Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
212 AM EST Wed Nov 17 2021
Valid 12Z Sat Nov 20 2021 - 12Z Wed Nov 24 2021
...Potential continues for a strong cold front and significant
storm to affect the East early next week...
...Overview...
The expected large scale pattern evolution from the weekend into
the middle of next week remains on track. Guidance consensus shows
flat mean flow as of early Saturday steadily becoming more
amplified as a deepening upper trough heads into the eastern U.S.,
downstream from a Pacific upper ridge that reaches western Canada
and the northwestern U.S. by Monday. Within the eastern trough
most models continue to show an upper low closing off and taking a
path somewhere within an area from the Great Lakes/southern Canada
into New England, supporting fairly strong Southern Canada into
Northeast U.S. low pressure which could undergo some redevelopment
during the overall system's life span. This storm remains a
prominent weather focus due to its timing right before
Thanksgiving but it will likely still take a while to resolve the
details. Meanwhile the models and ensembles still have an upper
low developing southwest/south of California during the weekend
and early next week. This feature should open up by Wednesday as a
Pacific shortwave reaches the West by Tuesday and additional
energy could arrive thereafter. Northern Plains into western U.S.
troughing should amplify at least a little around midweek as
another upper ridge builds over the Eastern Pacific. Eastern North
America low pressure and its trailing front will usher in a brief
cold spell over the East during the first half of next week while
the West and parts of the High Plains will see variable but
generally above normal temperatures through Tuesday.
...Guidance/Predictability Assessment...
Relative to typical guidance spread and error for forecasts 5-7
days out in time, most models and ensemble means were fairly well
clustered for the eastern North America evolution as of the
12Z/18Z cycles. A somewhat less extreme/northward trend in ECMWF
upper low track, six-hourly GFS runs shuffling around the guidance
average, and a deeper trend in the ensemble means (leading to at
least an implied upper low if not a closed one such as depicted in
the new 00Z GEFS mean) led to a forecast upper low track across
the upper/eastern Great Lakes and into New England. This evolution
reinforces a recent stronger trend for low pressure between the
Great Lakes and James Bay as of early Monday and a slight
northward adjustment from continuity for secondary development
over/near New England by Tuesday. CMC details have differed
somewhat. Taken as a whole, recent GFS/ECMWF/CMC runs illustrate
the possibilities of redevelopment and/or looping of low pressure.
The new 00Z ECMWF provides a significantly different adjustment
that would lessen effects over the Northeast. It is important to
note that the upper dynamics supporting this system are still
within flow to the north/west of an upper low over Alaska and
historically the models can have difficulty in resolving important
details of amplifying shortwaves from higher latitudes until much
closer to an event.
The GFS/ECMWF/UKMET along with the GEFS/ECMWF means are providing
the most common solution for the upper low that forms west/south
of California, with some opening/progression likely to occur after
early Tuesday. Including the new 00Z run, the CMC has been on the
eastern side of the guidance spread. A general model/mean average
is reasonable and consistent for the shortwave reaching the
Northwest and then the Plains Tuesday-Wednesday and for
downplaying uncertain shortwave details upstream (such as the
stronger 12Z ECMWF feature nearing the West Coast early Wednesday,
eliminated in the new 00Z run).
Based on the 12Z/18Z guidance comparisons, the updated forecast
started with the 12Z/18Z GFS and 12Z ECMWF along with some 12Z
UKMET for about the first half of the period and then transitioned
to a blend of those models along with the 18Z GEFS/12Z ECMWF means.
...Weather/Hazards Highlights...
The start of the weekend should be dry over much of the country
aside from rainfall persisting along the eastern Florida Peninsula
and some light snow over parts of the northern/central Rockies.
Expect precipitation to develop along and east of the Mississippi
Valley on Sunday ahead of a leading wavy Plains front and trailing
strong cold front (anchored by northern tier U.S./southern Canada
low pressure) that catches up. Most rain should be light to
moderate given the frontal progression but locally more intense
activity may be possible. Highest totals for Sunday-Sunday night
should be in the Ohio/Tennessee Valleys. From Monday onward the
most likely evolution of this system would bring more of an
enhanced rainfall threat to New England, ahead of the cold
front/secondary low development. The best snow potential would be
in the cold air behind the system, to the lee of the Great Lakes
and along westward-facing terrain from the central Appalachians
northeastward. There is still enough uncertainty in system
evolution to allow for other possibilities regarding precipitation
types and amounts. Confidence is somewhat higher in the potential
for a period of brisk to strong winds behind the cold front
sweeping through the Plains into the East, with such winds
continuing over the Great Lakes and Mid-Atlantic/Northeast into at
least Tuesday. The Pacific Northwest into northern Rockies may see
mostly light precipitation during the first half of next week with
a shortwave arriving from the Pacific. The combination of this
energy aloft and the opening/progression of the upper low
southwest of California may spread some moisture into parts of the
West by Tuesday/Wednesday, but with low confidence for
precipitation coverage/amounts at this time.
The East will see chilly temperatures (mostly 5-10F below normal)
lingering into the early weekend. The next surge of colder air
will spread across the eastern half of the lower 48 Monday-Tuesday
with the broadest coverage of highs 10-15F below normal expected
on Tuesday. Some locations may also see morning lows 10F or so
below normal. The West into the central/southern High Plains will
see mostly above normal temperatures through Tuesday with the
warmest anomalies of plus 10-15F for highs likely over the High
Plains on Saturday (lingering into Sunday near the Rio Grande) and
again on Tuesday. The upper trough over the West by Wednesday will
likely bring highs down to near or slightly below normal levels at
that time.
Rausch
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