Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Fri Aug 23 2019
Valid 12Z Mon Aug 26 2019 - 12Z Fri Aug 30 2019
...Overview and Guidance/Predictability Assessment...
Broad cyclonic flow aloft is expected across the northern tier of
the nation for much of the upcoming week as an upper level low
develops over southern Canada and then Hudson Bay. This is
expected to result in a broad upper level trough over the northern
Plains and extending to the Great Lakes with a Canadian surface
high in place. Upper level ridging extending from Texas to
California is expected to continue through midweek, and also
across the western Atlantic Ocean.
A heavily ECMWF-based blend was used as a forecast starting point
through much of the forecast period, with a minority component of
the CMC also used during days 3-4 (Mon-Tue). The GFS was much
weaker/closer to the coast relative to the ECMWF with an area of
low pressure off the Carolinas on Mon. In this case the ECMWF was
more closely aligned with the ensemble consensus. Farther north,
models showed significant timing differences even on day 3 with
northern stream energy crossing the Gulf of Alaska which
eventually dives southeast into the broad upper trough across the
north central U.S. The GFS was substantially faster with this
energy and as a result the GFS was discarded and not used in the
forecast blend for this update. By days 5-7 (Wed-Fri) models began
to show some timing differences with upper shortwave energy in the
broader trough across the north central U.S., and with the
progression of a couple additional cold fronts from the central
into the eastern U.S. Given these differences, ensemble means
(ECENS and GEFS) were used much more heavily later in the forecast
period.
...Weather Highlights/Threats...
Low pressure east of the Carolinas may produce areas of heavy
rainfall near or just offshore early Monday before the system
quickly moves northeast away from the coast. Meanwhile, a hint of
fall is expected across the north-central U.S. next week with high
temperatures generally 5 to 15 degrees below normal, which equates
to high temperatures in the 60s and 70s F for much of this region.
It should continue feeling like the middle of summer from Texas to
the Great Basin and the Desert Southwest with temperatures 5 to 15
degrees above normal, with the greatest anomalies expected early
next week across western Texas and New Mexico. High temperatures
could exceed 110 degrees across the lower elevations of southern
Arizona and southeast California on Monday and Tuesday.
Areas of moderate to locally heavy rainfall will be possible ahead
of the cold front crossing the Upper Midwest and Great Lakes
region early next week. For midweek and beyond, model uncertainty
reduces confidence in placement of heaviest rainfall areas.
However, there is the potential for a corridor of enhanced
rainfall across the Central Plains later in the week near the
trailing end of the front.
Hamrick/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