Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
1015 am EDT Tuesday Mar 11 2014
a low pressure system will begin to slide east out of the High
Plains today. This same low is expected to intensify over the next
two days as it approaches the region Wednesday afternoon, bringing
an Arctic cold front sweeping through the region Wednesday evening.
In the wake of the cold front, the region will be under the Arctic
air mass through the end of the work week. Low pressure in eastern
Canada this weekend will send another cold front into the Middle
Atlantic States by Sunday. Low pressure then develops along the
southeast coast of the United States next Monday.
Near term /until 6 PM this evening/...
a weak short wave/vorticity maximum is swinging into the southern portion
of the area this morning into the early afternoon, while the cold
front is sagging into the area from the north. This is leading to
the enhancement of cloud cover across the area, as well as some
reflectivity on the radar across the Delaware-Maryland-Virginia. We think most of
this is virga, so it would be hard for it to reach the ground
since it's falling out of the middle-levels, and even if it does, it
should only be sprinkles. So for now we have kept any mention of
showers/sprinkles out of the forecast for now.
Stat guidance has been under doing the forecast highs and the
numerical guidance has been too cold upstream at 925 and 850 mb.
Once again went above stat guidance today by a couple of degrees
in most spots so we should climb into the low-60s around the
philly metropolitan area and further south and east once the cloud cover
breaks apart this afternoon.
With the warmer temperatures and more low-level moisture advecting
into the region from the southwest, we should reach convective
temperatures allowing a stratocu deck to form. Across the coastal
plain it looks as though a sea breeze front will be able to move
inland with the lack of stronger low-level winds combating it.
Short term /6 PM this evening through 6 am Wednesday/...
we will be under the influence of deep layer warm air advection
tonight so we expect to see thickening clouds lowering with time
late tonight. Temperatures will not be able to drop off much after
sunset with ample middle and upper level cloud cover around and
advecting in. The winds do become light and variable so some
decoupling is expected but with a warmer starting point we should be
several degrees warmer than this morning...met/mavmos blend was
The current 00z guidance has trended a little further north
with the track of the surface low keeping our entire County Warning Area within the
warm sector and therefore an all liquid precipitation tonight. The
GFS has been running faster than the ec and the rgem and brings the
first shot of overrunning rain shortly after midnight. The 3 models
are in relatively good agreement with the strength and evolution of
the system as it enters the Ohio Valley late tonight so basically
used a blend of the 3 to get a better timing consensus for
increasing probability of precipitation. Chance to likely probability of precipitation across the north by before
daybreak Wednesday morning.
Long term /Wednesday through Monday/...
the primary concern through this period is the middle week system.
GFS, ECMWF, and UKMET are in relatively good agreement with the
track of the low. The NAM, on the other hand, is the outlier in
digging and intensifying the low. For now, it looks that a
majority of the associated precipitation will occur during the day
Wednesday as the region remains in the warm sector. The cold front
should sweep through the region late Wednesday afternoon into
Wednesday evening. One area of notable disagreement among the
models is the potential for wrap around, Post frontal
precipitation Wednesday evening, which the European model (ecmwf) shows, but the
GFS and UKMET show only a very brief window of opportunity. This
would make a big difference on impacts as the only window of
wintry precipitation would be behind the front. For now, given the
source region of the airmass, and the model dew-point temperatures
Post frontal, think that any Post frontal wintry precipitation
will be brief. The other concern is thunderstorms ahead of and
with the front. Though instability is very limited, mu cape values
less than 500 j/kg, given the impressive low and middle level flow,
gusty winds will be possible especially if a line of storms
develops along the front, and is able to translate the low level
jet down to the surface.
Behind the front, a period of Wind Advisory is possible, though
the timing of the front, generally after sunset, is
climatologically not the most favorable time of day for Wind
Thursday and Friday, temperatures will once again be well below
normal as the region remains under the influence of the Arctic air
mass. Some warm air advection is possible Friday as the surface
high shifts off the southeastern U.S.
Saturday, though the next low pressure system should be passing
well to our north, an attached trough could bring the next brief
chance for precipitation during the day Saturday.
To start the next week, another cold front pushes through on
Sunday and the region is once again under an Arctic air mass.
Aviation /14z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Today...VFR. A cold front will move into the region early this
morning and winds will back towards the west and remain under 10
knots. High level clouds will continue to stream in from the west
well ahead of the next low pressure system. Should be able to
reach convective temperatures later this afternoon so a few
stratocu are possible. Another chance for a seabreeze to develop
and move through Acy later this afternoon.
Tonight...mostly VFR. Winds become light and variable for a time
ahead of the next system....ultimately veering towards the
southeast. Will start to see the lowering cloud bases towards MVFR
Wednesday and Wednesday night...MVFR and even brief IFR conditions
will be possible under the heaver showers expected through the day
Wednesday. Isolated thunderstorms and rain is also possible Wednesday afternoon. A
cold front will bring an abrupt change with gusty westerly and
northwesterly winds Wednesday evening. A line of thunderstorms and rain with gusty
winds is possible with this front. However, conditions should
improve to VFR within hours of the frontal passage. Winds may stay
breezy through the night.
Thursday...VFR expected. Breezy northwesterly winds may continue
through Thursday afternoon.
Friday...VFR conditions expected.
Saturday...MVFR conditions possible in rain showers primarily at kabe and
krdg. Otherwise VFR conditions expected.
sub-advisory conditions will persist on the waters today and tonight. A
weak cold front will creep into the waters later this morning
backing winds to the west. Weak high pressure will move in ahead
of the approaching deepening low pressure across the Ohio Valley
this evening. Winds and seas will relax by then under a loose
pressure gradient. Winds then veer towards the southeast and
increase late tonight as the low pressure approaches our region.
Wednesday...winds increasing to Small Craft Advisory as a low approaches the region.
Wednesday night and Thursday...gale conditions expected behind a
cold front that will push through Wednesday evening. Winds not
expected to subside below gale until Thursday evening. Light
freezing spray likely on the ocean waters during the day Thursday.
Friday...conditions decreasing below sca, but may approach Small Craft Advisory
again on Friday night.
so far, quantitative precipitation forecast with the middle week system looks to stay less than one
inch. However, brief, heavy downpours under the thunderstorms are
possible leading to increased runoff, and possibly a higher threat
for minor flooding. However, where this will occur is still
uncertain. We will continue to monitor the situation.
Marine...gale watch from Wednesday evening through Thursday evening for
Small Craft Advisory from noon to 6 PM EDT Wednesday for