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Categorical Outlooks Day 3-7 Day 8-14

Experimental Probabilistic Outlooks (Information) Temperature Hazards Precipitation Hazards

Valid Friday December 21, 2018 to Tuesday January 01, 2019

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST December 18 2018

Synopsis: For the weekend, snow and rain are likely for both coasts, which could impact the travel for the upcoming holiday. Upper-level low pressure off the coast, as well as enhanced moisture, will likely cause prolonged unsettled weather for the Pacific Northwest for the next week. An amplified upper-level trough-ridge pattern is expected to develop early in Week-2, leading to unsettled weather over the central U.S. and cold temperatures in the west. A strong surface low is forecast to impact the Aleutian Islands early next week.


Hazards

• Heavy rain across portions of California and the Pacific Northwest, Mon, Dec 24.
• Heavy rain across portions of the Great Lakes, the Mid-Atlantic, the Northeast, and the Central Appalachians, Fri-Sat, Dec 21-Dec 22.
• Heavy snow across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Mon, Dec 24.
• Heavy snow across portions of the Southern Appalachians, the Great Lakes, the Mid-Atlantic, the Central Appalachians, and the Ohio Valley, Fri, Dec 21.
• Flooding occurring or imminent across portions of the Southeast, the Central Appalachians, the Mid-Atlantic, and the Southern Plains.
• Flooding likely across portions of the Southeast and the Pacific Northwest.
• High winds across portions of the Aleutians, Mon, Dec 24.
• Significant wave heights across portions of the Aleutians, Mon, Dec 24.
• Slight risk of much below normal temperatures for portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, and the Northern Great Basin, Thu-Sat, Dec 27-29.
• Moderate risk of heavy precipitation for portions of the Lower Mississippi Valley, and the Southern Plains, Wed-Thu, Dec 26-Dec 27.
• Slight risk of heavy precipitation for portions of the central and southern Plains, the Lower Mississippi Valley, the Tennessee Valley, the Central Rockies, the Middle Mississippi Valley, and the Southeast, Wed-Fri, Dec 26-Dec 28.
• Severe Drought across the Central Plains, the Central Rockies, the Central Great Basin, the Southern Plains, the Northern Great Basin, the Southern Rockies, California, the Alaska Panhandle, the Pacific Northwest, and the Southwest.

Detailed Summary

For Friday December 21 - Tuesday December 25: At the start of the 3-7 day period, a shortwave trough is expected to dig over the central U.S. and propagate eastward quickly. Ahead of this trough, a surface cyclone is forecast to be moving over the Mid-Atlantic and Northeast region along the associated frontal boundary. This system is expected to impact the region mostly with heavy rain leading into the weekend, due to temperatures warming prior to advancing storm. In addition to the rain, heavy snow (5+ inches) is likely for parts of the central and southern Appalachians Friday as the amplified upper-level trough shifts closer further east, causing a colder air mass to move over the region.

Upper-level troughing is forecast off the northwestern coast through much of the period. Possible atmospheric river activity is forecast for Sunday over the Pacific Northwest. This enhanced moisture flow is likely to cause heavy snow and rain on Monday. Widespread rainfall totals of 1 inch per 24 hour period are forecast along the coast and heavy snow totals over higher elevations are expected to exceed 8 inches. The heavy snow is likely to extend through from the Cascade range south through the Sierra Nevadas. Prior to the period, this region is expected to experience several periods of unsettled weather, so this additional rain and snow will likely cause issues with flooding and possibly mudslides, as well as a large accumulation of snow. As the system pushes further inland, heavy snow (6+ inches) is also expected for the Bitterroots and Sawtooth mountains in Idaho, Dec 24.

Weak upper-level troughing is forecast to persist over the Alaskan mainland throughout the week, with some ridging building in the Gulf of Alaska at the end of the period. An anomalous surface low (968 hPa) is expected to propagate out of the north central Pacific and over the Aleutians, causing high winds (50+ knots) and high waves (20+ feet) for Dec 24.

For Wednesday December 26 - Tuesday January 01: As we shift into Week-2, the upper-level trough is forecast to amplify over the western U.S., leading to a longwave trough-ridge pattern over the CONUS. Model guidance now indicates that the mean frontal boundary is likely to align along the Midwest and lower Mississippi Valley, shifted slightly further west than forecast yesterday. A moderate risk of heavy precipitation is forecast for parts of the lower Mississippi Valley and Southern Plains, Dec 26-28, where the GEFS and ECMWF ensemble means indicate several 24 hour periods of heavy rain exceeding 1 inch. A wider slight risk region is forecast over much of the central CONUS, Dec 26-29. Frozen precipitation is possible for parts of Central Rockies and High Plains. Both the GEFS and ECMWF probabilistic extremes tools show good agreement about the intensity and placement of this Week-2 storm system; however, the ECMWF solution continues to fall a little further west, with a maximum in rainfall rates over central Texas.

The amplified troughing over the western U.S. is likely to cause cold air from Canada to descend over parts of the Rockies, Great Basin and Northern Plains. A slight risk for much below normal temperatures is forecast for Dec 27-29. Temperature anomalies on the order of 10-15 degrees F below normal are possible, with many parts of the highlighted region forecast to fall below 0 degrees F.

The U.S. Drought Monitor, valid December 11th, indicates a slight decrease in severe to exceptional drought (D2 to D4) coverage from 11.34 percent last week to 11.20 percent this week. Improvement is noted across southern California where extreme drought (D3) has been removed. However coverage of severe drought (D3) has increased over Oregon and moderate drought (D1) has expanded over southern Florida.


Forecaster: Christina Maurin

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

Resources

Week-2 Probabilistic Extremes Tool

GFS Ensemble Forecasts