Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
208 PM EST Sun Nov 24 2019
Valid 12Z Wed Nov 27 2019 - 12Z Sun Dec 01 2019
...Several Major Storms for the Thanksgiving Holiday Period...
...Guidance/Predictability Assessment...
Model guidance in good agreement continues to indicate a tight
shortwave trough will spin up into a closed upper low in the Upper
Midwest by day 3/Wed, with good agreement in the 06Z GFS and GEFS
mean as well as the 00Z ECMWF and UKMET. The 12Z model cycle has
come in with this idea as well, so with good run-to-run
continuity, a multi-model blend was used for this feature which is
likely to spin up a strong surface low. These features track
east-northeast on day 4/Thu, affecting the Northeast.
The other main feature in the medium range is the anomalously deep
(3-4 standard deviations below normal for 500 mb heights) upper
low moving through the West midweek onward. The guidance is well
clustered through day 3-5 but start to diverge day 5-6 (Fri-Sat),
with the 06Z GFS and (to a lesser extent) the GEFS ejecting energy
eastward more quickly than other models like the EC. Given this
can be a common bias for the GFS suite, the forecast hedged more
toward the 00Z deterministic EC and EC mean, with some
incorporation of the GEFS to account for forecast uncertainty. The
new 12 ECMWF, however, seems to move the low farther eastward more
quickly than its previous run, matching fairly closely with the
12Z GFS. So the timing and the exact track remain uncertain at
this time for this feature, but it appears the the associated
surface low could end up around the same location (Upper
Midwest/Upper Great Lakes region) as the earlier low on Wed.
Overall these choices maintained good continuity with the previous
WPC forecast.
...Weather Highlights/Threats...
Precipitation is likely to continue from the short range (Tue)
into Wed into the Upper Great Lakes region and spreading into the
Northeast. Wintry precipitation is possible in northern/interior
New England as well as the Upper Great Lakes.
Meanwhile, heavy mountain snows are likely across the Great Basin
and Rockies Wednesday-Friday with snow levels expected to drop
significantly due to the anomalously low 700 mb heights. 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. Heavy
rainfall is also possible from portions of the Southern Plains to
the lower Ohio 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. A cold front could help
focus this rainfall as it treks eastward with the second main
surface low. The northwest edge of this precip shield across
eastern New Mexico/Texas Panhandle may also see accumulating
snowfall with surface high pressure and sufficiently cold air
diving southward across the Plains. With the next potential storm
system winding up in the Central U.S. into next weekend,
accumulating snows (potentially heavy or significant) are possible
to the north and west of this surface low from the Northern Plains
to the Upper Mississippi Valley next Friday and Saturday.
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
on Wednesday should moderate back to normal or slightly below
normal at the end of the week.
Tate/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