Prognostic Meteorological Discussion
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FXUS02 KWBC 052048

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
347 PM EST Thu Dec 05 2019

Valid 12Z Sun Dec 08 2019 - 12Z Thu Dec 12 2019

...Heavy rain potential for Mid-South to Northeast early next
...Well below normal temperatures over the northern Plains/Upper
Midwest early-mid week...

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

The main weather feature over North America through the medium
range is a sprawling Arctic low that anchors over Hudson Bay and
the broad cyclonic flow pattern around this deep low.
Deterministic guidance quickly diverges with timing of the main
trough rounding this low as it shifts southeast from the Pacific
Northwest in a rather positive tilt Sunday through Monday before
amplifying as it crosses the eastern CONUS Tuesday and Wednesday.
A southern stream low may spin off over the southwestern US
border, but the 00Z CMC has a much more cutoff solution on Day 3
which disrupts timing from then on and is thus ruled out from the
model blend. The GFS has been much faster than the UKMET/ECMWF
with the leading amplifying shortwave Monday into Tuesday over the
Midwest. Since the 06Z GFS is even faster than the 00Z GFS,
preference was given to the 00Z GFS starting on Day 4/Monday.
Details on shortwaves across the CONUS widely varies in
deterministic guidance by Day 6/Wednesday, so a majority of the
blend is given to ensembles for Days 6/7. Even the 00Z ECENS and
00Z GEFS diverge quite a bit by Day 7, so preference was given to
the 00Z ECENS which has less overall spread than the 00Z GEFS
(which had less spread than the 06Z GEFS). There is decent
agreement in this model preference for a western CONUS/Canadian
ridge coming ashore Monday and persisting through Thursday.

...Weather Highlights/Threats...

Areas of rain and higher elevation snow shift southeast with the
positively tilted trough from the Intermountain West Sunday night,
lingering over the southern Rockies through Monday. The building
ridge then keeps The West dry through midweek except for the
Pacific Northwest as shortwave energy rounds the northern part of
the ridge starting Tuesday night.

The precipitation forecast for low pressure developing in the lee
of the central Rockies Sunday night quickly becomes uncertain due
to timing and magnitude differences through the midweek. There are
signals for heavy rainfall over a broad area from the Mid-South to
the northern Mid-Atlantic and southern/eastern New England
Monday/Monday night. Locally heavy snow is possible in the comma
head of this low over the Upper Midwest Sunday night through
Monday with potential Lake Superior enhancement. Lake effect snow
should spread across the remainder of the Great Lakes Monday into
Tuesday in the wake of the low which is particularly cold.

The warm sector ahead of the this developing central Plains/Upper
Midwest shunts east with temperatures 10 to 25 degrees above
normal (the greatest anomaly with low temps) through Tuesday. A
polar plunge is expected behind the cold front with low
temperature anomalies of 15 to 20 degrees below normal over the
northeastern part of the Great Plains/Upper Midwest starting
Monday, peaking as much as 30 degrees below normal on Wednesday
over the eastern Dakotas and MN. Keep in mind this Day 6 forecast
is mainly based on ensemble means, so the details are yet to be
borne out. As of now the extent of this particularly cold air is
contained to the core of the Arctic low which remains centered
over Hudson Bay through the medium range timeframe, so low
anomalies are generally contained to the Great Lakes region.


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

- Heavy precipitation across much of the Washington Cascades,
Wed-Thu, Dec 11-Dec 12.
- Heavy rain across portions of the Olympic Peninsula, Tue-Wed,
Dec 10-Dec 11.
- Heavy rain across portions of coastal Maine, down across
southern New England into the northern Mid-Atlantic, as well as
parts of the interior Southeast, Sun-Mon, Dec 8-Dec 9.
- Heavy snow across portions of the higher elevations of the
northern and central Rockies, Sun, Dec 8.
- Heavy snow across portions of the central Great Lakes into
northern Wisconsin, Mon, Dec 9.
- Heavy snow across the central Great Lakes, Mon-Wed, Dec 9-Dec 11.
- Flooding occurring or imminent across portions of the Northern
- Much below normal temperatures across portions of the Great
Lakes, the Upper Mississippi Valley, and the Northern Plains,
Mon-Wed, Dec 9-Dec 11.

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

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