Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 AM EST Mon Nov 15 2021
Valid 12Z Thu Nov 18 2021 - 12Z Mon Nov 22 2021
...Overview and Guidance/Predictability Assessment...
Guidance generally agrees that progressive mean flow late this
week should eventually transition toward an amplifying mean trough
over the central-eastern U.S. as a ridge building over the eastern
Pacific during the weekend begins to move into the West early next
week. This evolution should promote a couple surges of chilly air
over the central/eastern states while temperatures over the West
remain near to above normal.
While most solutions offer similar ideas for the large scale
evolution, models and ensembles continue to be at odds over
various aspects of the forecast behind the well-clustered late
week upper trough crossing the East and associated cold
front/trailing high pressure. Before the latest 00Z cycle it had
appeared guidance was starting to cluster somewhat for the system
expected to track toward the Pacific Northwest late this
week--aside from the 12Z UKMET straying to the fast side and the
12Z/18Z GFS runs leaning a tad slow. Now the 00Z UKMET/CMC have
adjusted to a weaker and more open shortwave that leads to a much
weaker/slower/suppressed surface wave. The 00Z ECMWF is somewhat
weaker/slower but on a similar track and is now more similar to
the GFS. The latest manual forecast based on data through the
12Z/18Z cycles used a model composite at 12Z Thursday and then
phased out the UKMET contribution. Continued uncertainty with how
this system will evolve tempers confidence in details of the
general area of lower pressure and one or more fronts forecast to
reach the central U.S. by Saturday. Farther west, additional
diffuse energy may reach areas over and southwest of the
southwestern U.S. Models and ensembles vary considerably for what
may become of this energy (open trough or closed low) and its
location, with consecutive model runs also quite variable. In
addition there are meaningful differences for timing of the upper
ridge moving into the West early next week and upstream energy
(12Z CMC strayed well ahead of other guidance, 12Z ECMWF a bit
slow). At least the new 00Z CMC has adjusted closer to consensus.
These various issues affect how amplified the central-eastern U.S.
upper trough will be and the southern/eastern extent of surface
fronts by the end of the period. The lack of confidence in
specifics by the latter half of the period favored a fairly rapid
increase of 18Z GEFS/12Z ECMWF mean input, reaching 60 percent
total by day 7 Monday, with the rest composed of the 12Z ECMWF and
small parts of the 12Z/18Z GFS.
...Weather/Hazards Highlights...
Strong low pressure tracking over eastern Canada should bring
brisk winds to areas from the Upper Midwest into the Northeast
Thursday-Friday while mostly light to moderate precipitation
(mostly rain) will accompany the trailing front crossing the
eastern U.S. Northern and eastern portions of the Great Lakes
should see a period of lake effect precipitation after frontal
passage. Meanwhile the southern Florida Peninsula may experience
locally moderate to heavy rainfall around Thursday as moisture
interacts with a stalled front to the south and another front
approaching from the northwest. Continued spread and variability
in guidance for the system approaching the West Coast late this
week keep confidence lower than desired for the specifics of rain
and higher elevation snow that should spread across the
central/northern West Coast into the northern Rockies. Upstream
moisture may keep some precipitation over the northern Pacific
Northwest through the weekend. As shortwave energy continues
eastward, expect another area of rain to develop from the southern
Plains eastward/northeastward from the weekend into early next
week ahead of one or more cold fronts but again with low
confidence for coverage and amounts at this time. Precipitation
could change to snow over the farthest north latitudes with
reinforcing cold air.
Areas near the East Coast will see temperatures up to 10-15F above
normal on Thursday ahead of an advancing cold front. The cooling
trend behind this front will bring generally 5-12F below normal
readings from the Plains into the East Thursday-Saturday. Upstream
system progression may bring a brief episode of warmth from the
West into the High Plains late this week, peaking with some highs
10-15F above normal over the central High Plains on Friday.
Another push of cold air should extend southeastward from the
northern Plains Sunday-Monday with highs 5-10F below normal.
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