Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
219 AM EDT Sun Sep 08 2019
Valid 12Z Wed Sep 11 2019 - 12Z Sun Sep 15 2019
...Potential for record heat to continue for the lower Mississippi
Valley and Southeast...
...Overview and Guidance Evaluation/Preferences...
An upper-level ridge is forecast to remain rather entrenched from
the lower Mississippi Valley to the Southeast through much of the
medium range. Meanwhile, initially splitting upper flow from the
eastern North Pacific into western North America will consolidate
as a stream of relatively vigorous upper shortwaves traverse the
CONUS northern tier. Farther south, an easterly wave is forecast
to approach Florida and the coastal southeastern U.S. by the
upcoming weekend, traversing the southern periphery of the
expansive upper ridge.
A blend of the 12Z ECMWF/CMC/18Z GFS was used as a forecast
starting point during days 3-4 (Wed-Thu). This blend represented
consensus well with the vigorous upper shortwave moving from the
Rockies toward the central/northern High Plains during that time,
with surface cyclogenesis expected across the Northern Plains late
Wed into early Thu. Less GFS was used from day 5 (Fri) onward due
to growing differences from consensus on the timing of the central
U.S. system, as well as a faster than consensus solution with the
next energetic shortwave trough approaching the Pacific Northwest
by that time. This trough reaching the Northwest from Fri onward
poses the biggest forecast problem of the period, with models
diverging significantly on whether the wave moves inland quickly
(GFS) or closes off and progresses much more slowly (CMC). The
ECMWF was in the middle if not perhaps a bit toward the faster
side of the spread with this feature. Based on flow upstream
across the North Pacific and in the vicinity of Alaska that is
quite amplified, the preference was to lean away from the faster
GFS, toward the middle of if not even a bit toward the slower side
of the solution spread. Thus, while weighting of ensemble means
was boosted during days 5-7 (Fri-Sun) to account for the increased
spread, minority components of the ECMWF and CMC were also
included.
...Weather Highlights/Threats...
Heavy rain will be possible across much of the Northern Plains and
Upper Midwest Wed-Thu in association with a deepening surface low
pressure system crossing the region. Models suggest the potential
for at least a couple inches of rainfall across a relatively wide
area. The easterly wave approaching Florida and the Southeast U.S.
by late in the week is expected to increase shower and
thunderstorm activity, with locally heavy rains possible.
Increasing rain, potentially moderate to heavy, will also be
possible across the mountain ranges of the Pacific Northwest by
late week as upper troughing progresses into the region.
Hot conditions are expected to persist across portions of the
south central and southeastern U.S. through much of the upcoming
week. 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. Several record high
temperatures are possible each day, especially from the lower
Mississippi Valley to the Southeast where highs may soar well into
the 90s, perhaps approaching 100 deg at a couple locations.
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