Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
305 PM EST Thu Nov 15 2018

Valid 00Z Fri Nov 16 2018 - 00Z Sun Nov 18 2018

...Early season winter storm to bring ice and heavy snow from the
Ohio Valley to the Appalachians and interior Northeast through
Friday...

The big weather story across the continental U.S. through the end
of the work week will be the nor'easter affecting the northeastern
U.S.  Many locations from Virginia to New York have received
noteworthy early season snow, with some inland locations getting
in excess of six inches thus far.  This storm system will continue
lifting northward along the coast and bring widespread inland snow
from Pennsylvania to Maine, with the possibility of a foot or more
of snow accumulation for parts of upstate New York and northern
Pennsylvania.  Winter storm warnings are currently posted from
eastern West Virginia to western Maine, and winter weather
advisories extend westward to include parts of the Ohio Valley and
Great Lakes region.  Sleet and freezing rain are also likely
across parts of the central Appalachians to the Hudson Valley
region of New York.  Strong winds and coastal flooding will be
possible from Long Island to coastal Maine as the low pressure
intensifies overnight and into Friday.  Improving conditions will
return in time for the weekend.

Elsewhere across the U.S., a series of cold fronts dropping
southward from Canada will lead to snow across the northern tier
of the country as well.  By Thursday night, a front will approach
the northern Rockies and then the northern Plains, and light snow
will overspread those areas.  Snow is forecast to move southward
with the cold front across the central Rockies and central Plains
and then the middle Mississippi Valley by Friday evening.  The
northern and central Rockies could see 5 to 10 inches of snow, and
a swath of 2 to 4 inches of snow is expected from the northern
Plains to parts of the Upper Midwest Valley by Saturday morning,
followed by a fresh surge of very cold weather. 

D. Hamrick


Graphics available at
www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1254 AM EST Fri Nov 16 2018

Valid 12Z Mon Nov 19 2018 - 12Z Fri Nov 23 2018

...Overview...

Western ridging and eastern troughing will slowly translate
eastward next week in a mostly dry pattern for much of the lower
48. Much needed rain may finally work into California by later in
the week next week.


...Guidance/Uncertainty Assessment...

Forecast for Mon-Tue was formed from a multi-model deterministic
blend of the recent 12Z/18Z guidance. Weak clipper will dip
through the Great Lakes/Northeast as southern front sinks into the
Florida Straits. A weakening southern upper low will open up off
Southern California and push inland.

For Wed-Fri, ensembles still diverge on how quickly to lower
heights into the west. The ECMWF and its ensembles (with the 12Z
Canadian) remain quicker than the GEFS/Canadian ensembles while
the deterministic GFS runs were slowest. The FV3 and UKMET were
somewhat in between, near the 18Z GEFS mean. Trend over the past
several cycles has been slower in the ECMWF ensembles and just a
bit quicker in the GEFS members, which suggests leaning away from
the quicker members (12Z ECMWF included). It has been seen at
least a couple times in as many months that when the ECMWF
ensembles were quicker than the GEFS ensembles verification
favored the GEFS (though perhaps not completely). Opted to
continue with the previous shift preference with a slower
breakdown of the ridge along the line of the 18Z GEFS mean and
somewhat the 12Z FV3 GFS. Indeed the 00Z GFS was considerably
quicker than the previous runs, further exhibiting the
uncertainty. Either way, front should eventually make its way to
the coast, perhaps as soon as Wednesday but more likely early
Thursday. In the east, heights should slowly rise but there
remains differences across Canada with how much northern stream
troughing may linger in the Northeast. Leftover southern stream
reflection of the weakening upper low through the Southwest early
in the week may attempt to carry into the Gulf of Mexico later in
the week.


...Weather Highlights/Threats...

Rather quiet period early in the week dominated by NW flow out of
central Canada. Exception may be in the western Gulf along an
inverted trough to the north of a stationary boundary. Potential
for heaviest rain would be along the coast where instability is
highest. That will continue through the week with perhaps some
enhancement by the incoming southern stream shortwave. Entire west
coast will see in increase in the chance of rain and elevation
snow starting as soon as late Wednesday. Focus appears to be
around northern California but this may extend into SW Oregon
and/or central California given the ensemble spread. Amounts may
be in the 1-3 inch range Thu-Fri over favored areas and even about
a half inch in the Central Valley. 

Temperatures will be below average in the Great Lakes/Northeast
for much of the week as troughing will be slow to lift out.
Daytime highs in the 20s away from the coastal plain may be near
record cold values Tue/Wed with record lows overnight possible as
well especially Wednesday morning in New England with widespread
teens and single digits possible. Temperatures in the west will be
above average (by about 5-15F) along/inland from the central West
Coast through at least the first part of next week. Cooler
temperatures will accompany upper troughing and the frontal
approach, nudging the warmer than average temperatures into the
Plains by the end of the week.


Fracasso


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

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 PM EST Thu Nov 15 2018

Valid 00Z Fri Nov 16 2018 - 00Z Mon Nov 19 2018


...An Early-Season Winter Storm to bring ice and Heavy Snow from
the Midwest/Ohio Valley to the Appalachians and Interior New
Northeast through Friday...

...Northern Appalachians and Northeast...
Days 1-2...

A broad upper level low pressure moving out of the mid-Mississippi
Valley will be filling as it continues to translate
east/northeastward across the central Mid-Atlantic coast by later
tonight before it changes course and heads northeast along the New
England Coast on Friday. Widespread precipitation from the
central/northern Appalachians into New England will continue to be
fueled by a warm conveyor belt with extends well south into a
moisture-rich atmosphere.  At the surface, a cold high pressure
centered over New England will only slowly retreat to the east,
but maintain a cold air damming wedge down the east side of the
Appalachians into the overnight hours tonight. A wintry mix line
is expected near the I-95 corridor and inland with a swath of
mainly snow along the northern and western side of the
precipitation shield...with drier air eventually spreading
eastward and northward behind the system.

Uncertainty lingers as to the exact placement of the various
precipitation types even though the 12Z suite of model guidance
got into somewhat better agreement with respect to the extent and
strength of the warm nose.  The root of the uncertainty appears to
be rooted in the magnitude of vertical velocities within the deep
near-isothermal layer.

Farther north, eastern New England likely see a swath of freezing
rain along and just inland from the coast as a low pressure system
develops and becomes vertically stacked. A weaker warm nose in New
England should result in more sleet and snow at the onset and less
freezing rain. As the upper low moves moves from the Ohio valley
into the Mid-Atlantic, a surface low will enhance along the
Mid-Atlantic coast and lift northward to New Jersey this later
evening. The surface low is expected to hug the coast as it tracks
along the Northeast coast into Atlantic Canada on Friday. A TROWAL
is forecast to setup northwest of the low is expected to enhance
snowfall amounts from central PA to northern New England.

...North and Central Rockies and Great Plains...
Days 1-3...

A shortwave trough digging into the Northern Rockies, supported by
modest Pacific jet energy, will spread moisture out over the the
north-central CONUS. At the surface a cold front will dive
southward across the Northern Plains and bank against the
mountains producing a sharpening baroclinic gradient and upslope
low-level flow late tonight through Friday, then farther south to
the central Plains and CO Saturday. A wave of low pressure will
develop along this temperature gradient over the northern Plains
tonight and Friday while Pacific moisture leads to a steady
increase in column relative humidity. Lift associated with upper
diffluence and the developing surface low will produce widespread
elevation snows from the Northern Rockies near Glacier National
Park, southward toward the Laramie mountains of Wyoming. Also, a
band of snow can be expected to extend from MT/ND Friday to MN/IA
with a preference given to a roughly equal blend of the 15/12Z
ECMWF/GFS.

Bann

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1254 AM EST Fri Nov 16 2018

Valid 12Z Mon Nov 19 2018 - 12Z Fri Nov 23 2018

...Overview...

Western ridging and eastern troughing will slowly translate
eastward next week in a mostly dry pattern for much of the lower
48. Much needed rain may finally work into California by later in
the week next week.


...Guidance/Uncertainty Assessment...

Forecast for Mon-Tue was formed from a multi-model deterministic
blend of the recent 12Z/18Z guidance. Weak clipper will dip
through the Great Lakes/Northeast as southern front sinks into the
Florida Straits. A weakening southern upper low will open up off
Southern California and push inland.

For Wed-Fri, ensembles still diverge on how quickly to lower
heights into the west. The ECMWF and its ensembles (with the 12Z
Canadian) remain quicker than the GEFS/Canadian ensembles while
the deterministic GFS runs were slowest. The FV3 and UKMET were
somewhat in between, near the 18Z GEFS mean. Trend over the past
several cycles has been slower in the ECMWF ensembles and just a
bit quicker in the GEFS members, which suggests leaning away from
the quicker members (12Z ECMWF included). It has been seen at
least a couple times in as many months that when the ECMWF
ensembles were quicker than the GEFS ensembles verification
favored the GEFS (though perhaps not completely). Opted to
continue with the previous shift preference with a slower
breakdown of the ridge along the line of the 18Z GEFS mean and
somewhat the 12Z FV3 GFS. Indeed the 00Z GFS was considerably
quicker than the previous runs, further exhibiting the
uncertainty. Either way, front should eventually make its way to
the coast, perhaps as soon as Wednesday but more likely early
Thursday. In the east, heights should slowly rise but there
remains differences across Canada with how much northern stream
troughing may linger in the Northeast. Leftover southern stream
reflection of the weakening upper low through the Southwest early
in the week may attempt to carry into the Gulf of Mexico later in
the week.


...Weather Highlights/Threats...

Rather quiet period early in the week dominated by NW flow out of
central Canada. Exception may be in the western Gulf along an
inverted trough to the north of a stationary boundary. Potential
for heaviest rain would be along the coast where instability is
highest. That will continue through the week with perhaps some
enhancement by the incoming southern stream shortwave. Entire west
coast will see in increase in the chance of rain and elevation
snow starting as soon as late Wednesday. Focus appears to be
around northern California but this may extend into SW Oregon
and/or central California given the ensemble spread. Amounts may
be in the 1-3 inch range Thu-Fri over favored areas and even about
a half inch in the Central Valley. 

Temperatures will be below average in the Great Lakes/Northeast
for much of the week as troughing will be slow to lift out.
Daytime highs in the 20s away from the coastal plain may be near
record cold values Tue/Wed with record lows overnight possible as
well especially Wednesday morning in New England with widespread
teens and single digits possible. Temperatures in the west will be
above average (by about 5-15F) along/inland from the central West
Coast through at least the first part of next week. Cooler
temperatures will accompany upper troughing and the frontal
approach, nudging the warmer than average temperatures into the
Plains by the end of the week.


Fracasso


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