Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
140 AM EST Fri Nov 22 2019
Valid 12Z Mon Nov 25 2019 - 12Z Fri Nov 29 2019
...Guidance/Predictability Assessment...
The medium range period begins with a vigorous shortwave/surface
low exiting the Northeast, with various shortwaves rotating
through broad troughing across the remainder of the CONUS. For Day
3-4 (Mon-Tues), models overall show very good agreement and thus a
majority deterministic model blend sufficed, between the 18z/Nov
21 GFS, the 12z/Nov 21 ECMWF, and the 12z/Nov 21 UKMET. Beyond
this, one of the biggest forecast concerns continues to be
regarding the evolution of a shortwave as it digs southward
through the Western U.S. and ejects into the Central U.S. by day
4. Models continue to struggle with respect to both timing and
intensity of this system, which looks to spin up a surface low in
the south-central Plains on Tuesday, and track it quickly towards
the Northeast by Wednesday/Thursday. The GFS has been very
consistent in deepening this surface low early on as it tracks
through the Midwest, while the ECMWF shows much more run-to-run
variability ranging from a solution close to that of the GFS to
maintaining a rather weak wave through the Midwest and not showing
much intensification until the low reaches the Northeast on
Wednesday. The CMC also continues to exhibit high variability with
each new model run and the latest run barely shows a shortwave at
all and is so widely different from any other model solution and
the ensemble means that it was not considered. Timing of this
system also remains in question with the GFS on the faster side of
the guidance envelope and the ECMWF/UKMET on the slower side. In
addition, scatter plots of various ensemble member surface lows
shows better clustering towards the slightly faster GFS. Based on
this, and given the uncertainty remaining with this system beyond
day 5, a blend of the ensemble means (ECENS/GEFS) was preferred
which results in a moderately intense storm and a slightly faster
track much closer to that of the GFS. This also maintains very
good continuity with the previous WPC forecast.
Meanwhile, another vigorous shortwave follows on the heels of this
first one reaching the West coast by late Tuesday. Models agree
that this wave should become quite amplified by mid to late next
week, with some solutions wanting to close off an upper low
somewhere along the Southwest coast. Run-to-run variability in the
deterministic models, both with respect to timing and closed low
vs amplified troughing, remains high with this system and thus
more of the better clustered ensemble means was used in the blend
for days 6-7.
...Weather Highlights/Threats...
The developing surface low in the Central U.S. on Tuesday may
bring a period of heavy precipitation from parts of the Central
Plains to the Upper Midwest/Great Lakes, with snow possible on the
northern edge of the precipitation shield. As the surface low
lifts into the Northeast on Wednesday, potentially significant
snows are possible across northern/interior New England, though
given the uncertainty with this system, it is much too early for
specifics regarding snowfall amounts. Depending on surface low
intensity, a period of high winds may also accompany this system.
Rainfall will also be likely along the trailing cold front as it
moves from the lower Mississippi Valley to the the Southeast
Tuesday-Wednesday.
Across the West, heavy snowfall is likely in the higher terrain of
the Washington/Oregon Cascades, the Sierra Nevada range, and
eastward across the Great Basin/Rockies both with the initial
shortwave crossing the region Monday, and even more so with the
amplified troughing developing over the region by late in the
week. Another round of potentially heavy rainfall is possible
across the Southwest by mid to late next week, with rainfall also
developing by Friday eastward into the Southern Plains/Mid-Lower
Mississippi Valley ahead of the upper trough. Colder temperatures
across the West will also accompany this system, with daytime
highs averaging 10 to 20 degrees below normal across the Great
Basin and Northern Rockies.
Santorelli
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