Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
152 AM EST Tue Nov 19 2019
Valid 12Z Fri Nov 22 2019 - 12Z Tue Nov 26 2019
...Pattern Overview and Guidance/Predictability Assessment...
Models have continued to trend quicker over the past 24 hours with
ejecting an upper low eastward as a trough across the CONUS
southern tier. The ECMWF has continued to lag this trend for days,
while the GFS has led the trend, and the CMC/UKMET have often been
somewhere in the middle. Farther north, an active and progressive
northern stream flow regime will persist from the North Pacific
east across Canada and the CONUS northern tier. A vigorous
shortwave is expected to cross the lower Great Lakes and Northeast
on Fri, with a surface low pressure system deepening as it moves
across Quebec, and the trailing cold front moving off the Easter
Seaboard. Models continue to show variability on the degree of
interaction between the southern stream trough as it moves east
and the progressive northern stream. Faster timing of the southern
stream wave should reduce the potential for interaction with the
next northern stream wave, making for somewhat less phasing of
streams than indicated by the past few runs of the ECMWF. For a
wave of low pressure expected to develop along the trailing
surface front across the lower Mississippi Valley on Fri, the
result of the aforementioned interaction will be a quicker
eastward progression and perhaps somewhat less threat of a
significant coastal low off New England by Sun - although as with
any potential phasing interactions confidence in this aspect of
the forecast is quite low, particularly given that models have
seemed to struggle with timing of smaller scale northern stream
shortwave energy.
Farther west, models continue to depict the arrival of a strong
North Pacific jet to the northwestern U.S. by late Sat, which
should bring a series of shortwaves and frontal systems to the
region. Early on in the extended, models show consensus that an
upper low should separate from this flow and drift southward off
the West Coast before once again closing off near/off the
southwest U.S. coast over the weekend. The ECMWF was the lone
solution that takes the upper low westward, while almost all other
solutions eventually start bringing the feature eastward and merge
it with a trough amplifying across the western U.S. by early next
week. The latter idea was favored in this forecast, as the ECMWF
has recently shown a tendency to be too aggressive with taking
cutoff lows southwestward (this week's system was a similar
example). Given the favored solution and support from ensemble
means, expect to see amplification of a fairly significant upper
trough across the western U.S. by early next week, with a cold
front pressing across the West and into the central U.S., and
potential for development of a surface low somewhere across the
Plains by the end of the forecast period (next Tue).
The WPC forecast was based heavily on a blend of the 18Z GFS/12Z
UKMET/12Z CMC during days 3-5 (Fri-Sun). ECENS/GEFS ensemble means
comprised a majority of the forecast weighting during days 6-7
(Mon-Tue), with some continued use of the GFS/CMC.
...Weather Highlights/Threats...
The front crossing the Northeast on Fri will bring a mix of rain
and snow to the region, with the best potential for accumulating
snows in the higher terrain. Farther south, the wave of low
pressure expected to move from the lower Mississippi Valley to the
Southeast Fri-Sat is expected to produce widespread showers and
some thunderstorms. Locally heavy rain will be possible across
these areas, but the heavy rain threat appears at least somewhat
reduced compared to the past couple days, due to the trend to a
faster eastward progression of the system. Across the Pacific
Northwest, expect rain and mountain snow to begin by Sat with
rain/mountain snow expanding in coverage across much of the
interior western U.S. Sun and Mon as the upper trough amplifies.
Expect chilly temperatures to overspread the central U.S. Fri and
the eastern U.S. Sat-Sun, with highs 5 to 10 (and a few locations
as much as 15) deg F below average. A warming trend will bring
high temperatures 5 to 10 deg above average to the northern plains
and Upper Midwest over the weekend. Meanwhile, as the upper trough
digs into the West by early next week, expect temperatures to
drop, with highs 5 to 15 deg below average across much of the
interior western U.S.
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