The Weather Prediction Center

College Park, MD

Extended Forecast Discussion

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
322 PM EDT Sat Oct 23 2021

Valid 12Z Tue Oct 26 2021 - 12Z Sat Oct 30 2021

...Slow moving storms with heavy rain/runoff threats for the
Northeast Tuesday/Wednesday and from the south-central to eastern
U.S. Wednesday-Saturday...

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

The weather patern will remain quite active as potent systems
impact the nation next week. Guidance has trended less
progressive/more amplified with these systems.

An upper low developing over the lower Great Lakes Tuesday as
northern and southern stream energies phase will induce a
deepening cyclone with heavy wrap-around winds/rainfall and
flooding threats over the Northeast U.S. Guidance is now in better
agreement with this scenario and a well organized coastal storm is
expected to track toward the Canadian maritimes by Thursday.
Predictability with this feature is above average.

Meanwhile upstream, yet another potent trough will eject out of
the West/Rockies Tuesday-Wednesday. Guidance has swerved a bit and
now shows more separated flow with possible development of a
southern stream closed low and deeper surface low/frontal midweek
that offers a slowed and southward separated track from the
southern Plains to the Mid-Atlantic into next Saturday. Organized
moisture and rainfall will wrap around the low and accompany a
leading frontal system with rounds of heavy rainfall possible and
also offers a threat for runoff issues. Predictability with this
feature is less than average, but spread has decreased some in 12
UTC guidance.

Well upstream, a favorable jet and onshore track of shortwaves
pattern will also meanwhile fast track move into the Pacific
Northwest during the period to bring more heavy rain/mountain
snows to the Northwest that increasing run overtop lead/warming
upper ridge development underneath over the rest of the West
through late week. 

The WPC medium range product suite was primarily derived from a
composite blend of best clustered guidance from the 06 UTC GEFS
mean and 00 UTC ECMWF/ECMWF ensemble means, with emphasis
attributed to the GEFS mean into days 6/7 (next Friday/Saturday).
Growing forecast spread favors an ensemble mean focus and the GEFS
more separated stream flow and amplitude/slowed progression with
the main storms better fit latest guidance trends from recent
guidance including the newer 12 UTC runs.

...Weather/Hazards Highlights...

The leading closed low into the Northeast on Tuesday will allow
for heavy rainfall potential to continue from the end of the short
range period into much of New England early next week. Deepening
coastal low development may induce pockets of enhanced rainfall on
the backside of the low as it lifts northward towards the Canadian

The cold front reaching the Plains by Tuesday and onward into the
east-central U.S. Wednesday/Thursday is expected to present an
emerging and expanding lead rainfall/convective focus as ample
Gulf of Mexico and Atlantic moisture streams into the system.
Latest guidance suggests the best potential for heavy rainfall
will be across portions of the Gulf Coast states/Southeast and
inland Wednesday into Thursday and then into the Ohio
Valley/Appalachians/Mid-Atlantic/Northeast Thursday-Saturday with
closed upper low development and a slowed frontal system
progression. The Storm Prediction Center also is highlighting the
potential for severe weather across parts of the central/southern
Plains on Tuesday, and the central Gulf Coast on Wednesday.

Meanwhile, out west, a sustained moisture plume from the short
range period will continue into the Northwest U.S. on Tuesday
bringing a continued threat for heavy rainfall and mountain snows
to especially the Pacific Northwest/northern California but also
farther inland across the northern Rockies. Another system moving
towards the coast mid-later next week will continue the active
weather with another round of heavy rainfall and mountain snow
impacting parts of the Pacific Northwest.


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

Last Updated: 322 PM EDT SAT OCT 23 2021