Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
238 AM 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 low pressure
system is forecast to be pulling away from New England Fri
morning, moving northeast 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 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 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 blend of the 12Z
ECMWF/CMC/18Z GFS during days 3-5 (Fri-Sun). These solutions
showed relatively small differences during that time period, and a
blend fit the overall model consensus well. During days 6-7
(Mon-Tue), the forecast was based on a blend of the ECMWF along
with the ECENS and NAEFS ensemble means. The 18Z GFS showed some
significant differences from the ECMWF across the Midwest/Great
Lakes during this time frame, likely due to a less closed
upper-level system causing a leading surface to to move quickly
northeast into the Great Lakes. The ECMWF fit much better within
the center of the model/ensemble spread over the past few runs.
...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 height 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. 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 spread north across the Mississippi, Ohio,
and Tennessee Valleys Sun-Mon. Models suggest the potential for
1-2 inches of rain 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 by into the weekend,
with temperatures reaching 5 to 10 deg above normal by Sun-Mon.
Meanwhile, 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 east 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.
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