Threats Assessment Discussion

FXUS21 KWNC 131832
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 13 2019
SYNOPSIS: An expansive area of mid-level high pressure is forecast to result in 
above normal temperatures over much of the contiguous U.S. throughout week-2, 
with the hottest weather most likely in the southwest quadrant where the 
highest 500-hPa heights are projected. Mid-level low pressure over the Bering 
Sea favors a series of disturbances to move across the southern part of the 
mainland and Gulf of Alaska bringing increased chances for precipitation 
throughout week-2.
Moderate risk of excessive heat for portions of California and the Southwest, 
Wed-Fri, Aug 21-Aug 23.
Slight risk of excessive heat for portions of the Lower Mississippi Valley, the 
Southern Plains, the Southern Rockies, California, and the Southwest, Wed-Tue, 
Aug 21-Aug 27.
Slight risk of heavy precipitation for portions of the Alaska Panhandle and 
southern mainland Alaska, Wed-Tue, Aug 21-Aug 27.
FOR WEDNESDAY AUGUST 21 - TUESDAY AUGUST 27: Today's ECMWF reforecast run has 
substantially decreased probabilities for temperatures exceeding the 85th 
percentile of the climatological distribution in the Northeast and Great Lakes. 
While the probabilities have increased somewhat in the GEFS reforecast tool, 
the chances that actual temperatures reach hazards criteria is lower compared 
to yesterday. Therefore the excessive heat hazard is being removed from today's 
outlook. Generally, seasonably warm weather for late August over the Northeast 
and Great Lakes is plausible early in week-2 with temperatures in the 80s to 
near 90, with a cooling trend possible in the second half of week-2. 
The slight risk for excessive heat hazard is maintained for the Lower 
Mississippi Valley westward through the Southern Plains, Desert Southwest, and 
portions of the Central Valley of California throughout all of week-2. For 
parts of the Southern Plains and the Lower Mississippi Valley, high 
probabilities are shown for heat indices exceeding 105 deg F, with some parts 
of southeastern Texas and northern Louisiana showing the potential for heat 
indices reaching 110 deg F. Models also continue to suggest the potential for 
excessive heat over the Central Valley of California and the Desert Southwest. 
Both the GEFS and ECMWF probabilistic reforecast tools depict at least a 20% 
chance of maximum temperatures exceeding the 85th percentile compared to normal 
during week-2, with the ECMWF tool showing a large region of probabilities 
exceeding 40%. A moderate risk for excessive heat is posted over these areas 
where there is increased chances of max temperatures exceeding 105 deg F across 
southern parts of the Central Valley and possibly reaching 115 deg F over the 
Desert Southwest during the first half of week-2.
Troughing is forecast to build over the Bering Sea favoring increased storm 
activity over the western and southern parts of Alaska during week-2. A slight 
risk for heavy precipitation is posted over the southern mainland of Alaska and 
the Panhandle where impacts from surface low pressure are forecast early in 
week-2. There is a potential for 1-2 inches of precipitation (mainly rain, but 
some high elevation snow possible) in these highlighted areas. Another surface 
low may impact the same region later in the period. Increased wind speeds are 
also possible in association with these events but confidence is too low to 
issue a related hazard at this time.
For much of Week-2, the North American Monsoon is likely to remain suppressed 
as models favor overall dry conditions for the Southwest. However, the passage 
of a tropical disturbance over the eastern Pacific may trigger a gulf surge 
event late in week-2, leading to increased chances of precipitation. Related 
flooding and precipitation hazards may need to be introduced in future updates 
should this scenario become more probable.
FORECASTER: Thomas Collow