Prognostic Meteorological Discussion
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000
FXUS02 KWBC 262025
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
424 PM EDT Tue Oct 26 2021

Valid 12Z Fri Oct 29 2021 - 12Z Tue Nov 02 2021

...Major storm to focus a heavy rainfall/flood threat for parts of
the Mid-Atlantic and the Northeast Friday into Saturday...


...Overview and Guidance/Predictability Assessment...

An anomalously deep cyclone, capable of threatening October low
pressure records for some places in the Ohio Valley, will be
tracking into the Ohio Valley and Mid-Atlantic by the start of the
medium range period on Friday. This will likely bring a period of
active weather across the eastern U.S. for the latter part of this
week and into the weekend. Model guidance over the past few cycles
has generally shown good agreement on the large scale evolution
and track of this system, with some lingering uncertainties for
timing and details. A 00Z/06Z model composite represents consensus
well for this system and maintains good continuity with the
previous forecasts.

Behind this system the large scale flow aloft will trend toward a
strong ridge over western Canada and a mean trough eventually
covering the eastern half of Canada along with the northern lower
48. At the leading edge of the advancing upper trough, a frontal
system reaching the Pacific Northwest by early Friday will reach
the Plains this weekend. The models have recently come into better
agreement with the associated upper-level shortwave as it crosses
the northern tier of the U.S. and the upper trough core deepens
over central Canada. The front should reach the Northeast by early
next week.

Greater uncertainty exists over the eastern Pacific and western
U.S. A compact upper low should track into the
central/north-central West Coast by Saturday but the models are
still in the process of resolving the specifics. Latest GFS runs
have been a bit on the fast and/or south side of the guidance
spread. At least into Saturday the 12Z UKMET/CMC have gravitated
closer to the 00Z ECMWF versus their old 00Z runs that were
farther north. On the other hand the new 12Z ECMWF has adjusted
north of the new UKMET/CMC. The complexities increase by early
next week. Not only will there be lingering issues for the
weakening energy that remains from this upper low, but also
important detail differences for a reinforcing shortwave dropping
into the northern U.S. (some combination of the two possibly
supporting frontal waviness east of the Rockies). There is also
uncertainty over what proportion of eastern Pacific shortwave
energy may ultimately cut through a moderate upper ridge that the
ensemble means build along or inland from the West Coast--which
will play a significant role in the coverage and intensity of any
precipitation over the West. These issues tend to have lower
predictability by 6-7 days out in time, favoring a transition to a
model/ensemble mean blend that provides reasonable continuity
while awaiting more confident trends/clustering.

...Weather/Hazards Highlights...

The biggest weather threat during the medium range period will be
the deep low pressure system working across the Eastern U.S. later
this week into next weekend, bringing widespread moderate to heavy
rain. By Friday, the heaviest rainfall should be impacting much of
the Mid-Atlantic and into the lower Great Lakes, lifting into the
Northeast by Saturday. A sustained Atlantic moisture feed ahead of
the front anchored by the surface low will likely focus a possible
flood/runoff threat across parts of these regions.

The front moving through the Northwest will continue organized
rainfall and mountain snow across parts of the northern
Rockies/Pacific Northwest on Friday, with conditions finally
beginning to dry out by next weekend as upper ridging builds
behind the departing trough. This weekend the moisture should sink
southward into the Great Basin and the central Plains, with some
snow possible during the colder overnight hours even at the lower
elevations of the High Plains. From the weekend into early next
week there is lower confidence with the precise coverage and
intensity of precipitation from the West into the Plains, based on
the uncertainty of important details of flow aloft. Locations from
the southern Plains into the Midwest and Ohio Valley should see
expanding coverage of rainfall along the potentially wavy front by
early next week. Most of the rain should be on the light to
moderate side but a few pockets of heavier activity could be
possible. Some Pacific moisture may reach the northern half of the
West Coast early next week but again with low confidence in the
details at this time.

For temperatures, much of the Southeast/Central Gulf region should
see highs up to 10-15F below normal beneath the trough extending
south from the upper low tracking into the central Appalachians by
Saturday. Highs should trend back towards normal as the low lifts
into the Northeast. The central U.S. will see a wide variation of
temperatures over the course of the period. First an area of plus
10-15F or so anomalies for highs will drop from the northern
Plains/central High Plains into the southern High Plains and then
far southern Texas Friday-Sunday. This warmth will be replaced by
an expanding area of below normal readings--initially over Montana
on Saturday and then reaching at least as far as the central
Plains and Midwest by early next week, aided by advancing surface
high pressure. Areas from the northern High Plains through the
south-central Plains and east into the Middle Mississippi Valley
should see one or more days with highs 5-15F below climatological
values for this time of year. Anomalies for min temperatures
should be less extreme but could still be noteworthy for producing
the first freeze in some areas from the central Plains into the
Midwest around next Tuesday.

Rausch/Santorelli


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

Hazards:
- Heavy rain across portions of the Mid-Atlantic, the Northeast,
and the Great Lakes, Fri, Oct 29.
- Heavy rain across portions of New England, Sat, Oct 30.
- Flooding possible across portions of the Central Plains, the
Mid-Atlantic, and the Northeast.
- Flooding likely across portions of the Middle Mississippi
Valley, the Great Lakes, and the Ohio Valley.
- Much below normal temperatures across portions of the Central
Plains and Midwest, Tue, Nov 2.

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

$$




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