Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
512 PM EST Thu Nov 21 2019
Valid 12Z Sun Nov 24 2019 - 12Z Thu Nov 28 2019
...Guidance/Predictability Assessment and Weather
Highlights/Threats...
The first system of concern in the medium range is a surface low
pressure system that is expected to track near the Atlantic coast
of the Northeast on day 3/Sun. Overnight model runs, as well as
the new 12Z guidance, continued to show the low near the coast,
spreading precipitation into the Northeast. As a general rule, the
precipitation should fall more as rain near the coast and more as
snow for interior New England (following the usual pattern).
Probabilities for snow were increased in the Winter Weather
Outlook since model guidance (GFS, EC, and UKMET) has been
consistent for several runs now.
Broad troughing over the CONUS is expected for day 4-5/Mon-Tue
with embedded shortwaves with low predictability. Mon to Tue,
energy should drop through the western side of the broad trough
into the West Coast and then into the central U.S. The question
remains how much this energy spins up in the central U.S. on Tue
and northeastward on Wed. Deterministic GFS runs have been more
bullish with this energy than the deterministic ECMWF runs
especially Tue and even into Wed, which affects the depth of the
surface low that could spin up just ahead. The 00Z ECMWF was even
weaker than its 00Z mean with that surface low, but note that the
new 12Z EC has a much stronger low. Overall, with inconsistencies
from run to run in the deterministic guidance, mainly went with a
blend of the 00Z EC and 06Z GEFS means.
Uncertainty remains fairly high regarding the impacts of this
system. This surface low will sweep a front across the Plains,
Mississippi Valley, and into the Appalachians Mon-Wed. Models were
much lower with precipitation in the Mississippi, Tennessee, and
Ohio Valleys along this front compared to previous runs, so this
forecast package trended down with the QPF there, but the pattern
still looks favorable for precipitation. Additionally, behind the
low some wintry precipitation is possible, but ensemble
probabilities for snow are low and broad without much focus across
the Upper Midwest. Thus the areal extent and amount of wintry
weather remains in question.
Additional energy drops southward again on Wednesday, deepening
the trough in the eastern Pacific. With these multiple rounds of
trough reinforcement, cooler than average temperatures should be
present in the West for midweek with ample snowfall in favored
higher elevations.
Tate
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
Hazards:
- Heavy precipitation across portions of the Northeast, Wed-Thu,
Nov 27-Nov 28.
- Heavy precipitation across portions of the Upper Midwest into
the central Great Lakes, and for
parts of the Pacific Northwest into northwest California, Tue-Wed,
Nov 26-Nov 27.
- Heavy snow across much of the Sierra Nevada, Wed-Thu, Nov 27-Nov
28.
- Heavy snow across portions of northern New England, Sun, Nov 24.
- Heavy snow across portions of northern Idaho, Mon-Tue, Nov
25-Nov 26.
- Heavy rain across portions of the Southeast, the Southern
Appalachians, the Lower Mississippi
Valley and the Ohio Valley, as well as the Tennessee Valley,
Tue-Wed, Nov 26-Nov 27.
- Flooding occurring or imminent across portions of the Northern
Plains.
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