Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EST Fri Nov 12 2021
Valid 12Z Mon Nov 15 2021 - 12Z Fri Nov 19 2021
...Moderate to heavy precipitation over the Pacific Northwest
continues into Monday...
...Overview...
Expect the western ridge/eastern trough upper pattern as of the
start of the week to transition toward progressive and broadly
cyclonic mean flow. As this transition begins, Pacific energy
about to enter western North America will direct an axis of
moisture into the Pacific Northwest while under the eastern trough
aloft the chilly low level flow around a system tracking into the
Canadian Maritimes will promote lake effect/enhanced rain or snow.
Over the course of the week the Pacific shortwave will continue
across the continent, supporting a strong southern Canada surface
low and trailing cold front that ultimately crosses much of the
lower 48--along with some strong/gusty winds over parts of the
northern tier and later some rain over the east-central U.S. Well
above normal temperatures from the West into the Plains and cool
conditions over the East early in the week should moderate later
in the week with the flatter trend for the mean pattern.
...Guidance/Predictability Assessment...
A composite of 18Z/12Z operational models provided a good
representation of significant systems during the first half of the
period and incorporating some 18Z GEFS/12Z ECMWF mean input along
with the 18Z GFS/12Z ECMWF reflected the most common ideas of
guidance thereafter. There are still some finer-scale details to
be resolved for one or two low centers that will consolidate into
a stronger Canadian Maritimes system by early Tuesday. Some spread
persists for the upper trough crossing the continent and
associated Canadian low pressure/lower 48 cold front but
clustering has been gradually improving, with GFS adjustments over
the past 12 hours generally contributing to that trend. The new
00Z GFS/GEFS mean are a bit on the amplified side as the trough
comes into/through the West but the 00Z ECMWF has strongly
adjusted this way too. Later in the week there is decent agreement
for the cold front reaching the East Coast but not much support
for the 18Z GFS wave that would produce significant rainfall over
parts of Florida (so that aspect of the 18Z GFS was downplayed).
However the flatter mean flow farther west rapidly reduces
confidence in specifics, with individual model runs varying for
details of shortwaves that could be embedded within separate
Pacific streams. The preferred model/mean blend yielded a
conservative approach while awaiting improved agreement.
...Weather/Hazards Highlights...
A band of enhanced moisture along a wavy front heading into the
Northwest will produce locally heavy precipitation into the early
part of the week. Precipitation should be all rain even in higher
elevations of the Olympics and Cascades into Monday but snow
levels should lower Monday night as the cold front comes through.
Expect the highest precipitation totals over the Olympics and
Cascades with locally heavy rain possible. Some rain and higher
elevation snow should spill into the northern Rockies late Monday
through Tuesday. Meanwhile shortwaves moving through the southwest
side of the initial eastern U.S. upper trough and cold
northwesterly low level flow behind a consolidating system
tracking into the Canadian Maritimes will produce lake
effect/enhanced precipitation downwind of the Great Lakes through
Tuesday. The surface system itself could bring light rain and
higher elevation/interior Northeast snow. The surface low tracking
across southern Canada Monday onward will bring a period of
strong/gusty winds to northern tier areas, especially the northern
High Plains on Tuesday. Moisture should increase ahead of the
low's trailing cold front from midweek onward, leading to
increasing rain chances for portions of the Mississippi, Ohio, and
Tennessee Valleys along with additional precipitation for the
Great Lakes region. Most of this activity should be light to
moderate.
Well above normal temperatures will prevail over the West and into
the Plains Monday-Tuesday with a broad area of plus 10-25F
anomalies. Some readings could reach daily record values, perhaps
with a slightly better chance for warm lows relative to highs. At
the same time the East will be on the cool side, with Monday
seeing the best potential for some highs 10-15F below normal. Cold
frontal passage will spread slightly below normal temperatures
from west to east across the lower 48 mid-late week. Leading warm
anomalies reaching the South and East Wednesday-Thursday will be
only in the plus 5-15F range.
Rausch/Tate
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