Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 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.
The WPC forecast was initially based on a multi-model
deterministic blend of the 12Z ECMWF/CMC/18Z GFS during days 3-5
(Sat-Mon). 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. The CMC differed from consensus a
bit by day 5, closing off the upper low farther south than
consensus, and this solution was thus deemphasized at that time.
Uncertainty increases significantly by days 6-7 (Tue-Wed),
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 12Z ECENS/NAEFS ensemble means along with a continued minority
component of the deterministic ECMWF was used during this time
period. The ECMWF seemed to best reflect a trend seen among
several deterministic solutions toward faster (and stronger)
arrival of another shortwave in the Northwest on Tue, with further
amplification across the Rockies by Wed. Ensemble means show so
much spread, however, that it is difficult to discern any wave
pattern in the height contours, further emphasizing that forecast
confidence across the western U.S. by the middle of next week is
quite low.
...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. A frontal
boundary and wave of low pressure are expected to focus
potentially heavy rainfall along the immediate Gulf Coast and
perhaps 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
deepening 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.
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.
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