Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
130 AM EDT Wed Aug 11 2021
Valid 12Z Sat Aug 14 2021 - 12Z Wed Aug 18 2021
...Major West/Northwest heat wave to challenge daily high
temperature records through Saturday and weakening by Sunday...
...Tropical Storm Fred to bring a heavy rain threat from Florida
this weekend and the Southeast/Southern Mid-Atlantic early next
week...
...Pattern Overview...
Latest models and ensembles continue to show a strong upper level
ridge continuing over the Western U.S. on Saturday, with amplified
troughing moving into the region by Sunday and beyond which should
suppress the ongoing heat wave. Meanwhile, an upper level trough
will move through from the Great Lakes to the Northeast
Saturday-Monday, with heights beginning to rise again across much
of the East by later in the medium range period. Tropical Storm
Fred will likely affect Florida this weekend bringing heavy
rainfall, and then also into parts of the Southeast and southern
Mid-Atlantic early next week as tropical moisture feeds inland and
interacts with a stalled leading frontal zone.
...Guidance/Predictability Evaluation...
Across the board, the guidance shows above average agreement
through much of the period both with respect to incoming troughing
across the West and the exiting trough in the Northeast. Out West,
the 12z CMC gets a little fast late period so it was excluded from
the blend. A total deterministic model blend worked well for days
3-4, but after that WPC prefers a blend of the more agreeable
ECMWF/GFS and the ensemble means which helps mitigate the typical
timing and detail differences of various smaller scale features.
The models have also come into better agreement regarding the
eventual track of Tropical Storm Fred. Most guidance tracks the
system north just off the western Florida coast and eventually
into the Southeast early next week. There remain some minor timing
uncertainties, which should work themselves out as it gets closer
in time. Please refer to NHC guidance for the latest details and
official forecast track of Fred.
...Weather Highlights/Hazards...
The heat wave that will focus across the Northwest will continue
into Saturday with max/min temperatures of 10-20+ above normal
likely to reach or exceed daily records both for high temperatures
and warm overnight lows. Sunday temperatures should still be above
normal, but not nearly as extreme as previous days. By Monday,
troughing looks to quickly move into the region so temperatures
should actually rebound below normal across the Northwest. Some
portions of the Northern Rockies could be 10-15 degrees below
normal next Tuesday into Wednesday. Above to much above normal
temperatures will shift eastward Sunday and Monday into parts of
the Northern Plains as well.
Tropical Storm Fred should be impacting much of the Florida
peninsula by the start of the period on Saturday. Heavy rainfall
and gusty winds are likely through the weekend as Fred tracks very
near to the state. At this time, the heaviest rainfall looks to be
along the Gulf Coast of Florida Saturday and Sunday, but heavier
rain bands reaching the central and eastern parts of the state are
likely, especially if the storm were to track a little bit farther
east of its current projected path. Tropical moisture will also
spread into the Southeast and southern Mid-Atlantic early next
week, and well ahead of Fred. This moisture may interact with a
leading stalled frontal boundary and favorable Appalachian terrain
offering the potential for a pre event.
Elsewhere, as the western upper ridge axis shifts east with advent
of upper troughing over the Northwest, monsoon activity over the
Desert Southwest should increase and spread northward, again
bringing a localized flash flood risk. A cold front should reach
the East Coast and Southeast this weekend focusing periods of
showers and thunderstorms with areas of locally moderate to heavy
rain likely. Exact amounts and locations remain somewhat uncertain
at this time and may take until the short range period to resolve.
Daily scattered storm chances will also exist back into the lower
Mississippi Valley and Southern Plains within a warm and moist
environment. In addition, combined heat and humidity should
support heat index values into at least the low 100s for many
across the lower Mississippi Valley and areas along the central
and western Gulf this coming weekend.
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