Valid Saturday January 23, 2021 to Friday January 29, 2021
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EST January 15 2021Synopsis
: Mid-level low pressure remains
favored to develop across the western portion of the lower 48 to sustain an
elevated risk of potentially hazardous below normal temperatures, heavy
precipitation, and high elevation snow during week-2. Developing mid-level low
pressure is expected to increase chances of much below average temperatures
over portions of the eastern U.S. late in the outlook period. Offshore flow
forecast is likely to bring below-normal precipitation over southern Mainland
Alaska and the Panhandle. Hazards
- Moderate risk of much
below normal temperatures across parts of the Pacific Northwest, Northern
Intermountain, and Northern Rockies, Sat-Sun, Jan 23-24.
- Slight risk of much below normal temperatures across parts of the Pacific
Northwest, the Great Basin, Rockies, and the Great Plains, Sat-Wed, Jan 23-27.
- Slight risk of much below normal temperatures across parts of the Lower
Mississippi and Tennessee Valley, the Southern Appalachians, the Mid-Atlantic,
and the Southeast, Wed-Fri, Jan 27-29.
- Moderate risk of heavy precipitation for northern California, Mon-Wed, Jan
- Slight risk of heavy precipitation for parts of California and southern
Oregon, Sat-Fri, Jan 23-29.
- Moderate risk of heavy snow for the higher elevations of the Sierra Nevadas
of northern California, Mon-Wed, Jan 25-27.
- Slight risk of heavy snow for the higher elevations of the Sierra Nevadas
and the Cascades, Sat-Fri, Jan 23-29.
- Slight risk of heavy snow for the higher elevations of northern Arizona and
southern Utah, Sat-Mon, Jan 23-25.
- Slight risk of high winds over portions of the Great Basin, Southwest, and
Southern Rockies, Sat-Mon, Jan 23-25.
For Monday January 18 - Friday January 22: WPC Days 3-7 U.S.
Hazards For Saturday January 23 - Friday
Heading into the start of week-2, both the GEFS and ECMWF
ensemble means feature amplified ridging over the northern Pacific and Alaska,
and the deepening of a downstream anomalous trough with negative 500-hPa height
departures overspreading the western half of the CONUS. This latter mid-level
feature continues to be the focal point for hazardous weather concerns, as it
maintains elevated chances for much below-normal temperatures, and possible
heavy precipitation and high elevation snow throughout the West during week-2.
Towards the latter portion of the outlook period, ensembles suggest the
anomalous troughing and the associated hazard potential throughout the West may
begin to ease, however, ensembles also favor the strengthening of negative
500-hPa height departures across the eastern U.S. to elevate the risk of
hazardous cold towards the end of January. Although this better aligns with a
negative North Atlantic Oscillation to increase winter storm potential, there
continues to be uncertainty regarding the prospect of winter storms and
snowfall amounts, as models continue to struggle in maintaining continuity with
the location and timing of short wave troughs and any associated surface lows
tracking from the West.
Surface high pressure remains forecast to extend from western Canada and
advect cold Arctic air into portions of the northwestern CONUS by the start of
week-2. Reforecast guidance maintains a fairly broad region with at least a 20%
chance of minimum temperatures falling below the 15th percentile and less than
20 degrees F over many portions of the Rockies, Great Basin and Plains, and a
slight risk for much below normal temperatures remains posted for Jan 23 to 27.
This includes portions of the Pacific Northwest and northern California where
there is also a 20% chance minimum temperatures fall below the 15th percentile
and 28 degrees F. Compared to yesterday, higher probabilities (>40%) are now
indicated in both the GEFS and ECMWF reforecast tools over portions of northern
Washington to western Montana during early week-2. Thus, a moderate risk for
much below normal temperatures is issued for Jan 23 to 24 where today's 0z and
6z GEFS depicts ensemble mean minimum temperatures in the single digits and
teens across the region.
The anomalous troughing aloft over the West combined with surface low
pressure prevailing over the southwestern CONUS supports an increased risk for
heavy snow across parts of the Sierras, Great Basin, Rockies, and Southwest.
Reforecast tools continue to indicate at least a 20% chance that precipitation
accumulations exceed the 85th percentile and 1 inch over California, and a
slight risk of heavy precipitation is posted over the region, as well as a
slight risk of heavy snow over the higher elevations of Sierra Nevada's and the
Cascades for Jan 23 to 29. During the middle of the period, higher
probabilities are indicated in the reforecast guidance for precipitation
exceeding the 85th percentile, with the ECMWF reforecast tool notably showing
at least a 20% chance that 3-day precipitation amounts exceed 2 inches in parts
of northern California. As a result, a moderate risk of heavy precipitation and
a moderate risk for heavy snow are issued for the region for Jan 25 to 27.
Over the southwestern CONUS, a slight risk of heavy snow is also posted for
the higher elevations of northern Arizona and southwestern Utah as both the
GEFS and ECMWF reforecast tools indicate at least a 30% chance that 3-day
precipitation accumulations exceed the 85th percentile in the region for Jan
23-25. In addition, today's reforecast guidance also features enhanced
probabilities for sustained winds exceeding the 85th percentile over the Four
Corners region early in the outlook period, and a slight risk of high winds is
added over portions of the Great Basin, Southwest, and Southern Rockies for Jan
23 to 25.
Farther east, temperature guidance continues to favor the transition to
colder temperatures for many areas east of the Mississippi River by the middle
of the outlook period. The mid-level height ensembles have trended with a
deepening trough aloft extending southward across the eastern U.S. during this
time, and there is agreement between the GEFS and the ECMWF reforecast tools
which show increased probabilities for minimum temperatures falling below the
15th percentile and freezing over the southeastern CONUS during late week-2.
Thus, a slight risk for much below normal temperatures is posted for Jan 27 to
29. For precipitation, several deterministic solutions continue to favor
surface low development to bring increased precipitation east of the Rockies
early in week-2. Although reforecast tools depict elevated probabilities for
precipitation accumulations exceeding the 85th percentile throughout parts of
the Plains and the Mississippi Valley, these totals are expected to remain
below hazard criteria. Increased snowfall is possible on the backside of any
surface low, but there continues to be poor run to run continuity in the model
runs relative to the location and timing of surface lows that form, and with
probabilistic tools indicating snow amounts falling below hazards criteria
during week-2, no snow related hazards are issued at this time.
Over Alaska, much of the storm activity in the Bering Sea looks to lessen
and models are beginning to favor below-normal precipitation across the
southern Mainland and the Panhandle. Strong surface pressure gradients are
possible and reforecast tools maintain at least a 20% chance for sustained
winds to exceed the 85th percentile, however, there is not enough confidence
these winds will exceed the hazard criteria, and no hazards are issued for the
Forecaster: Nick Novella
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.
Week-2 Probabilistic Extremes Tool
GFS Ensemble Forecasts