Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
205 AM EDT Thu Sep 12 2019
Valid 12Z Sun Sep 15 2019 - 12Z Thu Sep 19 2019
...Heavy rain possible across portions of the Southeast...
...Overview and Guidance Evaluation/Preferences...
An amplified upper-level pattern is expected to develop across the
CONUS during the medium range, with an expanding and broad ridge
across the eastern third of the nation, and an
amplified/slow-moving trough digging into the West. A northern
stream shortwave is forecast to skirt the northeastern U.S.
through Canada Sun-Mon, pushing a cold front off the Eastern
Seaboard. The deepening western trough will also spread a cold
front east across the Great Basin and Rockies Mon-Tue and into the
central U.S. by Wed-Thu. Meanwhile, a tropical disturbance is
forecast to move northwestward across Florida and the eastern Gulf
of Mexico and into the southeastern U.S. Sun-Tue.
A blend of the 12Z ECMWF/CMC/18Z GFS (with more emphasis place
toward the ECMWF/CMC) was used as a basis for the forecast through
much of the medium range. The ECMWF/CMC showed relatively good
consensus on the track of the tropical disturbance affecting
Florida and the Southeast days 3-5 (Sun-Tue). The GFS was
eventually much farther west with this system, showing an
interaction between it and an upper low farther west across the
Gulf. Elsewhere, models showed relatively good consensus with the
northern stream shortwave crossing eastern Canada days 3-4, and
differences with the amplifying western trough were much less than
in recent days, primarily confined to relatively small timing and
amplitude differences (especially days 3-5). By days 6-7 (Wed-Thu)
as the western trough begins to emerge into the central U.S.
timing differences increased a bit with the GFS now among the
slower solutions and the ECMWF/CMC deeper but a bit quicker. Given
increased spread days 6-7, significantly boosted weight of
ECENS/NAEFS means during that time period to account for the
spread/uncertainty.
...Weather Highlights/Threats...
Numerous showers and thunderstorms are expected across Florida and
the Gulf Coast states this weekend into early next week in
association with the aforementioned tropical disturbance. Several
pieces of guidance suggest the potential for multi-inch rainfall
totals across several days over portions of Florida and southern
Alabama/Georgia from Sun through the first part of next week.
Widespread rains (and perhaps some snow at the highest elevations)
are expected across the Pacific Northwest by Sun as the upper
trough begins to amplify. Precipitation should spread inland
across the Great Basin and northern Rockies early next week.
Southerly flow east of the upper trough may revive monsoonal
showers and storms across the Four Corners region, with coverage
of precipitation increasing through early next week.
The broad upper ridge across the central and eastern U.S. will
result in above average temperatures across a wide area. Highs are
forecast to be 5 to 15 deg F above average for many areas from the
Plains to the Eastern Seaboard into next week. The upper trough
reaching the western U.S. Sun-Mon will bring much cooler
temperatures, with highs 10 to nearly 20 deg below average
expected to overspread the Northwest, Great Basin, and eventually
the northern Rockies. As mentioned above, with increasing
precipitation across the Northwest, temperatures should be cold
enough to support snow at the higher peaks.
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