687
FXUS02 KWBC 120712
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
212 AM EST Wed Feb 12 2025

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


...Heavy rain and flooding threat in the Tennessee/Ohio Valley and
nearby areas especially on Saturday along with another round of
snow from the Midwest to Northeast this weekend...


...Overview...

The upper trough associated with the wet/snowy western U.S. system
late this week should emerge into the central U.S. this weekend
and then lift rapidly northeastward. This trough and developing low
pressure will spread a broad area of heavy rain from near the
south-central Mississippi Valley through the Tennessee Valley into
the central Appalachians by Saturday, with the low continuing
through the Northeast into the Canadian Maritimes Sunday-Monday.
Areas from the Midwest through the Northeast will see the best
potential for significant wintry weather with this system. The next
Pacific system will track farther north than the first one,
confining the rain and higher elevation snow more to the Northwest
during the weekend. During the early to middle part of next week,
this system will likely spread some rain and mountain snow
southeastward into the central Rockies before expanding
precipitation across the central/southern Plains and points
eastward. Wintry weather will be possible in the northern part of
the moisture shield heading into midweek. The Plains will see the
most pronounced and persistent cold anomalies during the period and
the northern half of the East will also see a significantly colder
trend Monday-Wednesday after a brief warmer period ahead of this
weekend`s storm. Mean flow from the Pacific will tend to favor
moderate temperatures over the West with anomalies in the single
digits on either side of normal.

...Guidance/Predictability Assessment...

As has been the case lately, most guidance agrees well for the
large scale pattern evolution through the period but there are some
notable guidance differences for various details. Dynamical/machine
learning guidance through the 12Z/18Z cycles favored an operational
composite consisting of more 18Z GFS/12Z ECMWF relative to the 12Z
CMC/UKMET early-mid period, followed by incorporating some of the
18Z GEFS/12Z ECens means (reaching 50 percent total weight by next
Wednesday) while switching the ECMWF component to the old 00Z/11
run by Wednesday.

The developing system forecast to reach the upper Ohio
Valley/central Appalachians by around early Sunday is exhibiting
continued strength/detail differences in response to low-
predictability specifics of shortwave dynamics aloft. Recent
stronger trends in the GFS have brought it closer to the
ECMWF/UKMET (with the ICON/CMC weaker), while machine learning (ML)
guidance has so far been running weaker and a bit flatter/slower
with the surface low as of 12Z Sunday. However there is an
improving signal toward strong development as the system continues
northeastward, most likely reaching the 970s mb as it tracks a
little east of Maine by early Monday. An average of dynamical
guidance provides a reasonable starting point with good
continuity.

There are no pronounced differences with the system reaching the
Northeast by Sunday-Monday. Some typical spread develops as the
upper trough continues into the western/central U.S. early-mid
week. The 18Z GFS, 12Z ECMWF/CMC, and the ensemble means offered a
good clustering into Wednesday while the 12Z GFS was slow. ML
models show some variance with the timing and shape of this
trough, so some changes are certainly possible in future runs. It
will also be worth paying attention to the elongated
troughing/possible embedded low over the northern tier
U.S./southern Canada. A decent amount of spread develops for this
feature and it may ultimately have some influence on the upper
trough/surface evolution to the south beyond the end of the
forecast period.

Finally, guidance shows another system reaching the Northwest
around next Wednesday. The 12Z ECMWF/CMC closed off an upper low
that results in the surface system tracking southward of other
guidance. ML models strongly favored either an open upper trough or
an upper low path farther north than those ECMWF/CMC runs, which
would take the surface low on a path somewhere north/northwest of
Vancouver Island with the trailing front reaching the Pacific
Northwest. Thus preference went with the 18Z GFS and old 00Z/11
ECMWF plus the ensemble means. The new 00Z ECMWF/CMC runs appear to
have adjusted in that direction.

...Weather/Hazards Highlights...

The upper trough emerging from the Rockies at the start of the
weekend will support surface low development over the eastern half
of the country during the weekend. Guidance has been fairly
agreeable and consistent in depicting the potential for a broad
axis of heavy rainfall from over/near Arkansas northeastward into
the central Appalachians on Saturday, with anomalous moisture
representing a combination of Pacific and Gulf origins. Some
instability should help to enhance rain rates over at least
southern areas as well. In addition, a large portion of this region
will be sensitive due to prior significant rainfall. In the Day
4/Saturday Excessive Rainfall Outlook, a broad Slight Risk area
continues to extend from the ArkLaTex region to the central
Appalachians. While there is still some uncertainty in the exact
location of the max rainfall axis, overall guidance continuity and
excessive rainfall first-guess field trends from the past day now
offer support for an embedded Moderate Risk area covering portions
of Kentucky and Tennessee. There will likely be some nudges to this
area based on future guidance runs but overall it signifies the
relatively greater potential for flooding issues over this region.
The surrounding Marginal Risk area extends into the northern Mid-
Atlantic where guidance depicts less heavy but still significant
rainfall in combination with what will likely be wet ground from
short-term snow and rain. The Day 5/Sunday ERO currently depicts no
risk areas over the East as remaining rainfall becomes more
progressive. There may still be some locations with runoff issues
depending on rainfall distribution/amounts and antecedent
conditions, but it may take into the shorter term to resolve the
better defined areas of focus.

Meanwhile, areas from the Midwest into the Northeast will see the
best potential for significant snow extending through the weekend.
A transition zone of wintry mix may separate the rain/snow areas.
There is still some uncertainty in the precise strength and track
of the surface low, affecting the extent of various precipitation
types. There may be a period of strong and gusty winds over parts
of the East behind this system, along with some lake effect snow.

A northeastern Pacific system will likely spread rain and higher
elevation snow into the Pacific Northwest on Saturday. In general
expect moderate totals on Saturday and somewhat heavier activity on
Sunday. Still, current guidance suggests that Sunday`s rainfall
will stay below the Marginal threshold for the Day 5 ERO so no area
is depicted at this time. Rain and higher elevation snow will
progress east/southeast through the Rockies Sunday into the first
part of the next work week while tapering off over the Pacific
Northwest. Another system could begin to influence the region by
next Tuesday-Wednesday with light to moderate amounts.

As the upper trough crossing the West early next week heads into
the central U.S. by Wednesday, expect precipitation to develop
from the central/southern Plains eastward. Wintry weather will be
possible in the northern part of the precipitation shield with rain
over the South.

The northern and central Plains will be most persistently below
normal for temperatures during the period, with cold surges
reaching farther south around Sunday and Wednesday. Most days will
bring the potential for some highs to be 20-30F below normal.
Departure of the weekend storm in the East and the trailing upper
trough will bring below normal readings eastward early next week,
with highs 10-25F below normal from the Great Lakes/Ohio valley
into the Northeast Monday-Wednesday. Ahead of the storm, expect a
brief surge of warmer air over the East Saturday into Sunday. Some
highs over the Florida Peninsula could challenge daily records,
especially on Saturday. The Northwest should stay a little below
normal for most of the period, but the remainder of the West should
trend toward normal by Sunday and reach modestly above normal next
Monday. The Southwest may remain near to slightly above normal into
midweek.

Rausch


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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