Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 AM EST Thu Nov 21 2019
Valid 12Z Sun Nov 24 2019 - 12Z Thu Nov 28 2019
...Guidance/Predictability Assessment and Weather
Highlights/Threats...
Models over the past day have trended a bit toward a more phased
solution aloft, and a surface low track closer to the Northeast
U.S. coastline on day 3 (Sun). Solutions still show some
variability on the exact track of the surface low, but a
multi-model blend approach was favored for this system, including
the 12Z ECMWF/UKMET/GFS. These solutions were most closely
clustered within the center of the current ensemble spread. Given
the westward adjustment in the track of this system, potential
effects on the Northeast have increased somewhat. Increased QPF
amounts a bit for eastern/southern New England during the Sun-Mon
time period, and continue to indicate low end probabilities in the
Winter Weather Outlook. The surface low should quickly move
northeast into the Canadian Maritime Provinces by Mon morning,
bringing an end to any potential precipitation across the
Northeast.
A shortwave is forecast to cross the northern Rockies/northern
Plains Mon-Tue, with an associated cold front, and a surface low
tracking east across the Canadian prairies. Models have continued
to struggle with the details of this feature, both with respect to
timing and intensity. The 12Z GFS/UKMET were the closest solutions
to an ensemble-based consensus with this system. The 12Z ECMWF was
on the fast side of the spread, and has also shown the greatest
run-to-run variability with this feature, thus something more
along the lines of the GFS or 12Z UKMET was preferred. This system
will be relatively moisture starved, so any associated
precipitation is expected to be rather sparse and relatively
light. Warm advection ahead of the system will bring a period of
well above average temperatures to the Central/Northern Plains and
Upper Midwest Sun-Mon, with high temperatures forecast to be 5 to
as much as 15 deg F above average (especially on Sun).
Additional shortwave energy is forecast by all guidance to move
onshore in the Pacific Northwest Sun night/early Mon, bringing a
round of potentially heavy snow to the Washington/Oregon Cascades
and coastal ranges. While models agree on the existence of this
feature, solutions diverge significantly on the eventual evolution
as the system moves east toward the central U.S. As the shortwave
digs into the Rockies/Four Corners by late Mon, the ECMWF has
trended toward a much less amplified solution over the past few
runs. The GFS on the other hand, has backed off slightly on the
amplitude of the wave, but maintains a more amplified shortwave
exiting from the Four Corners into the Southern/Central Plains by
early Tue. The GFS has been highly consistent in developing a wave
of low pressure along the surface front across the
Southern/Central Plains Mon night/Tue, which then deepens
substantially as it moves northeastward, reaching the Great
Lakes/Upper Midwest by Tue night/Wed morning. The ECMWF/CMC, on
the other hand have both shown a large degree of run-to-run
variability, ranging from solutions similar to the GFS idea to a
much more suppressed system that does not begin deepening until it
reaches the Northeast on Wed. The ECENS/GEFS/NAEFS ensemble means
maintain a solution much more similar to the GFS than the ECMWF.
Further, a look at scatter plots of ensemble member surface lows
indicates a significant cluster of members from all three ensemble
systems, for at least 3-4 ensemble cycles, around the GFS idea.
Thus, despite what remains high forecast uncertainty, will
maintain a forecast along the lines of the GFS (and WPC
continuity). This scenario would bring the potential for a round
of significant winter weather from portions of the Central Plains
to the Upper Midwest Tue into early Wed. Depending on the eventual
intensity of the surface low, strong gusty winds would also be
possible, but confidence in the details of the forecast is too low
for such specifics at this point in time. In the wake of this
system, expect a colder air mass to overspread a wide area from
the Great Basin/Rockies east into the High Plains, with high
temperatures 5 to 15 deg F below average.
Lastly, models show another potentially significant shortwave
following quickly on the heels of the previous one, reaching the
West Coast Tue night/Wed. Models agree that this wave should
become quite amplified by Wed-Thu as it cross the Great
Basin/Rockies. Solutions differ on the timing, however, with the
ECMWF a bit faster than than most other guidance. The ECMWF has
also shown a high degree of run-to-run variability with this
feature. The GFS solution was closest to the ensemble means in
terms of the timing of this system, and has shown a higher degree
of run-to-run consistency relative to the ECMWF. Thus, favored a
blend of the GFS with ECENS/NAEFS ensemble means by the middle of
next week. This system is forecast to bring a significant increase
in rain/mountain snow along much of the West Coast and east into
the Great Basin and Rockies next week, along with another round of
cold temperatures (highs 10-20 deg F below average).
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