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Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Saturday January 23, 2021 to Friday January 29, 2021

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST January 15 2021

Synopsis: 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 25-27.
• 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.

Detailed Summary

For Monday January 18 - Friday January 22: WPC Days 3-7 U.S. Hazards

For Saturday January 23 - Friday January 29: 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 state.

Forecaster: Nick Novella

$$ Please consult local NWS Forecast Offices for short range forecasts and region-specific information.

Resources

Week-2 Probabilistic Extremes Tool

GFS Ensemble Forecasts