Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EDT Wed Sep 11 2019
Valid 12Z Sat Sep 14 2019 - 12Z Wed Sep 18 2019
...Overview and Guidance Evaluation/Preferences...
An upper-level ridge is forecast to expand across the central and
eastern U.S. through much of the medium range, centered across the
Mid-Mississippi and Ohio Valleys. Meanwhile, a relatively vigorous
shortwave is expected to move exit the Great Lakes into eastern
Canada on Fri, with a large upper-level trough expected to amplify
across the western U.S. from Sun onward. Farther south, a tropical
disturbance is expected to move westward across the Gulf of Mexico
Sat-Mon, with a number of solutions developing an area of low
pressure along the wave. The latest outlook from the National
Hurricane Center indicates a medium chance of tropical development
with this feature through the next several days.
A blend of the 12Z ECMWF/CMC/18Z GFS was used as a basis for the
forecast during days 3-4 (Sat-Sun). A blend of these solutions
should favorably depict features a series of shortwaves/surface
frontal systems crossing the CONUS during that time period. Model
consensus has improved quite a bit over recent days with respect
to the evolution of a large western U.S. trough early next week,
with the GFS now on board with the idea which was first shown by
the CMC a few days ago. Conceptually, this continues to make sense
given an amplified pattern in place upstream across the North
Pacific. Despite the improvement shown in the GFS, it continues to
show some degree of fluctuation on the eventual amplitude of the
upper trough by Tue-Wed, and is generally a bit less deep than the
ECMWF/CMC. Given the model trends across recent days, preferred to
lean more toward the ECMWF/CMC along with increasing ensemble
means (ECENS/NAEFS) through time during days 5-7 (Mon-Wed). Given
trends shown in ensemble members, cannot rule out an adjustment
toward an even slower/deeper upper trough solution across the West
in future forecast cycles.
...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