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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0651Z Sep 18, 2020)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
251 AM 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. Twin
tropical systems may have some affect on the Gulf (TD Twenty-Two)
and perhaps Maine (Hurricane Teddy) directly or indirectly. At the
least, Teddy will churn up waves in the Atlantic for the next
several days.


...Guidance/Predictability Assessment...

Through the 12Z/18Z guidance cycle, latest deterministic models
offered a reasonable cluster to start the forecast period, and a
blend was used as a starting point. Trend has been toward a more
amplified/deeper solution just offshore New England as has been
advertised by the ECMWF guidance the past several runs. This
involves Hurricane Teddy becoming encapsulated within the digging
trough in a striking extratropical transition over at least the
Canadian Maritimes/Nova Scotia. To the south, TD Twenty-Two is
forecast to meander off the Texas coast and then perhaps drift
northward/northeastward by the end of the week. Ensembles differ
on timing of this evolution, tied to the strength/path of ridging
to the north that initially holds it in place before troughing
from the northwest tugs at it later in the week. Consensus of the
ensembles was preferred (by WPC and NHC) until some more details
can be ascertained. Rest of the CONUS will be fairly quiet with
ridging stretched from the Southwest to the Plains.


...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. Over the Gulf, TD Twenty-Two
may bring heavy rain to at least parts of the coast on its
forecast track, even while remaining offshore. 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.

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

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