Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1025 AM EST Wed Nov 14 2018

Valid 12Z Sat Nov 17 2018 - 12Z Wed Nov 21 2018

...Overview...

Upper-level ridging is expected to persist across western Canada
through much of the extended period, with the potential for
Pacific shortwave energy to undercut the ridge and move into the
western/southwestern U.S. Farther east, a mean trough/upper low
will persist in the vicinity of Hudson Bay, with progressive flow
from north central Canada into the Great Lakes and northeastern
U.S. This flow pattern will favor continued cold air intrusions
into the central/eastern U.S. through much of the extended period
with the potential for fast-moving, clipper-like systems to
traverse the U.S. northern tier (with potential cyclogenesis
along/off the Northeast U.S./eastern Canada coast.


...Guidance/Uncertainty Assessment...

For days 3-5 (Sat-Mon), models showed relatively good consensus,
allowing use of a deterministic multi-model blend
(ECMWF/GFS/UKMET). An elongated shortwave (and associated surface
frontal boundary) crossing the Rockies/central U.S., and an
additional more vigorous shortwave (associated with a surface
clipper-like system) diving south across the Great Lakes during
the time period are both well represented by this model blend. By
day 5, as some degree of phasing begins to occur between these two
waves across the Appalachians, models show general agreement that
a surface low should begin to develop somewhere along the northern
Mid-Atlantic or Northeast coast, and then move quickly
northeastward from there. By days 6-7 (Tue-Wed) model spread
begins to increase across the board. Timing and intensity
differences emerge with respect to the developing low along the
Northeast coast (which should quickly exit the picture as it moves
into Canada Mon night-Tue). More significant differences emerge
along the West Coast, with the ECMWF breaking down the ridging
much more quickly than the consensus of models/ensembles, and
moving a significant upper-level trough into the Great Basin by
Wed morning (when the GFS still has the trough axis several
hundred miles off the West Coast). This obviously has significant
implications in terms of both temperatures and precipitation for
the western U.S. by early next week. Ensemble means (including the
ECENS) have been fairly persistent and support something a good
bit slower than the deterministic ECMWF. Given these
considerations, weighting of ECENS/GEFS means was increased
substantially during days 6-7, with the deterministic ECMWF
eliminated from the blend entirely by day 7.


...Weather Highlights/Threats...

Expect the forecast pattern during the Sat-Wed period to yield
fairly low coverage of meaningful precipitation across the CONUS.
Some rain will likely develop over parts of southern Texas by late
Sun in association with a western Gulf Coast surface trough and
approach of the cold front pushing southward through the Plains.
Rainfall coverage/intensity may increase over the southern Plains
and vicinity toward Tue-Wed as the southern stream upper system
approaches. Also by next Tue-Wed the West Coast states should see
some increase in moisture but with considerable uncertainty
regarding the timing/coverage/magnitude of associated
precipitation. Farther eastward the north-central Rockies/High
Plains will see some early-period snow generated by shortwave
energy aloft and low level upslope flow. Some of the moisture may
extend eastward as light rain/snow.  The Great Lakes region will
see periods of lake effect snow with this activity tending to
favor northern and eastern areas. The wavy front potentially
settling over the area next week may promote some synoptic-scale
snow as well.

Over the course of the period the emphasis for coldest anomalies
(some highs at least 15-20F below normal) should shift from the
central U.S. during the weekend into the Northeast by Tue-Wed.
Behind the cold air there will be a moderating trend that
progresses from the Plains into east-central U.S. Anticipate
warmest highs relative to normal over and near northern
California/southern Oregon during the first half of the period
with some plus 10-15F anomalies.

Ryan/Rausch


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