Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
218 AM EDT Tue Aug 06 2019
Valid 12Z Fri Aug 09 2019 - 12Z Tue Aug 13 2019
...Overview and Guidance/Predictability Assessment...
An upper low is expected to drift slowly eastward across James Bay
and northeastern Canada through the medium range, with broad
cyclonic flow south of the low across the eastern third of the
CONUS. Several shortwaves will traverse this zone of cyclonic
flow, the most pronounced of which should move a cold front to the
Eastern Seaboard on Fri, with the trailing end of the front
lingering from the Southern/Central Plains to the Southeast into
the weekend. Farther west, an upper low/trough initially off the
West Coast is forecast to push inland by Sat, moving into the
northern Great Basin and northern Rockies by Sun, and the Northern
Plains by Mon. Meanwhile, broad upper ridging is expected to
remain centered across the south central U.S. An additional
shortwave appears likely to approach the Pacific Northwest early
next week, although models differ quite a bit by that time on the
flow pattern across western/central Canada and the northwestern
U.S.
The 18Z GFS along with the 12Z ECMWF/CMC showed sufficient
consensus to use a blend of these solutions as a basis for the
forecast during days 3-4 (Fri-Sat). Starting on day 5 (Sun),
models continue to show differences with the handling of the
shortwave crossing the U.S. northern tier, and more generally with
the character of the northern stream flow from the Gulf of Alaska
into the northwestern U.S. The ECMWF shows more progressive flow
by the weekend into early next week across these areas, while the
GFS is somewhat less progressive, with a more amplified trough
offshore of British Columbia, and a slower eastward progression of
incoming additional shortwave energy as well as the shortwave
crossing the northern Rockies. Based on teleconnections associated
with a Rex block from the western Bering Sea into the Arctic
Ocean, preferred to lean toward the somewhat more amplified and
slower solution shown by the GFS by early next week. The 00Z GFS,
which ran after completion of the WPC forecast, doubled down on
this idea even further. Given these considerations, the forecast
for days 5-7 (Sun-Tue) leaned gradually more heavily on the 18Z
GFS/GEFS, with less weight on the 12Z ECMWF and ECENS.
...Weather Highlights/Threats...
The central U.S. will be the focus for potentially multiple rounds
of showers and thunderstorms with heavy rainfall. Models continue
to show a signal for an area of organized convection with heavy
rainfall across portions of the Mid-Mississippi Valley on Fri,
along the trailing end of the frontal boundary moving off the
Eastern Seaboard. The Pacific shortwave will initially bring areas
of heavy rain to portions of the interior Pacific Northwest
Fri-Sat before bringing showers and thunderstorms to the
Northern/Central Plains and portions of the Midwest Sun-Mon.
Convection appears likely to focus along a warm front, with models
showing the potential for heavy rainfall from portions of the
Central Plains east into the Midwest. Meanwhile, monsoonal
moisture will keep diurnal showers and thunderstorms active across
the Four Corners region and the southern/central Rockies through
the forecast period, with a daily threat for locally heavy
rainfall.
The upper ridge will keep hot conditions in place for the Southern
Plains, with high temperatures forecast to be 5 to 10 deg F above
average through early next week. Cooler temperatures will
accompany the Pacific shortwave inland, with highs forecast to be
5 to 15 deg below average from the Pacific Northwest to the
northern Rockies Sat-Sun, spreading into the Northern Plains by
Mon-Tue.
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