Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
233 AM EDT Thu Oct 28 2021
Valid 12Z Sun Oct 31 2021 - 12Z Thu Nov 04 2021
...Overview and Guidance/Predictability Assessment...
The start of the medium range period on Sunday will feature a deep
upper low and associated surface low tracking into the Northeast,
though the bulk of the weather hazards associated with this system
should be into the Canadian Maritimes by then. Model guidance has
maintained reasonably good agreement and continuity over the past
few days and once again a consensus model blend provides a good
starting point.
Behind this, a shortwave should drag across the northern tier of
the U.S. as multiple shortwave perturbations rotate southward
providing reinforcement for mean troughing across the central into
parts of the Eastern U.S. through the middle of next week. Models
show good agreement with the initial shortwave through the Great
Lakes Sunday-Monday, but timing and detail differences quickly
begin to arise regarding energy rotating through the base of the
mean trough Tues-Thur next week.
Out West, the guidance continues to suggest a compact upper closed
low may slide into the Pacific Northwest, but diverge after day
3/Sunday regarding what happens to the remnant energy as it drops
southward through the Great Basin. The GFS has consistently been
south and faster with this system, while the ECMWF/UKMET/CMC
maintain better continuity with the ensemble means. There remains
enough run to run variability in this though across all the models
that confidence is low in the specifics. The next shortwave moves
into the Western U.S. by Monday of next week as another amplified
trough digs upstream becoming established off the West Coast by
Thursday.
The WPC progs for this cycle used a blend of the deterministic
solutions days 3-5 (with slightly more weighting towards the ECMWF
and CMC). After that, increased contributions from the ensemble
means in order to help mitigate the timing and detail differences
which arise mid to late period for several systems. This approach
also maintained reasonable continuity with the previous WPC
forecast as well.
...Weather/Hazards Highlights...
Heavy rains associated with the departing Eastern U.S. cyclone
should lift into the Canadian Maritimes by the start of the period
on Sunday with only some lingering light showers across Maine and
interior New England. Elsewhere, light rain/mountain snows will be
possible Sunday across parts of the central Rockies and High
Plains. Some snow may be possible during the colder overnight
hours even at the lower elevations of the High Plains. Confidence
in the precise coverage and intensity of precipitation across
these regions remains low, based on the uncertainty of important
details of flow aloft. Locations from the southern Plains into the
Midwest/Ohio Valley and eventually the interior northeast should
see expanding coverage of rainfall along the potentially wavy
front early next week. Most of the rain should be on the light to
moderate side but there is a developing signal for some heavier
totals over or near the southern Plains/Lower Mississippi Valley
by next Tuesday or Wednesday. Pacific shortwave energy and
moisture may bring one or more episodes of precipitation to
central/northern parts of the West Coast Monday-Wednesday, still
with fairly modest confidence in the details at this time.
The only notable temperature hazard is below to much below normal
temperatures expanding across much of the Central U.S. through
much of the period. Areas from the northern High Plains through
the south-central Plains and east into the Middle Mississippi
Valley/Ohio Valley should see one or more days with highs 10-20F
below climatological values for this time of year. Anomalies for
min temperatures should be less extreme but could still be
noteworthy for producing the first freeze in some areas from the
central Plains into the Midwest around next Tuesday.
Santorelli/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