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Prognostic Meteorological Discussion (PMD)

Issued by National Weather Service (NWS)

                            
000
FXUS21 KWNC 221809
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 22 2019

SYNOPSIS: Amplified mid-level high and low pressure is forecast over the 
western and central  U.S. for the beginning half of week-2. Warm temperatures 
are likely over the interior West Coast, especially over the Pacific Northwest. 
As the mid-level low shifts south, colder than normal temperatures are expected 
with the descending air mass from Canada. A shift in the mid-level pattern 
could give Southern Alaska a good shot of much needed heavy rainfall, as they 
have had a much drier than normal August. 

HAZARDS

Moderate risk of excessive heat for portions of California, and the Pacific 
Northwest, Fri-Sun, Aug 30-Sep 1.

Slight risk of excessive heat for portions of the western U.S., Fri-Mon, Aug 
30-Sep 2.

Slight risk of much below normal temperatures for portions of the central and 
northern Rockies, the northern Great Plains and the Upper Mississippi Valley, 
Fri-Sun, Aug 30-Sep 1.

Slight risk of heavy precipitation for portions of the Alaska Panhandle and 
mainland Alaska, Mon-Wed, Sep 2-Sep 4.

DETAILED SUMMARY

FOR SUNDAY AUGUST 25 - THURSDAY AUGUST 29: 
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR FRIDAY AUGUST 30 - THURSDAY SEPTEMBER 05: The mid-level pattern forecast 
over the CONUS for week-2 has continued to weaken in the model solutions. 
Ridging is still forecast for the western CONUS for part of week-1 moving into 
week-2, with a  trough developing over the central Plains late in week-1. This 
amplified wave pattern is expected wane mid-period, with the trough shifting 
northeastward and deamplifying and weak ridging spreading eastward toward 
central CONUS. 



For the western CONUS, the widespread excessive heat that has been a theme in 
our forecasts this week has been scaled back, with lower probabilities now 
forecast. The majority of the extreme temperatures over the Southwest are now 
firmly in the week-1 time period, so the extent of the slight and moderate 
risks for excessive heat have been trimmed westward.  Over the Pacific 
Northwest, temperatures exceeding 90 degrees F, are still forecast for the 
first few days of the period. A moderate risk of excessive heat (40% chance) 
extends from northern California northward through the interior Pacific 
Northwest, 8/30-9/1. A wider slight risk hazard (20-40%) of excessive heat 
continues over most of the western CONUS through Labor Day, 9/2.

 

With the mid-level trough descending southward over the Plains region, 
anomalous northerly flow out of Canada is likely to cause cooler temperatures 
to overspread the region. A slight risk for much below normal temperatures has 
been forecast for parts of the Rockies and High Plains, extending toward the 
Upper Mississippi Valley. Minimum temperatures could fall below 50 degrees F 
for much of the highlighted region, with temperatures around freezing (32 
degrees F) at the higher elevations of the Northern Rockies. Early frosts are 
not expected on the eastern half of the highlighted region, but cooler 
temperatures could impact growing degree days for some of the northern and 
western crop regions.



Frontal activity over the eastern U.S. late in week-1 could cause some 
lingering heavy rainfall through the beginning of week-2. The GEFS mean 
indicates chances for daily rainfall rates of 1-1.5 inches on 8/30-31. The 
ECMWF model falls flat on this rainfall though, with very disperse and minimal 
rainfall totals reflected in the 00z model run. Due to this discrepancy, heavy 
rainfall is not currently forecast at this time.



Mid-level conditions over Alaska are forecast to become more favorable for 
precipitation along the South Coast. Conditions along the coast have been 
unseasonably dry, with rainfall totals falling much below normal over the past 
month. Model guidance is currently indicating the possibility of a strong storm 
system moving through after Labor Day, toward the end of Week-2. Rainfall 
totals from this storm could near 2 inches. A slight risk for heavy 
precipitation has been forecast for parts of the South Coast for 9/2-9/4.

FORECASTER: Christina Maurin 

$$