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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1944Z Sep 17, 2020)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
344 PM 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 presents a late period threat to the Canadian
Maritimes/parts of New England...

...Guidance/Predictability Assessment...

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 some
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. The
ECMWF has been very consistent over the past day or two with a
more amplified Northeast U.S. trough, helping to steer a still
powerful Teddy (likely beginning extra tropical transition by
then) into the Canadian Maritimes by day 6/Wednesday. The past run
or two of the GFS/CMC/UKMET has also trended increasingly towards
the ECMWF, albeit a bit slower, which boosts confidence a bit on
Teddy's eventual track compared to recent days. The ensemble means
also would support a track into the Canadian Maritimes as well.

The WPC blend for today relies heavily on the agreeable
deterministic models for days 3-4, with increasing contributions
from the ensemble means to help mitigate the differences with
Teddy, a shortwave traversing the Canadian/US border day 6-7, and
also potential tropical development in the Gulf of Mexico (which
shows considerable spread late in the period). This approach also
helps maintain good continuity from the previous overnight WPC

...Weather/Hazard Highlights...

Confidence has increased some in the recent days on what is
currently Hurricane Teddy over the central Atlantic eventually
tracking towards/into the Canadian Maritimes. This would increase
the threat for wrapback moderate rains/unsettled conditions into
parts of New England by next Tuesday into Wednesday.

A cold front pushing offshore to begin the period will allow
strong high pressure to settle into the East bringing well below
normal temperatures and a few record values this weekend into
early next week, with gradual moderation from west to east. This
should also help to confine deeper moisture and convective rain
potential mainly to Florida and the Gulf Coast near a slow moving
trailing front which looks to linger over the next week. In
addition, NHC continues to monitor likely development of a slow
moving tropical disturbance in the southwest Gulf of Mexico, and
models seem set to bring moderate to heavy rainfall into the
western to west-central Gulf Coast beginning as early as this
weekend. It should be noted there remains considerable spread
amongst the models and ensembles on whether this potential low
tracks more towards south Texas or farther north into the
west-central Gulf Coast, of course having strong implications on
if/when/where the heaviest rainfall sets up. The potential is
there and so the coastal threat continues to be highlighted both
in the latest WPC medium range QPF and day 3-7 hazards chart.

Elsewhere, temperatures should remain above to much above normal
throughout the period across the north-central U.S. and back into
the interior West. There is also growing signal in the models and
ensembles to develop an amplified northeast Pacific upper trough.
The resultant deepened storm could force a lead plume of much
needed and welcomed moisture and enhanced rainfall into the
Pacific Northwest mid to later next week.


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
- Heavy rain across portions of the Pacific Northwest and the
Northern Great Basin, Wed-Thu, Sep 23-Sep 24.
- Heavy rain across portions of the Southern Plains, the Lower
Mississippi Valley, and the Southeast, Sun-Thu, Sep 20-Sep 24.
- Flooding possible across portions of the Southeast, the
Mid-Atlantic, the Southern Appalachians, and the Tennessee Valley.
- Flooding occurring or imminent across portions of the Southeast.
- Flooding likely across portions of the Southeast.
- Much below normal temperatures across portions of the Great
Lakes, the Ohio Valley, the Central Appalachians, the
Mid-Atlantic, and the Northeast, Sun-Tue, Sep 20-Sep 22.

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: