Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
1104 am EST Friday Dec 13 2013
high pressure will gradually weaken and shift off the coast of
the southeastern states through today. A cold front is expected to
cross our region today into tonight from the north, then strong
high pressure builds across southeastern Canada into the northeast
tonight and Saturday. Low pressure developing over the lower
Mississippi Valley Friday night into Saturday will track
northeastward to off the middle Atlantic coast early Sunday. This
system will strengthen as it quickly moves toward the Canadian
Maritimes during Sunday. A quick moving clipper system will then
track to our north Tuesday, pulling a cold front across our area.
High pressure will then build to our south Wednesday and Thursday.
Near term /until 6 PM this evening/...
for the morning update...cloud cover has shown a decreasing trend
this morning and temperatures are running a bit higher than forecast.
Therefore the sky cover grids were adjusted downwards some...
especially south of phl...and temperatures were nudged up a bit. The afternoon
maximum temperature was increased a couple of degrees so forecast maximums are now
middle 20s in the Poconos to near 40 over southern Delaware. West winds
have increased and become somewhat gusty with daytime mixing and
downward momentum Transfer...so wind gust grids were adjusted
Otherwise...high pressure centered over the southeastern states this
morning will continue to influence our region today. The center of
the high is expected to pass off the North Carolina coast this
afternoon. Meanwhile, a cold front extended from low pressure over
the Saint Lawrence River valley to the northern part of the Great
Lakes region. The low is expected to move eastward and the front
should drop through New York state and New England today.
Short term /6 PM this evening through 6 am Saturday/...
the cold front is expected to progress southward through most of
our forecast area between about 900 PM and 100 am. Meanwhile, cold
high pressure is anticipated to build across southern Quebec,
northern New York and northern New England.
The frontal passage is expected to be barely noticeable in much
of our region. A light north to northeast wind may develop on the
coastal plain and on the adjacent waters with the wind remaining
light and variable well inland.
Clouds will be on the increase as the weekend system begins to
approach. High clouds should become broken to overcast this
evening with middle level clouds arriving mainly late tonight.
The latest guidance indicates that some light snow may begin to
spread across our northern counties from the west toward daybreak
Long term /Saturday through Thursday/...
a quick hitting winter storm to affect our area Saturday into
early Sunday, potentially with significant impacts for parts of
the County Warning Area.
The overall synoptic setup is comprised of an upper-level trough
that starts to lift out allowing the flow to turn more zonal
Saturday for a time. Southern stream energy is forecast to eject
eastward across the southern states with a surface low developing
into Saturday. The upper-level feature should become involved with
northern branch energy amplifying across the Great Lakes,
resulting in a deepening surface low off of our coast to start
Sunday. As cold surface high pressure shifts across New England
and then the Canadian Maritimes, cold air damming becomes
involved. This sets the stage for an overrunning event that may
morph into perhaps some wrap around as the coastal storm deepens
offshore. Our confidence remains very high that the storm will
occur, however there is still some lower confidence regarding the
precipitation types as we are anticipating a transition zone to
become established. This appears to be right along the Interstate
95 corridor. The model guidance is pointing toward mostly snow
across the western and northwest zones overall as the cold air
Given enough model support and other guidance along with
increasing confidence and extensive collaboration with our
neighboring offices, a Winter Storm Watch has been issued for much
of the western and northern areas. It appears that a trough then
settles into the east for early next week before weakening
Wednesday into Thursday. We used a model blend approach Saturday
through Monday, then blended in some wpc guidance with continuity
thereafter. Most of the focus was placed on Saturday and Sunday.
For Saturday and Sunday...a quick hitting but potentially
significant storm will affect our area Saturday into early Sunday.
The model guidance is in good agreement overall with the main
features of the system, however there are some differences still
with the thermal fields and to some degree the quantitative precipitation forecast. Overall though
there will be a transition zone of precipitation types. The upper-
level setup has northern and southern stream energy shifting
eastward, however the surface low looks to be mostly tied to the
southern energy initially. There will be a battle between warmer
air trying to advect northward with cold air sitting in the lower
levels. The cold air damming signature continues and this looks to
lead to a Transfer of energy from a weak low in the Ohio Valley to
what becomes the main surface low off of the New Jersey coast
Saturday night and Sunday morning. The challenges with this event
include the location and duration of mixed precipitation, which
can certainly play havoc with the snowfall forecast, and the
potential for any banding that may setup.
A comparison of forecast soundings for several points across the
County Warning Area shows overall the cold air wanting to win out. This is due to
the low-level flow being driven by the ageostrophic component
which is from the northeast, and plus this should be enhanced some
as the coastal low gets going. This allows the colder air to
remain longer especially down in the lower levels. The forecast
soundings and model mass fields strongly suggest that an
increasing isentropic overrunning/upglide event unfolds ahead of
the surface low deepening off of our coast. The associated
increase in flow, warm air advection and moisture influx should lead to a warm air advection-
type Wing arriving somewhere in the County Warning Area from west to east. This
should tend to fan out with time as large scale lift begins to
arrive but the isentropic upglide seems to be the driving
forecast, as seen in the 295-305 isentropic levels. The actual warm air advection
and associated tightening thermal gradient should then shift the
overrunning precipitation into more of dynamic/large scale lift
scenario. Prior to this though, the precipitation may have some
lulls in it.
The model guidance still hinting at a band of frontogenetic
forcing becoming established generally between about 700 mb and
800 mb. This may end up being north and west of Interstate 95, but
this lift which could result in a mesoscale type banded feature is
difficult to pinpoint this far out. It will ultimately depend on
the surface low track and associated thermal gradient that sets
up. Some of the forecast soundings also indicate that some sleet
and freezing rain may occur somewhere from about the Interstate 95
corridor to perhaps nearing the Lehigh Valley. There will probably
be some snow still on the ground from the Interstate 95 corridor
on north and west leading into this event and the ground is now
colder, therefore this may help to keep the low-level cold air in
longer than what some of guidance suggests.
There is also some suggestion via the forecast soundings that
there could be bursts of snow. This may mostly be in the form of
an overrunning burst, then the precipitation lessens as we go
through later Saturday night with perhaps some drizzle before
ending. Based on the above, we held onto the snow longer near
Interstate 95 and then introduced a bit longer mix. From about the
New Jersey Turnpike to Interstate 95, this corridor carries more
questions as several events have shown the cold air remains stuck
longer than modeled. Farther northwest, while some mixing is
anticipated we trended toward more snow. The Winter Storm Watch
was placed where high enough confidence resides in warning snow
criteria being reached. The tier of zones south and east of the
watch is a bit trickier as there should be some mixing involved.
Our current snowfall forecast is close to warning criteria,
however confidence was not high enough and we are leaning toward
an advisory event here and a bit farther south and east. These
areas will continue to be highlighted in the hazardous weather
outlook. As of now, ice accumulations are a tenth of an inch or
less in the corridor where mixing is forecast. If the milder
solutions end up being correct, snowfall amounts decrease but then
the icing threat increases. As of now, we leaned toward more snow
and less icing thinking the thermal profiles remain cold enough.
For Monday and Tuesday...a narrow and weak area of high pressure
builds in Monday then shifts to our east by Tuesday, as energy
dives southeastward from the Midwest. This is within an upper-
level trough and looks to drive a clipper type system from the
Midwest to southeastern Canada Monday. An associated cold front is
forecast to slide across our area during Tuesday, however there is
not much moisture forecast to accompany it. However given the warm air advection
and isentropic lift, there could be a small shield of
precipitation that zips across our central to northern zones.
Overall, probability of precipitation are on the low side and favor more north than south.
A cold day Monday, then moderation should take place Tuesday ahead
of the aforementioned cold front. No major changes were made to
this time frame as more focus was placed on Saturday and Sunday.
For Wednesday and Thursday...as an upper-level trough may take on
a negative tilt Wednesday, surface low pressure is forecast to
deepen near the Maine coast then track into the Canadian
Maritimes. This allows a shot of colder air to arrive on the heels
of increased cold air advection. This combined with a northwest flow could allow
for some lake effect snow showers to near the Poconos Wednesday.
The flow looks progressive therefore the trough lifts out some and
surface high pressure slides to our south. This allows the surface
flow to back some to the west and even southwest with some air mass
modification underway. An upper-level trough digging across the
west is forecast to allow for a surface low to organize Thursday
in the Central Plains. This should allow for increasing warm air advection into
the Ohio Valley along with the development of a warm front.
Overall, looks like our area is protected still by departing high
pressure therefore a dry forecast. No major changes were made to
this time frame as more focus was placed on Saturday and Sunday.
Aviation /16z Friday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
VFR conditions are expected through the taf period. Any ceilings
should remain above 7000 feet.
Wind speeds should increase to 8 to 12 knots today with gusts
well into the teens from middle morning until early afternoon. The
gusts are forecast to subside for the middle to late afternoon with
the wind expected to become light and variable for this evening.
Saturday and Sunday...MVFR to IFR conditions developing during
Saturday as snow arrives. The snow will mix with sleet and then
change to rain from the south and east. Kabe and krdg should have
mostly snow, however some sleet or freezing rain cannot be ruled
out. Kacy and kmiv should have the shortest duration of snow. The
precipitation looks to taper off early Sunday morning. A return to
VFR may occur as early as midday Sunday. Northeast to east winds
around 10 knots Saturday, shifting to the northwest 10-20 knots
Sunday as the storm moves away. Confidence is high regarding
precipitation occurring, however it is still lower with the timing
and especially with the precipitation types.
Monday and Tuesday...mainly VFR. A low chance of some light rain
or snow Tuesday, especially from about kphl on northward.
Northwest winds 5-10 knots Monday becoming light and variable,
then turning to the south-southeast Tuesday 5-10 knots.
the wind speeds a few thousand feet off the surface will be in
excess of 35 knots at times this morning. While the strongest of
the winds should not mix to the surface near land due to the
temperature inversion, an occasional gust around gale force is
possible near the outer edge of our marine area.
Based on the latest guidance, we have extended the Small Craft
Advisory until 400 PM for our waters off the New Jersey coast. The
pressure gradient between the high to our south and the low and
cold front to our north should keep wind gusts near or above 25
knots through that time. The gradient is anticipated to relax a
bit sooner on our southern waters than in the north. As a result,
we will keep the expiration time on the Small Craft Advisory for
our waters to the south of Cape May at 100 PM.
It appears as though the temperature inversion will limit 25 knot
plus gusts on upper Delaware Bay, so we have discontinued the
Small Craft Advisory there. The same is true for much of lower
Delaware Bay. However, we remain concerned that the influence of
the temperature inversion will be less pronounced near the mouth
of the Bay and enough mixing could take place to result in
frequent gusts near 25 knots there until early afternoon. We have
extended the Small Craft Advisory for lower Delaware Bay until 100
Wind speeds and wave heights will likely be below the Small Craft
Advisory criteria on our ocean waters and on Delaware Bay for
Saturday and Sunday...a low pressure system is expected to near
our waters during Saturday and strengthen as it moves away from
the New Jersey coast Sunday. Wind gusts and seas are expected to
reach Small Craft Advisory criteria Saturday night through Sunday.
Monday and Tuesday...weak high pressure will build across our
area Monday, then shift to the east into Tuesday. A warm front
should lift north Tuesday with a quick moving cold front moving
through later Tuesday. The winds and seas should be lowering to
below advisory criteria Monday.
PA...Winter Storm Watch from Saturday morning through late Saturday
night for paz054-055-060>062-101-103-105.
New Jersey...Winter Storm Watch from Saturday morning through late Saturday
night for njz001-007>010.
Marine...Small Craft Advisory until 4 PM EST this afternoon for
Small Craft Advisory until 1 PM EST this afternoon for anz431-