Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
251 AM EDT Sun Oct 17 2021
Valid 12Z Wed Oct 20 2021 - 12Z Sun Oct 24 2021
...Overview...
A fairly typical late October weather pattern is expected to be in
place across the majority of the nation during the medium range
forecast period. A surface low and trailing cold front will track
from the Midwest to the Northeast, with the front exiting the East
Coast by the end of the week. A rather active pattern will be in
place across the eastern Pacific with a persistent upper level
trough off the West Coast that will support several shortwaves
rotating around it along with a couple of cold frontal passages
and rounds of precipitation. Meanwhile, a ridge axis is expected
to briefly build over the Intermountain West and then be
positioned over the Plains by next weekend.
...Guidance Evaluation/Preferences...
Model guidance for the 00Z cycle is in good agreement with the
overall synoptic scale pattern through about Thursday, and the
operational guidance agrees that the upper low tracking across the
Plains and Midwest will likely become absorbed by northern stream
flow over the Great Lakes region. Model differences remain across
Ontario and Quebec with a closed low dropping south, with the 00Z
CMC appearing to be a slower and slightly weaker solution, and the
past few runs of the GFS remaining more progressive. Out West,
the models remain in decent agreement on the overall large scale
trough, with the typical differences in shortwave timing and
magnitude, with the GFS more amplified with the shortwave reaching
California Friday night. The WPC fronts and pressures were
primarily derived from a multi-deterministic blend through
Thursday, and then increasing contributions from the ensemble
means by next weekend owing to increasing model spread and less
weighting to the GFS.
...Weather/Hazards Highlights...
Across the West Coast region, multiple rounds of showers are
expected from northern California to western Washington, and some
of these will likely be moderate to heavy at times with moist
onshore flow ahead of multiple Pacific storm systems. Some
locations may get on the order of 2 to 5 inches of rain over the
entire forecast period, particularly across windward terrain.
Snow is possible for the highest mountains of the Cascades and
Sierra Nevada, mainly above pass level. There is some signal in
the model guidance for an atmospheric river event next weekend
ahead of an energetic Pacific storm system across the coastal
ranges of northern California and southwest Oregon, with rainfall
reaching as far south as the greater San Francisco metro area.
Strong winds are also possible near the coast. Across the eastern
U.S., a broken swath of showers and perhaps some thunderstorms can
be expected ahead of the advancing cold front, but the relative
dearth of deep moisture ahead of it will serve to limit the heavy
rainfall potential.
In terms of temperatures, it will likely feel more like mid
November across much of the Northern Plains in the wake of the
departing storm system on Wednesday, with high temperatures
running about 10 to 15 degrees below average. There should be a
gradual moderation in temperatures going through the end of the
week and into the weekend as an upper-level ridge begins to build
in, and then readings up to 10 degrees above average for the
western High Plains by next Sunday. Generally cooler than normal
temperatures are forecast for the West Coast, influenced by the
upper trough and mainly overcast skies. Ahead of the cold front,
it will likely feel more like late September from the Great Lakes
and Ohio Valley to the East Coast with highs on the order of 5-10
degrees above average for the middle of the week, although
humidity levels should remain in check.
Hamrick
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