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Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Saturday December 10, 2022 to Friday December 16, 2022

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST December 02 2022

Synopsis: The high-latitude blocking pattern in place continues to favor below normal temperatures over much of the West and the northern tier of the CONUS. Today's dynamical models depict more (less) amplified mid-level low pressure across the West (East) compared to yesterday, translating to increased (decreased) spatial and temporal coverage of much below normal temperatures across the West (East). Multiple models continue to depict surface low pressure systems tracking across the East, although there is increased model divergence compared to yesterday. This pattern would generally favor episodes of heavy precipitation across portions of the eastern CONUS, heavy snow across northern portions of the East, and high winds across parts of the Great Lakes, Northeast, and upper Mid-Atlantic.

Hazards

• Slight risk of much below normal temperatures across portions of the West and north-central CONUS, Sat-Fri, Dec 10-16.
• Slight risk of much below normal temperatures across the northeastern CONUS, Sat-Mon, Dec 10-12.
• Slight risk of episodes of heavy precipitation across parts of the Lower Mississippi, Ohio, and Tennessee Valleys, Appalachians, Mid-Atlantic, and Northeast, Sat-Wed, Dec 10-14.
• Slight risk of episodes of heavy snow across parts of the Middle Mississippi and Ohio Valleys, Great Lakes, upper Mid-Atlantic, and Northeast, Sat-Wed, Dec 10-14.
• Slight risk of periods of high winds across parts of the Great Lakes, Northeast, and upper Mid-Atlantic, Sat-Wed, Dec 10-14.
• Slight risk of much below normal temperatures across parts of the Alaska Panhandle, Sat-Mon, Dec 10-12.

Detailed Summary

For Monday December 05 - Friday December 09: WPC Days 3-7 U.S. Hazards

For Saturday December 10 - Friday December 16: Both the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) are forecast to be in a negative phase at the outset of week-2, favoring below normal temperatures across portions of the CONUS. Multiple models show mid-level troughing at the beginning of week-2 shifting eastward over time, although there is significant variation among models regarding its evolution resulting in increased uncertainty, compared to yesterday, with regards to associated hazards potential.

Multiple model ensemble means indicate more (less) amplified troughing across the West (East), translating to tools favoring more (less) expansive extreme cold compared to yesterday, both in time and space. Therefore, the slight risk of much below normal temperatures is increased (decreased) in coverage across the West (East) compared to yesterday's outlook. A broad area of a slight risk of much below normal temperatures is highlighted across the West and north-central CONUS throughout the entirety of week-2, with the anticipation of this risk initiating across West Coast states and the Great Basin, expanding eastward to the north-central CONUS over time. Both the GEFS and ECMWF reforecast tools show at least a 20% chance of minimum temperatures in the designated risk area falling to the lowest 15th percentile and freezing.

Recent deterministic GFS runs and ensemble means indicate the potential for an initial surface low forming near the East Coast, with a secondary surface low and associated fronts forming over the central CONUS and shifting eastward ahead of the aforementioned mid-level trough. There is, however, significant uncertainty regarding the precise timing and evolution of these features and resulting in relatively high uncertainty regarding possible hazards. Given these differences, the hazards posted have broad coverage in both time and space to account for the two possible disturbances from the beginning to middle of week-2. A slight risk of episodes of heavy precipitation is posted across parts of the Lower Mississippi, Ohio, and Tennessee Valleys, Appalachians, Mid-Atlantic, and Northeast, with a slight risk of periods of heavy snow designated across parts of the Middle Mississippi and Ohio Valleys, Great Lakes, upper Mid-Atlantic, and Northeast, Dec 10 to 14. The reforecast tools both indicate at least a 20% chance of 3-day accumulated liquid equivalent exceeding the 85th percentile and one inch during the 8 to 10 day period (Dec 10 to 12), with the GEFS showing this signal extending into Dec 12 to 14. The snow water equivalent (SWE) reforecast tool continues to show above normal 3-day snow totals in the 8 to 12 day period across the Northeast, although with less coverage compared to yesterday. Despite the weaker signal, the broad heavy snow area is highlighted based on recent deterministic model guidance and areas that would be favored to have heavy snow based on possible storm tracks. If this snow materializes, this could be the first snowfall of the season for some areas of Kentucky, West Virginia, and southern Ohio. Additionally, high winds may accompany the storm tracks. A slight risk of periods of high winds is designated for parts of the Great Lakes, Northeast, and upper Mid-Atlantic, Dec 10 to 14, primarily based on recent deterministic model guidance and anticipated areas most likely favored for high winds based on surface disturbances.

Both reforecast tools have diminished in the extreme cold signal over the Northeast compared to yesterday. Despite the reforecast tools no longer indicating at least a 20% chance of minimum temperatures falling to the lowest 15th percentile and freezing early in the period, a potential storm system in the vicinity of the Northeast could increase the risk of anomalously cold temperatures, especially if snowfall occurs. Therefore, a slight risk of much below normal temperatures is highlighted across the Northeast, Dec 10 to 12.

Forecast mid-level low pressure over the Alaska Panhandle at the beginning of the period supports a slight risk of much below normal temperatures over the region, Dec 10 to 12. The reforecast tools show at least a 20% chance of minimum temperatures falling to the lowest 15th percentile and 20 Deg F over the highlighted risk area. Mean surface low pressure centered over the Gulf of Alaska favors periods of gusty winds and increased precipitation over the southern coast of Mainland Alaska and the Panhandle. However, precipitation totals are not forecast to reach hazards thresholds. While the ECMWF and GEFS reforecast tools indicate at least a 20 percent chance of wind speeds across parts of southern coastal Alaska and the Panhandle exceeding the 85th climatological percentile and 40-mph, these wind speeds are favored to be more intermittent, and not necessarily hazardous given these regions are climatologically stormy this time of year. As a result, no related wind or precipitation hazards are posted across Alaska.

Forecaster: Melissa Ou

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

Resources

Week-2 Probabilistic Extremes Tool

GFS Ensemble Forecasts