767
FXUS21 KWNC 271957
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST February 27 2025

SYNOPSIS: There is better model agreement relative to previous model guidance.
A strengthening area of mid-level low pressure forecast supports increased
chances of heavy precipitation and high elevation snow across much of the West
Coast. Flooding is possible for parts of northern California and southwestern
Oregon. As the mid-level pressure shifts eastward later in week-2,  high winds
and heavy snow are also possible for much of the Interior West.  Farther east,
the potential for multiple surface low pressure systems increases the risk of
heavy precipitation over the southeastern contiguous U.S which may exacerbate
flooding conditions being felt across much of Mississippi Valley. Over Alaska,
enhanced onshore flow supports increased chances for above-normal precipitation
and episodes of high winds across parts of the southern Mainland and the
Southeast, although conditions are not anticipated to reach hazards criteria at
this time.

HAZARDS

Moderate risk of heavy snow for portions of the northern Sierra Nevada,
Klamath, and southern Cascade Mountains, Sat-Sun, Mar 8-9.

Moderate risk of heavy precipitation for portions of northern California and
southwestern Oregon, Sat-Sun, Mar 8-9.

Slight risk of episodic heavy snow for the Cascades, Klamath, and Sierra Nevada
Mountains for Fri-Tue, Mar 7-11.

Slight risk of episodic heavy precipitation for parts of western Oregon,
western Washington, and much of California, Fri-Tue, Mar 7-11.

Slight risk of episodic high elevation snowfall over portions of the Great
Basin, Intermountain West and the Rockies, Sat-Thu, Mar 8-13.

Slight risk of periodic high winds for much of the western CONUS, Fri-Thu, Mar
7-13.

Slight risk of heavy precipitation for portions of the Southeast, Fri-Sat, Mar
7-8.

Slight risk of high winds for portions of the Mid-Atlantic and the Northeast,
Fri, Mar 7.

Slight risk of episodic heavy precipitation for portions of the Ohio,
Tennessee, Middle and Lower Mississippi Valleys, and the Southeast, Tue-Thu,
Mar 11-13.

Possible Flooding for parts of southwestern Oregon and northern California.

DETAILED SUMMARY

FOR SUNDAY MARCH 02 - THURSDAY MARCH 06:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR FRIDAY MARCH 07 - THURSDAY MARCH 13: A fairly progressive pattern appears
to be on tap for week-2. At the start of the period, 500-hPa height forecasts
from the GEFS and ECMWF are in agreement featuring broad troughing over eastern
North America, with a shortwave disturbance centered over the Four Corners.
This latter mid-level feature looks to induce additional surface low formation
in the lee of the Rockies, where increased and possibly heavy precipitation is
favored to return over parts of the southeastern CONUS in the wake of a frontal
system exiting the eastern seaboard late in week-1.  Upstream, models come into
better agreement compared to yesterday in bringing more anomalous troughing
from the eastern Pacific, consistent with enhanced onshore flow and an
increased risk for heavy precipitation and high elevation snow over the West
Coast next weekend. Over time, much of the anomalous troughing is favored to
push eastward over the Interior West while gaining latitude over western North
America, allowing for more mid-level ridging downstream. While such a longwave
transition would likely promote unseasonably warming temperatures for much of
the central and eastern CONUS, a trough/ridge pattern also supports the
potential for a high wind and high elevation snow risk across much of the West,
with a renewed heavy precipitation risk east of the Rockies towards the middle
of March.



Tied to the shortwave energy ejecting out of the southwestern CONUS, and
surface low development favored over the Great Plains, there is better
agreement in the Probabilistic Extremes Tools (PETs) indicating 3-day
precipitation amounts exceeding the 85th percentile and 1 inch focused over the
lower Eastern Seaboard. Therefore, a slight risk (20-40% chance) of heavy
precipitation is issued for the Southeast for Mar 7-8.  Based on increased
signals in the Snow Water Equivalent (SWE) PET from the GEFS, accumulating
snowfall is possible on the backside of the surface low over parts of the
eastern U.S. However, given the potential for a more suppressed storm track,
with little to no support in the ECWMF ensemble or deterministic solutions, no
corresponding snow hazard is issued.  Ahead of this potential surface system,
ensembles remain bullish on a potent mean surface low (<990mb) over the
Canadian Maritimes. While any precipitation and/or snow risks will have timed
off for week-1 associated with this low, ensembles continue to show strong
pressure gradients at play early in week-2 which supports the continuation of a
slight risk of high winds over the northeastern CONUS for Mar 7.



With the aforementioned high latitude troughing upstream and additional surface
low formation favored in the lee of Rockies later in week-2, there is an
impressive amount of agreement in both the uncalibrated and calibrated
precipitation model guidance in the late Week-2 leads for a heavy precipitation
risk returning over the southeastern CONUS.  In particular, the raw ECWMF
depicts 20-40% chances for 3-day amounts exceeding an inch on days 12-14 (Mar
11-13) from the lower Great Lakes southward to the Lower Mississippi Valley.
Based on the coverage of these signals, and the PETs which favor a slightly
more southerly placed threat, a slight risk of heavy precipitation is posted
just north of the Ohio River Confluence to the Gulf Coast.  Antecedent
saturated ground conditions in the Mississippi Valley, as well as potentially
heavy precipitation forecast in week-1 (based on WPC QPF) and week-2, may
trigger localized flooding in the highlighted region.



Over the western CONUS, the anomalous troughing and enhanced onshore flow
forecast supports the continuation of slight risks for heavy precipitation and
high elevation heavy snow over the West Coast. Relative to yesterday`s hazard
outlook, these slight risk areas are combined and are now valid for Mar 7-11
for a broader area of the West Coast based on the heavy precipitation risk
arriving earlier than previously advertised, with this potential still expected
to shift southward with time. Accompanying the existing moderate risk of heavy
snow, a moderate risk of heavy precipitation is added over the parts of
Northern California and southwestern Oregon (both valid for Mar 8-9), based on
improved model agreement, stronger signals in the ECMWF Integrated Vapor
Transport (IVT) tool, and both the GEFS and ECMWF PETs for 3-day amounts
exceeding the 85th percentile and 2 inches.  Due to soil moisture content
registering in the upper percentiles, and possibly heavy precipitation favored
during week-1 in the highlighted moderate risk region, a possible flooding
hazard has been added across the lower elevations of northern California and
southwestern Oregon.



As the mean trough axis is favored to shift inland by day 11 (Mar 10),
ensembles show a reduction of the precipitation totals over much of the West
Coast, however much of the enhanced onshore flow shifting inland increases the
risk for high elevation snowfall over the Interior West. This is supported in
the ECWMF ensemble and GEFS SWE PET which depicts the snow risk expanding
eastward by the middle of week-2, thus a slight risk of periods of heavy snow
is posted for Mar 8-13 a broad area over the Interior West.  Along the base of
the trough, a slight risk of episodic high winds is also posted Mar 7-13 for
much of the western CONUS where there is increased support in both the PETs
depicting 20-30% chances for wind speeds exceeding the 85th percentile mainly
west of the Mississippi.  A moderate risk for high winds was considered,
however due to the weak IVT signature in the GEFS, and low chances for winds
exceeding 25mph onshore, a higher risk designation is not included in the
updated outlook.



Over Alaska, much of the enhanced onshore flow also looks to extend over the
Gulf of Alaska to bring above-normal temperatures and precipitation for most of
week-2. Although PETs show increased chances for both precipitation totals and
winds exceeding the 85th percentile, there is not enough confidence actual
amounts will exceed hazard thresholds, and no hazards are posted.

FORECASTER: Nick Novella

$$