The Weather Prediction Center

College Park, MD

Extended Forecast Discussion

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EST Tue Jan 22 2019

Valid 12Z Fri Jan 25 2019 - 12Z Tue Jan 29 2019


Latest models and ensembles maintain the theme of an amplified
West Coast/eastern Pacific ridge and expansive central-eastern
U.S. mean trough.  Later in the period the D+8 multi-day means
still suggest the ridge could retrograde just enough to allow for
a little more southwestward elongation of shortwave energy over
the West, per teleconnections relative to the ridge's
corresponding positive height anomaly center.  Another aspect that
persists is spread/variability for some important embedded
details--thus tempering confidence in the sensible weather
forecast over some areas in spite of an overall mean pattern that
has much higher predictability.

...Guidance Evaluation/Preferences...

Based on the array of 12Z/18Z guidance the preferred blend started
with an average of operational model solutions days 3-4 Fri-Sat
followed by a transition to mostly 18Z GEFS/12Z ECMWF means late
in the period as model details diverged from each other and the
less volatile ensemble means.

Forecast specifics still appear to be in flux to some degree with
various differences/trends of note.  Waviness expected to track
across the northern Plains/Midwest early in the period has trended
weaker over the past day in response to a slightly more sheared
appearance of shortwave energy, with a general trend toward the
faster side of prior spread which was represented by the ECMWF. 
On the other hand the upper low initially over Hudson Bay has
trended farther south in latest model runs.  This brings a
stronger surface reflection/frontal system into the Great Lakes
during the weekend.  Note that the new 00Z CMC has corrected from
its 12Z run that had brought the upper low as far south as the
Ohio Valley.

Farther west and south there is loose agreement that shortwave
energy should drop southward across the west-central U.S. into the
southern Plains/Mexico by around Sat and then continue around the
southern/eastern periphery of the mean trough.  Differences in
shortwave detail as well as how flow to the north may interact
with this energy lead to a variety of possible outcomes for
potential Gulf/western Atlantic low pressure during days 5-7
Sun-Tue.  For timing and track the GEFS/ECMWF ensemble means have
provided the most consistent starting point for this possible
development.  Better guidance agreement in future runs may help to
guide deeper adjustments if needed.

During the latter half of the period there is decent agreement
that the combination of Pacific energy rounding the ridge and some
Canadian flow will support northern U.S. low pressure and a
trailing cold surge that will again focus over the central and
eventually eastern states but also bring somewhat more cooling to
the Rockies/Interior West than preceding fronts.  Prefer a
model/mean blend that tilts more to the means late in the forecast.

...Weather Highlights/Hazards...

Locations from the northern Plains into the Great Lakes may see a
series of snow events associated with surface waves tracking over
or just south of this area.  Confidence is not yet particularly
high for pinpointing exact intensity/coverage of snow due to
guidance spread and run-run variability/trends.  Over the Great
Lakes there will likely be periods of pure lake effect snow as
well as some lake enhancement with the aforementioned systems. 
Difficulties in resolving important details aloft complicate the
forecast across the South and East from the weekend into next
week.  Best consensus would have the Gulf Coast region receiving
at least some rainfall during the weekend.  Any moisture extending
farther north could meet enough cold air to produce snow at times.
 Moisture may then extend along the East Coast depending on
western Atlantic low pressure evolution.  Best potential for some
snowfall exists over the Northeast.  Over the Rockies expect two
primary episodes of locally enhanced snow progressing from north
to south, one late this week and another from late weekend into
early next week.

The Midwest/Upper Mississippi Valley should continue to be the
most prominent focus for coldest temperature anomalies during the
period with some readings at least 20F below normal Fri-Sat and
then again by next Tue.  The rest of the eastern half of the
country should see near to below normal temperatures.  On the
other hand expect the West Coast states to remain warmer than
average with plus 10-15F anomalies in some cases.  The northern
Rockies/High Plains will likely be on the warm side as well
Fri-Sun, including much above normal morning lows during the
weekend.  The Rockies and vicinity should see a pronounced cooling
trend early next week.


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

Last Updated: 158 AM EST TUE JAN 22 2019