The Weather Prediction Center

College Park, MD


Extended Forecast Discussion




Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
418 PM EDT Mon Oct 19 2020

Valid 12Z Thu Oct 22 2020 - 12Z Mon Oct 26 2020

...Guidance Evaluation/Preferences...

Guidance is agreeable and consistent with the large scale forecast
of broad troughing aloft over North America, evolving toward a
somewhat sharper and more amplified trough over the western U.S.
during the weekend.  Within this pattern an operational model
consensus provides a good depiction of significant features in the
days 3-5 Thu-Sat period--initial energy ejecting from the
Northwest and corresponding Plains to Upper Great Lakes wave Thu
into Fri, followed by strong shortwave energy dropping into the
Northwest along with an arriving Northeast Pacific surface wave
(12Z CMC somewhat fast with this wave).  By late weekend and early
next week models/ensemble members diverge for the details of the
amplifying energy and its eventual ejection along with the
associated surface evolution (00Z/06Z GFS runs straying to the
fast side of the spread).  As a result the forecast rapidly
increases weight of the similar 06Z GEFS/00Z ECMWF means during
days 6-7 Sun-Mon, yielding a wave that reaches the western Great
Lakes by next Mon.  The new blend provides good continuity in
principle relative to the previous cycle.


...Weather/Hazard Highlights...

The shortwave initially ejecting from the West from Thu onward and
associated low pressure will produce periods of moderate
precipitation from the northern Rockies/Plains into the Upper
Mississippi Valley and Upper Great Lakes regions Thu-Fri.  Highest
totals are most likely over parts of the Upper Mississippi Valley
and Upper Great Lakes, while locations from the northern Rockies
through the Dakotas and northern Minnesota will see the best
potential for meaningful snowfall.  At least over northern areas
the precip should trend lighter as the front trailing from the
surface low continues into the eastern U.S. 

The Northeast Pacific wave reaching the Northwest late this week
will bring more moisture across the northern third of the West,
with the rain/higher elevation snow extending southward with time
as upper troughing amplifies/sharpens.  Toward the start of next
week another Plains/Great Lakes wave may develop, spreading
potential for some snow from the Rockies across parts of the
northern half of the Plains into the Upper Midwest along with
rainfall farther southeast along the front trailing from the
surface low.  Overall expect best potential for heavy snow to be
over the northern Rockies while details farther east will take
additional time to come into focus. 

Persistence of cold Canadian air will keep the core of most
extreme below normal temperature anomalies centered over the
northern Rockies/High Plains during the period.  Parts of Montana
may see readings up to 25-35F below normal for multiple days while
only moderately less extreme anomalies push southward over the
Plains in response to the individual shortwaves/surface systems
expected early and late in the period.  Expect some of this cold
air to extend into the West as well, with temperatures 5-20F below
normal over the Interior Northwest/northern Great Basin from the
start of the period Thu, followed by expansion/southward push of
minus 10-20F anomalies over the West as upper troughing amplifies.
 Farther east, the warm sector ahead of the first front will
contain well above normal temperatures (up to plus 10-20F
anomalies).  Frontal passage will bring highs close to normal over
the East but morning lows will remain above normal over the
southeast third of the lower 48 as the boundary lingers over the
Florida Peninsula and adjoining waters.

Meanwhile expect moisture and rainfall of varying intensity
currently over and near Florida to reach the eastern Gulf of
Mexico by the start of the period.  This rain may lift northward
across the Southeast in advance of the leading front that reaches
the East.  There is still fairly high uncertainty regarding the
amount of moisture that ultimately reaches the Southeast and how
it may interact with the front.  In addition there has been a
signal for additional moisture that could reach Florida and the
east-central Gulf states in association with low pressure that
could emerge from the western Caribbean Sea.  However guidance has
been diverse and inconsistent with this aspect of the forecast.

Rausch/Schichtel


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

Hazards:
- Heavy rain across portions of the Central Plains, the Lower
Mississippi Valley, the Tennessee Valley, the Great Lakes, the
Middle Mississippi Valley, the Southern Plains, and the Ohio
Valley, Sun-Mon, Oct 25-Oct 26.
- Heavy rain across portions of the Southeast, the Great Lakes,
and the Upper Mississippi Valley, Thu-Fri, Oct 22-Oct 23.
- Heavy rain across portions of the Northeast and the Great Lakes,
Mon, Oct 26.
- Heavy snow across portions of the Northern Plains, the Northern
Rockies, and the Northern Great Basin, Fri-Sat, Oct 23-Oct 24.
- Heavy snow across portions of the Central Rockies, the Central
Plains, the Central Great Basin, the Northern Plains, and the
Northern Rockies, Sat-Sun, Oct 24-Oct 25.
- Heavy snow across portions of the Upper Mississippi Valley and
the Northern Plains, Thu, Oct 22.
- Flooding occurring or imminent across portions of the Lower
Mississippi Valley.
- Much below normal temperatures across portions of the Central
Plains, the Central Great Basin, the Northern Rockies, the Central
Rockies, the Northern Great Basin, and the Pacific Northwest,
Sun-Mon, Oct 25-Oct 26.
- Much below normal temperatures across portions of the Northern
Plains, the Northern Rockies, and the Northern Great Basin,
Thu-Mon, Oct 22-Oct 26.
- Much below normal temperatures across portions of the Central
Plains, the Northern Plains, the Northern Rockies, the Central
Rockies, the Middle Mississippi Valley, and the Upper Mississippi
Valley, Fri-Mon, Oct 23-Oct 26.
- Heavy precipitation across portions of mainland Alaska and the
Aleutians, Thu-Fri, Oct 22-Oct 23.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sun-Mon, Oct 25-Oct 26.
- Heavy snow across portions of mainland Alaska, Thu-Fri, Oct
22-Oct 23.
- High winds across portions of mainland Alaska and the Aleutians,
Thu-Fri, Oct 22-Oct 23.

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





Last Updated: 418 PM EDT MON OCT 19 2020