302
FXUS21 KWNC 101831
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT March 10 2025

SYNOPSIS: An area of redeveloping mid-level low pressure favored over the
western contiguous U.S. (CONUS) is expected to renew the risk of heavy
precipitation for parts of the southwest, and high elevation snowfall for much
of the Interior West.  While not as potentially impactful as a frontal system
favored during week-1, additional surface low formation in the lee of Rockies
increases the risk for heavy precipitation and localized flooding over parts of
the Mississippi, Tennessee and Ohio Valleys, lower elevation heavy snow over
the northern tier of the U.S, with episode of high winds to the east of the
Four Corners. The combination of elevated winds, above-normal temperatures and
very dry conditions is expected to lead to an increased wildfire risk with
Rapid Onset Drought (ROD) over parts of the Southern Plains. Across Alaska,
enhanced onshore flow supports an increased risk of heavy precipitation for
many parts of the Southern Mainland and the Southeast.

HAZARDS

Moderate risk of high winds for the Central and Southern High Plains, and
Rockies, Tue-Thu, Mar 18-20.

Slight risk of high winds for many portions of the central and eastern CONUS,
Tue-Fri, Mar 18-21.

Slight risk of heavy precipitation for parts of southern California and the
Desert Southwest, Tue, Mar 18.

Slight risk of heavy precipitation for parts of the Ohio, Tennessee, and
Mississippi Valley, Appalachians and the Southeast, Wed-Thu, Mar 19-20.

Slight risk of high elevation heavy snow over portions of the Great Basin,
Rockies, Central and Northern Plains, and the Upper Mississippi Valley,
Tue-Thu, Mar 18-20.

Slight risk of heavy precipitation for Southeast Alaska, Wed-Thu, Mar 19-20.

Slight risk of much above normal temperatures for southern Texas, Tue, Mar 18.

Possible flooding for parts of the southern Ohio, Tennessee, and Lower
Mississippi Valleys.

Rapid onset drought risk across portions of the Southern Plains.

DETAILED SUMMARY

FOR THURSDAY MARCH 13 - MONDAY MARCH 17:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR TUESDAY MARCH 18 - MONDAY MARCH 24: Following a very active weather pattern
tied a strong frontal system during week-1, the GEFS and ECMWF ensembles have
been consistent in favoring a reloading mean trough over the western CONUS,
with the rebuilding 500-hPa heights downstream east of the Mississippi early
next week.  While the GEFS favors the mean trough axis over the Great Basin at
the start of week-2, the ECMWF maintains a more westerly solution of this
feature, signaling the persistence of enhanced onshore flow over the West Coast
which is reflected in the raw and calibrated precipitation tools between the
models.  Based on 2-day trends in the 500-hPa height forecasts from the GEFS
which show lower heights covering the eastern Pacific and West Coast, deference
is given to the ECMWF Probabilistic Extremes Tool (PET) which depicts increased
chances of 3-day amounts exceeding the 85th percentile and 1 inch early in the
period. Therefore, a slight risk of heavy precipitation is posted over southern
California and into southern Arizona for day 8 (Mar 18).  Beyond this time,
anomalous 500-hPa ridging is predominately favored over the eastern Pacific,
where more northwesterly flow is expected to bring a period of drier conditions
for the West Coast. Although any additional rainfall during week-2 may
exacerbate saturated grounds over parts of northern California and southwestern
Oregon ( where soil moisture content continues to register in the upper
percentiles), the flooding possible hazard is removed from this part of West
Coast as the return of enhanced onshore flow would be limited further north in
the Pacific Northwest later in week-2.



With mean troughing favored to overspread the Interior West, surface low
development is again favored in the lee of Rockies early in week-2.  For this
iteration, the strength of the shortwave energy and lee cyclogenesis does not
appear to be as robust as previously advertised with the surface low now
favored during week-1, however there is good support in the ensembles depicting
a favorable synoptic setup consisting of a heavy precipitation threat returning
in the warm sector, high winds, as well as accumulating and potentially heavy
snow on the backside of the low as it tracks into the Upper Midwest. A slight
risk of heavy precipitation issued from the Ohio Valley southward to the Gulf
for days 9 and 10 (Mar 19-20) where there is agreement in the PETs indicating
at least a 20% chance for 3-day amounts exceeding the 85th percentile and 1
inch.  Given the return of increased signals for dewpoints exceeding 60 deg F
in ECWMF ensemble and deterministic solutions, thunderstorm activity is also
possible over parts of the Mississippi and Tennessee Valleys.  Due to the
renewed precipitation threat with antecedent saturated conditions over portions
of the Mississippi Valley associated with enhanced precipitation favored during
week-1 (based on WPC QPF), a possible flooding hazard remains issued over a
broad area that includes the southern Ohio, Tennessee, and Lower Mississippi
Valleys. Upstream, the return of enhanced tropospheric moisture tied to the
advancing trough aloft also supports a slight risk of heavy snow posted for the
higher elevations of the Interior West, and extending eastward into the Upper
Midwest for Mar 18-20 based on the predicted track of the mean surface low.
Within the highlighted region, the GEFS based Snow Water Equivalent (SWE) PET
indicates at least 20% chances for 3-day amounts exceeding the 85th percentile,
with increased chances in the raw ECMWF for accumulations upwards of 4 inches
early in week-2.



Strengthening surface pressure gradients are again featured in the
deterministic and ensemble means associated with the surface low. Both the GEFS
and ECMWF depict a fairly deep trough axis extending into Southern High Plains
and northern Mexico on days 9 and 10, where PETS also show the greatest chances
(30-40%) for wind speeds exceeding the 85th percentile and 25mph. Given this,
and in consideration of a continued wildfire risk with drought conditions
registered (ranging from D0 to D4), with suppressed precipitation favored
during week-1 in the Southern High Plains, a moderate risk of high winds is
posted for Mar 18-20.  A broader slight risk area for high winds is posted (Mar
18-21) over much of the central and eastern CONUS based on increased wind
signals in the PETs as the surface low tracks northeastward. Prior to the low
shifting into the Mississippi Valley to bring cooler temperatures into the
Plains, a slight risk of much above normal temperatures is posted over southern
Texas for Mar 18, where there is support in the PETs for temperatures exceeding
the 85th percentile and 90 degrees F by the outset of week-2.  The combination
of very little precipitation, above-normal temperatures, and gusty winds, also
supports the continuation of a Rapid Onset Drought (ROD) hazard over portions
of the Southern Plains.



Over Alaska, enhanced onshore flow associated with amplified troughing over the
Bering Strait and anomalous ridging favored over the Gulf of Alaska early in
week-2 is expected to bring above-normal precipitation for much of the southern
Mainland and Southeast. While the ECMWF is more pronounced with this pattern
than the GEFS, both PETs indicate 20-40% chances for 3-day amounts exceeding
the 85th percentile over, with amounts nearing hazard thresholds over the
Southeast to support the addition of a slight risk of heavy precipitation for
Mar 19-20. Episodes of high winds are possible within the highlighted region,
however a corresponding hazard is not issued due to PETs being less supportive
of this risk.



By day 11 (3/21) and beyond, ensemble guidance generally favors a more
deamplified, and zonal pattern over the CONUS. One exception may be the Pacific
Northwest, where there is modest support for the potential return of enhanced
onshore flow with heavy precipitation and high elevation snow over the
northwestern CONUS, though there is a good amount of disagreement in the
ensembles regarding the location and evolution of a 500-hPa ridge near the West
Coast.  Another notable aspect of the extended range guidance concerns the
shortening of wavelengths featured in some of the latest solutions. This is
typical of height patterns entering boreal spring, but it does pose
predictability challenges in terms of how well models can adjust, which further
adds to forecast uncertainty moving forward.

FORECASTER: Nick Novella

$$