Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
251 AM EDT Fri Oct 15 2021
Valid 12Z Mon Oct 18 2021 - 12Z Fri Oct 22 2021
...Overview...
A large surface high should be in place across much of the eastern
U.S. for the beginning of next week, bringing more fall-like
weather compared to the unseasonable warmth and humidity in recent
days. A storm system is expected to organize across the central
Plains by Tuesday and track in the general direction of the Great
Lakes by around Thursday, with a trailing cold front that should
reach the East Coast by the end of the week. Meanwhile across the
western U.S., a large scale trough over the northeast Pacific will
increase the potential for unsettled weather for the West Coast
region, with a weakening storm system by midweek and potentially a
stronger storm system to close out the week.
...Guidance Evaluation/Preferences...
Model guidance for the 00Z cycle is in good agreement on the
synoptic scale pattern on Monday. One of the more uncertain
aspects of the forecast has been the timing of the upper low
crossing the Rockies, and the guidance has come into better
agreement compared to the 12Z Thursday guidance in portraying a
slower solution. The 00Z ECMWF has trended in line with the
slower GFS/UKMET. At the time of this discussion, the 12Z ECENS
is the most progressive solution here, although this is also
likely to adjust farther to the west. By the end of the forecast
period, the GFS is more offshore with the West Coast storm system,
and farther east with a closed low over southern Canada compared
to the ECMWF/CMC. The WPC forecast was primarily derived from a
GFS/ECMWF/CMC blend through the majority of the forecast period,
with increasing contributions from the ensemble means by Thursday
and Friday.
...Weather/Hazards Highlights...
Valley rain and mountain snow associated with the potential closed
low over the northern Rockies on Monday is expected to reach the
northern Plains by midweek, with perhaps up to an inch of rain on
the north side of the surface low across portions of northern
Nebraska and South Dakota, and several inches of snow across the
higher terrain of Idaho, Montana, and Wyoming. Showers and storms
will likely increase in coverage ahead of the trailing cold front
from Texas to the Ohio Valley, and this should reach the
Appalachians by Thursday as the front steadily progresses
eastward. The next storm system reaching the West Coast by the
end of the week will likely bring moderate to locally heavy rain
for coastal areas from northern California to the Olympic
Peninsula, although placement specifics for heaviest rainfall
remain uncertain.
Temperatures are expected to be 10-20 degrees below normal to
start the week across portions of California and Nevada in the
wake of the cold front, and about 5-15 degrees above normal across
the northern Plains and Upper Midwest. The cooler readings reach
the Plains by midweek with the Ohio Valley and East Coast region
getting a modest warm-up ahead of the next front, with anomalies
only on the order of 5 to 10 degrees above normal. By the end of
the week, most locations should be within 5 degrees of their
climatological averages with perhaps some greater warm anomalies
over portions of Montana and Idaho.
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