493
FXUS21 KWNC 061934
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST November 06 2024

SYNOPSIS: By the middle of next week, a departing frontal system is expected to
bring increased precipitation with the potential for high winds over the
northeastern contiguous U.S. Upstream, the favored return of mid-level low
pressure and enhanced onshore flow may lead to heavy precipitation and high
elevation snowfall across the Pacific Northwest, as well as an increased risk
of heavy snow and high winds for many parts of the Interior West later in
week-2.  Additional surface low formation in the lee of the Rockies may also
bring another round of heavy precipitation across the central U.S. Surface low
pressure shifting eastward from the Bering Sea may lead to episodes of strong
winds for much of coastal Alaska.

HAZARDS

Slight risk of heavy precipitation across portions of the Pacific Northwest,
Thu, Nov 14.

Slight risk of heavy precipitation for parts of the Great Plains and
Mississippi Valley, Sat-Tue, Nov 16-19.

Slight risk of heavy snow across northern portions of the Cascades, Thu, Nov 14.

Slight risk of heavy snow across many portions of the Rockies, Thu-Mon, Nov
14-18.

Slight risk of high winds across many portions of the northeastern CONUS,
Thu-Fri, Nov 14-15.

Slight risk of high winds for much of the western CONUS and Central and
Northern High Plains, Thu-Mon, Nov 14-18.

Slight risk of high winds across coastal portions of Southeast Alaska, western
and southern Mainland Alaska, and the Aleutians, Thu-Mon, Nov 14-18.

DETAILED SUMMARY

FOR SATURDAY NOVEMBER 09 - WEDNESDAY NOVEMBER 13:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR THURSDAY NOVEMBER 14 - WEDNESDAY NOVEMBER 20: Since yesterday, 500-hPa
height forecasts from the 0z ECMWF, GEFS and Canadian ensembles have come into
better agreement favoring a more progressive pattern towards the middle of next
week. Leading into week-2, shortwave troughing remains favored to lift out of
the Midwest with positive mid-level height departures overspreading the central
and eastern CONUS in its wake. The ECWMF favors a slightly stronger and faster
evolution of this mid-level shortwave feature compared to the GEFS and
Canadian, resulting in differences in the synoptic surface pattern early in the
period. Meanwhile, an amplifying trough remains favored to shift eastward from
the northeastern Pacific into the Interior West, increasing chances for
enhanced onshore flow over the Pacific Northwest, as well as maintaining the
potential for high elevation heavy snow and high winds for many parts of the
West. Later in the period, ensembles show the pattern becoming less progressive
and amplified, however the mean longwave trough/ridge pattern is expected to
promote above-normal, and possibly heavy precipitation for parts of the Plains
and Mississippi Valley later in the November.



Tied to shortwave troughing forecast, the Weather Prediction Center (WPC) day 7
(Nov 13) surface analysis features a frontal system with the low pressure over
the upper Midwest. By day 8 (Nov 14), ensembles diverge on the storm track,
with the ECWMF favoring a more southerly solution and stronger mean surface
low, promoting higher precipitation amounts over the Ohio Valley, Mid-Atlantic,
and Northeast in both the raw and calibrated tools compared to the GEFS and
Canadian. As a result of these differences, no corresponding precipitation
hazard is issued related to this system, as any increase in precipitation would
bring welcomed ground moisture for many areas along the East Coast,
particularly for the Northeast where moderate and severe drought (D1 to D2)
conditions are currently being registered. Although outlook confidence is
lowered in regards to heavy precipitation, there is better agreement in the
ensembles depicting strong surface high pressure over the Canadian Maritimes
which may induce periods of strong winds. Therefore, a slight risk of high
winds is issued over the northeastern CONUS for Nov 14-15, where there is
support in the ECWMF and Canadian Probabilistic Extremes Tools (PETs) with wind
speeds exceeding the 85th percentile. For the Great Lakes, it is worth noting
that Lake temperatures have been substantially above normal (and in some cases
record breaking) during the past month or so according to GLSEA/NOAA. As such,
parts of the Great Lakes look to be primed for lake-effect precipitation/snow
should any anomalously cold air work its way into the region.



Upstream, the encroaching mid-level trough remains favored to bring enhanced
onshore flow to many parts of the West Coast by the middle of next week, though
there is added uncertainty in regards to timing, given the more progressive
pattern featured in the ensembles. With the mean trough axis favored to be
along or just offshore of the West Coast, Integrated Vapor Transport (IVT)
tools from the GEFS show the greatest moisture signal spanning both the week 1
and 2 periods (days 7 and 8). While both the latest GEFS and ECMWF PETS have
backed off from the heavy precipitation potential depicting lesser chances for
3-day amounts exceeding the 85th percentile over western Washington and Oregon,
there is continued support in the raw precipitation tools on day 8 before drier
conditions are favored to prevail. Therefore, the slight risk of heavy
precipitation and high elevation snowfall over Cascades remains posted, but is
now valid only for day 8 (Nov 14) to capture any lingering heavy precipitation
potential.



By days 9 and 10, much of the anomalous troughing is favored to be centered
over the Interior West. The combination of continued Pacific moisture with
cyclonic flow aloft over the higher elevations is expected to remain conducive
for periods of potentially heavy snow over the Rockies, and a slight risk area
for heavy snow is maintained in the updated outlook. This slight risk is now
valid through day 12 (Nov 18) based on the deamplification of anomalous
troughing favored aloft in the 0z ECMWF ensemble and 6z GEFS. Similarly, the
broad slight risk area of high winds remains posted, where there is continued
support in the PETs for winds exceeding the 85th percentile through Nov 18.



The longwave troughing pattern also supports the potential for additional lee
cyclogenesis towards the middle of week-2. Mean surface pressure analyses from
the ensembles are rather tenuous with surface low formation at this lead,
however there is good agreement among the models in favoring surface high
pressure centered over the Southeast and Mid-Atlantic to help promote moist
return flow from the Gulf of Mexico into the central CONUS. While the
uncalibrated GEFS is on the drier side, PETs show increased chances for 3-day
precipitation amounts exceeding the 85th percentile, where both the raw and
calibrated ECWMF indicate 20% chances for amounts exceeding 1 inch. Given the
increased support in the tools, a slight risk of heavy precipitation is posted
from the Southern Plains eastward to the Ohio and Tennessee Valleys for Nov
16-19.



In the wake of a departing shortwave trough, anomalous 500-hPa ridging is
favored to develop over the northeastern Pacific to bring more tranquil
conditions towards the middle of next week. However, ensembles continue to show
additional trough and surface low pressure development over the Bering Sea,
with a secondary mean low forming in the Gulf of Alaska which may bring another
period of strong pressure gradients over much of southern and western Alaska.
Based on continued support in the PETs, a slight risk of high winds remains
posted, and is expanded to include the western Mainland, and the Southeast, as
well as extended through Nov 18 based on elevated signals persisting in the
PETs.

FORECASTER: Nick Novella

$$