Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
120 AM EDT Sat Sep 07 2019
Valid 12Z Tue Sep 10 2019 - 12Z Sat Sep 14 2019
...Record heat to continue for the Lower Mississippi Valley/Gulf
Coast next week...
...Overview and Guidance Evaluation/Preferences...
The medium range period begins on Wed with an upper trough in the
west and a rather strong ridge (for September) in place across the
Southeast. An upper ridge initially in place across west central
Canada will result in split flow across western North America, but
this ridge should break down by Wed-Thu, eventually allowing
stronger westerlies from the higher latitudes to dig southward
into the northwestern U.S. Shortwave energy crossing central
Canada Tue-Wed is forecast to move a wave of low pressure and
associated frontal system across the Upper Midwest and Great
Lakes, reaching the Northeast Wed night. The more significant
upper trough initially across the West is forecast to move east
across the Rockies by Thu and into the Central/Northern Plains Thu
night/Fri. Model consensus shows relatively vigorous cyclogenesis
occurring across the Northern Plains Wed night/Thu morning ahead
of the shortwave crossing the Upper Midwest and moving into
central Canada by Thu night/Fri as the surface cold front
continues to sweep eastward into next weekend. Finally, as upper
flow consolidates by mid to late next week as described above,
models show some consensus that another shortwave should dig
toward the Pacific Northwest by next Fri, with an associated
surface frontal system pressing inland, reaching the Great
Basin/Rockies by Sat.
A blend of the 12Z ECMWF/18Z GFS served as a forecast starting
point during days 3-4 (Tue-Wed), although with more weight toward
the ECMWF relative to the GFS due to a low track from the Upper
Midwest into the Great Lakes more aligned with the overall
model/ensemble consensus. Starting on day 5 (Thu) and continuing
through day 7 (Sat), weight of ECENS/GEFS ensemble means was
gradually increased, although the deterministic ECMWF maintained
some utility at least into day 6 with a solution relatively in
line with the ensemble consensus. Model spread with respect to
both timing and structure of the shortwave reaching the Northwest
late next week was relatively high, with even some timing
differences noted between the ECENS/GEFS means which were
slower/faster, respectively. Given these, differences, a blend
primarily of the ensemble means was thought to represent a
reasonable compromise approach by the end of the forecast period.
...Weather Highlights/Threats...
Moderate to locally heavy rains will accompany the leading low
pressure system across the Upper Midwest/Great Lakes Tue-Wed. The
second low pressure system developing across the north central
U.S. Wed-Fri may result in a threat for somewhat more widespread
heavy rains. Models show a signal for an area of strongly forced
rainfall to the north of the surface low track across portions of
the northern High Plains, with at least a couple inches of rain
possible. Farther south, additional areas of showers and
thunderstorms will be possible across much of the central U.S.
along/ahead of the cold front, with the potential for at least
localized heavy rainfall.
Persistent hot temperatures across portions of the south central
and southeastern U.S. will be the other significant story in the
medium range. High temperatures are forecast to be 5 to 15 deg F
above average across a large area from the Ohio Valley to the
lower Mississippi Valley and the Southeast through most of next
week. Several record high temperatures are possible, especially
from the lower Mississippi Valley to the Southeast
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