Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 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...
...Potential Tropical Cyclone Sixteen to exit away from the North
Carolina coast on Sunday...
...Pattern Overview and Guidance/Uncertainty Assessment...
Amplified and relatively progressive pattern is expected to
continue across the CONUS during the medium range. The National
Hurricane Center has recently started advisories on Potential
Tropical Cyclone Sixteen in the western Gulf of Mexico that is
forecast to exit North Carolina Sunday 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
Sunday, with another behind it late Monday into Tuesday. Another
significant trough is forecast to quickly amplify across the
Rockies Sunday before reaching the High Plains on Monday. Most
deterministic model solutions suggest development of a closed
upper low and deep surface cyclone across the Upper Midwest by
Monday, which should then lift northward into western Lake
Superior and Ontario on Tuesday. 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 aide in producing widespread modest rains with
locally heavier amounts that will then shift eastward into the
Mid-Atlantic and Northeast by Tue and 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.
Models continued to shift over the Southeast with respect to
Potential Tropical Cyclone Sixteen. Latest 00Z/06Z models offered
a reasonable clustered starting point that was close to the NHC
official forecast. The 00Z/06Z GFS runs became much quicker with
the upstream central U.S. trough than all other ensemble members,
which allowed it to wrap PTC16 back to the north into New
England--but this was considered a low probability solution.
Overwhelming consensus would take PTC16 more eastward than
northward and get somewhat stuck beneath building heights before
dissipating, per the ECMWF and somewhat like the UKMET. Trend both
upstream (over the Pacific) and downstream (over the north
Atlantic) was toward more amplification and a slower eastward
progression of the cold front Mon-Wed to the East Coast. This was
captured well in the ECMWF ensemble mean and also the ECMWF and
GEFS mean, which formed the basis for the forecast next Wed-Thu.
...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--SPC has noted both Sunday and Monday
for a chance of severe weather over eastern OK and the northern
Gulf Coast, respectively. Advancing frontal boundary and PTC16 are
expected to focus isolated 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 eastward, widespread and locally heavy
rains will also be possible across portions of the Northeast. The
heavy rain threat may be enhanced/exacerbated by a slower-moving
frontal system. GEFS reforecast (M-climate percentile forecasts)
highlight this well Tue into Wed (above the 99th percentile).
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. Another surge of
cooler than average temperatures may spread into the West with
additional troughing by next Thu.
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