWF and most ML models which
maintain an open upper trough until forming a closed low farther
eastward. For the most part, the trailing cold front should not
have a lot of moisture to work with so any associated rainfall over
the central and eastern U.S. should be on the lighter side.

Some light precipitation could reach parts of the West Coast by
late Monday or Tuesday, depending on very uncertain specifics of
incoming Pacific shortwave energy. Monitor future forecasts as
guidance eventually refines the details aloft and associated
precipitation.

In terms of temperatures, the Western U.S. into northern High
Plains will see a warm pattern for the end of the week with both
daytime highs and overnight lows about 10-20 degrees above average,
with some readings potentially 25 degrees above average across
portions of Wyoming and Montana. The warmth will reach the central
and northern Plains by the weekend as the upper ridge shifts
eastward, with widespread 15-25 degree positive anomalies across
the Central Plains and Midwest by Sunday. The amplified upper
trough initially over the East will keep highs at some locations up
to 10-15 degrees or so below normal Friday-Saturday before a
steady warmer trend to moderately above normal readings early next
week. Cooler air behind the Northwest U.S. front eventually pushing
into the East will bring temperatures down to near or slightly
below normal. A trailing upper ridge building into the West early
next week should support a rebound to somewhat above normal
temperatures over the region.

Rausch/Hamrick


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