Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1055 AM EST Sun Nov 11 2018

Valid 12Z Wed Nov 14 2018 - 12Z Sun Nov 18 2018

16Z Update: The main forecast challenge during the medium range
period is the evolve and speed of the southern stream closed low
over the southern plains, beginning Wednesday morning.  The 6Z GFS
and the 6Z GEFS mean are about 250 miles east of the
UKMET/ECMWF/CMC/EC mean solutions at that time, and these
differences become greater by Friday morning as this feature
reaches the East Coast, with the GFS ahead of the GEFS mean by
Thursday morning.  The 00Z GFS, while still faster than the model
consensus, is not quick as progressive and depicts a more
reasonable solution.  Given the greater model agreement on a
slower track, more weighting was given to this scenario in the WPC
forecast, with only a small percentage of the 00Z GFS being
incorporated and no 6Z GFS.  There has also been a trend for a
stronger East Coast surface low by the end of the week compared to
earlier model guidance, and the latest WPC forecast reflects that.


By the end of the forecast period next weekend, model spread
increases near the West Coast as Pacific shortwave impulses begin
to break down the upper level ridge over the western third of the
nation.  The ensemble means provided a good baseline for the
forecast, with the EC mean slightly faster than the GEFS mean
regarding trough axis placement.  The previous forecast discussion
follows for reference.

D. Hamrick


...Overview and Guidance/Uncertainty Assessment...

Over the past day a lot of the guidance has been exhibiting
significant changes/trends within an expected re-establishment of
a western ridge-eastern trough pattern after a brief flattening of
mean flow very early in the forecast period.  The two primary
trends of note involve greater separation and concentration of
residual shortwave energy reaching the southern Plains/northern
Mexico during the latter half of the short range period followed
by a sharper/more amplified depiction of the rebuilding West Coast
ridge and amplifying downstream trough.  The combination of
stronger southern tier energy and a sharper/slower trough
amplifying to its northwest is significantly increasing the
potential for a widespread precipitation event over the eastern
states and low pressure development along the East Coast.  Deeper
trends with the amplifying eastern trough lead to colder
temperatures over the East by next weekend.  On the other hand the
strengthening trend for the West Coast ridge is delaying the
arrival of previously advertised precipitation and prolonging the
duration of above average temperatures.  In addition once the
moisture arrives into the Pacific Northwest, amounts are trending
lower as some solutions hint at a possible split in supporting
trough energy. 

Looking at guidance comparisons through the 12Z/18Z cycles, there
was decent clustering from day 3 Wed into early day 4 Thu.  After
that time specifics diverged regarding the southern/eastern energy
and upstream amplifying trough along with corresponding surface
evolution.  The scenario that seems to be in the process of being
reinforced in the 00Z guidance had generally been a minority
solution seen in a small percentage of ensemble members along with
a run or two of the ECMWF a number of days ago.  The signal
started to reappear a bit yesterday and the past couple ECMWF runs
plus the 12Z CMC and 18Z GFS/GEFS mean added to it.  At the time
of forecast preparation it appeared best for a single
deterministic forecast to incorporate both operational model and
ensemble mean ideas which would provide some degree of trending
while awaiting agreement on the specifics.

Regarding northern U.S./southern Canada flow, GFS runs have
generally been at odds with the majority cluster of guidance over
the past couple days.  Consensus has held fairly steady with the
system forecast to track over/near the upper Great Lakes late in
the week aside from perhaps some modest timing adjustments. 
However latest GFS runs have been straying to the north of
established consensus while 24 hours ago they were accelerating
ahead of consensus.  At the same time earlier GFS runs had been
among the most eager to bring upstream Pacific energy into the
western North America ridge late in the week.  More recent GFS
runs are much closer to the guidance majority.

Regarding the eastern Pacific trough nearing the West Coast late
in the period, ensemble spread has improved out to about days 5-6
Fri-Sat.  During the weekend there is a pronounced divergence of
models/ensemble members regarding how much flow separation may
occur and where.  Preference leans toward the means late in the
period given the uncertainty in specifics but with the
understanding that the trough may ultimately be less phased.

Based on the combination of forecast considerations, the forecast
started with a blend of 12Z operational models (more GFS/ECMWF
than CMC/UKMET) early in the period and then increased 18Z
GEFS/12Z ECMWF mean input so that the means comprised 70 percent
of the blend by day 7 Sun.


...Weather Highlights/Threats...

Very deep storm system reaching eastern Canada by early Wed may
bring strong winds to parts of New England around midweek.  Also
expect some lingering lake effect snow on Wed, mainly over the
northeastern half of the Great Lakes region.  Then during the
latter half of the week guidance is showing greater potential for
an area of moisture to overspread portions of the eastern U.S. in
association with a mid level feature lifting out of the southern
Plains and surface low pressure near the East Coast.  Some wintry
precipitation may be possible in western/northern portions of the
moisture shield.  Confidence is currently below average regarding
specifics as guidance has been changing considerably over recent
runs.  There is somewhat better confidence in persistence of some
rainfall over/near Florida until low pressure to the north pushes
the trailing front eastward.  Late this week into the weekend low
pressure tracking near the Canadian border will bring an episode
of mostly light precipitation to the Great Lakes, followed by
another period of lake effect snow.  Shortwave energy reaching the
West Coast by early Wed will produce a brief period of rain and
higher elevation snow over the Pacific Northwest and northern
Rockies.  Some moisture may reach the Pacific Northwest coast this
weekend but it is likely that precipitation amounts will be on the
light side.

The midweek time frame will see the most extreme cold anomalies
over the east-central U.S. with some highs 20-25F below normal and
possibly cold enough for daily records.  Records for morning lows
appear to be somewhat less likely/widespread.  There should be
some moderation late in the week but the East may trend colder
again next weekend with expanding coverage of highs 10-15F below
normal.  Western warmth will cover a decent proportion of the area
from the West Coast states into the northern High Plains with some
pockets of plus 10-15F anomalies for min/max readings.

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