Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
238 AM EDT Wed Aug 07 2019
Valid 12Z Sat Aug 10 2019 - 12Z Wed Aug 14 2019
...Overview and Guidance/Predictability Assessment...
A zone of relatively progressive flow across the CONUS northern
tier will be squeeze between blocked flow at higher latitudes in
Canada, and a persistent upper ridge anchored across the south
central U.S. The GFS differed from the consensus of model guidance
even on day 3 (Sat) in this area of progressive flow with respect
to a shortwave crossing the Canadian prairies and skirting the
Great Lakes, showing a solution nearly out of phase with the 12Z
ECMWF/CMC/UKMET as well as the 12Z ECENS/18Z GEFS ensemble means
(although the 00Z run of the CMC did join the GFS). Model
consensus was relatively good with an upper low reaching the U.S.
West Coast on Sat. Solutions showed relatively small
timing/amplitude differences Sun-Mon as the system opens into a
broader upper-level wave and crosses from the northern Great Basin
to the Northern Plains. Models show this feature continuing to
lose amplitude and broaden out as it moves east Tue-Wed across the
Great Lakes, moving a cold front east into the Ohio Valley.
Finally, models seem to have settled more on a less progressive
trough off the U.S./Canada west coast by the early to middle
portion of next week - a solution that was previously preferred
due to an upstream blocking ridge across Alaska.
Given these considerations, the WPC forecast was initially based
heavily on a blend of the 12Z ECMWF and CMC during days 3-5
(Sat-Mon). A gradual increase in weighting of ECENS/GEFS ensemble
means was shown through time, with the majority of the forecast
comprised of ensemble means by days 6-7 (Tue-Wed).
...Weather Highlights/Threats...
Showers and thunderstorms will focus along a cold front moving
from the northern Rockies into the Northern/Central Plains
Sat-Sun, with areas of heavy rainfall possible. Convection with
locally heavy rains will continue to focus from the Central Plains
east to the Mid-Mississippi and Ohio Valleys early next week as
the frontal system moves east. Meanwhile, the weakening remains of
another frontal boundary will focus scattered showers and
thunderstorms across the Southeast through the forecast period.
Below average temperatures will accompany the upper low/trough
into the northwestern U.S., where high temperatures Sat-Sun are
forecast to be 5 to 15 deg below average. These temperatures
anomalies are forecast to spread east into the Northern Plains by
early next week. Meanwhile, hot conditions will persist across the
Southern Plains beneath the upper-level ridge, with high
temperatures forecast to be 5 to 10 deg above average into next
week.
Ryan
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards 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