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Extended Forecast Discussion
(Latest Discussion - Issued 2030Z Sep 27, 2021)
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Abbreviations and acronyms used in this product
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
430 PM EDT Mon Sep 27 2021

Valid 12Z Thu Sep 30 2021 - 12Z Mon Oct 04 2021

... Heavy rain threat over portions of the southern Plains late
this week into the weekend...


An amplified upper pattern will likely continue across the U.S.
mainland for much of the medium range period.  This pattern will
encourage returning moisture from the Gulf of Mexico to interact
with a stalled frontal boundary across the Plains.  A threat of
heavy rain appears most likely over portions of the southern
Plains late this week into the weekend as a shortwave is forecast
to lift across the region.  Meanwhile, a broad upper low will be
slow to exit the Northeast, as the next cold front is forecast to
move through the Northwest and then into the northern Plains by
early next week.

...Guidance Evaluation/Preferences...

Latest models and ensembles show fairly good agreement on the
slowly exiting broad upper low over the Northeast, as well as the
next trough moving into the Pacific Northwest.  It is the weaker
and more subtle upper trough extending across the Plains that the
models continue to indicate noticable differences and run-to-run
variabilities.  While the GFS exhibits run-to-run jumps, the ECMWF
has generally trended faster toward the mean GFS position of this
upper trough over the central Plains during the weekend.  A
general model blend of the 00Z ECMWF/EC mean, 06Z GFS/06Z GEFS,
and some contribution from the 00Z CMC/CMC mean was used to handle
this system.

Elsewhere, some additional timing issues are seen later this
weekend into early next week across the Ohio Valley where the
upper trough from the Pacific Northwest is forecast to reach the
northern Plains.  Meanwhile, near/just off the California coast,
models are indicating the formation of an upper low during this
period. Models are showing fairly good agreement on its position.
More of the ensemble means from the 00Z EC, 06Z GEFS, and the 00Z
CMC were used to compose the Days 6 and 7 forecasts.

...Weather and Potential Hazards...

Portions of the southern Plains can expect multiple days of
widespread and potentially heavy rainfall for the latter half of
the week, as the upper low in the Southwest exits into the Plains
and interacts with tropical moisture coming from the Gulf of
Mexico. The specifics of rainfall duration and intensity remain
somewhat uncertain given model differences in the timing and track
of the upper low, but the current forecast calls for 2 of 4 inches
of rain with locally higher amounts extending through a good
portion of Texas.  Periods of rain can be expected farther north
across the rest of the Plains and the Midwest in the vicinity of
the stalled front, with a tendency for the rain to slowly shift
eastward into the Ohio Valley later in the weekend and early next

The Pacific Northwest should see precipitation lasting through
Thursday before drier and cooler air usher in behind a cold front.
Orographically enhanced rain is expected in the Olympics with some
snow is possible at the highest elevations of the northern
Cascades. Scattered showers and thunderstorms are likely across
southern Florida as a couple of fronts move through. The rest of
the East should be generally dry under the influence of a high
pressure system other than showers in the Northeast due to the
nearby upper low on Thursday.

Warmer than average temperatures are forecast across the
north-central tier of the CONUS through the period, with highs in
the 70s, even 80s in the Midwest underneath upper ridging. On the
other hand, high temperatures 10-20F below average should be
centered over the central/southern portions of the Rockies and
High Plains given the clouds and precipitation over the region.
The West Coast should warm up above normal by the weekend, while
slightly below average temperatures are expected for the Northeast
with the lingering upper low.


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

- Heavy rain across portions of the Pacific Northwest and the
Northern Great Basin, Thu, Sep 30.
- Heavy rain across portions of the Southern Rockies, the Central
Plains, the Lower Mississippi Valley, and the Southern Plains,
Thu-Sat, Sep 30-Oct 2.
- Flooding occurring or imminent across portions of the Great
- Heavy precipitation across portions of mainland Alaska, Thu-Fri,
Sep 30-Oct 1.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Thu-Sat, Sep 30-Oct 2.
- High winds across portions of the Alaska Panhandle, mainland
Alaska, and the Aleutians, Thu, Sep 30.

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