WPC Medium Range Archive
Medium Range Products Valid On:
(Day 7 PMDEPD: Valid 09/16/2020 to 09/20/2020)
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Sun Sep 13 2020
Valid 12Z Wed Sep 16 2020 - 12Z Sun Sep 20 2020
...Sally is expected to spread heavy rainfall northward into the
Tennessee Valley and the southern Appalachians Wednesday night
through Friday morning...
...Welcome rains for the Northwest, northern California, and the
northern Rockies this week into the weekend...
Lead by the 12Z ECMWF from yesterday, models today have generally
trended toward a more inland recurvature of Sally across the
interior Southeast during the latter part of this week. Both the
00Z UKMET and 00Z CMC still preferred a sharper recurvature of
Sally across southern Alabama and southern Georgia but their
latest 12Z runs have also shifted toward a more gradual turn to
the northeast in agreement with the GFS and ECMWF. The latest 12Z
ECMWF did track Sally slight farther south but that is within the
realm of reasonable uncertainty.
Elsewhere, models have come into better agreement on the timing of
an upper low moving into the Pacific Northwest late this week.
The GFS appears to be too fast in bringing upper level energy into
the High Plains by this weekend, leading to too much surface
trough/low pressure development. Therefore, the morning WPC
medium-range prognostic charts were derived using an even blend of
the 06 UTC GFS and the 00Z ECMWF with some of their ensemble
means, then gradually eliminating the GFS solution toward day 7,
and with larger weights toward the 00Z EC mean.
...Overview and Weather/Hazard Highlights...
With Sally's forecast track farther inland into the interior
Southeast after landfall, the threat of heavy rainfall has
increased across the Tennessee Valley and the southern
Appalachians. The timing of the heavy rain should be from
Wednesday night through Friday morning. Sally is expected to
become extratropical over the interior Southeast as it merges with
a stationary front there. A longwave trough forecast to dip into
the eastern U.S. should then push the system relatively quickly
off to the east and into the Atlantic by the weekend.
Meanwhile, a complex evolution over the northeastern Pacific
should yield a fairly deep trough/closed low that is slated to
slowly bring some welcome rainfall and cooler temperatures into
the fire stricken Northwest and northern California by midweek,
lingering into late week in a wet late summer pattern.
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: