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Categorical Outlooks Day 3-7 Day 8-14

Experimental Probabilistic Outlooks (Information) Temperature Hazards Precipitation Hazards

Valid Monday February 18, 2019 to Friday March 01, 2019

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST February 15 2019

Synopsis: A strong area of upper-level low pressure is likely to persist over western North America through late February, while upper-level high pressure remains anchored across the Southeast. During the next two weeks, arctic surface highs are forecast to shift south from western Canada into the western and north-central U.S. Multiple surface low pressure systems are expected to track across the Ohio and Tennessee Valleys during the next two weeks. Periods of enhanced onshore flow are forecast to affect the Aleutians, Alaska Peninsula, and western mainland Alaska through the end of February.

Hazards

• Heavy snow across parts of Arizona, southern Colorado, and northern New Mexico, Mon, Feb 18, and Thu-Fri, Feb 21-22.
• Heavy snow changing to sleet and freezing rain for parts of the central Appalachians and mid-Atlantic, Tue-Wed, Feb 19-20.
• Heavy snow for parts of the mid-Atlantic and Northeast, Tue-Wed, Feb 19-20.
• Periods of heavy rain for parts of the Southeast, Mon-Fri, Feb 18-22.
• Much below-normal temperatures for much of the western U.S. and Great Plains, Mon-Fri, Feb 18-22.
• High winds across portions of mainland Alaska and the Aleutians, Mon-Tue, Feb 18-19.
• High significant wave heights for coastal portions of mainland Alaska and the Aleutians, Mon-Tue, Feb 18-19.
• High risk of much below normal temperatures for the interior Pacific Northwest, northern California, northern Great Basin, northern Rockies and northern Great Plains, Sat-Fri, Feb 23-Mar 1.
• Moderate risk of much below normal temperatures for much of the western U.S. and Great Plains, Sat-Fri, Feb 23-Mar 1.
• Slight risk of much below normal temperatures for portions of the Central and Western U.S., Sat-Fri, Feb 23-Mar 1.
• Slight risk of much below normal temperatures for the Alaska Panhandle, Sat-Sun, Feb 23-24.
• Slight to moderate risk of heavy precipitation across parts of the mid-Atlantic, Southeast, and Ohio Valley, Sat-Fri, Feb 23-Mar 1.
• Flooding occurring, imminent, likely, or possible for parts of the Ohio and Mississippi Valleys along with Arizona.
• Severe Drought across parts of the western U.S., Alaska Panhandle, and Hawaii.

Detailed Summary

For Monday February 18 - Friday February 22: A vigorous shortwave trough is forecast to eject from the Southwest early next week, promoting the development of a frontal wave of low pressure along the western Gulf Coast by Feb 19. Moderate to heavy snow (3 to 6 inches) is expected across the interior mid-Atlantic and Northeast due to strong warm air advection. A strong surface high (around 1032-hPa) centered across northern New England is likely to result in cold air damming. The GFS and ECMWF ensemble means on Feb 15 indicate 0.10 to 0.25 inches, or more, of ice accumulation in the depicted hazard area. Heavy snow at the onset of the precipitation and a changeover to freezing rain is probable for the Shenandoah Valley north to south-central Pennsylvania on Tuesday night into Wednesday. Please refer to local NWS offices for the latest forecasts on this potential winter storm.

Periods of heavy rain are likely throughout this 5-day period across the Southeast and southern mid-Atlantic as waves of low pressure track along a nearly stationary front from the western Gulf Coast northeast to the southern mid-Atlantic. The deterministic 0Z ECMWF model indicates a large area of 3 to 8 inches of rainfall during this 5-day period within the depicted hazard area. Soil moisture is currently above the 90th percentile and the upcoming pattern of frequent, heavy rainfall may eventually lead to small stream and river flooding.

On February 18, heavy snow is expected to be ongoing across parts of Arizona, southern Colorado, and northern New Mexico. Another shortwave trough is forecast to cross the Four Corners region on Feb 21 and 22, renewing the risk of heavy snow across these same areas. Snow levels are likely to be relatively low (below 4,000 feet) with these winter storms and total snowfall accumulations during this 5-day period of more than a foot can be expected.

The high amplitude pattern over the north Pacific and western North America is likely to favor arctic high pressure shifting south from western Canada. Therefore, much below normal temperatures (-12 degrees F or more) are forecast to affect the interior West along with the Great Plains. The coldest temperatures (below -10 degrees F) are forecast across the northern Rockies and northern high Plains on Tuesday morning. Subzero minimum temperatures may extend as far south as the central high Plains. Since this region is free of protective snow cover, the bitter cold increases the risk of winterkill to exposed wheat. Although negative temperature anomalies are forecast to be smaller across California, the Central Valley is included in the much below normal temperature hazard since the pattern supports a frost or light freeze during this outlook period.

The deterministic GFS remains consistent that a surface low (around 968-hpa) enters the Bering Sea on Feb 18. Significant waves and high winds are likely with this low pressure system across the Aleutians, Alaska Peninsula, and southwestern mainland Alaska on Feb 18 to 19. The consistent, enhanced onshore flow is likely to cause coastal flooding and exacerbate erosion along the coastline of southwest Alaska. Another round of significant waves and high winds may return to the Aleutians by Feb 22.

For Saturday February 23 - Friday March 01: A high amplitude pattern with an upper-level ridge (trough) over the north Pacific (western North America) is likely to persist through the final week of February. A high risk (60 percent chance or higher) of much below normal temperatures (lowest 15th percentile of the climatological minimum temperature distribution) is forecast across the interior Pacific Northwest, northern Great Basin, northern Rockies, and northern Great Plains throughout Week-2. The GEFS probabilistic temperature tool indicates that there is more than a 40 percent chance of record cold temperatures across the interior Pacific Northwest and northern Great Basin early in Week-2. Even later in Week-2, this tool indicates a 20 to 40 percent chance of record cold across these areas.

The 6Z GEFS ensemble mean features negative daily temperatures anomalies of -20 to -30 degrees F, each day of the Week-2 period, across the northern Great Plains. This persistent amplified pattern and successive arctic highs shifting south from Canada support an expansive slight and moderate risk area of much below normal temperatures across much of the western U.S. and Great Plains throughout Week-2. Although a moderation in temperatures is expected across the Southwest later in Week-2, the eastward shift of the anomalous upper-level trough over western North America continues to be delayed. Therefore, little relief from the bitter cold is anticipated across the northern Rockies and northern Great Plains through the end of Week-2. Arctic high pressure centered over western Canada supports a slight risk of much below normal temperatures across the Alaska Panhandle on Feb 23 and 24.

Downstream of the longwave trough axis, multiple low pressure systems are forecast to continue tracking along a strong temperature gradient across the Tennessee Valley. Abundant Gulf inflow favors the likelihood of heavy rainfall with each low pressure system. Although there may be a 2 to 3 day break from the heavy rainfall during the middle part of Week-2, the slight to moderate risk areas cover the entirety of Week-2 due to uncertainty in the timing of individual shortwave troughs at this time range. The moderate risk coincides with where the GFS and ECMWF ensemble means indicate more than 2 inches of precipitation during Week-2 and also where rainfall is likely to be heaviest prior to Week-2. The slight and moderate risk areas cover the entirety of Week-2 due to uncertainty on the timing of individual waves of low pressure.

The U.S. Drought Monitor, valid Feb 12, indicates severe to exceptional drought (D2-D4) coverage throughout the CONUS decreased to 6.23 percent. Improvements were made across parts of Arizona, Colorado, and Utah. This is the lowest coverage of D2-D4 since December 2017.

Forecaster: Brad Pugh

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

Resources

Week-2 Probabilistic Extremes Tool

GFS Ensemble Forecasts