Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EDT Thu Oct 17 2019
Valid 12Z Sun Oct 20 2019 - 12Z Thu Oct 24 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. An area of low
pressure originating in the western Gulf of Mexico is forecast to
across the southeastern U.S. Sun-Mon after merging with a frontal
boundary. Meanwhile, a series of rapidly moving shortwaves and
frontal systems are forecast to affect the northwestern U.S.,
spaced approximately 24-48 hours apart through the middle of next
week. One such Pacific shortwave will cross the Great Lakes on
Sun, with a trailing cold front weakening across the
Mid-Mississippi Valley. Another significant Pacific trough is
forecast to quickly amplify across the Rockies Sun before reaching
the central U.S. on Mon. Most deterministic model solutions
suggest development of a closed upper low and deep surface cyclone
across the Upper Midwest by Mon, which should then lift northward
into the Great Lakes and 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.
Uncertainty increases significantly by Tue-Wed across the Pacific
Northwest and the northeastern Pacific, 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. This uncertainty extends east
into the Rockies and central U.S. by Wed-Thu.
Model differences were quite significant even early in the period
with respect to the low crossing the Southeast and moving. The
UKMET and GFS have consistently shown more interaction with an
additional area of upper-level energy currently across western
Texas, causing the eventual surface low across the Southeast to be
deeper and move faster. Meanwhile, the 12Z ECMWF/CMC showed no or
limited interaction, respectively, with a slower and weaker
surface low across the Southeast. Preferred to lean toward the 12Z
ECMWF/CMC solution until interaction of these feature becomes more
certain, as this also maintains a forecast close to continuity.
These solutions also fit relatively well within the model/ensemble
consensus elsewhere and a 12Z ECMWF/CMC blend was used for the WPC
forecast during days 3-5 (Sun-Tue). The aforementioned blend leans
toward a somewhat deeper and slower solution (relative to the GFS)
with the low pressure system crossing the Midwest/Great Lakes
Mon-Tue. By days 6-7 (Wed-Thu) as uncertainty quickly increased,
preferred to stick relatively close to forecast continuity, which
aligned fairly well with the 12Z CMC. Thus, used a blend of the
12Z CMC along with 12Z ECENS/NAEFS ensemble means by late in the
forecast period.
...Weather Highlights/Threats...
Potentially heavy precipitation is expected across the Pacific
Northwest into next week as the series of shortwaves and surface
fronts move inland. Snow levels should rise some as the upper
trough axis shifts to the central/eastern U.S., but some
significant snows will still be possible at the higher elevations
of the Cascades and portions of the northern Rockies. 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 across the western U.S. as
the upper trough moves inland. High temperatures 5 to 15 deg below
average are forecast on Sun 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.
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