Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
353 PM EST Monday Dec 22 2014
a frontal boundary will stall near the southeast coast through
Tuesday. This stalled boundary will eventually lift northward into
the area as a warm front by Wednesday. Meanwhile, low pressure
developing near the northern Gulf Coast late Tuesday is expected to
strengthen as it tracks northward into the Midwest Wednesday and
Great Lakes Wednesday night. A cold front will move through early
Thursday. High pressure builds over the eastern United States for
Thursday and Friday. Another cold front is forecast to arrive late
Saturday into Sunday.
Near term /until 6 am Tuesday morning/...
an amplifying upper-level trough is across the central portion of
the nation. The water vapor imagery shows quite a bit of short wave
energy moving through the trough. One of the short waves is helping
to develop a surface low off the Carolina coast this afternoon, and
this feature will add some lift to produce some rain across our area
through tonight. This process overall though has been slow thus far.
An upper air analysis revealed decent warm air advection at 850 mb focused near the
Delaware-Maryland-Virginia area. This warm air advection will continue northward through the night
along with a 250 mb jet riding across the area. The combination of
these features will generate enough lift to produce periods of rain.
While a surface high is wedged down into the area, the airmass is
modifying and the cool air damming is expected to give way and the
forecast soundings indicate a sharpening inversion through the
night. The dew points at the surface are rising however some areas
across the Poconos and into Sussex County, New Jersey are still below
freezing. This may allow for some freezing rain or sleet to occur
for a time tonight, although given the east or even southeast wind
this afternoon enough warming may prevent much in the way of
freezing precipitation. Since there is still some potential
especially in the higher terrain and the advisory is already out, we
will keep the Freezing Rain Advisory for now. Any icing will be
light. Elsewhere in eastern Pennsylvania, there could be a little
sleet due to a lingering dry layer. Otherwise, some rain will
overspread the area. The pop increase from south to north was slowed
down some more into this evening.
As the airmass moistens and the inversion strengthens, some drizzle
may occur along with some fog. Some fog has already been noted this
afternoon in the Poconos. Given the warm air advection forecast, despite the light
flow, the temperatures were shown to mainly be steady or even slowly
rise during the night. Low temperatures were mostly a blend of the
NAM/GFS MOS. The lamp/lav guidance was used to assist with the
hourly temperatures and dew points.
Short term /6 am Tuesday morning through 6 PM Tuesday/...
the synoptic setup is comprised of an upper-level trough to our
west. This will drive low pressure toward the Great Lakes with
continued low-level warm air advection across our area. A weak surface low sliding
northward off of the middle Atlantic coast should tend to weaken
further during the course of the day as some ridging builds. The
combination of this weakening feature, a low-level inversion and
plenty of moisture beneath it spells unsettled weather.
The forecast soundings overall show as the large scale lift wanes,
some drying occurs aloft. This would tend to lead to some drizzle
and fog across much of the area, assuming the low clouds do indeed
lock in. The more focused rain may be across our northern zones
before lifting north of our cwa, then leftover drizzle around. We
trended the probability of precipitation to reflect this idea as much of the quantitative precipitation forecast should be
on the light side.
As for the high temperatures, we used an even GFS/NAM MOS blend but
then adjusted these down some due to the expected low clouds and
Long term /Tuesday night through Monday/...
the large scale pattern during midweek will feature a high amplitude
trough over the central Continental U.S. And downstream ridge over the western
Atlantic. Cyclogenesis is forecast to occur along the northern Gulf
Coast late Tuesday. The surface low is then forecast to track
northward through the deep south Tuesday night and Midwest on
Wednesday. This track means the warm sector will become established
up the eastern Seaboard owing to a deep southerly fetch on the east
side of the storm system. Forecast soundings are only supportive on
one ptype with this event: rain. There is an opportunity for a
brief lull in the rain Tuesday evening when the initial band of
isentropic lift passes to our north. A large precipitation shield over the
southeast states is expected to expand northward into the northern
middle Atlantic late Tuesday night and Wednesday morning. Rain should
become widespread and moderate at times on Christmas evening both during
the day and at night. Added a slight chance of thunderstorms near
the Delaware and southern New Jersey coast as well. Not expecting a lot of Hydro
issues with quantitative precipitation forecast between 0.75 and 1.50 inches for this event.
Models have trended slightly slower with the cold front, indicating
a late Wednesday night/early Thursday morning frontal passage with today's 12z
guidance compared to a Wednesday evening passage from yesterday's
runs. We favored a more progressive solution where the precipitation ends
by Christmas morning since the upper trough is expected to
Delaware-amplify, allowing for more of an eastward push with the frontal
Westerly winds behind the front will advect colder and drier air
into the region on Thursday. Skies may clear in the morning before
Post-frontal stratocu is expected to develop by the afternoon. It
will be breezy during the day as BUFKIT momentum Transfer from the
12z GFS shows the potential for 30 to 40 miles per hour gusts. Maximum temperatures in the
upper 40s and lower 50s may occur in the early morning just prior to
frontal passage before dropping thruout the day. A few flurries may reach the
Poconos during the afternoon.
Broad area of high pressure builds along the eastern Seaboard
Thursday night and Friday. Quiet conditions and slightly above
normal temperatures are in store to end the week.
There is still a large forecast spread with the latest 12z models
for this weekend. The European model (ecmwf) remains the most bullish with low
pressure developing over the Gulf Coast states this weekend as a
result of a strong shortwave trough tracking across the southern
states. This model then simulates the low strengthening as it moves
northeastward toward the Middle-Atlantic States and starts to phase
with northern stream energy Sunday and Sunday night. Meanwhile, the
GFS shows a much more progressive pattern with the above mentioned
shortwave trough pushing a weak cold front through the middle Atlantic
earlier in the weekend. Sided closely with wpc for the weekend
forecast, which was weighted more toward the naefs/European model (ecmwf) ensemble
mean. Low chance probability of precipitation for rain near the coast and rain/snow inland
were included for late in the weekend.
Aviation /21z Monday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Rest of this afternoon...VFR/MVFR ceilings, with the lowest ceilings
north of the kphl area. As moisture increases, the ceilings are
anticipated to lower into this evening. Some light rain south of
kphl will slowly lift north. Some local visibility restrictions due
to some fog especially around the kttn area. Northeast or
east-northeast winds mainly 5-8 knots.
Tonight...MVFR ceilings are anticipated to lower to IFR/LIFR with
light rain and drizzle. Some fog will occur as the ceilings lower,
which will bring the visibilities down. Northeast winds generally
around 5 knots.
Tuesday...the ceilings should remain around IFR as the low-level
moisture becomes trapped. This will also combine with some light
rain or drizzle. Northeast winds 5 knots or less, possibly turning
locally southeast in the afternoon.
Tuesday night...may be a lull in precipitation Tuesday evening before
more rain moves back into the area overnight. Still expecting MVFR
and IFR conditions.
Wednesday and Wednesday night...mainly IFR conditions in low clouds
and rain. The rain will be steady and moderate at times. Southerly
winds are forecast to increase to near 50 knots less than 1000 feet above ground level
Wednesday afternoon and evening. May see low level wind shear during this time
especially if wedge of colder air near the surface is more stubborn
to erode than models indicate, making the thermal inversion stronger.
Thursday...MVFR lingering early, improving to VFR during the day.
Breezy west winds gusting 30-25 knots.
Thursday night through Saturday...VFR and light winds.
a weak surface low off the Carolina coast will track northward
tonight and then weaken off the New Jersey coast during Tuesday. The
pressure gradient will tighten some tonight, and therefore the winds
will also increase some. The forecast soundings indicate that as
warming occurs aloft, the thermal profile becomes less conducive for
mixing. While the onshore flow will allow the seas to build, it
appears marginal enough at this time and we will hold off on a Small Craft
Advisory. The winds should lessen Tuesday as the surface low weakens.
Tuesday night...south winds below Small Craft Advisory criteria. Waves forecast to
build to 4 feet off the Delaware coast.
Wednesday and Wednesday night...widespread Small Craft Advisory conditions expected
with seas building to 5-8 feet and southerly winds sustained 20-25 knots.
Stable lapse rates will limit mixing potential so not expecting much
higher gusts, except if convection is able to develop over the ocean.
Thursday...west wind gusting in excess of 25 knots expected. Gales
are even possible during for a period. 5 to 8 feet seas forecast for
Thursday night through Saturday...no marine headlines expected at
PA...Freezing Rain Advisory until 4 am EST Tuesday for paz054-055.
New Jersey...Freezing Rain Advisory until 4 am EST Tuesday for njz001.