Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
246 AM EDT Mon Oct 14 2019
Valid 12Z Thu Oct 17 2019 - 12Z Mon Oct 21 2019
...Series of storm systems to bring heavy precipitation to the
Pacific Northwest later this week...
...Pattern Overview and Guidance/Uncertainty Assessment...
Amplified and relatively progressive pattern is expected to
continue across the CONUS during the medium range. A deepening
coastal storm is forecast to be ongoing along the New England
coast Thu morning, moving northeast into the Canadian Maritimes by
Thu night/Fri morning, with models now reasonably well-clustered
on the forecast track. Meanwhile, a series of rapidly moving
shortwaves and frontal systems are forecast to affect the
northwestern U.S. from Thu onward, the first of which should be
moving inland across the Rockies on Thu, with systems reaching the
Pacific Northwest spaced approximately 24-36 hours apart through
the rest of the week and into the weekend. 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. Solutions continue to differ on
the timing of this last trough, as well as how far south it digs.
Teleconnections associated with the strongest hemispheric 500 hPa
anomaly centers during the medium range provide somewhat
conflicting guidance. A strong and persistent upper high across
central Russia supports a continuation of relatively progressive
upper flow across the CONUS, but positive height anomalies in the
northeast Pacific support stronger height falls into the
southwestern U.S. (more like the 00Z GFS of 12Z CMC). Given these
considerations a compromise seemed in order, and decided to lean
toward solutions in the middle of the model/ensemble spread.
Based on an evaluation of ensemble members, a blend of the 12Z
ECMWF/GFS/CMC appeared to best represent a consensus for the track
and intensity of the surface low as it moves along the New England
coast and into the Canadian Maritimes Thu/Thu night, and this
blend was followed across the CONUS during days 3-4 (Thu-Fri). The
CMC was dropped from the blend starting on day 5 as it began to
stray from consensus with the central/eastern U.S. shortwave and
was among the deepest/slowest solutions with the strong Pacific
trough across the West on Sun. The forecast during days 5-7
(Sat-Mon) reflected continued use of the 12Z ECMWF/GFS solutions,
with a gradual trend toward greater weighting of ensemble means
(ECENS/NAEFS) through time consistent with increasing guidance
spread.
...Weather Highlights/Threats...
Lingering rain is likely across northern New England on Thu as the
deep low pressure system moves along the coast. Sufficient cold
air may be in place for rain with mix with or change to snow at
the highest peaks of the White Mountains. Widespread and
potentially heavy precipitation is expected across the Pacific
Northwest later this week 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.
By the weekend, moisture return across the central U.S. ahead of
the western trough could generate showers and thunderstorms across
much of the Mississippi Valley. A weakening warm front may focus
potentially heavy rainfall across portions of the Gulf Coast, from
southeast Texas to the Florida Panhandle. As the warm front along
with deeper moisture drift northward across Sun-Mon, heavy rain is
possible across portion of the lower/mid-Mississippi Valley.
Relatively cool conditions are forecast across the eastern third
of the CONUS on Thu, with highs 5 to 10 deg F below average for
many locations. These areas should warm up by Fri and 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.
Highs are forecast to be 5 to 10 deg below normal across the
Northwest on Thu. By the weekend, colder conditions should
overspread much of the West as the main upper trough axis moves
inland and amplifies, with highs forecast to be as much as 15 deg
below normal for some areas.
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