Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
925 PM EST Friday Jan 30 2015
low pressure will intensify into the Canada Maritimes through
Saturday. Meanwhile, high pressure builds toward our area tonight
before settling overhead later Saturday into Sunday. A low
pressure system will then track across the middle Atlantic Sunday
night and Monday. High pressure builds back in again for Tuesday
before quickly shifting offshore Tuesday night into Wednesday,
then a cold front pushes through Wednesday into Thursday as low
pressure may develop along it. High pressure then builds from the
plains toward the Ohio Valley late Thursday and Friday.
Near term /until 6 am Saturday morning/...
strong low pressure off the New England coast and high pressure
centered over the Ohio River valley will maintain a tight pressure
gradient in our region tonight. As a result, it will remain very
windy and a Wind Advisory remains in place.
The strong northwest to north flow will bring very cold air into
our region. Temperatures are forecast to fall into the single
digits and teens by morning. Also, a Wind Chill Advisory is in
effect for the Poconos and far northern New Jersey.
The area of scattered snow showers that affected parts of
southeastern Pennsylvania and southern New Jersey this evening had
all but dissipated and moved off the coast by 900 PM. Other than
scattered flurries in the Poconos, no additional snow is
anticipated for our region tonight.
Short term /6 am Saturday morning through 6 PM Saturday/...
as high pressure continues to build into the area from the Ohio
Valley...the pressure gradient over our region will gradually
relax during the afternoon. While strong gusty winds will continue
through the morning hours...there will be improvement by late
afternoon. With morning temperatures starting out very cold...even
for this time of year, wind chill factors will make outside
conditions feel bitter cold. Actual temperatures will peak out
only in the low teens up north...and in the middle 20s elsewhere.
With dew point temperatures in the single digits...relative
humidities will be very dry.
Long term /Saturday night through Friday/...
the overall synoptic setup is comprised of a negative tilt trough
lifting through the Canadian Maritimes Saturday night and Sunday,
as energy across the southwest generally hangs back before sliding
into Mexico. The associated upper-level trough sharpening up
Sunday and Monday will drive low pressure across the middle Atlantic
on Monday. There remains though some uncertainty on the exact
track of the surface low. The upper-level trough relaxes some
Tuesday into Wednesday before short wave energy in the northern
stream amplifies another trough from the Midwest and Great Lakes
region Wednesday into Thursday. This should drive a cold front
eastward with the potential for cyclogenesis to occur along the
front as it reaches or moves off the coast. We used a model blend
approach through Monday, then generally utilized the 12z wpc
guidance thereafter. Some adjustments were then made based on
additional collaboration with our neighboring offices and also
For Saturday night and Sunday...a negative tilt trough is
forecast to be lifting northeastward into the Canadian Maritimes
during this time frame. This will take a deepening surface low
with it. As the troughing eases in the east Sunday, high pressure
builds over our area before shifting off the southeastern U.S.
Coast by late in the day. As this occurs, energy diving across the
Midwest to middle Mississippi Valley will amplify an upper-level
trough. A developing and probably strengthening confluence zone
downstream should help high pressure build across southeastern
Canada. All of this will assist in sharpening a baroclinic zone
draped from west to east, where developing low pressure will track
along it. Before we get to that though, Saturday night will be
rather cold although the low-level flow should be shifting to
light southerly. As low pressure organizes from the middle
Mississippi Valley to the Ohio Valley Sunday, clouds are expected
to increase across our area. The ramping up of warm air advection aloft and the
start of increasing isentropic lift may result in some light snow
developing from west to east by late afternoon Sunday. The bulk of
the precipitation though is anticipated to remain to our west
during the day Sunday.
For Sunday night and Monday...the amplification of the
aforementioned upper-level trough will continue into Monday. This
drives surface low pressure across the middle Atlantic Monday
morning. The digging short wave in the Ohio Valley may be such to
allow the surface low to track a bit farther northward, and this
track will be key in determining the location of the heavier axis
of precipitation along with the precipitation types. Despite a
fair amount of spread in the guidance regarding the track of the
surface low, there is a general consensus of a storm affecting our
area. An accumulating snowfall, potentially significant, is
anticipated for at least parts of our County Warning Area. Where this occurs along
with a mixed precipitation/rain zones is less certain attm,
although the trend is pointing to our northern zones having the
highest snowfall amounts.
The lift associated with this feature should rapidly increase
Sunday night into Monday from west to east north of the surface
low track where a tightening thermal gradient develops. The
associated frontogenetic forcing and warm air advection should result in some
west-east bands of some heavier precipitation. The farther north
one GOES, the better chance of all snow. The picture becomes less
certain southward across the County Warning Area as the injection of warmer air
for a time should result in a zone of mixed precipitation with
even an area of rain. Where this occurs carries lower confidence
at this time as the difference in some of the model guidance keeps the
door open for a some northward adjustment. The NAM/sref is more
amplified, however there is more support from the well clustered
GFS and European model (ecmwf) with even stronger agreement with their ensemble
means. Despite the lower confidence, based on the GFS/European model (ecmwf) we
added more mixed precipitation from around the Philadelphia metropolitan
southward with even some rain across the Delaware-Maryland-Virginia. We did not go
as far north though with the mixed precipitation like the NAM
would suggest. Any rain and mixed precipitation should change to
snow before ending through Monday afternoon as colder air wraps
into the departing system. Since we are not real confident on
where the mixed precipitation zone may occur, we elected to hold
off on a Winter Storm Watch at this time. Our storm total snowfall map
/through 7 PM Monday/ leaned toward the wpc/westward guidance after
earlier collaboration, and it should be noted that this map is
highly subject to change.
For Monday night...the surface low pulls away into the Canadian
Maritimes during Monday night with high pressure building in from
the west. The atmosphere dries out pretty quickly and any light
precipitation should end just as fast. Clearing skies in the wake
of the low along with a fresh snow pack in many areas will
contribute to very cold overnight temperatures. Temperatures will
plummet and we will see temperatures drop into the single digits, both
plus and minus, across the region. With winds remaining up through
the overnight period, wind chills will become a concern,
especially across the southern Poconos and northwestern New
Jersey. Wind chill advisories may be needed for Monday night into
Tuesday morning across our northern counties.
For Tuesday...high pressure will build across the region Tuesday
with a shift to our east Tuesday night. Plenty of cold air will
accompany this high pressure system with 850mb temperatures in the
-10c to -15c range. Maximum temperatures will struggle to rise and
generally remain in the 20s, with teens across the Lehigh Valley,
southern Poconos and northwestern New Jersey.
The high will slide across the region and move offshore Tuesday
evening. The air mass will modify a bit as it slides eastward and
minimum temperatures will not be as cold Tuesday night as they are
on Monday night. It will still be cold with lows ranging from the
single digits across the north to near 20 across the southern part
of the forecast area.
For Wednesday and Thursday...the exits to the east of the area
with a surface low pressure system located to the north of the
area and another one developing to our south. The northern low
gets pushed more to the northeast and the attached cold front
looks to approach the region on Wednesday. The southern low moves
toward the Carolina coast and then tracks to the northeast. There
is differing opinions amongst the guidance as to which low will
win the battle and in turn will affect the region around Thursday.
Will keep the mention of precipitation in the forecast at this
time as we monitor the trends of these systems over the coming
days. Maximum temperatures will generally be in the upper 20s to
upper 30s across the region.
For Friday...high pressure looks to build into the region from
the west. Highs on Friday look to be cold once again as the high
pushes to the east. Temperatures into the 20s can be expected
across the forecast area.
Aviation /03z Saturday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
VFR conditions with a mainly clear sky are anticipated through
the taf period.
A gusty northwest will will persist for tonight into Saturday.
Sustained speeds of 15 to 25 knots are expected with gusts into
the 30s and perhaps around 40 knots.
Saturday night...VFR. Northwest to west winds diminishing to
around 5 knots.
Sunday and Monday...VFR to start Sunday with increasing clouds,
then ceilings may lower to MVFR by Sunday evening. The conditions
should then quickly lower to IFR/LIFR Sunday night and Monday as a
storm moves across the middle Atlantic. All snow looks to occur for
kabe, krdg and kttn /lowest visibilities/, then some mixed
precipitation may occur for a time Monday morning at kphl, kpne
and kilg, with even a change to rain for a time at kmiv and kacy.
Any mixed precipitation or rain should change to snow Monday
afternoon as colder air arrives. The transition of mixed
precipitation and the timing carries lower confidence at this
time. There is the potential though for a significant accumulating
snowfall for at least some of our terminals, and as of now the
higher chance of this is north of kphl.
Monday night...improving conditions from MVFR to VFR across the
terminals. Diminishing winds becoming light out of the northwest
through the overnight period.
Tuesday through Wednesday...VFR conditions expected. Light
westerly winds becoming southwest on Wednesday.
no changes to the marine headlines. Gale Warning continue. Expect
gusts in the 35-40 knots through tonight...and into Saturday. Seas in
the 5 to 9 foot range can be expected. Conditions will start to
gradually improve later Saturday afternoon.
Saturday night...the winds and seas should quickly decrease as
high pressure builds in.
Sunday and Monday...an east to northeast wind should increase
late Sunday into Monday before turning north as low pressure
slides across the middle Atlantic. This should result in at least
Small Craft Advisory criteria being met, however at this time the
quick movement of the storm should limit winds to less than gale
Monday night through Tuesday... Small Craft Advisory conditions
expected through Tuesday. Winds will decrease and fall below 25
knots Tuesday morning. Seas will remain elevated and fall below 5
feet around Tuesday afternoon/evening. .
Wednesday...both winds and seas should remain below Small Craft
PA...Wind Advisory until 6 am EST Saturday for paz054-055-060>062-
Wind Chill Advisory until 11 am EST Saturday for paz054-055.
New Jersey...Wind Advisory until 6 am EST Saturday for njz001-007>010-
Wind Chill Advisory until 11 am EST Saturday for njz001.
Delaware...Wind Advisory until 6 am EST Saturday for dez001>004.
Maryland...Wind Advisory until 6 am EST Saturday for mdz008-012-015-019-
Marine...freezing spray advisory until 6 PM EST Saturday for anz430-431-
Gale Warning until 6 PM EST Saturday for anz430-431-450>455.