Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
223 PM EDT Tue Sep 14 2021
Valid 12Z Fri Sep 17 2021 - 12Z Tue Sep 21 2021
...Overview and Guidance Evaluation/Preferences...
The synoptic pattern will be amplifying during the extended period
with troughing over the West and ridging over the East. As the
week progress so will the trough, shifting from the western states
into the central U.S. The flow in between will help move along
frontal systems across the Plains and New England. There is an
area of convection near the Bahamas in the short term that the
models have organizing by or before the day 3 period and persists
through day 7 as it tracks further out over the Atlantic Ocean.
The National Hurricane Center is monitoring this disturbance as a
possible tropical system so refer to NHC's Tropical Weather
Outlooks for additional information. The intensity and placement
of QPF will be dependent on the strength and path of this feature,
however at this time the heaviest rainfall should remain offshore,
perhaps well offshore.
The clustering of the models is relatively good initially, but
they continue to struggle with the overall evolution of a few key
features that by day 5- particularly the timing of the trough axis
and individual shortwave perturbations within the mean trough
begin to arise as it enters the Western U.S. The ensemble means
have a better handle on this especially for days 6 and 7 therefore
were utilized heavily. WPC favored an increasing blend towards the
ensemble means days 5-7 with some modest contributions from the
ECMWF and GFS for added flow definition. This blend of guidance
maintained continuity with the previous WPC forecast.
...Weather/Hazard Highlights...
Tropical cyclone Nicholas will likely be recent memory by this
start of this forecast but remnant tropical moisture may stick
around over the Gulf states but with low confidence in the
details. Back along the gulf coast, this moisture may also
interact with a potential convergence boundary to produce
additional mainly light to moderate rainfall across parts of
southeast Texas and Louisiana. Localized instances of flooding may
be possible, depending on resulting heavy rain impacts as Nicholas
moves through that region during the short range.
Fronts tracking through the northern tier of the country will
bring areas of rain, possibly moderate to heavy across portions of
the Upper Mississippi Valley and Great Lakes. With greater
confidence, areas of moderate to locally heavy rainfall will
likely reach the Pacific Northwest Friday into the weekend with
some snow possible in the highest elevations of the Washington
Cascades and into the Northern Rockies. The majority of rainfall
with the possible system off the East Coast should remain offshore
but areas from the Mid-Atlantic into New England should continue
to monitor forecasts closely due to the western periphery of the
moisture shield being very sensitive to exact track.
The amplified troughing over the West will allow for cooler
temperatures over the Northwest. Daily highs will average 10-15F
below normal beyond Friday with gradual expansion into the
Northern Rockies as surface high pressure settles in behind the
front. A warming trend in temperatures is expected from the
Southern/Central Plains to the Northeast within a building ridge
aloft. Warmest anomalies of possibly +10-20F should shift from the
Northern Plains on Saturday into the Upper Midwest and Great Lakes
Sunday-Monday.
Campbell
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook 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