Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
135 AM EDT Thu Nov 01 2018

Valid 12Z Sun Nov 04 2018 - 12Z Thu Nov 08 2018

...Pattern Overview...

A mean upper-level trough is expected to persist across the
central U.S. through the extended period. The trough will be
reinforced by a series of shortwaves originating in the north
Pacific (north of a persistent upper-level ridge), potentially
interacting with additional shortwave energy of polar/arctic
origin as they cross the Canadian Rockies before amplifying as
they encounter the area of broad and persistent cyclonic flow
across the central U.S. A series of frontal/low pressure systems
will accompany these shortwaves across the CONUS, generally 1-2
days apart, resulting in a period of relatively active weather.


...Guidance/Predictability Assessment...

A solution close to the consensus of the ECENS and GEFS means was
preferred with the low pressure system crossing the
Mid-Mississippi Valley/Upper Midwest during days 3-4 (Sun-Mon).
The ECMWF was the closest deterministic solution to the ensemble
consensus with the track/timing of the surface low, followed by
the GFS. Thus, a ECMWF/GFS (more weight toward the ECMWF) blend
served as a starting point for the WPC forecast during that time
frame. By Mon the next shortwave should be exiting the Rockies
into the Plains, with development of yet another surface low
pressure system expected. Spread among the deterministic guidance
seems to have increased rather substantially with this second
system by day 5 (Tue) and onward. The last few runs of the GFS
have deviated from the ensemble consensus and quickly move the
surface low northward into Canada, with another frontal wave
developing farther south along the front. The ECENS mean and even
the GEFS/NAEFS means continue to look much more like the ECMWF,
which remains reasonably well centered within the ensemble spread.
Thus, something along the lines of the means and the ECMWF remains
the preferred solution, which also keeps reasonably good
continuity with the previous forecast. This scenario shows a
strengthening surface low moving north-northeastward across the
Great Lakes on Tue and continuing to deepen (perhaps quite
substantially) as the low moves into eastern Ontario toward James
Bay on day 6 (Wed). Through late in the forecast period, the same
general pattern looks to hold strong, with additional shortwaves
continuing to reinforce the mean trough (although the details of
these systems are quite murky). Given the increased spread
especially after day 5, weighting was shifted toward the ensemble
means during the day 6-7 time period, with a bit of continued
emphasis on the ECMWF through day 6.


...Weather Highlights/Threats...

The Mississippi and Ohio Valleys will see the potential for rain
to become more widespread late in the weekend into next week as a
couple frontal systems traverse the region and tap into Gulf of
Mexico moisture. An expanding areas of rain will spread
northeastward ahead of the midweek system (Tue-Wed) with an inch
or more of rain possible over a large area, particularly over the
Ohio/Tennessee Valleys northward to the Lower Great Lakes. Off the
coast, the lingering stationary front may push back toward the
coast and bring the potential for precipitation back to the
Northeast. Heavy rain and higher elevation snow will be possible
across the Pacific Northwest into early next week as upper
shortwaves/surface fronts traverse the northern periphery of the
upper ridge. These features will also bring rounds of snow to the
higher elevations of the northern and then central Rockies early
next week. Chilly conditions will be prevalent across portions of
the central U.S. beneath the persistent upper trough, where highs
will generally be 5 to 10 deg F below average. Chilly air may make
its way into the eastern U.S. by later next week behind a strong
cold front. Meanwhile, above average temperatures will persist
across portions of the West, from California into the Great Basin
(especially Sun-Tue), where high temperatures may reach 5 to 15
deg F above average downstream of persistent upper-level ridging.


Ryan


WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found 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