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Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Wednesday February 08, 2023 to Tuesday February 14, 2023

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST January 31 2023

Synopsis: Surface low development over the Great Plains, combined with enhanced moisture flow from the Gulf of Mexico increases the risk of heavy precipitation over the southeastern Contiguous U.S. (CONUS) early in the period. Forecast mid-level low pressure is expected to bring below (above)-normal temperatures (precipitation) over many portions of the West, with slight chances for heavy precipitation and high elevation snowfall late in week-2 across some areas.

Hazards

• Slight risk of heavy precipitation for portions of the Great Plains, Mississippi, Tennessee and Ohio Valleys, Southeast, and Mid-Atlantic, Wed-Fri, Feb 8-10.
• Slight risk of heavy precipitation for portions of California, Sat-Tue, Feb 11-14.
• Slight risk of high winds for portions of the West Coast and the Great Basin, Sat-Tue, Feb 11-14.
• Slight risk of heavy snow for the higher elevations of the Sierra Nevada, Sat-Tue, Feb 11-14.
• Slight risk of heavy snow for the higher elevations of the Four Corners, Sun-Tue, Feb 12-14.

Detailed Summary

For Friday February 03 - Tuesday February 07: WPC Days 3-7 U.S. Hazards

For Wednesday February 08 - Tuesday February 14: There is generally good agreement among the dynamical model mean week-2 500-hPa height anomaly forecasts featuring amplified ridging overspreading the central and eastern CONUS, with weak troughing upstream over the West. Relative to yesterday, both the GEFS and ECMWF ensembles have backed off on the strength of the mean troughing across the West. Analysis of the daily evolution of mid-level heights now shows a more transitional pattern over this part of the country, though models still hint at troughing becoming more pronounced towards the end of the period. Notably, the strongest negative mid-level height departures in the ensembles are favored over the higher latitudes of North America, with the mean pattern becoming more in-line with both positive phases of the Arctic Oscillation (+AO) and North Atlantic Oscillation (+NAO). As a result, the favored absence of Arctic air intruding upon the CONUS is likely to reduce the risk of much below normal temperatures, with much of the mean flow favored to be more of Pacific origin for much of the country during week-2.

By the middle of next week, a shortwave trough ejecting from the Rockies is expected to induce a surface low over Southern Plains. With surface high pressure likely established over the eastern CONUS to promote ample return flow from the Gulf, conditions remain conducive for heavy precipitation over the southeastern CONUS early in the period. A slight risk of heavy precipitation remains issued for Feb 8-10, and is expanded to include parts of the Mid-Atlantic where reforecast tools show at least a 20% chance of 3-day totals exceeding the 85th percentile. Deterministic solutions continue to depict dewpoint temperatures reaching the lower to mid-60s in the warm sector of the surface low, suggesting an increased potential of thunderstorm activity over portions of the Gulf States. These solutions also show the potential for accumulating snow on the backside of the low over the Plains and Midwest, however reforecast guidance remains less supportive of liquid-equivalent amounts exceeding hazard criteria, precluding a heavy snow hazard. Regardless of whether heavy precipitation is realized, the anomalously wet conditions favored over the Lower Mississippi Valley during week-2 may exacerbate saturated ground conditions and trigger localized flooding, particularly over southern Louisiana and Mississippi where soil moisture content and streamflow are registering in the highest percentiles. Later in the period, both the GEFS and ECMWF ensembles show signs of additional surface low formation in the lee of the Rockies. While this is evidenced by the return of increased precipitation amounts across the Plains and Mississippi Valley in the raw guidance, particularly in the ECMWF, no corresponding precipitation hazards are issued due to a lack of support from the GEFS reforecast tool.

Relative to previous guidance, more anomalous mid-level ridging is depicted over western CONUS in the GEFS and ECMWF ensembles early in the week-2. While some anomalously cold air is favored downstream of the ridge axis, several temperature tools have become more modest with the predicted negative temperature departures. Similarly, both the GEFS and ECMWF reforecast tools depict less spatial coverage of probabilities for temperatures falling below the 15th percentile across the West throughout the period compared to yesterday, prompting the removal of the slight risk of much below normal temperatures in today's outlook. Some of the cold signals in percentile space return over the parts of the southwestern U.S associated with more anomalous troughing and northwesterly flow favored later in the period, but remain marginal at this time.

In regards to heavy precipitation potential in western CONUS, there is considerable disagreement in the latest reforecast guidance. The GEFS reforecast tool remains muted with the heavy potential until the middle portion of week-2, whereas the ECMWF reforecast tool introduces a robust signal from the Pacific Northwest to California, likely tied to a deeper trough extending from the Gulf of Alaska in the ECMWF ensemble mean on day 9 (Feb 9). Later in the period, there is slightly better agreement, depicting increased chances of 3-day amounts exceeding the 85th percentile over portions of California and the Great Basin. Given the increased alignment among the ensembles' mid-level height pattern during this time, a slight risk of heavy precipitation is posted over parts of California, as well as a slight risk of heavy snow over the higher elevations of the state, from Feb 11-14. At the base of an amplifying trough, a broad slight risk area of high winds remains posted over the West, which is also supported by the ECMWF reforecast tool's increased chances of winds exceeding the 85th percentile. A slight risk of heavy snow also remains posted over the Four Corners region (Feb 12-14) where there is continued support from Snow Water Equivalent (SWE) reforecast tool indicating totals exceeding the 85th percentile late in week-2.

Over Alaska, periods of increased onshore flow are expected to promote above-normal precipitation over the southern Mainland and Panhandle throughout the period. Both the GEFS and ECMWF reforecast tools show increased chances for 3-day accumulations exceeding the 85th percentile, however amounts are not expected to exceed hazard criteria, therefore no corresponding hazards are issued.

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