Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Wed Sep 11 2019
Valid 12Z Sat Sep 14 2019 - 12Z Wed Sep 18 2019
...Overview and Guidance Evaluation/Preferences...
...15Z update...
...Heavy rain possible from Florida westward to the central Gulf
states...
A couple of upper-level shortwaves moving eastward across the
northern tier states will give way to a more substantial long wave
trough settling into the western U.S. during the medium-range
period. Meanwhile, an upper-level ridge is forecast to slide off
the eastern U.S. as an upper-level low tracks westward across the
Gulf of Mexico. This upper low will likely interact with a
tropical wave which is forecast to track westward across the
Bahamas into Florida during the weekend. The ECMWF has joined the
rest of the global models in predicting a surface low developing
in the eastern Gulf on Sunday but its track has shifted from a
western outlier yesterday to being one of the eastern-most
solution. The UKMET is the eastern outlier, showing a track up
the east coast of Florida. On the other hand, the GFS and
Canadian models show a more consistent track toward the central to
eastern Gulf coast before turning northward further inland. The
interaction between the upper low and the tropical wave would
likely result in a sheared system with limited potentials for
rapid intensification. Nevertheless, the threat of heavy rain is
becoming more likely from Florida westward to the central Gulf
states from the weekend to early next week, possibly spreading
northward farther inland into mid-week next week.
The WPC morning grid package is a general compromise of the 00Z
ECMWF, 00Z EC mean, 06Z GFS, 06Z GEFS, 00Z CMC and its ensemble
mean. More weights have been given to the 00Z EC mean for Day 7
to keep a slower solution for the deep trough over western U.S.
Kong
...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 westward moving tropical disturbance. Several
cycles of model runs have consistently focused the greatest chance
of heavy rainfall along the immediate coastline from Louisiana
east to Florida, where several inches of rainfall are possible,
spread across multiple days. Moisture from this system may spread
farther north into portions of the lower Mississippi Valley by
early Mon-Tue. Widespread rains (and perhaps some snow at the
highest elevations) are expected to reach the Pacific Northwest by
Sat night into 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 this
weekend 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 degrees for many areas from the Plains to
the Eastern Seaboard this weekend into next week. The upper trough
reaching the western U.S. Sun-Mon will bring much cooler
temperatures, with highs 5 to 15 deg below average expected to
overspread the Northwest and northern Great Basin. 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