Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
359 PM EDT Tue Oct 15 2019
Valid 12Z Fri Oct 18 2019 - 12Z Tue Oct 22 2019
...Series of storm systems to bring heavy precipitation to the
Pacific Northwest...
...Pattern Overview and Guidance/Uncertainty Assessment...
Amplified and relatively progressive pattern is expected to
continue across the CONUS during the medium range. A deep low
pressure system is forecast to pull away from New England Fri
morning, moving into the Canadian Maritimes. Meanwhile, a series
of rapidly moving shortwaves and frontal systems are forecast to
affect the northwestern U.S., spaced approximately 24-36 hours
apart into early next week. The first of the Pacific shortwaves
will move into the central U.S. Fri-Sat, pushing a cold front
across the Plains and the Upper Midwest. By Sat-Sun, another
significant Pacific trough is forecast to quickly amplify across
the Great Basin and Rockies before reaching the central U.S. by
Mon, with a number of model solutions suggesting development of a
closed upper low and deep surface cyclone across the Upper
Midwest, which should then lift northward into western Ontario on
Tue. A strengthening subtropical ridge off the Southeast U.S.
coast early next week will contribute to a highly amplified flow
setup, with a broad region of moist southerly flow across the
Mississippi, Tennessee, and Ohio Valleys.
The WPC forecast was initially based on a multi-model
deterministic blend of the 00Z GFS/ECMWF/CMC/UKMET and 06Z GFS
which showed good clustering. The UKMET and Canadian departed from
the consensus over the central CONUS (UKMET was quicker) by Sat
and over the Gulf (UKMET/Canadian were farther east with a
potential area of low pressure) by Sun, so they were dropped from
the blend. The GFS/ECMWF and their ensemble means were utilized
for the Sun/Mon period with an increased weight on the ensemble
means as the north-central system next Monday showed typical
spatial spread (ECMWF to the NW, GFS to the SE, but both ensemble
means in between). 00Z Canadian was actually closer to the
ensemble mean consensus than the ECMWF or GFS runs, but
slower/deeper. This was certainly in the realm of possibility and
the Canadian has had at least a recent history of better handling
deepening systems out of the West the past few weeks.
...Weather Highlights/Threats...
Widespread and potentially heavy precipitation is expected across
the Pacific Northwest through the weekend as the series of
shortwaves and surface fronts move inland. Snow levels are
forecast to fall as colder air and lower heights press inland. The
heaviest precipitation is expected to fall across the coastal
ranges as well as the Cascades, with multi-inch liquid equivalents
possible. Farther inland, rain and mountain snows are likely
across the northern Great Basin, northern Rockies, and eventually
the central Rockies this weekend. As the central U.S. trough
amplifies this weekend into early next week, moisture return from
the Gulf of Mexico will increase. A warm front is expected to
focus potentially heavy rainfall along the immediate Gulf Coast
this weekend, before deeper moisture and widespread
showers/thunderstorms (possibly severe) spread northward across
the Mississippi, Ohio, and Tennessee Valleys Sun-Mon. Models
suggest the potential for widespread 1-2 inches of rain (areal
average) across a large area from the Gulf Coast to the Ohio
Valley, with embedded areas of heavier rainfall.
Relatively cool conditions are forecast across the eastern third
of the CONUS on Fri, with highs 5 to 10 deg F below average for
many locations. These areas should warm up into the weekend, with
max temperatures reaching 5 to 10 deg above average by Sun-Tue
ahead of the cold front (and mild overnight temperatures as well).
Below average temperatures are forecast to spread across the
western U.S. as upper troughing moves inland. High temperatures 5
to 15 deg below average are forecast to spread from the Northwest
and Great Basin eastward to the central/northern Rockies as the
main upper trough axis moves inland and amplifies. As the trough
reaches the central U.S. next Mon-Tue, highs may be 5 to 10 deg
below average across the central/northern Plains.
Fracasso/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
Hazards:
- Heavy precipitation across portions of the Pacific Northwest and
the Northern Great Basin, Sat-Sun, Oct 19-Oct 20.
- Heavy rain across portions of the Southeast and the Central Gulf
Coast, Sat-Mon, Oct 19-Oct 21.
- Heavy rain across portions of the Central Appalachians, the
Tennessee Valley, the Middle/Lower Mississippi Valley, the
Mid-Atlantic, the Southern Appalachians, the Southeast, the Great
Lakes, and the Ohio Valley, Mon-Tue, Oct 21-Oct 22.
- Heavy snow across portions of the Pacific Northwest and the
Northern Great Basin, Fri-Sun, Oct 18-Oct 20.
- Heavy snow across portions of the Central Rockies, the Northern
Rockies, and the Northern Great Basin, Sat-Sun, Oct 19-Oct 20.
- Flooding possible across portions of the Northern Plains.
- Flooding occurring or imminent across portions of the
Northern/Central Plains, the Upper/Middle Mississippi Valley, and
the Great Lakes.
- Flooding likely across portions of the Northern Plains.
- High winds across portions of the Central Rockies, the
Northern/Central Plains, the Central Great Basin, the Upper
Mississippi Valley, Sat-Mon, Oct 19-Oct 21.
- High winds across portions of the Great Lakes and the Upper
Mississippi Valley, Sun-Tue, Oct 20-Oct 22.
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