Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Saturday December 28, 2024 to Friday January 03, 2025

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST December 20 2024

Synopsis: Anomalous mid-level low pressure shifting eastward from the northeastern Pacific favors an elevated risk of Atmospheric River (AR) activity to bring heavy precipitation, periods of high winds, and high elevation snowfall for many parts of the West Coast and the Interior West. Given the timing of the AR potential, any realization of the associated hazard risks is likely to disrupt Holiday travel next week and may lead to localized flooding over parts of the West Coast. Surface low formation in the lee of the Rockies may lead to heavy precipitation amounts mainly over the Mississippi, Tennessee and Ohio Valleys.

Hazards

• Moderate risk of heavy precipitation for the Pacific Northwest and portions of California, Sat, Dec 28.
• Slight risk of heavy precipitation for the Pacific Northwest and portions of California, Sat-Sun, Dec 28-29.
• Moderate risk of high elevation snowfall for the Cascades, Klamath, and Sierra Nevada mountains, Sat, Dec 28.
• Slight risk of high elevation snowfall for the Cascades, Klamath, and Sierra Nevada mountains Sat-Sun, Dec 28-29.
• Moderate risk of high winds for the Pacific Northwest, northern half of California, Northern and Central Intermountain, and Northern High Plains, Sat, Dec 28.
• Slight risk of high winds for the West Coast and Interior West, Sat-Sun, Dec 28-29.
• Moderate risk of high elevation snowfall for parts of Northern Intermountain and Northern Rockies, Sat, Dec 28.
• Slight risk of high elevation snowfall for many parts of the Northern Intermountain, Great Basin, Southwest, and the Rockies, Sat-Mon, Dec 28-30.
• Slight risk of heavy precipitation for parts of the Southern Plains, Lower and Middle Mississippi, Ohio, and Tennessee Valleys, and the Southeast, Sat-Tue, Dec 28-31.
• Possible Flooding for parts of central and northern California.

Detailed Summary

For Monday December 23 - Friday December 27: WPC Days 3-7 U.S. Hazards

For Saturday December 28 - Friday January 03: Since yesterday, dynamical models continue to favor the persistence of 500-hPa troughing over the Gulf of Alaska with anomalous ridging to the south over the eastern Pacific late next week. These mid-level features remain consistent with a continued risk of Atmospheric River (AR) activity extending from week-1 into early week-2, before much of the enhanced onshore flow pattern breaks down heading into the following weekend. Both raw and calibrated model guidance have tracked the evolution and strength of this AR activity quite well during the past several days, and there are little changes in the updated hazard outlook.

Based on the latest Integrated Vapor Transport (IVT) tools, there is an increased AR risk on day 8 (Dec 28), followed by a weakening of this potential on day 9 (Dec 29) which is supported in the Probabilistic Extremes Tools (PETs) with lesser chances (20-30%) for 3-day precipitation amounts exceeding the 85th percentile early in week-2, compared to yesterday. Although the ECWMF favors a slightly stronger AR signal lingering into day 9 compared to the GEFS, the deamplification of mid-level troughing favored in both ensembles is reflective of the ongoing AR risk winding down. Therefore, the moderate risk of heavy precipitation issued over much of the West Coast remains valid through Dec 28, with the broader slight risk area for heavy precipitation valid through Dec 29. As before, the timing of the enhanced onshore flow supports an accompanying moderate risk and slight risk for high elevation heavy snow over the West Coast, also valid through Dec 28 and Dec 29, respectively. Lastly, strengthening surface pressure gradients associated with AR activity is expected to bring an elevated risk of high winds throughout the West. While PETS suggest more of a persistent wind threat compared to previous guidance early in week-2, raw ensembles show surface pressure gradients weakening by day 9, and a moderate risk of high winds for Dec 28, along with broader slight risk area for Dec 28-29 remains issued.

The continued AR risk next week is likely to disrupt holiday travel for the West Coast and may lead to localized flooding over parts of central and northern California, where a possible flooding hazard is posted. Soil moisture content in the highlighted region is currently registering in the upper percentiles based on well above-normal precipitation observed during the past 30 days. Given the increased potential for heavy precipitation during week-1, portions of southwestern Oregon may also be susceptible to localized flooding during week-2.

As much of the enhanced Pacific moisture works its way into the Interior West, a moderate risk of high elevation heavy snow remains posted over the Northern Intermountain and Northern Rockies (Dec 28), with a broader slight risk of high elevation heavy snow extending into the Four Corners. Based on the raw snow tools and the Snow Water Equivalent (SWE) PET, the slight risk area now excludes the Southwest and the Southern Rockies, and is extended through Dec 30 where ensembles continue to favor shortwave troughing over the Interior West.

This shortwave energy is expected to lead to surface low formation in the lee of the Rockies, however the ECMWF and GEFS remain at odds in regards to the timing and strength of surface low development and associated tracks through the middle of the period. The GEFS has been bullish on developing a surface low by the outset of week-2, whereas the ECWMF continues to favor a stronger ridge axis through the Plains, limiting any surface low development until closer to day 10 (Dec 30). These differences are reflected in both the raw and calibrated precipitation tools between the GEFS and ECMWF with the latter being less supportive of a heavy precipitation risk early in week-2. Despite this, the prevalence of shortwave troughing to induce lee cyclogenesis and good return flow from the Gulf of Mexico warrants the continuation of a slight risk of heavy precipitation centered over the Mississippi Valley. Due to well above-normal precipitation received near the Mississippi-Ohio River confluence in recent weeks, any heavy precipitation realized along the northern periphery of the slight risk area may lead to flash flooding during week-2.

A notable change since yesterday concerns an increase in signals for accumulating lower elevation snowfall over the Northern Plains in the GEFS SWE PET and in the uncalibrated ECMWF. A slight risk of heavy snow was considered for inclusion, however due to the added uncertainty regarding surface low development, as well as strong chances for above-normal temperatures favored over the Midwest where daytime temperatures may struggle to fall below freezing, no corresponding snow hazard is issued but will continue to monitor this potential moving forward.

Much of the troughing favored over the northeastern Pacific is favored to deamplify and retrograde to the west, with anomalous ridging developing over northwestern Canada during week-2. Even with this transition, the pattern is still favorable for continued onshore flow, resulting in above-normal precipitation and periods of high winds predominantly favored over southern Alaska. The continued unsettled weather is reflected in the PETs, though there is more uncertainty on whether precipitation amounts and wind speeds will exceed hazard thresholds during the pattern transition, and no corresponding hazards are posted over Alaska in the updated outlook.

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