Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
909 PM EDT Wednesday Mar 12 2014
strong low pressure will continue to move to our northeast tonight
pulling a strong cold front across the area this evening. High
pressure will build to our south through Thursday night and Friday.
A warm front may lift across the area Friday as an area of low
pressure moves through southeastern Canada. A cold front associated
with this low will move across the area early Saturday, followed by
another early Sunday. High pressure will try to build out of south
central Canada and down across the northeast late in the weekend
into early next week while another coastal storm may develop off the
southeast coast and move near the middle Atlantic region Sunday night
Near term /until 6 am Thursday morning/...
we cancelled the rest of the Severe Thunderstorm Watch as the line has
weakened and moved offshore.
In its wake, strong and gusty winds from the northwest will
continue through late evening and overnight, with rapidly falling
temperatures. Some adjustments were made to temperatures/dewpoints over
the next few hours, as well as timing of the line of strong
showers through the area, but overall tonights forecast looks in
Otherwise, band of showers moving through the County Warning Area now in the 23z-
02z time frame with possible isolated low topped thunderstorm with the
cold front. Ml cape is almost non existent but shear is large and
its a very dynamic intensifying system so we cannot rule out a
gust 50 knots with the remnants of whatever sqline passes through our
County Warning Area early this evening. Please see Storm Prediction Center discussions and further
updates from our office this evening.
The Special Weather Statement continues with updates for a relatively extreme change with
wind driven cold arriving by midnight. 20 degree drops foreseen in
In essence... the 12z/12 NAM follows suit from prior multi model
consensus. That means we did not use the 12z/12 met MOS guidance
which at midnight for phl shows a surface temperature of 41f (5c) while the
12z/12 NAM bl temperature of 14c at 00z/13 (might be 2c too low? Unless
its raining at 00z) plunges to -7c at 06z (a drop of 21c or 38f in
6 hours). Our previous numbers and trend remain the same as
posted in the 330 am forecast.
Otherwise...the gusty south-southwest winds to 25-35 miles per hour phl area southward will
shift northwest this evening.
Tonight...cold front will sweep through the region between roughly
23z and 02z with a probable 1 hour delay of temperatures falling substantially
after the wind shifts from SW to northwest. Any remaining severe risk
will be primarily over Delmarva, far southeast PA, and S New Jersey and this
should end no later than 02z.
Winds: the strongest winds on land are expected in the Poconos,
where terrain effects will help mixing, and in the southern half
of the County Warning Area where the isallobaric wind component is expected to be
the strongest. The Wind Advisory continues as posted early this
morning. There is risk of High Wind Warning criteria for mds
Eastern Shore and Delaware where the combination 8 to 10mb 3 hrly
pressure rises and winds of 45-50 knots develop down to near 925 mb with
very steep lapse rates developing by 03z. An upgrade may occur
here near 02z if upstream evidence supports.
Snow: not much quantitative precipitation forecast expected after 00z and so snow accums will be
minimal in the flash freeze area of Monroe County and possibly
elevated Sussex County New Jersey. First it has to turn cold enough to snow
and that may not occur in Monroe and Sussex counties until 9 or 10 PM
when most of the quantitative precipitation forecast has ended. Stay tuned for updates on this snow
section as this is a fine line line between less than one half inch
snow and 2 inches in 2 hours. Otherwise flurries possible overnight
elsewhere in New Jersey and PA
Short term /6 am Thursday morning through 6 PM Thursday/...
Thursday...blustery and cold with northwest wind gusts 35 to near 45 miles per hour and
temperatures nearly 20 degrees colder than normal, wind chill values
will remain in the single digits, possibly below zero across the
northern zones, for most of the day.
Long term /Thursday night through Wednesday/...
fairly good agreement with the long term models for the end of
the week into the weekend, followed by some model spread and larger
differences starting Monday into the middle of the week.
High pressure will briefly build across the area Thursday night then
will push to the south of the area Friday while a warm front
lifts north of the area. We expect dry conditions Thursday night
into Friday as vertical moisture will be limited and no strong
short waves/vorticity maxes are expected to affect the area.
As we move into Friday night and Saturday, a cold front will be
approaching the area from the west as an area of low pressure swings
through southeastern Canada. Again, moisture is mostly lacking with
the frontal boundary, and the main short wave/vorticity maximum is expected
to pass north of the area. So for now we do not expect much
widespread precipitation, although there could be enough for some
light showers to affect the northern half of the area Friday night
into Saturday morning. Model guidance has any precipitation
dissipating as it moves into our area.
Another relatively dry frontal boundary is expected overnight
Saturday into Sunday morning, so for now, we will keep the
weather dry for this period. However, precipitating chances
increase once later Sunday into Monday as models are indicating a
coastal storm developing and moving near the middle-Atlantic coast.
There are some timing and placement differences with all of the
model guidance which would have an effect on precipitation chances.
The Canadian is the farthest northwest, with the European model (ecmwf) farthest
offshore, and the GFS in between with their respective
precipitation fields. As usual, this is all dependent on the track
of the low and how far northwest the moisture pushes. For now we
keep the highest probabilities across the southern half of the
area for Sunday night into Monday. Canadian high pressure will be
sliding to our north Sunday night into Monday, which will usher in
colder air across the area. This would make the p-type mostly snow
for much of the period.
Precipitation chances diminish as we move into Monday night and
Tuesday as the aforementioned storm moves to our northeast and high
pressure briefly builds across the area. Then models diverge with
what to do with the next potential system. For Tuesday night into
Wednesday, the GFS brings a low across southeastern Canada, while
the European model (ecmwf) brings a low right over our area. With the spread and
uncertainty, we followed closely to wpc which is somewhere in
between and introduces chance probability of precipitation across the area around Wednesday.
Aviation /01z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Remainder of today...VFR ceilings were degrading to MVFR conds in showers.
There is a risk for isolated thunderstorms and rain with a west gust 30-40 knots between
22z and 01z near the cold frontal passage. Otherwise the gradient
south-southwest wind gusts to near 25 knots kphl region southward. Light variable
mainly NE winds vicinity kabe and krdg.
Tonight...the cold front this evening will bring an abrupt wind shift
to northwest with gusts 34-44 knots. Conditions should improve to mainly VFR
ceilings at or above 3000 feet within a few hours of the wind shift to northwest.
Thursday...VFR. Gusty northwest winds 30 to 38 knots.
Thursday night-Friday...VFR. Gusty SW 20-25 knots winds Friday.
Friday night-Saturday...mostly VFR. Sight chance of rain or snow
showers north. Gusty SW winds 20-25 knots Saturday.
Saturday night-Sunday...VFR. Gusty northwest winds 15-20 knots Sunday.
Sunday night-Monday...MVFR/IFR conditions possible with snow
possible for southeast/coastal areas. Mostly VFR expected for
central and northwestern tafs outside of snow. Gusty NE winds
glw and stw posted for tonight. Funneling effect of Delaware Bay may
assist in gusty northwest winds reaching 50-55 knots for a time near 06z
Northwest gales of 35-40 knots through the day Thursday and subside Thursday
Thursday night...Gale Warning continuing early, then diminishing to
Small Craft Advisory conditions overnight.
Friday-Saturday...conditions may drop below advisory levels early,
then increase to advisory conditions later in the day Friday and
continue into Saturday.
Saturday night-Sunday...conditions may drop below advisory levels,
but still remain gusty around 20 knots.
Sunday night-Monday...return to Small Craft Advisory conditions
possible with possible coastal low.
after coordinating with partners, we will likely issue a fire
weather Special Weather Statement for Thursday overnight tonight/Thursday morning, and
possibly another Friday morning for a portion of the area. Areas
north of I-195 in New Jersey as well as eastern Pennsylvania are
expected to have moist enough fuels to preclude the need for any
Special Weather Statement statements. Areas south of I-195 in New Jersey as well as
Delaware and the Eastern Shore of Maryland could be dry enough to
issue an Special Weather Statement for enhanced fire weather concerns as it will be
quite windy Thursday and relative humidity values lower close to 30 percent.
Similar relative humidity conditions may occur on Friday, but winds are not
expected to be as strong.
operational quantitative precipitation forecast amounts are not expected to make any of our rivers
reach flood levels, there will be rises due to the combined rainfall
and melt off of the snow across the northern half of the area.
With any thunderstorms late in the day, there could also be some
heavy rainfall that could produce urban and poor drainage type
Something to keep an eye...per the Hydro ensembles, which are running
with more rainfall than operational solutions, 1.00 to 1.50 inches
of basin-wide rainfall would increase the flood threat across the
northern half of the hsa.
Luckily, we do have some wiggle room with 0.50 to 0.75 inches in the
PA...Wind Advisory until 11 am EDT Thursday for paz054-055-060>062-
New Jersey...Wind Advisory until 11 am EDT Thursday for njz016>018-021>025.
Delaware...Wind Advisory until 11 am EDT Thursday for dez001>004.
Maryland...Wind Advisory until 11 am EDT Thursday for mdz008-012-015-019-
Marine...Gale Warning until 1 am EDT Friday for anz450>452.
Storm Warning until 11 am EDT Thursday for anz431-453>455.
Gale Warning until 8 PM EDT Thursday for anz430.