Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
242 AM EDT Mon Aug 09 2021
Valid 12Z Thu Aug 12 2021 - 12Z Mon Aug 16 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. 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, while a tropical wave could bring enhanced
precipitation to parts of Florida by next weekend.
...Guidance/Predictability Evaluation...
Main issues to note are regarding details around individual
shortwaves through the mean troughing over the Great Lakes and the
Northeast. The GFS and CMC become much faster with the trough axis
than a lot of the guidance next Sunday and beyond. The ECMWF is
slower but also more in line with the ensemble means. Out west,
there remains spread and variability for how quickly an incoming
shortwave to the Northwest erodes the ridge late period, with
again, the GFS on the fast side of the envelope. The WPC blend
consisted of a mix of the deterministic runs for the first half of
the period, leaning more heavily on the ECMWF and the ensemble
means late period for the reasons outlined above. This approach
also maintained good continuity with the previous shift.
...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 Thursday 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 and Monday the highs may moderate somewhat as the
ridge gets eroded by incoming shortwave energy, but morning lows
should remain quite warm. 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 details on
exactly where remain very uncertain. 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 tropical wave/surface low and
some upper level energy moving into the area from the
Bahamas/Cuba. The National Hurricane Center is currently
monitoring this system for potential tropical development in the
next few days.
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 Thursday-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 Northeast later in the 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.
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