Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Thu Oct 31 2019
Valid 12Z Sun Nov 03 2019 - 12Z Thu Nov 07 2019
...Overview and Guidance/Uncertainty Assessment...
Expect mean ridging aloft over the eastern Pacific and Alaska to
support persistence of a mean trough over the lower 48 states. On
any particular day troughing will vary in precise amplitude and
shape in response to individual shortwaves progressing within the
mean flow. As has been the case over recent runs, guidance shows
decent clustering in principle into about early Tue. Then
model/ensemble spread and variability increase due to differences
in how ridging over and near Alaska evolves and this influence on
shortwave energy reaching northwestern North America early in the
week.
A blend emphasizing the latest operational guidance through the
12Z/18Z cycles provided a reasonable starting point for the
updated forecast during the first half of the period when guidance
is fairly agreeable (Sun into early Tue) aside from detail
differences that tend to have fairly low predictability 3-5 days
out in time. After a leading shortwave/cold front departs from
the East Coast early Sun the primary feature of interest will be
the next amplifying shortwave that should bring low pressure
across the northern tier/Upper Great Lakes region. The trailing
front will drop south/southeast across the central U.S. while the
western part settles along the eastern slopes of the Rockies.
From later Tue into Thu the central U.S. front should continue
south and east. Based on the current array of guidance there is
greater confidence in progression of the northern part of the
front, with upstream issues leading to the uncertainty in
progression/southward extent of the remainder of the front. As
for the upstream evolution there is decent agreement in principle
that another surge of cold air dropping through western Canada
should reach the northern U.S. around Tue-Tue night. However by
Wed-Thu there is rapid divergence in solutions for evolution of
the supporting dynamics aloft in response to the shape of the
Pacific/Alaska ridge--leading to some dramatic differences in
southward/westward extent of cold air. At the very least the 12Z
ECMWF appeared less likely than most other solutions as its
western trough amplification (and resulting cold push through most
of the West) exceeded nearly all of the ECMWF ensemble members.
Reviewing remaining guidance, 12Z ECMWF ensembles as a whole have
trended toward somewhat lower Northwest U.S. heights aloft than
continuity or latest GEFS/CMC means while latest GFS runs show a
hint of the ECMWF mean scenario. Teleconnections relative to
strong positive height anomalies over/near Alaska offer potential
for lower heights over this region as well, either within or
beyond the forecast period. On the other hand the 12Z CMC
ensembles adjusted to the GEFS scenario after previously showing
more potential for lower heights over the Northwest. Based on the
sum of these considerations the forecast ultimately trended toward
a blend of the 18Z GEFS/12Z ECMWF means and minority input from
the 12Z/18Z GFS and old 00Z/30 ECMWF.
...Weather Highlights/Threats...
The northern tier/Great Lakes system and the cold cyclonic flow in
its wake will bring periods of snow and rain to the Great Lakes
region. Lighter precipitation will extend across the northern
Plains. Portions of the northern Rockies/High Plains should see a
couple periods of snow on the cold side of fronts dropping south
from Canada. Meanwhile there is still considerable debate in the
guidance over the coverage and intensity of rainfall over the
eastern U.S. ahead of the front that should reach a Great Lakes to
southern Plains orientation by Tue. Southern stream energy aloft
may support more rain than forecast by the GFS/GEFS but
ECMWF/ECMWF ensemble runs could be overdone. Depending on
uncertain details at the surface and aloft, rainfall could begin
to increase over or near the southern Plains around or shortly
after next Thu. Easterly low level flow and one or more fronts
will likely bring periods of mostly light rain to the Florida
Peninsula.
From Sun into Mon southern Plains into eastern U.S. high pressure
will bring a brief period of below normal temperatures, with
decent coverage of minus 10-15F anomalies on Sun. After this high
departs the focus for coldest air will extend from the northern
High Plains southeast into the central Plains and east into the
Upper Midwest/Great Lakes. The northern High Plains region has
the best potential for multiple days with highs 15-25F below
normal while other areas should see one or more days at least
10-15F below normal. Western areas should remain modestly above
normal with plus 5-10F anomalies for highs, while the East will
see readings vary with system progression Tue-Thu.
Rausch
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