Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 PM EST Sat Nov 23 2019
Valid 12Z Tue Nov 26 2019 - 12Z Sat Nov 30 2019
...Several Major Storms for the Thanksgiving Holiday Period...
...Guidance/Predictability Assessment...
The latest models and ensembles show much better than average
forecast clustering/predictability regarding the evolution of a
shortwave trough exiting the Central Rockies at the beginning of
the period (Tuesday), and steadily deepening as it lifts into the
Great Lakes and the Northeast by Wednesday. This should spin up an
impressive cyclone at the surface which tracks across the Central
Plains and Midwest Tuesday into early Wednesday, exiting New
England by late Wednesday. Model agreement and run-to-run
variability on the timing and intensity of this system has
improved within the past 24-36 hours, with most deterministic and
ensemble means showing a similar track for this feature. The
preferred blend for this system was a composite of the GFS/ECMWF,
the GEFS/ECMWF ensemble means and the National Blend of Models.
This maintains good continuity with the previous WPC forecast and
depicts a significant storm system to message.
Another focus for highly threatening weather is also out West
where models and ensembles continue to show very amplified
troughing and an upper level low dropping down the West Coast
starting by mid next week that persists and is reinforced into
later next week. The sharp digging of a potent upper jet and
height falls that acts to carve out the anomalously amplfied
system is well supported by upstream mid-upper ridge amplification
pumped up by farther upstream potent northeast Pacific and Bering
Sea low/storm activity. This is a high predictability event at
medium range time scales, especially for the day 3-5 time frame.
There remains some question on how quickly the energy begins
ejects though by days 6-7 (Fri-Sat). Suspect lead main energies
will eject through the Rockies into the Central U.S. with some
vigor heading into next weekend, but reinforcing energy will still
maintain a base trough over the West. The progressive 12 UTC GFS
seems the outlier compared to most other models and ensembles
including the 12 UTC GEFS.
...Weather Highlights/Threats...
Moderate to locally heavy precipitation is likely Tuesday into
Wednesday on the north and west side of the surface low from the
Central Plains to the Upper Midwest. Some accumulating snows may
be possible on the northern edge of the precipitation shield. As
the storm lifts into the Northeast on Wednesday, wintry
precipitation is possible across northern/interior New England.
Depending on surface low intensity, a period of high winds may
also accompany this system.
Heavy mountain snows are likely from the Washington/Oregon
Cascades and the Sierras on Tuesday, spreading into the Great
Basin and Rockies Wednesday-Friday as the upper trough amplifies
along the coast. Snow levels will drop significantly. Another
round of potentially heavy rainfall with a threat of runoff
problems are also especially likely across Southern CA and the
Southwest (including burn scars) by mid to late next week.
Rainfall is also likely from portions of the Southern Plains to
the Tennessee Valley as multiple shortwave impulses round the base
of the Western U.S. trough and tap some mid-upper level moisture
from the tropical east Pacific.
The biggest temperature anomalies will focus across the Western
U.S. through much of the period with daytime highs averaging 10 to
20 degrees below normal. In response to this trough, ridging
builds across the south-central U.S. which should bring a return
of above normal temps (anomalies +5 to +10 degrees) to this region
by the end of the period. In the East, above normal temperatures
Tuesday and Wednesday should moderate back to normal or slightly
below normal at the end of the week.
Schichtel
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