Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
247 PM EDT Wed Oct 30 2019
Valid 12Z Sat Nov 02 2019 - 12Z Wed Nov 06 2019
1530 UTC Update...
Models showed above average consensus during the medium range, and
have become more consistent run-to-run with many of the details,
particularly through day 5. Thus, while the forecast was updated
to reflect a blend of the latest deterministic guidance, no major
changes were made to the ongoing forecast from overnight. The 00Z
ECMWF/UKMET/CMC/06Z GFS were used during days 3-5 (Sat-Mon), with
somewhat more emphasis on the ECMWF/UKMET solutions given that
they were slightly better centered within the ensemble spread, and
have shown the most consistency. During days 6-7 (Tue-Wed),
emphasis was gradually shifted to majority ECENS/NAEFS ensemble
means, with some continued use of the deterministic ECMWF and CMC.
The GFS became the solution with the most significant differences
by the middle of next week, as it was somewhat less amplified with
the broad upper trough across central/eastern Canada and the north
central U.S., resulting in warmer temperatures across much of the
CONUS than the guidance consensus. With the upper ridge along the
West Coast building north across Alaska by next week,
teleconnections favor continued incursions of cold air into the
north central U.S., with perhaps an intensification of the cold
from the northern Rockies into the northern plains near and beyond
the end of the forecast period. The discussion from overnight
follows below.
Ryan
Previous Discussion (issued at 0658 UTC)...
...Overview and Guidance/Uncertainty Assessment...
Most guidance maintains good agreement and continuity with the
expectation of a persistent and fairly amplified east-central U.S.
mean trough downstream from a northeastern Pacific ridge. There
has been some typical spread/variability with details, which
favors maintaining a model/ensemble mean approach to represent the
most common ideas of guidance while downplaying less confident
specifics.
Within the forecast mean pattern expect a leading shortwave over
the Mississippi Valley early day 3 Sat to lift northeastward
during the weekend and push a leading cold front through the East.
Upstream shortwave energy over southern mainland Alaska and the
Gulf of Alaska on Sat will quickly drop southeast and support a
northern tier surface low and trailing front late weekend into
early next week. The primary consideration for this feature is
that recent GFS runs (including the new 00Z run) carry their
surface wave slower and farther south than the majority cluster
that consists of the ECMWF/UKMET and the ensemble means. The CMC
is on the weaker side of the spectrum but the low track in the 00Z
run is close to consensus as well. An operational model blend to
start the period and then by day 5 Mon adding some 18Z GEFS/12Z
ECMWF mean input and reducing the 12Z/18Z GFS a bit reflects
consensus early and a non-GFS solution for the early week northern
tier system.
Later in the period the GEFS/ECMWF means have been very agreeable
and stable at least for their mass fields. Thus prefer to trend
the more distant part of the forecast 50-60 percent in their
direction. The 12Z ECMWF compared better to the means than the
18Z GFS in particular, though the new 00Z GFS has trended
favorably into early Wed. As a result the late-period operational
component used more 12Z ECMWF than 12Z GFS while phasing out the
18Z run. This solution brings the next upper trough into the East
by next Wed with the leading front reaching the East Coast by
then. The greater model/ensemble discrepancy involves moisture
return ahead of the late-period eastern front. Localized ECMWF
precip amounts and ECMWF mean coverage could be a little
aggressive but consensus suggests that GFS runs could be too fast
with southern stream energy that could play a role in developing
at least some areas of rain.
As suggested by variability in recent operational model runs,
there is still a fair degree of uncertainty over wave details
along the mean frontal boundary likely extending from southwest
Canada southeastward into the Rockies/High Plains. This is
connected to various potential solutions for what happens with
shortwave energy expected to be over Alaska/northwestern Canada
early in the week. Thus far GEFS/ECMWF ensembles are narrowing
their spread toward keeping the northeast Pacific ridge rather
strong and not allowing the shortwave energy to elongate westward
like the 18Z GFS and to some degree 12Z GFS. On the other hand
CMC ensembles continue to show lower heights over the northwestern
states--seemingly a low probability scenario at this time given
the robust consensus of the GEFS/ECMWF means and operational
GFS/ECMWF/CMC runs. Finally, GFS/ECMWF runs and their means have
come into good agreement for the weak upper low offshore
California.
...Weather Highlights/Threats...
From the weekend into the start of next week the majority of
precipitation will be with areas of mostly light-moderate snow and
rain between the northern Rockies/High Plains and Great Lakes.
Cool cyclonic flow around a leading system tracking away from the
Upper Great Lakes will promote a period of lake effect activity
while the next wave tracking into the northern tier and trailing
front will provide another precipitation focus. From Mon into Wed
the passage of the northern wave will generate more precipitation
over the Great Lakes with lower confidence in precip coverage to
the west. Rainfall coverage may increase somewhat over the
eastern half of the country as the northern tier system's front
heads toward the East Coast, but to what extent remains uncertain
and dependent on evolution/timing of upper dynamics. One other
localized area of rainfall during the period will be over the
Florida Peninsula in association with a stalling front and
easterly low level flow.
The eastern periphery of the Pacific ridge aloft will provide
modestly warm temperatures to most of the West during the period,
with most anomalies for highs around plus 10F or less. One chilly
airmass will progress across the central/eastern states during the
weekend with most locations seeing at least one day with highs
10-20F below normal. Well below normal anomalies should be less
extensive with the next push of cool air Mon-Wed, with areas from
the northern High Plains into the Upper Midwest/Great Lakes most
likely to see highs 10-20F below normal.
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
- Flooding possible across portions of the Middle Mississippi
Valley, the Great Lakes, and the Northern Plains.
- Flooding occurring or imminent across portions of the Middle
Mississippi Valley, the Upper Mississippi Valley, and the Northern
Plains.
- Much below normal temperatures from portions of the Central
Rockies to the Southern/Central Plains, and Lower Mississippi
Valley, Sat-Sun, Nov 2-Nov 3.
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