Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1042 AM EST Thu Nov 15 2018

Valid 12Z Sun Nov 18 2018 - 12Z Thu Nov 22 2018

...Overview...

Upper-level ridging across western North America will be slow to
weaken/break down during the extended period, which will support a
continued downstream mean trough across the Great Lakes and
Northeast. Polar shortwave energy will traverse the mean trough
through much of the period, with some potential for phasing
interactions with southern stream/Pacific energy undercutting the
western ridge and crossing the CONUS southern tier. The position
of the mean trough will delay any potential phasing interactions
such that any developing coastal low pressure systems will likely
be quickly pulling away from the northeastern U.S. before
significant deepening begins.


...Guidance/Uncertainty Assessment...

Model consensus was sufficient during days 3-5 (Sun-Tue) to base
the forecast on a multi-model deterministic blend (including the
ECMWF/GFS/FV3/UKMET). Differences among these solutions were
relatively small with respect to shortwave energy rounding the
base of the mean trough across the Great Lakes during that time
period, and with an elongated/trailing shortwave extending back
into the Plains and Four Corners regions. As these features
partially phase, an area of low pressures is expected to develop
along the Northeast U.S. coast Wed night/Thu morning, which will
very quickly move northeastward into Canada. By day 5 (Tue)
another shortwave crossing the Great Lakes and a trailing frontal
boundary will develop yet another surface wave which is expected
to quickly move northeastward into Canada by day 6 (Wed). From day
6 onward, models continue to show some significant differences,
especially with respect to any potential weakening of the western
ridge and the arrival of height falls in the West associated with
Pacific shortwave energy. The ECMWF was rejected during this time
period yesterday as it was by far the fastest solution. A general
trend toward a slower ridge breakdown/slower arrival of height
falls was observed across the model/ensemble suite, and despite a
significant trend in that direction, the ECMWF remains among the
faster solutions to spread height falls into the West by the
middle of next week. Ensemble means (especially the GEFS mean)
have remained much more stable than deterministic solutions. At
this time, the slower solutions will continue to be favored.
Weighting of ensemble means was boosted substantially by days 6-7,
with a bit more weight toward the GEFS mean relative to the ECENS
mean, with very little emphasis on the deterministic ECMWF by that
time.


...Weather Highlights/Threats...

The trend toward a slower arrival of significant height falls in
the western U.S. appears likely to delay any chances of
precipitation across the West Coast states until at least Tue
night/Wed. Locations near the central and northern West Coast
should have the best potential for seeing some rain and possibly
high elevation snow. A couple southern stream shortwaves along
with a region of potential isentropic upglide north of a Gulf of
Mexico frontal boundary will support an expansion of rainfall
across the western Gulf Coast during the period, with the
potential for some areas of heavy rainfall, especially close to
the coast where instability will be best. Periods of lake effect
snow should be most favored over northern and eastern portions of
the Great Lakes. Some of the snow over this area may also be
focused near one or more frontal boundaries/embedded waves.

During Sun-Mon expect coldest temperature anomalies of 10-20F or
so below normal to be most common from the southern Plains into
Midwest/Great Lakes. Then as the core of upper troughing adjusts
into the Northeast, that region should see coldest readings versus
normal Tue-Thu, 10-25F below normal. Such anomalies may challenge
daily record values at some locations. Confidence remains above
average for plus 5-15F high temperature anomalies along/inland
from the central West Coast through at least the first part of
next week, after that time, forecast confidence decreases given
the uncertainty regarding the arrival of height falls from the
Pacific.

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