Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
401 PM EDT Thu Aug 13 2020

Valid 12Z Sun Aug 16 2020 - 12Z Thu Aug 20 2020

...Heat Wave with Record Temperatures for the dry West...

...Guidance/Predictability Assessment...

For the medium range period (Sunday - Thursday), there is
relatively good agreement in the deterministic and ensemble
guidance for a persistent western U.S. ridge and eastern U.S.
trough. Days 3-5 were well clustered on this idea and a near equal
weighting of the 00Z ECMWF, 06Z GFS, 00Z UKMET, and 00Z CMC were
utilized. Beyond that time frame, increasing uncertainty to the
strength and blocking of the western U.S. ridge, particularly how
it extends northward into the Intermountain West and Pacific NW
(flatter GFS versus more amplified ECMWF) led to more inclusion of
the ECENS and GEFS means at this time.

...Weather/Hazard Highlights...

A closed upper high centered over the southern Great Basin along
with upper ridging from the southern Plains through the West will
continue to inhibit monsoonal flow. Much above normal temperatures
will build north with the ridge over the Intermountain West and
West Coast to the Pacific Northwest. Heat wave temperatures will
range 5-15 degrees above normal, upwards to 15-20 degrees above
normal in the Northwest Sun-Tue. Record high max and min
temperatures will persist over the Desert Southwest with
temperatures up to 115-120 degrees. Additional record highs will
spreads over the rest of the West in tandem with upper trough
amplification off the West Coast. This will foster a continued dry
pattern with frontal relief expected only for the Pacific
Northwest late in the forecast period.

Meanwhile, amplified upper troughing settles over the east-central
U.S.. Widespread showers and thunderstorms will gradually push
through the Eastern Seaboard early next week as a wavy lead
front/modest coastal low and a temperature moderating main front
digs robustly down through the central and eastern U.S.. This may
produce some local runoff issues, especially early period for the
recently wet Mid-Atlantic. A wavy trailing front will settle
through the Southeast and Gulf of Mexico back to the
Plains/Rockies and focus additional bouts of convection, some
strong to severe, into mid-later next week as impulses dig to the
lee of the western ridge.  

Schichtel/Taylor

Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

Hazards:
- Heavy rain across portions of the Southeast, the Mid-Atlantic,
and the Southern Appalachians, Tue-Wed, Aug 18-Aug 19.
- Heavy rain across portions of the Mid-Atlantic, Sun, Aug 16.
- Flooding possible across portions of the Upper Mississippi
Valley.
- Excessive heat across portions of California, the Lower
Mississippi Valley, the Pacific Northwest, the Northern Great
Basin, and the Southern Plains, Sun, Aug 16.
- Excessive heat across portions of the Central Great Basin,
California, and the Southwest, Sun-Thu, Aug 16-Aug 20.
- Excessive heat across portions of the Lower Mississippi Valley
and the Southern Plains, Sun-Mon, Aug 16-Aug 17.
- Much above normal temperatures across portions of the Central
Great Basin, the Northern Plains, the Northern Rockies, the
Central Rockies, California, the Northern Great Basin, and the
Pacific Northwest, Sun-Tue, Aug 16-Aug 18.
- Much above normal temperatures across portions of the Central
Rockies and the Central Great Basin, Sun-Wed, Aug 16-Aug 19.
- Heavy rain across portions of the Alaska Panhandle, Sun-Mon, Aug
16-Aug 17.

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml