Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Thursday December 19, 2024 to Wednesday December 25, 2024

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST December 11 2024

Synopsis: There is significantly increased uncertainty regarding possible mid-level low pressure forming over the Northeast Pacific. In general models depict a weaker low and less favorable orientation for hazardous impacts to the northwestern contiguous U.S. (CONUS) compared to yesterday. Given these changes, the slight risks for heavy precipitation has been decreased in spatial and temporal coverage relative to yesterday’s outlook. A slight risk for high winds is maintained across the Pacific Northwest and northern California, and the moderate risk is removed in today’s forecast. Surface lows and associated fronts tracking across the Northern Plains may support episodic high winds. A series of storms over the Gulf of Alaska could bring high winds to coastal portions of south-central and southeastern Alaska during week-2. Potential amplified troughing across the East increases the risk for much below normal temperatures across the southeastern CONUS.

Hazards

• Slight risk of periodic heavy precipitation across coastal parts of the Pacific Northwest and northern California, Sat-Mon, Dec 21-23.
• Slight risk of periodic heavy snow for high elevation portions of the Cascades, Sat-Mon, Dec 21-23.
• Slight risk of episodic high winds across portions of the Pacific Northwest and northern California, Thu-Mon, Dec 19-23.
• Slight risk of episodic high winds across parts of the Northern and Central Plains, Sat-Mon, Dec 21-23.
• Slight risk of much below normal temperatures across parts of the Southeast U.S., Tennessee Valley, and Lower Mississippi Valley, Sun-Mon, Dec 22-23.
• Slight risk of periodic heavy precipitation across parts of south-central and southeastern Alaska, Thu-Mon, Dec 19-23.
• Slight risk of episodic high winds across parts of south-central and southeastern Alaska, Thu-Mon, Dec 19-23.

Detailed Summary

For Saturday December 14 - Wednesday December 18: WPC Days 3-7 U.S. Hazards

For Thursday December 19 - Wednesday December 25: There is significantly increased uncertainty among models regarding the potential formation of amplified mid-level troughing over the Northeastern Pacific during week-2. In general models’ depiction of the amplitude and orientation of this feature is less favorable to impacts across the northwestern CONUS compared to yesterday. The GEFS and Integrated Vapor Transport (IVT) tools indicate enhanced IVT values (250 kg/m/s) limited to upper portions of the Pacific Northwest at the start of the period, whereas the ECENS counterpart indicates a broader area extending into northern California. These differences translate to the probabilistic extremes tools (PETs) having decreased signals of precipitation and winds reaching hazardous criteria compared to yesterday, temporally and spatially. Therefore, a slight risk of heavy precipitation is limited to coastal portions of the Pacific Northwest and northern California, Dec 21-23, with a slight risk of heavy snow designated only for the Cascades for the same period. These hazards are highlighted based on uncalibrated guidance and PETs. Today’s PETs indicate only localized areas expected to receive 3-day precipitation totals exceeding the 85th percentile climatologically and 2 inches.

There is no moderate risk of high winds designated for today due to the decreasing probabilities in model tools of weather conditions reaching hazardous criteria. A slight risk is continued for portions of the Pacific Northwest and northern California, Dec 19-23, where PETs show at least a 20% chance of wind speeds exceeding the 85th percentile and 40 mph (lower speeds anticipated further inland). Across the West, multiple models indicate the potential for surface high pressure building over the Great Basin with the potential for an inverted surface trough over coastal southwestern California. This pattern is conducive for increased chances of periods of Santa Ana winds across the southwestern coast of California which would increase wildfire risk, especially where there are already active wildfires.

A series of surface lows and associated fronts are forecast to track across the Northern Plains by the middle of week-2. Combined with adjacent surface high pressure over the Great Basin, there is increased potential for tight pressure gradients supporting high winds. A slight risk of episodic high winds is posted for the Northern and Central Plains, Dec 21-23, where PETs show at least a 20% chance of wind speeds exceeding the 85th percentile and 20 mph.

There is multi-model depiction of amplified troughing forming over the eastern CONUS towards the middle of week-2. This pattern favors surface high pressure building across the East, bringing colder arctic air southward. A slight risk of much below normal temperatures is designated for the Southeast U.S., Tennessee Valley, and Lower Mississippi Valley, Dec 22-23, where PETs indicate at least a 20% chance of minimum temperatures falling to the lowest 15th percentile and sub-freezing.

The mid-level trough in the Northeast Pacific may support multiple storms forming over the Gulf of Alaska. A slight risk of episodic high winds is maintained for coastal portions of south-central and southeastern Alaska, Dec 19-23, where the PETs indicate at least a 20% chance of wind speeds exceeding the 85th percentile and 40 mph. Enhanced onshore flow supports a slight risk of periodic heavy precipitation and high winds across southeastern Alaska, Dec 19 to 23. PETs indicate at least a 20% chance of 3-day precipitation exceeding the 85th percentile and 2 inches. While this is somewhat lower than typically used hazards criteria, this enhanced precipitation potential combined with high winds and expected high elevation freezing levels (anomalously warm at lower elevations) increases the risk of flooding and landslides for localized areas.

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