Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
203 AM EDT Sat Oct 12 2019
Valid 12Z Tue Oct 15 2019 - 12Z Sat Oct 19 2019
...Pattern Overview and Guidance/Uncertainty Assessment...
A relatively amplified and progressive pattern will be in place
across the CONUS during the extended period. A shortwave crossing
the northern Plains/Midwest on Tue and the Great Lakes/Northeast
Wed/Thu is forecast to gradually amplify, with a closed upper low
developing across New England by Wed night/Thu morning. At the
surface, a low pressure system and surface frontal system are
forecast to sweep across the eastern half of the nation. Formation
of a new low is expected across the Mid-Atlantic region on Wed,
which is forecast to intensity into a relatively deep cyclone
along the New England coast Wed night/Thu as the upper low
intensifies. Farther west, an intense Pacific upper-level jet is
forecast to reach the Northwest along with a train of shortwaves
starting Wed night, which should quickly lower heights across the
Northwest and eventually the northern Rockies as the series of
systems moves inland. By the end of the forecast period (day
7/Sat), models show quite a bit of spread, but there is some
consensus that the primary trough axis should be centered across
the Rockies with a rapid succession of energy ejecting eastward
across the central U.S.
The 12Z ECMWF/UKMET were heavily used in the forecast during days
3-5 (Tue-Thu). Additionally, the GEFS mean was preferred over the
GFS as the mean was not quite as fast as the operational run to
lower heights farther inland across the Northwest/northern
Rockies. There has been a trend toward faster progression of
energy inland compared to this time last night, but the GFS
remains on the fast side of the model/ensemble spread and was not
preferred at this time. Later in the forecast period model spread
and run-to-run variability increases significantly. The evolution
of shortwave energy across the central U.S./Canada, as well as the
amplitude of additional shortwave energy reaching the West Coast,
during this time frame, are very much in question. Confidence in
the specifics of the forecast by days 6-7 is quite low, especially
across the western and central U.S. A blend of the ECENS/GEFS
ensemble means was primarily used for the day 6-7 forecast to
accommodate the lack of confidence in the specifics.
...Weather Highlights/Threats...
A weakening/stalled frontal boundary lingering over the Lower
Mississippi Valley to Southeast on Tue will be the focus for
widespread showers and thunderstorms. Model consensus suggests
that 2-3 inches of rain are possible across portions of these
areas. As the area of low pressure develops mid-week over the
Great Lakes and moves into the Northeast U.S., deeper moisture
will be pulled northward, resulting in widespread rainfall, some
of which could be locally heavy. Meanwhile, precipitation is
expected to gradually increase across the Pacific Northwest
starting Tue and continuing through the the week as multiple
shortwaves/frontal systems affect the region. Snow-levels are
forecast to gradually fall through the week as lower heights begin
to press inland. Heavy precipitation is expected to become
widespread across the coastal ranges of the Northwest as well as
the Cascades by later in the week, with multi-inch liquid
equivalents possible. By late next week, moisture return across
the central U.S. ahead of the western trough could generate
scattered showers across portions of the Mississippi Valley,
meanwhile, a lingering surface front in the Gulf of Mexico may
focus showers along the western/central Gulf Coast.
Temperatures are forecast to initially be well below average
across the north central U.S. in the wake of the current winter
storm. Highs are forecast to be 15-25 deg F below average across
portions of the northern Plains and Upper Midwest Tue-Wed, before
temperatures begin to moderate somewhat later in the week. Cooler
temperatures are forecast to spread into the eastern half of the
nation by the middle next week as Canadian high pressure
overspreads the region behind the cold front. High temperatures 5
to 15 deg below average are forecast across a large area from the
Mississippi River to the Eastern Seaboard Wed-Thu, before
temperatures return to near seasonal norms by next Fri-Sat. Below
average temperatures will spread into the Pacific Northwest and
northern Great Basin by late next week as upper-level heights fall
and the series of frontal systems affect the region.
Ryan
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards 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