Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
105 AM EDT Thu Jul 15 2021
Valid 12Z Sun Jul 18 2021 - 12Z Thu Jul 22 2021
...Heatwave from the northern Great Basin to the northern
Rockies/northern Plains...
...Weather Highlights/Hazards...
A potent upper ridge building from the Great Basin to the northern
Rockies/northern Plains this period will lead to a heatwave across
these regions. Temperatures are forecast to be the 90s and lower
100s at lower elevations this weekend into next week, which are
10-20 degrees above normal. Expect some record values will be
observed.
The above normal area expands east into MN and WI next week as
well. Temperatures in the Southwest will be slightly below normal
due to increased cloud cover and higher rainfall chances. Showers
and storms will be scattered across AZ/NM/southern CO/southern UT
during the afternoon and early evening. The monsoon season has
been more active than last summer in AZ and the favorable
conditions are forecast to persist across AZ. The GFS forecasts a
moisture plume from the Gulf of California north into AZ, with
above normal precipitable water values of 1.5-1.75 inches
persisting through Thu 22 July supporting the monsoonal convective
activity.
A wavy and slow moving front will move south down across the
Plains, MS Valley, OH Valley, Appalachians, Mid-Atlantic, and
settle over the South/Southeast this period, with scattered to
numerous showers and thunderstorms, with locally heavier amounts
where thunderstorms locally train. High temperatures are forecast
to fall below normal into early next week due the clouds and
showers in the vicinity of the modestly cooling front.
...Guidance/Predictability Evaluation...
The bulk of guidance seems in good agreement through early next
week in a pattern with overall above normal predictability, due to
both the large, sprawling anticyclone out west, plus slow movement
of the eastern US upper trough. The WPC medium range product suite
was primarily derived from the best clustered 12 UTC ECMWF/12z
ECMWF ensembles. Minor timing differences exist with the upper
trough in the east and anticyclone out west, but these differences
are less than usually observed days 5-7. The favored WPC blend
generally maintains good continuity.
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
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