Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Thu Jan 13 2022
Valid 12Z Sun Jan 16 2022 - 12Z Thu Jan 20 2022
...Wintry weather is expected across the Appalachians and parts of
the Eastern Seaboard, but uncertainty remains with precipitation
type in some areas and with the location of the heaviest snow...
...Overview...
The most notable weather feature during the medium range period is
a developing surface low pressure system that slides across the
Southeast on Sunday and then pivots northward near/along the East
Coast ahead of a potent mid-upper shortwave trough. This will
cause impactful wintry weather to spread across much of the East
early next week. However, uncertainty remains with the exact low
track, which leads to low confidence in precipitation type (which
could change in some locations as the event progresses), the axis
of the heaviest snow, and the westward extent of the snow.
Meanwhile in the West, mainly dry conditions are expected
underneath surface high pressure, with the exception of light
precipitation in the Pacific Northwest and Northern Rockies as
weak fronts track through.
...Guidance/Predictability Assessment...
Model guidance for the 12/18Z cycle (and the incoming 00Z cycle)
shows a similar idea to the previous couple of cycles of model
guidance with the impactful surface low in the East--that is, a
low track as described above staying onshore in the deterministic
guidance, but with the GEFS and EC ensemble means showing a track
offshore, while individual ensemble members demonstrate
considerable spread in low placement, with certainly some ensemble
members along the coast or inland. Again, while model guidance
exhibits good agreement for a strengthening low, the exact low
track will create significant differences in weather type at
certain locations, especially near the Atlantic coast to about the
I-95 corridor. The 12/18/00Z GFS along with the 12/00Z CMC all
show a track basically on the western side of the guidance
envelope (well inland over the northern Mid-Atlantic Monday
morning), with the 12Z ECMWF a bit farther east but still onshore.
Felt it prudent to continue with a low track in between the
western deterministic guidance and the eastern EC and GEFS
ensemble mean guidance, but somewhat favoring the fairly well
clustered deterministic models. The best proxy for this at the
time of forecast creation was close to the 12Z ECMWF and the 12Z
CMC ensemble mean; the 12Z CMC ensemble mean along with the
incoming 00Z CMC mean are the first ensemble means to show an
onshore track with the low. This led to a surface low basically
over Philadelphia on Monday morning, which was a very minor shift
farther inland/west compared to continuity. Hopefully models will
converge and confidence will increase within the next day for the
exact low track, as the shortwave energy leading to this system
will enter the Pacific Northwest/southwestern Canada and have
better data sampling for input into the model guidance.
Elsewhere, ridging is likely to have hold of the West early this
week, other than a meandering southern stream upper low centered
offshore of California. The southern stream low is forecast to
track eastward and join with the main flow by around Tuesday, with
some possible interaction with northern stream energy in the
eastern Pacific, but most guidance other than the 12Z CMC now has
more separation with these pieces of energy. Then there is good
model agreement for an amplified ridge over the West Coast by
Thursday drifting eastward from the Pacific, with some model
differences in possible embedded shortwaves particularly shown by
the GFS runs. Meanwhile, additional shortwaves reinforce troughing
centered over the Great Lakes Wednesday-Thursday, for which there
is reasonable model agreement.
The WPC medium range forecast was based on an initial blend of the
12/18Z deterministic guidance favoring the ECMWF somewhat, adding
the CMC ensemble mean by day 4 with its favorable low position in
the East and increasing the proportion of the EC ensemble mean for
the latter part of the period as well, to help temper minor
variations in model guidance.
...Weather Highlights/Hazards...
The surface low lifting from across the Southeast and up the East
Coast will bring with it a full array of weather hazards and
threats. Concerning chances for heavy snow are likely in the
southern Appalachians beginning Saturday, spreading northward
along/on the backside of the deepening low and generally west of
the I-95 corridor in the Mid-Atlantic and the Northeast. Chances
are also increasing for a potentially significant icing event,
especially from parts of far northern Georgia, through the western
Carolinas, and into south-central Virginia where warm and moist
Gulf of Mexico inflow will overrun initially cold air at the
surface. WPC has both the snow and ice threats highlighted in the
most recent hazard outlook. To the south, moderately to locally
heavy rain could move across the Southeast this weekend, and
possibly up the immediate East Coast Sunday into Monday. Gusty
winds surrounding the surface low may create high waves off the
coast leading to possible coastal flood concerns along the
Northeast coastline. Of course, all of this is highly dependent on
the exact track of the surface low and even small adjustments back
to the east could bring heavy snow potential into even coastal
parts of the East.
Elsewhere, after the main low exits northeast of the U.S. on
Tuesday, another (weaker) low pressure system is forecast to track
over the north-central U.S. in response to a mid-upper shortwave
and bring some snow to the Upper Midwest and Great Lakes, with
possibly higher totals near typical lake enhancement areas. The
West could see some light precipitation during the period,
particularly over the Pacific Northwest and into the Northern
Rockies early next week, with mainly dry conditions elsewhere.
Rain could develop again across the Lower Mississippi Valley and
Southeast beginning Tuesday and increasing Wednesday with return
flow from the Gulf of Mexico.
Cooler than normal temperatures are expected generally over the
eastern U.S. early next week, with particularly chilly lows below
0F in the interior Northeast Sunday morning, about 15-25 degrees
below average. The West to the Plains can expect generally above
average conditions through Tuesday. Then as a cold surface high
drops into the north-central U.S. Wednesday and Thursday,
temperatures will fall to 10-20F below normal in the Upper Midwest.
Tate/Santorelli
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
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