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Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Wednesday August 24, 2022 to Tuesday August 30, 2022

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 16 2022

Synopsis: Dynamical models are in good agreement regarding mid-level high pressure across the western contiguous U.S. (CONUS), and mid-level low pressure downstream over the central and eastern CONUS during week-2. This pattern favors increased chances for excessive heat across portions of the western CONUS and Northern High Plains. An unseasonably cool pattern is forecast across much of the interior eastern and central CONUS during late-August with enhanced rain chances over parts of the Middle and Lower Mississippi, Ohio, and Tennessee Valleys tied to the aforementioned mid-level low pressure. The southwest monsoon is forecast to remain active early in the period, although weakening mid-level high pressure combined with decreasing East Pacific tropical cyclone activity is expected to reduce the amount of moisture over the region later in the period. Mid-level low pressure forecast across Alaska favors enhanced southerly flow and rounds of heavy precipitation along the southern coastal Mainland during week-2.

Hazards

• Moderate risk of excessive heat across portions of the Pacific Northwest, Intermountain West, Northern Rockies, and northern California, Wed-Fri, Aug 24-26.
• Slight risk of excessive heat across portions of the Pacific Northwest, Intermountain West, Northern Rockies, Northern Plains, and California, Wed-Sun, Aug 24-28.
• Slight risk of heavy precipitation across portions of the Central Great Basin, Desert Southwest, and Central and Southern Rockies, Wed-Sat, Aug 24-27.
• Slight risk of heavy precipitation across portions of the Southern Plains, Middle and Lower Mississippi Valley, Ohio and Tennessee Valleys, Southern Appalachians, and Southeast Wed-Fri, Aug 24-26.
• Slight risk of heavy precipitation across portions of southern Mainland Alaska and the northern Panhandle, Wed-Sun, Aug 24-28.
• Rapid-onset drought possible for portions of the Northern Plains, Northern Great Basin, and Northern Rockies.
• Flooding possible for portions of the Four Corners Region and Southwest.

Detailed Summary

For Friday August 19 - Tuesday August 23: WPC Days 3-7 U.S. Hazards

For Wednesday August 24 - Tuesday August 30: The 0z ECMWF, GEFS, and Canadian ensembles are in very good agreement during week-2, depicting an amplified 500-hPa height pattern across western North America, with troughing favored over Alaska, and ridging over the western CONUS and Canada. While a slight reprieve in the heat over the Northwest CONUS is possible late in week-1, relatively higher temperatures are likely to return to the region early in week-2, corresponding with an increased heat signal in the reforecast tools. The GEFS reforecast tool depicts some areas of the Pacific Northwest and Intermountain Region having at least a 40 percent chance of maximum temperatures exceeding the 85th climatological percentile. Both the GEFS and ECMWF reforecast tools indicate at least a 30 percent chance of maximum temperatures exceeding the 85th climatological percentile extending southward from the Northwest to the northern half of the California Central Valley. The reforecast tool probabilities indicating extreme heat are higher today compared to yesterday, and the National Blend of Models also indicates the potential for daily record high temperatures over parts of the Northwest early in week-2 (mid to upper 90s deg F). Therefore a moderate risk for excessive heat is highlighted across the Pacific Northwest, Intermountain Region, and northern California for 8/24-8/26.

A broader area of slight risk for excessive heat includes all of the California Central Valley where temperatures may exceed 100 deg F, and also extends eastward through the Northern High Plains where temperatures may exceed 95 deg F. The ridge is forecast to weaken later in week-2 allowing the slight risk to be discontinued after 8/28. The persistent anomalously warm temperatures, combined with little precipitation forecast during the next week, favors an elevated risk for rapid onset drought across portions of the Pacific Northwest, Northern Great Basin, and Northern Plains, as well as enhanced wildfire threat over these areas.

For much of the central and interior eastern CONUS, an unseasonably cooler pattern is forecast due to intermittent troughing forecast over the region downstream of the ridging to the west. Enhanced precipitation is also likely, especially over parts of the Southern Plains, Middle and Lower Mississippi Valley, and the Ohio and Tennessee Valleys, where a shortwave impulse is forecast to develop along the mean trough axis around day-7 (8/23), bringing the potential for heavy rainfall persisting into the early part of week-2. Over time, the greatest chances for heavy precipitation are forecast to shift eastward and southward, reaching the Central Gulf of Mexico Coast by day-10 (8/26). The ECMWF and GEFS reforecast tools indicate at least a 20 percent chance of 3-day precipitation totals exceeding the 85th percentile and 1-inch early in the period over parts of these areas, further justifying a slight risk of heavy precipitation from 8/24-8/26.

The southwest monsoon is forecast to remain moderately active during the first half of week-2. The ECMWF reforecast tool depicts probabilities of at least 20 percent for 3-day precipitation exceeding the 85th climatological percentile and a half-inch through the middle of the period across much of the Four Corners Region and Southwest. The GEFS reforecast tool depicts weaker probabilities and less coverage compared to the ECMWF, with many areas falling below 20 percent by the middle of the period. The slight risk of heavy precipitation is maintained from Aug 24-27 over much of the Southwest and Four Corners region given good continuity with yesterday and the more robust ECMWF guidance. A flooding possible hazard remains posted over much of the highlighted region as the convective nature of precipitation could result in localized flash flooding. It is worth noting that flash flooding can occur even in areas that do not directly receive rainfall due to runoff of precipitation upstream. The monsoon is expected to weaken toward the end of week-2 due to a flattening of the ridge over the West, along with a decrease in tropical cyclone activity over the East Pacific.

Mid-level troughing forecast across Alaska, along with associated surface low pressure south of the Aleutian Islands, favors enhanced onshore flow over the southern Mainland throughout week-2. The amplitude of the troughing is weaker compared to yesterday’s guidance, with the GEFS and ECMWF reforecast tools indicating reduced coverage of areas having at least a 20 percent chance of 3-day precipitation totals exceeding the 85th percentile and 2 inches. However, uncalibrated guidance from the GEFS, ECMWF, and Canadian ensembles continue to indicate localized totals of over 4 inches for week-2 as a whole, with the greatest precipitation chances during the early and middle parts of the period. Several inches of snow is also possible across higher elevation areas of the Alaska Range. A slight risk for heavy precipitation is maintained over much of the southern coastal Mainland and northern Panhandle from 8/24-8/28 despite the weaker probabilities in the reforecast tools relative to yesterday, given that the uncalibrated guidance continues to depict high precipitation totals over the region.

Forecaster: Thomas Collow

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

Resources

Week-2 Probabilistic Extremes Tool

GFS Ensemble Forecasts