Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
229 AM EDT Mon Oct 29 2018

Valid 12Z Thu Nov 01 2018 - 12Z Mon Nov 05 2018

...Pattern Overview and Guidance/Predictability Assessment...

A mean upper-level trough is expected to remain generally in place
across the central U.S. during the medium range, while ridging
dominates off the West Coast. Shortwave energy crossing the North
Pacific will interact with additional energy from higher latitudes
across western Canada, resulting in a series of amplifying
shortwaves crossing the central/eastern U.S., maintaining the mean
trough. While model guidance continues to show good agreement on
the large scale pattern, significant differences exist (and grow
through time) with respect to the details of individual features.

Models show timing differences starting on day 3 (Thu) for the
frontal wave crossing the lower Mississippi and Tennessee Valleys.
Additional shortwave energy is expected to quickly dive southeast
across the northern plains and Midwest on Thu-Fri, reaching the
Ohio Valley by day 5 (Sat). Models show both timing and amplitude
differences with this system, with the GFS and UKMET representing
the more amplified side of the spread and the CMC the least
amplified. To resolve these differences, a multi-model blend
approach was used for days 3-5, based primarily on the
ECMWF/GFS/UKMET. By days 6-7 (Sat-Sun) model differences continue
to quickly escalate, with deterministic guidance losing much of
its usefulness. Shortwave energy appears likely to reach the
Pacific Northwest on day 5 which should then amplify as it crosses
the Rockies and central U.S. days 6-7. Deterministic model
solutions range from a very amplified wave (and surface low
pressure system) across the north central U.S. by day 6-7 on the
ECMWF, to flow quickly tending toward zonal on the CMC. As a
result of the model spread and variability, leaned heavily toward
ECENS/GEFS ensemble means by days 6-7.


...Weather Highlights/Threats...

Potentially heavy convection may be ongoing at the beginning of
the extended period on Thu across portions of the Tennessee and
Ohio Valleys in association with the frontal wave. Additional
areas of locally heavy rain will be possible through the end of
the week in association with this frontal system as it reaches the
Eastern Seaboard. Heavy rain also be possible across the Pacific
Northwest through the weekend 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 perhaps the central Rockies. Chilly conditions will
be prevalent through most of the forecast period across much of
the central U.S. beneath the persistent upper trough, where highs
will generally be 5 to 10 deg F below average. Meanwhile, above
average temperatures will persist across portions of the West,
from California into the Great Basin, where high temperatures may
reach 5 to 15 deg F above average.


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