Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1147 AM EDT Wed Oct 16 2019
Valid 12Z Sat Oct 19 2019 - 12Z Wed Oct 23 2019
...Heavy rain and mountain snow expected for portions of 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 series of
rapidly moving shortwaves and frontal systems are forecast to
affect the northwestern U.S., spaced approximately 24-36 hours
apart through early next week. The first of the Pacific shortwaves
will cross the central U.S. Sat, pushing a cold front into the
Great Lakes, Mississippi Valley, and southern Plains. By Sun,
another significant Pacific trough is forecast to quickly amplify
across the Rockies before reaching the central U.S. by Mon. Most
deterministic model solutions suggest development of a closed
upper low and deep surface cyclone across the Upper Midwest/Great
lakes by Mon, 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. This will
then shift eastward into the Mid-Atlantic and Northeast by Tue and
out to sea on Wed.
The WPC forecast was initially based on a multi-model
deterministic blend of the 00Z GFS/ECMWF/CMC/UKMET during days 3-4
(Sat-Sun). Differences among these solutions with respect to
precisely how much and how quickly the upper low deepens across
the Upper Midwest Sun-Mon result in differences on the track and
intensity of the surface low. In the Gulf, the surface low
(potentially with tropical characteristics) is forecast to lift
northeastward into coastal AL or the FL panhandle and eventually
be absorbed into the front to its north. 06Z GFS was farther east
and faster than the 00Z model consensus and was discounted. 00Z
UKMET departed from the consensus over the Plains/Great
Lakes/southern Canada by Sun into Mon with a slower shortwave
progression owing to its emphasis on trailing rather than leading
height falls.
Uncertainty increases significantly by days 6-7 (Tue-Wed) off the
Pac NW coast, particularly with respect to a building upper ridge
off the West Coast and how it impacts the active train of
shortwave energy from the north Pacific to the northwestern U.S.
Models have shown quite large timing and amplitude variations from
run-to-run, which significantly reduces forecast confidence by
that time. A blend of the 06Z GFS and 00Z ECMWF/Canadian and the
ensemble means was used during this time period to minimize future
timing/amplitude shifts.
...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. Significant snows are
possible across the Cascades as well as portions of the northern
Rockies, with heavy rains possible at the lower elevations. As the
central U.S. trough amplifies this weekend into early next week,
moisture return from the Gulf of Mexico will increase, along with
a chance of severe weather (see SPC's Day 4-8 outlook for a
highlighted area on Sunday in eastern OK and northeastern TX).
Advancing frontal boundary and Gulf low pressure are expected to
focus potentially heavy rainfall into portions of the Southeast
this weekend, before deeper moisture and widespread
showers/thunderstorms spread northward into the Mississippi, Ohio,
and Tennessee Valleys ahead of the strengthening central U.S. low
pressure system early next week. By Mon-Tue as the system moves
east, widespread and locally heavy rains will also be possible
across portions of the Northeast. Maine will be the last to see
the rain exit into the Atlantic on Wednesday.
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 push 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. Pre-frontal
warmth will be limited to about 5-10 degrees above average for
maxes but 5-15 degrees above average for overnight mins. Some
records may be possible across Texas on Sunday and over Florida
next Mon/Tue.
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
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