Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
223 AM 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 onward...
...Overview...
Most of the guidance agrees that upper level ridging will
consolidate from the Northwest U.S. into the Southern Plains
during the extended range period, while upper troughing (and
embedded shortwaves) moves through the Upper Great Lakes and
Northeast. These features should support a heat wave affecting 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 latest cycle of model guidance continues to show above average
agreement regarding both the upper ridge across the West and
South, and mean troughing settling from the Great Lakes to the
Northeast. There remains some typical uncertainties in the details
of individual shortwave impulses embedded within the larger scale
trough, but a general blend of the guidance seems like a good
starting point right now. Elsewhere, there continues to be some
timing differences regarding a shortwave dropping out of the Gulf
of Alaska as well as another shortwave moving towards the West
Coast day 7/Sunday which could act to suppress the Western U.S.
ridge a bit. Given the late period timing, opted to lean on a
50/50 blend of the deterministic models and ensemble means which
gave a reasonable middle ground solution.
...Weather Highlights/Hazards...
For the Northwest heat wave, daytime high anomalies of plus 10-20F
or so should prevail over Washington/Oregon and surrounding areas
from Wednesday through the weekend. A decent number of locations
across the Northwest and into northern California may see readings
approach if not reach/exceed daily records for highs and warm lows
over multiple days. Current forecasts suggest this heat wave will
not be nearly as extreme as the all-time record event in late June
but could still be the second most significant of the summer for
the Portland-Seattle corridor.
Meanwhile, shortwaves/moisture tracking around the southern side
of the consolidating upper ridge over the Great Basin/central
Rockies will likely provide a steady increase of coverage and
rainfall amounts in monsoon activity over and near the Four
Corners states. As a result high temperatures over this area may
decline a bit after near or slightly above normal values early in
the period.
To the east of the Rockies, a leading Plains into Great Lakes
front (dissipating by Thursday) and a trailing front that reaches
the East Coast and south-central U.S. around the start of next
weekend will provide the best-defined focus for showers and
thunderstorms during the period. Some areas of locally moderate to
heavy rain will be possible from the Midwest and Great Lakes/Ohio
Valley into the Mid-Atlantic/Northeast but it will likely take
into the short-range time frame to resolve the finer details of
magnitude and location of highest totals. Farther ahead of the
fronts, daily scattered storm chances will also exist over the
East and Deep South within a warm and moist environment. A
tropical wave may also increase rainfall over southern Florida
next Friday into Saturday.
Very warm to hot conditions should prevail to the east and south
of the two cold fronts. The highest anomalies of plus 10F or
greater should be for morning lows over and near the Great Lakes
especially Wednesday-Thursday plus the Northeast late in the week,
though a few isolated plus 10F anomalies for highs could be
possible. Even with single-digit anomalies for highs over most
areas, the combined heat and humidity could support heat index
values into the low 100s for many in at least the southern half 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.
Santorelli/Rausch
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