Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EST Mon Nov 25 2019
Valid 12Z Thu Nov 28 2019 - 12Z Mon Dec 02 2019
...Increasing confidence in a major winter storm impacting the
Thanksgiving Holiday and following weekend from the Northern
Plains to Upper Great Lakes...
...Patter overview and Guidance/Predictability Assessment...
The medium range period begins Thursday with the first major storm
system exiting the Northeast while anomalous (500mb heights 3 to 4
standard deviations below normal) and very amplified
troughing/closed upper low is situated along the West Coast. The
Western U.S. system is the main system of interest in the medium
range period as it slowly meanders over the Western U.S.,
eventually shifting eastward as vigorous short wave energy ejects
out of the Rockies around Day 5/Saturday. This will induce
cyclogenesis in the lee of the Rockies and a rather potent surface
low tracking from the Central Plains to the Northeast, day 5-7.
This system appears to take a very similar, albeit slower, track
to the previous system now in the short range period.
For days 3-4, the models show good enough agreement to warrant a
majority deterministic model blend. After this, questions begin to
arise mainly on timing of the shortwave energy ejecting into the
Central Plains and a tightly closed upper low forming and tracking
towards the Upper Great Lakes. The GFS has been the most
consistent both with timing and intensity of this feature. The
latest run (12z/Nov 24) of the ECMWF is quite a bit faster than
the GFS, though run-to-run continuity is poor. It atleast appears
the ECMWF may be trending in the direction of the GFS though,
which lends to a bit more confidence in the GFS solution.
Meanwhile, the CMC and UKMET (which of course is not available
past day 5) are much slower than the GFS, not actually ejecting
the energy into the Plains until almost a full day later. The
ensemble means are generally in good agreement and support a
solution closer to that of the faster GFS/ECMWF and is a good
middle ground between the two as well. After day 5, the WPC progs
trended towards the ECENS/GEFS means, with small contributions
from the GFS to help give some definition to the typically washed
out look of the means. This maintains good continuity as well with
the previous WPC forecast.
...Weather Highlights/Threats...
Heavy mountain snows are likely across parts of the Central Great
Basin and Rockies Thursday and Friday, with snow levels expected
to drop significantly. Additionally, another round of heavy
rainfall with a threat for runoff problems is likely across
Southern CA and the Southwest (including burn scars) late in the
week. As the surface low moves out into the Plains, a swath of
heavy to potentially significant snowfall is possible to the north
and west of the low from the Northern Plains to the Upper
Mississippi Valley and Upper Great Lakes Friday and Saturday.
Depending on surface low intensity, a period of heavy winds could
accompany this system making for hazardous travel across this
region during the upcoming busy Holiday weekend. Across the South,
shortwave energies tapping into mid-upper level moisture from the
tropical East Pacific will result in heavy to possibly excessive
rains shifting from the Southern Plains to the Ohio Valley along
and ahead of the main cold front.
The biggest temperature anomalies will focus across the Western
U.S. through much of the period with daytime highs averaging as
much as 20 to 25 degrees below normal for some places. In
response, temperatures will be warm and above average from Texas
to the Gulf Coast states and into the Ohio Valley Friday and
Saturday, trending cooler by the end of the period following the
cold front passage.
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