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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0658Z Sep 19, 2020)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Sat Sep 19 2020

Valid 12Z Tue Sep 22 2020 - 12Z Sat Sep 26 2020

...Heavy rainfall threat for the northwest Gulf Coast next week
from Beta...

...Hurricane Teddy will move through Atlantic Canada just east of
Maine on Tue/Wed...


...Overview...

Deep upper lows near the Gulf of Alaska and just off the Northeast
will favor ridging over the Rockies and toward the Plains next
week. In the Gulf, tropical storm Beta is forecast to become a
hurricane and move along/near the Texas coast toward Louisiana at
a slow pace, leading to the potential for heavy rain along the
coast but with a large degree of uncertainty. In the western
Atlantic, major hurricane Teddy will lift northward into Nova
Scotia and merge with a non-tropical low, spreading some
wrap-around rain and wind across Maine. In addition, Teddy will
continue to churn up waves along the East Coast which could cause
life-threatening surf and rip current conditions.


...Guidance/Predictability Assessment...

The latest 12z/18z deterministic guidance offered a reasonable
cluster to start the forecast period outside the UKMET in the
Gulf) and a general model blend was used for Tue/Wed. Afterwards,
there remains significant spread with respect to the timing/depth
of troughing through the Lower Mississippi Valley out ahead of
Beta which will influence its path. Preference was for a slow
motion northeastward later next week in between the quicker
solutions and slowest ones that linger the system in the Gulf,
depicted by the 03Z NHC forecast and coordinated yesterday past
day 5. Ensembles show good agreement off the West Coast with an
incoming trough and attendant surface cold front. Blended solution
trending toward a majority ensemble mean solution sufficed. In the
Northeast, post-Teddy upper pattern will bring in another trough
across the Great Lakes Thursday in quasi-zonal flow.


...Weather/Hazard Highlights...

Tropical storm Beta will likely take a path near enough to the
coast to spread some heavy rain around its circulation next week,
slowly migrating northward/northeastward in time. Range of
solutions remains large, from light accumulations along the coast
to in excess of 10" over the multi-day period. Given the official
track, leaned toward the higher end along the coast and over the
open water but this will be refined several more times before it
will become more specific. Over Maine, Teddy will bring some rain
to Maine but heavier amounts will be over Canada. A cold front
will bring several days of rainfall to western WA/OR, focused
mostly around Wednesday as the front moves inland. Showers will
continue behind the front eastward to northern Idaho. Temperatures
will remain above to much above normal throughout the period
across the north-central U.S. and back into the interior West
(through Thu/Fri). Highest anomalies will lie over the
Dakotas/Upper Midwest where 70s/80s will be common. Approaching
cold front next weekend will start to cool temperatures over the
Rockies. Cooler than normal temperatures in the Lower Mississippi
Valley will be in response to abundant clouds and showers/rain
around Beta.


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