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Chris Swafford on August 28, 2023

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FXUS21 KWNC 281825

PMDTHR

US Hazards Outlook

NWS Climate Prediction Center College Park MD

300 PM EDT August 28 2023

SYNOPSIS: The seemingly endless summer heat that is forecast in the Great

Plains and Midwest during week-1 is forecast to continue across much of the

central contiguous United States (CONUS) during week-2 with the greatest

potential for excessive heat favored early in the period. However, a surface

high pressure over the Southern Appalachians may limit excessive heat chances

across the southern Atlantic States. Strong surface pressure gradients favored

in the Southwest may bring chances for high winds to the region.

HAZARDS

Moderate risk of excessive heat for portions of the Great Plains, Mississippi

Valley and Great Lakes, Tue-Thu, Sep 5-7.

Slight risk of excessive heat for portions of New Mexico, the Great Plains, the

Mississippi, Tennessee, and Ohio Valleys, and the Southeast, Tue-Mon, Sep 5-11.

Slight risk of high winds for portions of the Southwest, Tue-Mon, Sep 5-11.

Rapid onset drought risk for parts of the Southern Plains and Lower Mississippi

Valley.

DETAILED SUMMARY

FOR THURSDAY AUGUST 31 - MONDAY SEPTEMBER 04:

https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR TUESDAY SEPTEMBER 05 - MONDAY SEPTEMBER 11: Mid-level riding is forecast to

redevelop across the Great Plains during week-1 and is then forecast to persist

through much of the week-2 period. The highest 500-hPa height anomalies look to

be centered across the Great Lakes but may retrograde back into the Great

Plains during the period. As has been the case for much of the summer, the

ridging forecast is expected to bring much above-normal temperatures with

elevated chances for excessive heat. The Probabilistic Extremes Tools (PET)

from the GEFS and ECMWF both show enhanced chances (>50%) of temperatures

reaching the 90th climatological percentile across portions of the Great Lakes

and Upper-Mississippi Valley and temperatures near 90 deg F. Further south

across the Central and Southern Plains, elevated chances (>40%) of temperatures

exceeding the 90th climatological percentile and 100 deg F. Along the Gulf

Coast, the ECMWF PET has weaker chances of exceeding the 90th climatological

percentile compared to previous forecasts and relative to the GEFS PET.

However, the National Blend of Models (NBM) also indicates a wide area of near

record and record breaking temperatures on days 9 and 10 in areas along the

Gulf Coast. The calibrated heat index tools also indicate heat indices above

110 deg F in these areas consistent with the Weather Prediction Center’s (WPC)

day 7 heat index forecast. Therefore, a moderate risk of excessive heat is

posted for portions of the Gulf Coast, Central and Southern Plains, along with

the Middle and Lower Mississippi Valley for Sep 5-7. Meanwhile, a broader

slight risk of excessive heat covers portions of the Southeast, Gulf Coast,

Tennessee and Ohio River Valleys and Great Plains where the PETs from the GEFS

and ECMWF continue to show at least a 20% chance of temperatures exceeding the

90th percentile for much of the week-2 period and approaching hazards criteria.

In the Southwest, surface high pressure over the Colorado Plateau and surface

low pressure in northern Gulf of California lead to increased chances for high

winds. The PETs from the ECMWF and GEFS support this region and the overall

pattern looks unchanged throughout much of the week-2 period. Therefore, a

slight risk of high winds is posted for the entire week-2 period.

Along the Pacific Northwest, mid-level troughing is favored and PETs from the

GEFS and ECMWF indicate slight (>20%) chances of 3-day precipitation amounts

exceeding the 85th climatological percentile and half an inch of precipitation.

However, these amounts would not reach hazardous thresholds in the region and

raw precipitation guidance indicates lower amounts as well. Therefore, no

corresponding precipitation hazard is posted.

A Rapid Onset Drought (ROD) risk remains posted across parts of the Southern

Plains and Lower Mississippi Valley due to the combination of increased 30-day

precipitation deficits, little to no rainfall favored in the next week, and

high evapotranspiration rates associated with the above-normal temperatures.

In Alaska, positive 500-hPa height anomalies are forecast south of the Aleutian

Islands. North of this ridge above-normal precipitation is favored across

portions of western Alaska, however, this is unlikely to reach hazardous

thresholds. There is also a threat of a recurving tropical cyclone to impact

western Alaska during the week-2 period. However, uncertainty remains very high

and no corresponding hazard is posted. In Southeast Alaska, below-normal

precipitation is favored and unlikely to reach the 85th climatological

percentile with northwest flow aloft.

FORECASTER: Ryan Bolt