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FXUS02 KWBC 041836
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
236 PM EDT Wed Aug 4 2021

Valid 12Z Sat Aug 7 2021 - 12Z Wed Aug 11 2021

18Z Update: With the quasi-zonal flow pattern becoming established
over the north-central U.S. and southern Canada, more noteworthy
model differences are becoming apparent compared to the recent
amplified pattern.  The trough entering the Pacific Northwest this
weekend is now in better agreement with the arrival of the 12Z
ECMWF, compared to the much slower and amplified 00Z ECMWF.  By
the middle of the week, the 12Z CMC becomes out of phase by having
a ridge over the Midwest states, whereas the model consensus
favors a building trough over this region.  More of the ensemble
means were incorporated for the middle of next week to account for
the increasing spread in the deterministic guidance.  Temperatures
for the most part should be near to above average for the majority
of the nation, with the only exception being across the Pacific
Northwest early in the forecast period with the upper level trough
moving through.  The previous forecast discussion is appended
below for reference. /Hamrick

...Overview...

Troughing will persist in the West and along the Canadian border
next week as upper ridging builds into the East and South. This
favors a drier pattern than in recent weeks with rainfall focused
mostly over the Upper Midwest/Great Lakes as systems converge in
the center of the continent and lift northeastward. Mid-summer
heat will return with a vengeance by the middle of next week in
much of the East.

...Guidance/Predictability Evaluation...

Through the 12Z/18Z cycle, the deterministic models largely paired
well with the ensembles except that the GFS runs were largely
quicker than the consensus. Though there has been a several day
trend of a quicker pace, and the quasi-zonal pattern may support
it, did not favor this quicker solution as many GEFS members were
as slow as the ECMWF (on the slowest end) while the UKMET/Canadian
fell in the middle. Opted to rely more on the ensembles means by
the end of the period in order to compensate for uncertainty in
timing by next Tue/Wed in the west-to-east flow. Favored a bit
higher heights over the Southeast per the GFS/GEFS just over the
5940m threshold.

...Weather Highlights/Hazards...

Rainfall will expand across the Upper Midwest and Great Lakes this
weekend ahead of a shortwave trough south of a main low over
BC/Alberta. Some rainfall could be locally heavy over MN into
WI/MI. As the system lifts into southern Ontario/Quebec, rainfall
coverage and intensity may wane a bit before perhaps picking up
again next Tue/Wed along the tail-end of the boundary again over
the Corn Belt into the Great Lakes. In the Southwest, some monsoon
rains may flare up over southeastern AZ as some moisture rides up
the west side of mainland Mexico.

Temperatures will be about 5-15 degrees cooler than normal in the
Pacific Northwest behind the lead cold front this weekend with a
slow moderation next week. Ahead of this lead cold front,
temperatures will be 5 to 12 degrees above normal over the Plains
this weekend and expanding to the East Coast next week. A ridge
over the Southeast sets up by Sunday and looks to persist well
into next week bringing an extended heat wave over the Lower
Mississippi Valley eastward to the Mid-Atlantic. Temperatures will
climb into the 90s with heat indices into the low 100s as dew
points rise into the 60s and low 70s.

Fracasso


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

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



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