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Area forecast discussion 
National Weather Service Mount Holly New Jersey
349 PM EST sun Dec 21 2014

Synopsis...
weak high pressure across the region this evening will remain
through Monday. Weak low pressure will move northward up along the
East Coast Monday afternoon through Tuesday night. This low will be
followed by another stronger system for Wednesday and Wednesday
night. High pressure will build in for the end of the week. A cold
front will cross the area next weekend.

&&

Near term /until 6 am Monday morning/...
the center of a 1030 mb surface high over eastern Canada will
continue to ridge southward down the eastern Seaboard through
tonight. A middle- to upper-level ridge axis will progress toward the
East Coast this evening.

Some hi clouds will spill into the area overnight as the ridge
shifts downstream of our area and is quickly replaced by a leading
shortwave trough that is expected to move in from the southwest.

The south to north low-level thermal gradient associated with a warm
front is forecast to strengthen as it moves toward southeast Virginia late
tonight. Isentropic lift will develop across the boundary and expand
northward into the Delaware-Maryland-Virginia before daybreak. The airmass on cold
side of this baroclinic zone will be initially dry so expect the
thicker low clouds and precipitation to remain south of Delaware and the
Eastern Shore of Maryland tonight.

&&

Short term /6 am Monday morning through 6 PM Monday/...
southerly flow aloft around the backside of a western Atlantic ridge
and ahead of a trough over the northern plains will become
established over the eastern Continental U.S. On Monday. Warm air advection develops over the
forecast area in this pattern, especially in the low and middle levels.
At the surface, high pressure is forecast to continue nosing
southward down the eastern side of the Appalachians even with the
center of the aforementioned high retreating toward the Canadian
Maritimes. A cad setup will keep the colder air anchored over our
region with a northeasterly wind near the surface. This should
suppress the advancement of the surface warm front as it approaches
from the south on Monday.

The latest 12z NAM continues to be quicker and farther north with
the development of overrunning precipitation compared to the other
operational models. The airmass will be very dry initially, so
precipitation may not reach the ground as quickly as the NAM indicates.
However, there appears to be enough moisture return and isentropic
lift for precipitation to eventually overcome this initial dry air,
allowing the precipitation shield to expand northward through the day.
Chance probability of precipitation first advertised in the morning for the Delmarva, midday
for the southern philly suburbs and late afternoon for the I-80
corridor.

Ptype will predominately be rain on Monday, but there could be a
light wintry mix (snow/sleet) away from the coast at the onset due
to evaporational cooling. There is still not enough forecast
certainty to publish a snowfall map, but the setup appears limited
for any accumulations even in the higher elevations of PA. We
preferred to use the cooler NAM/European model (ecmwf) for thermal profiles in this
cad setup to diagnose potential ptypes and for forecasting temperatures.

&&

Long term /Monday night through Sunday/...
unsettled weather will be across the region at the beginning of the
long term. A 500 mb trough will be deepening across the middle part of
the country...and its effects will be for clouds and mostly rain
beginning Monday night and persisting into Wednesday night and
Thursday morning. The first batch of rain will come as weak low
pressure develops along the Carolina coast Monday and moves
northward along the New Jersey/Delaware coast Tuesday. We have chance probability of precipitation north/west and
likely probability of precipitation S/east with this feature. On Tuesday...the higher probability of precipitation move
to the northern areas as onshore flow and residual moisture from the
low remain across the area.

There will not be much of a break in the wet pattern Tuesday
night...although some weak ridging will be across the area. Another
stronger low pressure system will be developing across the southern
states and it will move northward Tuesday night through Wednesday
night. The low will track to the west...resulting in a surge of
warmer air across the area. We have gone along with the high probability of precipitation
offered by wpc...which are in the categorical range Tuesday night through
Wednesday night.

A strong cold front will cross the area overnight Wednesday night into Thursday.
It appears that most of the showers will be done by 12z...but we
have left in a chance for a few showers Thursday morning...in case the
system slows up at all. It appears that the p-type will still be
rain...since the colder air will not arrive fast enough for a
changeover.

The rest of the extended period will feature drier weather as a surface
high extends across the area. The 500 mb trough will still be to the
west...so temperatures will remain above normal both Friday and Sat.
Another low and developing front may begin to affect the area Sat
night into next Sunday...low chance probability of precipitation are in the grids for this.

&&

Aviation /21z Sunday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.

Leftover overcast stratocu continues to erode late this afternoon over most of
the terminals. At 20z, ttn and ridge lie on the edge of these clouds,
which are yielding MVFR ceilings between 2500 and 3000 feet. Expect the
clouds to clear out at these two terminals by sunset at the latest.

VFR this evening. VFR anticipated for most terminals overnight and
Monday morning though the 12z NAM develops stratus from phl north
and east, resulting in MVFR and even IFR ceilings between between
09-15z. This may be a bit overdone, but will still need to monitor
the potential as the marine layer tries to become established in
concert with a light northeast wind in the boundary layer.
Otherwise, middle clouds will quickly overspread the area from
southwest to northeast in the morning. Rain is also forecast to
develop in vicinity of ilg/miv during the middle to late morning,
phl/pne/rdg during the early to middle afternoon and Abe/ttn late in
the day.

Outlook...

Monday night through Tuesday...mostly VFR. Increasing clouds late Tuesday night.
Tuesday night through Thursday morning...occasional low ceilings/visibilities in rain and fog.
Thursday afternoon through Friday...mostly VFR. Gusty west/northwest winds.

&&

Marine...
light northwest winds 5-15 knots this afternoon will become northerly
tonight and northeasterly on Monday. Seas generally 1-2 feet during
this time.

Outlook...
Monday night...sub Small Craft Advisory conditions.
Tuesday/Tuesday night...Small Craft Advisory conditions on the ocean..sub-sca over del Bay.
Wednesday through Friday...Small Craft Advisory conditions with low end gale gusts Wednesday night and Thursday.

&&

Phi watches/warnings/advisories...
PA...none.
New Jersey...none.
Delaware...none.
Maryland...none.
Marine...none.

&&

$$
Synopsis...O'Hara
near term...Klein
short term...Klein
long term...O'Hara
aviation...Klein/O'Hara
marine...Klein/O'Hara

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