Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
227 PM EDT Mon Aug 09 2021
Valid 12Z Thu Aug 12 2021 - 12Z Mon Aug 16 2021
...Major Northwest heat wave to challenge daily high temperature
records into the weekend...
...Tropical moisture brings a heavy rain threats to the Southeast
and Mid-Atlantic this weekend...
...Overview...
Deterministic guidance maintains good agreement and continuity for
the overall pattern of a rather amplified ridge drifting east
across the West Coast and a deep low over lifting north from
Hudson Bay Thursday through Saturday. There are then some timing
and strength differences for the next through entering the Pacific
Northwest/BC on Sunday into Monday. A tropical wave approaches
Florida or the eastern Gulf of Mexico this weekend with preceding
tropical moisture reaching into the Mid-Atlantic. The strong
ridging over the West will support an extreme heat wave that
affects the Northwest and parts of the Great Basin/California into
or through the weekend with temperatures over some areas expected
to reach or exceed daily records for both highs and especially
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, particularly ahead of
an approaching cold front and any tropical low center that
develops.
...Guidance/Predictability Evaluation...
Acceptable agreement with the 00Z ECMWF/CMC/UKMET and 06Z GFS was
sufficient to allow a simple blend of these through Day 5. The GFS
continues to be much more amplified with the next wave into BC
Saturday into Sunday which draws a low off the Pacific Northwest
inland while the ECMWF/CMC leave a low offshore. Based on this
difference and related downstream differences, went heavier toward
a GEFS/ECENS mean blend for Days 6 and 7. Regarding a tropical
wave approaching the CONUS from the Antilles this weekend, there
is decent agreement to having a cyclone develop and track toward
FL or the eastern Gulf of Mexico.
...Weather Highlights/Hazards...
The Northwest heat wave should produce several days with max/min
temperatures of 10-20F above normal across the Pacific Northwest
through Saturday night. Anomalies for daytime highs should be the
greatest around Thursday/Friday when a few locations, perhaps
centered over Portland, OR could reach 20-25F above normal. Dozens
of locations across the Northwest and into northern CA/NV should
see readings reach/exceed daily records for high max and mins. 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 still be
of major significance for the Portland-Seattle corridor.
As the upper ridge axis shifts east, monsoon activity over the
Desert Southwest should increase again which brings a localized
flash flood risk. The current guidance consensus indicates Arizona
as the main threat area.
A cold front pushes southeast from the northern Plains/Great Lakes
Thursday night, reaching the East Coast and Southeast this
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, particularly
Saturday as tropical moisture meets the front from the central
Appalachians to the TN Valley and then east to the Mid-Atlantic
and Southeast on Sunday. Farther ahead of the front, daily
scattered storm chances will also exist over the East and Deep
South within a warm and moist environment. Florida sees 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 continues to monitor
this system for further tropical development in the next few days.
Very warm to hot conditions prevail to the east and south of the
cold front through Friday. The highest anomalies of plus 10-15F
are most likely for morning lows from the Great Lakes into the
Northeast Thursday night-Friday night with several more daily
records possible Friday morning. The combined heat and humidity
should support heat index values into at least the low 100s for
many lower elevations on Friday from the Mid-Atlantic to eastern
TX. High heat index values are suppressed by the cold front
Saturday and Sunday to over the lower Mississippi Valley along
with areas along and inland from the Gulf Coast.
Jackson
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