Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
207 PM EST Thu Nov 11 2021
Valid 12Z Sun Nov 14 2021 - 12Z Thu Nov 18 2021
...Moderate to heavy precipitation for the Pacific Northwest
Sunday-Monday...
...Overview...
The medium range period begins Sunday with upper ridging over the
Southwest, with onshore flow bringing enhanced precipitation to
the Pacific Northwest, and general troughing over the eastern half
of the country with lake effect/enhanced snow/rain showers. This
pattern moves fairly progressively after that, as troughing comes
into the Northwest by Tuesday, flattening the western pattern and
producing a notable surface low moving across south-central
Canada. This low should spread gusty winds to the north-central
U.S. behind it as well as eventually some rain to the
Mississippi/Ohio/Tennessee Valleys along its trailing cold front
for midweek onward.
...Guidance/Predictability Assessment...
00/06Z model guidance is in generally good agreement with the
large-scale pattern described above, though there are some
differences in the details. Aspects of the pattern where models
vary include the potential for and timing of a mid-upper low to
close off within the eastern trough and potentially more impactful
differences in strength/depth of shortwave energy coming into the
Pacific Northwest tracking eastward. The latter creates
differences in the surface pattern with the track of the southern
Canada low--the ECMWF and CMC runs have appeared more consistent
with the low track, while GFS runs have varied more, with the 00Z
run farther north compared to consensus and the new 12Z run
perhaps a bit quick/east. This has trended south a bit compared to
the previous forecast. But overall deterministic models were
fairly well clustered, and for the WPC medium range forecast, a
mainly deterministic blend of the 06Z GFS and 00Z ECMWF/CMC/UKMET
was able to be used, phasing out the UKMET and phasing in small
proportions of the GEFS and EC ensemble means by the latter part
of the period.
...Weather/Hazards Highlights...
For the Pacific Northwest, a long fetch of Pacific moisture
directed onshore will lead to moderate to locally heavy
precipitation during the early part of the week. Precipitation
should begin as all rain even in higher elevations of the Olympics
and Cascades on Sunday into Monday, but snow levels should lower
Monday night as a cold front comes through. Precipitation will be
enhanced over the Olympics and Cascades, with locally heavy rain
possible. Some precipitation is likely to spill into the northern
Rockies late Monday into Wednesday, with higher elevation snow.
Shortwave troughs moving through the southwest side of a broad
trough over the eastern half of the U.S. and cold northwest flow
will lead to lake effect/enhanced precipitation downwind of the
Great Lakes through around Tuesday, along with a weak low pressure
system bringing light rain and higher elevation/interior Northeast
snow. Then as the surface low pressure system moves through
southern Canada Monday through Thursday, gusty winds are expected
on its backside, especially for 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.
Mild temperatures that are considerably above normal are forecast
for the western half of the country early next week, with
temperatures 15-25F above normal for the High Plains on Monday and
Tuesday. After the cold frontal passage, cool high pressure will
drop into the western and central parts of the CONUS and drop the
temperatures to slightly below normal. Meanwhile, expect
temperatures of 5-15F cooler than average in the eastern U.S.
early in the week to push eastward Tuesday and offshore by
Wednesday, replaced by normal to slightly above normal
temperatures.
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