Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
442 PM EDT Thu Jul 15 2021
Valid 12Z Sun Jul 18 2021 - 12Z Thu Jul 22 2021
...Hot temperatures from the northern Great Basin to the northern
Rockies/northern Plains...
...Overview...
An upper high over the Four Corners region Sunday will drift into
Wyoming, maintaining well above normal temperatures for much of
the region into the northern Plains. A deep upper low off the
coast of British Columbia/Haida Gwaii will very slowly wobble
inland over the period as the upper ridge over the Rockies
flattens a bit. In the East, upper troughing will be reinforced
from Canada and likely detach its southern extent westward across
the Lower Mississippi Valley and southern Plains toward the upper
Rio Grande, favoring a wetter and cooler than average pattern.
...Guidance/Predictability Evaluation...
The ensembles remained in very good agreement overall with the
longwave evolution and saw no reason to deviate from the
consensus. The 00Z ECMWF and somewhat the Canadian offered the
best clustering with the means as the GFS runs were more
aggressive with troughing in the east as a result of a more
amplified pattern upstream (slower with the upper low off BC).
Multi-day ensemble trend was slightly more amplified overall but
it is unclear if this will continue or if it was just within the
typical noise of a day 6-7 forecast.
...Weather Highlights/Hazards...
The potent upper ridge building from the Great Basin to the
northern Rockies/northern Plains will sustain very warm to hot
temperatures across these regions--90s and lower 100s at lower
elevations this weekend into next week, which are 10-20 degrees
above normal. This may near or exceed daily records especially
Sun/Mon. The above normal area will expand eastward into MN and WI
around midweek. Temperatures in the Southern Tier/Southwest will
be slightly below normal due to increased cloud cover and higher
rainfall chances as a cold front settles (and dissipates) into the
Southeast Mon-Wed. Rainfall could be locally heavy in parts of the
central to southern Plains and MS Valley Sun-Tue along and ahead
of the front. Farther west, the monsoon season has been more
active than last summer in AZ and nearby areas with conditions
favorable to persist, enhanced by the retrograding weakness aloft.
Rainfall could even extend into Southeastern California later in
the week.
A wavy and slow-moving lead front will through the Northeast with
its tail-end over the southern Mid-Atlantic as it settles over the
South/Southeast, with scattered to numerous showers and
thunderstorms and locally heavier amounts where thunderstorms
locally train. High temperatures are forecast to fall below
normal into early next week due the clouds and showers but near
normal in its wake. Another cold front out of Canada could provide
additional cooling of a few degrees around the eastern Great
Lakes/Northeast later next week in what is typically they hottest
time of the year.
Fracasso/Petersen
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
Hazards:
- Heavy rain across portions of the central Plains to the lower
Mississippi Valley, as well as portions of the Mid-Atlantic,
Sun-Mon, Jul 18-Jul 19.
- Heavy rain across portions of the southwestern Texas, Tue-Thu,
Jul 20-Jul 22.
- Heavy rain across portions of the Mogollon Rim to the Gila
Mountains in Arizona, Sun-Thu, Jul 18-Jul 22.
- Flooding possible across portions of the mid-Mississippi Valley,
the Great Lakes, and the Ohio Valley.
- Flooding occurring or imminent across portions of Florida and
the mid-Mississippi Valley.
- Excessive heat across portions of the Snake River valley and
northern Rockies, Sun-Mon, Jul 18-Jul 19.
- Excessive heat across portions of the northern High Plains,
Sun-Tue, Jul 18-Jul 20.
- Much above normal temperatures across portions of the central
and northern Great Basin, the northern to central Rockies, across
the northern Plains, and into the upper Midwest, Sun-Thu, Jul
18-Jul 22.
- Much above normal temperatures across portions of the central
Rockies, northern Plains, and into the upper Midwest, Wed-Thu, Jul
21-Jul 22.
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