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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2030Z Sep 18, 2020)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
430 PM EDT Fri Sep 18 2020

Valid 12Z Mon Sep 21 2020 - 12Z Fri Sep 25 2020

...Heavy rainfall threat for the Western and Northwest Gulf Coast
next week from TD Twenty-Two...

...Hurricane Teddy may pose a threat to the Canadian
Maritimes/parts of New England...


...Overview...

Upper ridging will build into the Rockies and toward the Plains
next week as troughing eases into the Pacific Northwest and
eventually the East. Confidence is increasing on twin tropical
systems having an affect on the Gulf (TD Twenty-Two) and perhaps
Maine (Hurricane Teddy) directly or indirectly. At the very least,
Teddy will churn up waves in the Atlantic into the medium range
period before it races into Eastern Canada as an extra Tropical
low.


...Guidance/Predictability Assessment...

The latest 00z/06z deterministic guidance offered a reasonable
cluster to start the forecast period, and so a general model blend
was used for days 3-4. After this, the trend continues towards a
more amplified/deeper solution offshore New England which should
eventually encapsulate/absorb Hurricane Teddy around day 5 over
the Canadian Maritimes/Nova Scotia. To the south, what is
currently TD Twenty-Two is forecast to meander near/off the Texas
coast (as a TS or Hurricane) and then perhaps drift slowly
northward/northeastward by the end of the week. Especially after
mid-week, there remains significant spread with respect to timing
of this evolution (both with deterministic and ensembles), which
is tied to strength/path of ridging to the north initially holding
it in place before troughing from the northwest pulls it
northward. The ECMWF is the fastest of the deterministic solutions
with this scenario, while the CMC keeps the system out over the
Gulf waters. The latest forecast from the NHC appears to prefer a
consensus of the GFS/ECMWF deterministic solutions along with
ensemble means.


...Weather/Hazard Highlights...

Teddy will bring heavy rain and strong winds to at least Atlantic
Canada but perhaps back to Maine depending on its
track/interaction with the upper trough. High pressure initially
over New England on Monday will bring in much cooler temperatures
to the East with a slow moderation the rest of the week. Over the
Gulf, models show good agreement that TD Twenty-Two should bring
heavy rain and coastal flood concerns to at least parts of the
western to northwestern Gulf coast, regardless of the current
spread in eventual track. Over the Pacific Northwest, a cold front
will bring in rainfall to western WA/OR later in the week.
Elsewhere, temperatures will remain above to much above normal
throughout the period across the north-central U.S. and back into
the interior West. Highest anomalies will lie over the
Dakotas/Upper Midwest where 70s/80s will be common.

Santorelli/Fracasso


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

Hazards:
- 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 Southeast, the Lower
Mississippi Valley, and the Southern Plains, Mon-Fri, Sep 21-Sep
25.
- Flooding possible across portions of the Southeast, the
Mid-Atlantic, and the Southern Appalachians.
- Flooding occurring or imminent across portions of the Southeast
and the Southern Appalachians.
- Flooding likely across portions of the Southeast and the
Southern Appalachians.
- Much above normal temperatures across portions of the Central
Plains, the Upper Mississippi Valley, the Northern Plains, and the
Northern Rockies, Thu-Fri, Sep 24-Sep 25.
- Much above normal temperatures across portions of the Central
Plains, the Middle Mississippi Valley, the Upper Mississippi
Valley, and the Northern Plains, Mon-Tue, Sep 21-Sep 22.
- Much below normal temperatures across portions of the Northeast,
the Central Appalachians, the Tennessee Valley, the Mid-Atlantic,
the Southern Appalachians, the Southeast, the Great Lakes, and the
Ohio Valley, Mon-Tue, Sep 21-Sep 22.
- Much below normal temperatures across portions of the
Mid-Atlantic, the Northeast, and the Great Lakes, Mon, Sep 21.
- High significant wave heights for coastal portions of the
Southern Plains, Mon-Thu, Sep 21-Sep 24.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon-Tue, Sep 21-Sep 22.
- Heavy precipitation across portions of the Alaska Panhandle,
Tue-Wed, Sep 22-Sep 23.
- High winds across portions of the Alaska Panhandle, Tue-Wed, Sep
22-Sep 23.

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