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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0659Z Sep 29, 2022)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Thu Sep 29 2022

Valid 12Z Sun Oct 02 2022 - 12Z Thu Oct 06 2022

...Ian forecast to bring heavy/excessive rainfall and brisk to
strong winds to parts of the Appalachians and Mid-Atlantic through
this weekend and possibly into early next week...

...Overview...

The pattern over the lower 48 will be fairly blocky to start the
forecast on Sunday, with a trough/upper low over the East
(supporting the extratropical reflection of Ian) and a slow moving
northern Rockies into Plains upper low between ridges over the
West Coast and central-southern Plains/Midwest.  After Tuesday the
guidance suggests that flow will begin to transition toward larger
scale mean troughing over eastern North America in response to
more robust upper ridging that extends into far western Canada and
the Alaska Panhandle.  Meanwhile the models and ensemble means are
starting to show a more common evolution over the eastern
Pacific/West Coast by Wednesday-Thursday, with some weak southern
stream trough energy approaching California while the northern
part of the initial trough weakens as it approaches the core of
positive height anomalies over the Alaska Panhandle/British
Columbia.  The two main areas of precipitation focus will be over
the Mid-Atlantic where moisture from Ian may continue to produce
locally heavy rainfall into early next week, and parts of the
Rockies/High Plains where the upper low and associated trough will
produce areas of precipitation. 

...Guidance Evaluation/Predictability Assessment...

Guidance still shows considerable disagreement for the specifics
of the evolution over the East from Sunday onward, involving the
extratropical reflection of Ian and potential interaction of one
or more northern stream impulses.  UKMET runs remain on the open
and progressive extreme but even among other solutions there is a
range between incoming energy reinforcing the initial upper low or
ejecting it (then possibly forming a new low).  Given the typical
sensitivity of surface evolutions to small shortwave details,
confidence is even lower for the details of low pressure near the
East Coast after Sunday.  Meanwhile the GFS continues to gravitate
back toward prior remaining guidance for the Rockies/Plains upper
low that should open up by midweek or so.  A blend/ensemble
approach looks best for resolving the varying shortwave details
within southern Canada flow that begins to amplify into the
northern tier states, gradually pushing a cold front into the
region Tuesday-Thursday.  For the moment guidance is converging
over the eastern Pacific after at least a couple days of
significant spread, with consensus now showing a weak southern
stream trough nearing California by next Thursday after the rest
of the initial trough erodes while downstream mean ridge.  The
updated forecast started with the 18Z GFS/12Z ECMWF and a little
12Z CMC to represent the overall preferred solution for features
of interest early in the period, followed by some inclusion of the
18Z GEFS/12Z ECMWF means as detail spread increases over some
areas.   

...Weather/Hazards Highlights...

The potentially slow moving eastern U.S. upper low and associated
low pressure (whether initially from Ian or eventual redevelopment
just off the East Coast) may continue to produce locally heavy
rainfall over parts of the Mid-Atlantic/Appalachians through the
weekend and possibly into early next week.  Brisk to strong winds,
aided by high pressure to the north, should be most noticeable
over coastal areas.  The experimental Day 4 Excessive Rainfall
Outlook valid Sunday-Sunday night shows a Slight Risk over parts
of the central/south-central Appalachians east into the
Mid-Atlantic, with a small embedded Moderate Risk over northwest
North Carolina and southwest Virginia.  The Day 5 outlook focuses
a lingering Slight Risk closer to the coast.  Guidance differences
for specifics at the surface and aloft at this time frame lead to
lower confidence in rainfall coverage and amounts, so monitor
upcoming forecasts for possible adjustments.

The upper low emerging from the northern Rockies and the trough to
its south should produce areas of precipitation from the Rockies
into the High Plains.  Slow movement of the upper low and
sufficient moisture could support some areas of locally moderate
to heavy rainfall mainly Sunday-Monday.  Highest elevations under
the upper low may see some snow.  Expect precipitation to end from
the northwest and become lighter/more scattered as the upper low
opens up and the shortwave continues onward.

Much of the East will see below normal highs during the first half
of next week.  Parts of the Mid-Atlantic into Southeast may be
10-15F below normal into early next week with the clouds/rain
originally associated with Ian, while cool high pressure prevails
over New England.  Temperatures should finally trend closer to
normal by Wednesday-Thursday.  On the other hand, persistent upper
ridging will keep much of the West warmer than average with some
locations in the Northwest potentially seeing highs 10-15F above
normal for most of the period.  Modestly above normal temperatures
over the Plains will eventually settle more over southern areas
while locations over the far northern tier may trend colder than
normal by next Thursday. 

Rausch


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, experimental excessive rainfall
outlook, 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/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml