Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Sun Aug 08 2021
Valid 12Z Wed Aug 11 2021 - 12Z Sun Aug 15 2021
...Northwest heat wave likely to bring temperatures that challenge
daily records from midweek into the weekend...
...Overview...
Most guidance maintains good agreement and continuity for the
overall pattern through the period. Expect upper level ridges from
the eastern Pacific and southern Rockies/High Plains to
consolidate over the Great Basin and central Rockies by Friday
with a strong ridge remaining in place through the weekend.
Downstream the axis of a mean upper trough (with embedded
shortwaves) will move from the Upper Midwest into the Great Lakes
and Northeast. The strong ridging over the West should support a
heat wave that affects the Northwest and parts of the Great
Basin/California with temperatures over some areas likely to
approach or reach daily records for highs/warm lows from midweek
onward. Across the eastern third of the country, associated fronts
and leading warm/moist flow will produce areas of rain and
thunderstorms, some locally heavy at times.
...Guidance/Predictability Evaluation...
The primary guidance issues continue to involve details around the
periphery of the western U.S. ridge and shortwaves embedded within
the mean trough that settles into the Great Lakes/Northeast. Over
the West, 00Z/06Z GFS runs were generally stronger than other
guidance with shortwave energy on the south side of the upper high
while the new 12Z run has adjusted more to the model/mean average.
Also latest GFS runs have been on the aggressive side of
operational guidance in how much Pacific shortwave energy reaches
the northwestern U.S. and western Canada by next Sunday. Even
among remaining guidance there is some spread and variability for
how quickly heights aloft may decline. The GFS scenario is within
the full ensemble spread albeit in the minority while the new 12Z
ECMWF erodes the ridge a bit more than the prior run. An
intermediate model/mean solution looks reasonable at this time. A
blend approach maintains stability for the eastern trough aloft
and leading surface front details with embedded shortwave
specifics having lower predictability. The updated forecast
started with an operational model composite early and then trended
toward a half model (more 00Z ECMWF than 00Z CMC) and half mean
(06Z GEFS/00Z ECens) blend by day 7 Sunday.
...Weather Highlights/Hazards...
The Northwest heat wave should produce several days with max/min
temperatures of 10-20F above normal over Washington/Oregon and
some surrounding areas from Wednesday through the weekend.
Anomalies for daytime highs should be the greatest around
Thursday-Friday when a few locations could reach 20-25F above
normal. A decent number of locations across the Northwest and into
northern California/Nevada may see readings approach if not
reach/exceed daily records for highs and warm lows over multiple
days. By Sunday the highs may moderate somewhat but morning lows
should remain quite warm. Coverage and character of any smoke will
provide short-term wild cards for temperature forecasts. Currently
expect this heat wave to be less extreme than the all-time record
event in late June but it may still be the second most significant
of the summer for the Portland-Seattle corridor.
Shortwaves and moisture to the south of the consolidating upper
ridge over the Great Basin/central Rockies will likely promote an
increase of coverage and rainfall amounts in monsoon activity over
the Four Corners states and parts of the Southwest. There is still
some uncertainty in the relative strength of the shortwaves and
ridge to the north so the precise coverage/intensity of rain will
take additional time to resolve. High temperatures over the
southern Rockies/Southwest should be within a few degrees on
either side of normal Wednesday followed by the warmer areas
trending to modestly below normal by late week/weekend.
A weakening Plains into Great Lakes front and then a trailing
front that reaches the East Coast and south-central U.S. by next
weekend will focus areas of showers and thunderstorms over the
eastern third to half of the lower 48 during the period. Some
areas of locally moderate to heavy rain are likely but it may take
until the short-range time frame to refine the most favored areas
for highest totals. Current indications suggest some potential
from the Upper Ohio Valley and portions of the Appalachians to
areas along the East Coast. The southern Mid-Atlantic/Carolinas
and areas to the west could see increasing potential for
training/repeat activity later in the weekend as the front
decelerates. Farther ahead of the fronts, daily scattered storm
chances will also exist over the East and Deep South within a warm
and moist environment. Florida should see rainfall increase Friday
into the weekend due to a surface trough and some upper level
energy moving into the area from the Bahamas/Cuba. This feature
originates from a tropical wave initially near 50W longitude.
Very warm to hot conditions should prevail to the east and south
of the two cold fronts. The highest anomalies of plus 10-15F are
most likely for morning lows from the Great Lakes into the
Northeast Wednesday-Friday with a few daily records possible. Plus
5-10F anomalies will be more common for highs but there may be a
few isolated higher readings over the central Plains around
midweek and to the east/northeast mid-late week. The combined heat
and humidity could support heat index values into at least the low
100s for many lower elevations across the southern half to
two-thirds of the central/eastern U.S.. Highest heat index values
are likely over the lower half of the Mississippi Valley and
vicinity along with areas along and inland from the Gulf Coast.
The cold front reaching the East toward the end of the week will
likely bring high temperatures down to within a few degrees on
either side of normal over the central U.S. by Friday and farther
east by the weekend. This will suppress 100F+ heat indices more
toward the Gulf Coast region by Sunday.
Rausch/Santorelli
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