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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0659Z Sep 17, 2020)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Thu Sep 17 2020

Valid 12Z Sun Sep 20 2020 - 12Z Thu Sep 24 2020

...Heavy rainfall threat for the western to west-central Gulf
Coast next week associated with likely slow moving tropical system
development in the Southwest Gulf of Mexico...

...Hurricane Teddy looming over the Atlantic...


...Guidance/Predictability Assessment and Weather/Hazard
Highlights...

Model forecasts remain pretty well clustered across the CONUS days
3/4 (Sun-Mon) and a blend of the latest GFS/ECMWF/CMC/UKMET seemed
reasonable. Guidance remains in reasonable agreement with the flow
pattern for much of the nation days 5-7, but there remains
question on how much a shortwave moving through the Great Lakes
amplifies across the Northeast, which has implications for the
eventual track of Hurricane Teddy during the day 5-7 frame. Recent
ECMWF runs and 12 UTC ECMWF ensembles offer a much slower and more
amplified Northeast U.S. trough than guidance consensus. This
would help to phase/steer a still powerful Teddy (likely beginning
extra tropical transition by that point) to the Canadian Maritimes
or New England next midweek. The past few GFS/CMC/GEFS runs have
been faster to progress the upper trough over the Northeast which
forces Teddy more offshore.  The older 12 UTC ECENS ensemble mean
was mostly used to create the WPC medium range product suite as it
offered a compromise between the aforementioned guidance camps and
best fit 03 UTC NHC guidance. This trended Teddy westward toward
the Canadian Maritimes and allowed midweek moderate
rains/unsettled conditions to wrap back into New England.
However, the newer 00 UTC GFS and especially the 00 UTC
UKMET/Canadian have trended increasingly toward the ECMWF,
substantially increasing threat potential in model space.

In the wake of Sally, strong high pressure settling into the East
will bring well below normal temperatures including a few record
values this weekend into early next week, with gradual moderation
from west to east. The high pressure surge will also act to
confine deeper moisture and heavy convective rain potential mainly
to Florida and the Gulf of Mexico near a slow moving trailing
front to linger over the next week. NHC is monitoring likely
development of a slow moving tropical disturbance now over the
southwest Gulf of Mexico and broad deep moisture seems set to fuel
widespread moderate to heavy rainfall that could feed in onshore
flow into the western to west-central Gulf Coast all next week.
The latest 00 UTC UKMET/Canadian and especially the 00 UTC ECMWF
now shows much more threatening development up into the western
Gulf off southern TX.  

There is also a growing later period model and ensemble signal to
develop an amplified northeast Pacific upper trough whose deepened
storm could force a lead plume of moisture and enhanced rainfall
into the Pacific Northwest mid-later next week in active flow.

Santorelli/Schichtel

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