Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Mount Holly New Jersey
356 PM EDT Thursday Apr 24 2014

Synopsis...
a narrow area of high pressure will slide across our region through
tonight, then weaken and shift offshore Friday. Low pressure is
forecast to track across the Ohio Valley and middle Atlantic region
Friday into Saturday morning. High pressure will then follow for
late Saturday into Monday. Low pressure, developing over the middle
of the nation Monday and Tuesday, will be a slow mover and it should
result in an extended period of unsettled weather for our region
starting Tuesday and lasting through Thursday.

&&

Near term /until 6 am Friday morning/...
another sunny but windy day is in progress as we are still on the SW
edge of low pressure east of New England...and diurnal mixing brings down
momentum from aloft. The air mass is very dry and skies are cloud
free at this time. In spite of sunshine it looks like temperatures this afternoon will
not quite reach the forecast maximums except over northern Delaware-Maryland-Virginia.

For tonight...the forecast area will come under the influence of a surface
high pressure ridge which will extend from Hudson Bay southward across the
middle-Atlantic region to off the southeast US coast. As a result the gusty
winds will diminish by sunset this evening and then become light and
variable overnight. Generally good radiational conds will allow min
temperatures to reach near or below freezing at many places north of phl and
possibly also in the southern New Jersey Pine barrens. A freeze warning has been
issued for parts of New Jersey generally north of ttn where sub-freezing temperatures are
expected to last for three or more hours. Because of the dry air...
little frost is expected even in colder areas. Also, as the upper
ridge axis now over the Midwest move east...some hi level cloudiness
should begin to spill over and may limit cooling over eastern PA.

&&

Short term /6 am Friday morning through 6 PM Friday/...
for Friday...the surface ridge axis will be over the area in the morning
but will move offshore in the afternoon. Winds will pick up from the southeast
to S by middle day. Hi and middle level clouds will be increasing ahead of
an upper shortwave trough approaching from the Ohio Valley. Even so sunshine
through the cloudiness and low level warm advection should result in
maximum temperatures a few degrees warmer than today.

No precipitation is forecast for Friday morning but by afternoon probability of precipitation begin to increase
from west to east across the forecast area. Scattered rain showers are expected by
late afternoon west of phl but any rainfall amounts should be just a few
hundredths at most.

&&

Long term /Friday night through Thursday/...
the overall synoptic setup is comprised of an initially progressive
flow with a short wave trough sliding across the area Friday night
and Saturday. The flow then looks to undergo significant
amplification as blocking develops. This allows for a closed low to
develop southeast of New England Sunday, with another one ejecting
out into the plains. An initial sharp/narrow ridge between these two
systems should get knocked down as energy rotates across the Great
Lakes before being absorbed by the closed system from the plains.
Due to blocking downstream, a large closed low should result during
next week from the Great Lakes to the Tennessee Valley. This places
our area on the east side of the closed low, which is typically a
favorable area for moisture to surge northward. If this pans out
like this, a period of unsettled and cooler weather will occur
across our area. We used a general model blend from Friday night
into Sunday, then favored wpc guidance thereafter. Some adjustments
were then made following additional collaboration with our
neighboring offices.

Friday night and Saturday...a short wave trough is forecast to move
through during at least a portion of this time frame. The axis of
the trough looks to start taking on a negative tilt for a time
before it weakens and gives way to increasing height falls with the
closed low near the upper Great Lakes. This will provide a ribbon of
enhanced lift across the area, which should result in a decent
amount of showers during at least Friday evening. There may be
enough instability Friday evening combined with the forcing for some
thunder especially across portions of the southern zones. As low
pressure slides across the area later Friday evening then shifts to
our northeast Saturday morning, the showers are expected to clear
our County Warning Area. However, the main upper-level trough/closed low is then
forecast to drop down across the northeast and northern middle Atlantic
region. This along with colder air aloft and a secondary frontal
boundary should trigger some additional showers. While there could
be some instability to allow for thunder, we may have just some
low-topped convective cells during Saturday afternoon and thus no
thunder was added for Saturday at this time. We went with likely probability of precipitation for a
portion of Friday night, then lowered to the chance/slight chance range
through Saturday. The shower threat should end very early Saturday
evening as the system shifts to the east and the airmass stabilizes.

Sunday and Monday...it appears that this time frame will feature our
area being between weather systems. A closed low to our northeast is
forecast to shift farther out to sea, however this feature may also
help in the main flow becoming slower. A narrow area of high
pressure looks to wedge down into the middle Atlantic during Sunday
into Monday. The main surface high is forecast to be centered well
to our north as a system closes off across the central parts of the
nation. As downstream blocking looks to get going, the systems as a
whole should move rather slowly eastward. This would initially place
our region within an area of general subsidence, however the flow
turning onshore especially later Monday should allow for more clouds
to start developing. This time frame is dry, with the exception of
some slight chance to low chance probability of precipitation late Monday night as the onshore
flow becomes better established along with the development of a warm
front to our south and gradual isentropic lift.

Tuesday through Thursday...a significant closed low drifting
eastward from the middle Mississippi Valley Tuesday will spell an
unsettled period of weather for our entire region. This will drive a
surface low that will probably occlude into the middle Atlantic later
Wednesday and Thursday. There may be a lingering surface wedge of
high pressure down into the middle Atlantic initially, allowing for a
better overrunning surface to be present. The initial warm air advection and
associated isentropic lift coupled with moisture advection should
allow for rain to develop especially later Tuesday. This looks to
continue right through Thursday, however the rain may move through
in stages and be heavy at times especially when tied to the arrival
of embedded short wave energy ejecting ahead of the closed low. The
initial surge in moisture though may have a tougher time getting to
our northern zones for a time as some drier air may hang on. We
therefore went with high chance to likely probability of precipitation through much of this
time frame. The low-level flow looks to be onshore which tends to
limit or subdue the instability /especially surface-based/, however
some elevated instability is possible which could lead to some
embedded thunder. Given it being this far our in time and lower
confidence regarding the thermal profiles and therefore associated
instability, thunder was not included at this time.

&&

Aviation /19z Thursday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.

VFR conds are expected for the next 24 hours or so. The gusty northwest
winds will diminish by early evening and becoming light and variable
overnight as a surface high pressure ridge moves overhead. Light winds will
continue tomorrow morning but will start to pick up from the southeast to S
by midday. High to middle-level clouds will be increasing during the day
but ceilings will remain well above MVFR limits until late in the day.

Outlook...
Friday night and Saturday...MVFR to at times IFR with showers /some
thunder cannot be ruled out especially south of kphl Friday
evening/, then conditions improving to VFR early Saturday morning.
Some additional showers are anticipated for Saturday afternoon.
Southerly winds generally 5-10 knots, becoming west and northwest
10-15 knots Saturday.

Sunday and Monday...overall VFR. Northerly winds 10-15 knots Sunday,
becoming light and variable Sunday night and Monday. Clouds increase
Monday night with a chance of some MVFR ceilings toward daybreak
Tuesday.

Tuesday...MVFR ceilings should develop with an onshore flow
especially later in the day. Some rain should overspread the area
from south to north late in the day and at night with sub-VFR
conditions anticipated.

&&

Marine...
winds are still a bit gusty over the forecast waters but should be
rapidly diminishing during the next few hours as low pressure well east of
New England moves farther out to sea and the pressure gradient relaxes.
Seas should also subside this evening. The Small Craft Advisory has been dropped for
Delaware Bay but continues until 6 PM for the New Jersey and Delaware coastal
waters. Winds should be rather light overnight and Friday morning
but will begin to increase from the southeast to S during the afternoon tmrw.

Outlook...
Friday night and Saturday...southerly winds ahead of a cold front
will shift to the northwest later Friday night and Saturday. The
flow may be strong enough to allow for advisory level gusts to
occur. The seas will build in the southerly flow, however the
southerly direction is not anticipated to be prolonged. The
conditions should then be mainly sub-advisory Saturday night.

Sunday and Monday...generally sub-advisory northwesterly flow
Sunday, then winds diminish and start to turn onshore Monday.

Tuesday...easterly winds increase, however probably remaining below
advisory criteria. The seas however should build to near 5 feet
during this time frame.

&&

Fire weather...
the red flag warning remains in effect through early this evening,
as gusty winds continue along with rather low relative humidity
values. The fuels have dried out more today as well, and some
brush fires have been reported. Radar showed a large smoke plume
earlier from a fire in Ocean County. The winds are expected to
diminish this evening. For Friday, there may be an enhanced risk
of fire weather /sps issuance possible/ especially for
eastern Pennsylvania and New Jersey, however the winds are
anticipated to be much less compared to the last few days.

&&

Phi watches/warnings/advisories...
PA...red flag warning until 8 PM EDT this evening for paz054-055-
060>062-070-071-101>106.
New Jersey...red flag warning until 8 PM EDT this evening for njz001-
007>010-012>027.
Freeze warning from 2 am to 8 am EDT Friday for njz001-007>010.
Delaware...red flag warning until 8 PM EDT this evening for dez001>004.
Maryland...red flag warning until 8 PM EDT this evening for mdz008-012-
015-019-020.
Marine...Small Craft Advisory until 6 PM EDT this evening for
anz450>455.

&&

$$
Synopsis...gorse
near term...amc
short term...amc
long term...gorse
aviation...amc/gorse
marine...amc/gorse
fire weather...gorse