Scientific Forecaster Discussion
fxus61 kphi 172229
Area forecast discussion
National Weather Service Mount Holly New Jersey
629 PM EDT Mon Jun 17 2019
a frontal boundary will remain stalled near the area through
the week with several waves of low pressure moving along the
boundary. The front will finally push to our south Friday as an
area of low pressure strengthens and moves to our northeast.
High pressure briefly affects the area Saturday before shifting
offshore and to our south Saturday night into Sunday. A warm
front will lift across the area Sunday night into Monday as an
area of low pressure moves across the Great Lakes region into
Near term /until 6 am Tuesday morning/...
severe threat has ended for the southeast Pennsylvania counties
in the Severe Thunderstorm Watch, so will go ahead and remove
those counties from the watch. Severe Thunderstorm Watch remains
in effect until 11 PM otherwise. Damaging straight-line wind
gusts is the main threat.
A mesoscale analysis early this evening indicates that surface-
based instability is in the 1000-2000 j/kg range from roughly
the Philadelphia Metro southward with mixed-layer instability of
500-1500 j/kg. While the midlevel lapse rates are not all that
steep, the low-level lapse rates are steep across the southern
zones where temperatures are in the 80s. The flow aloft is
mostly westerly at 40-50 knots of flow between 700 mb and 500
mb. This results in 30-40 knots of shear, which should tend to
favor organized clusters or line segments. The airmass is rather
warm with the freezing level near 13,000 feet with midlevel
lapse rates on the weak side, therefore severe hail may be less
of a threat unless some convection can develop some midlevel
Given the parameters in place, any organized and stronger
convection is anticipated to be south of I-78 and especially
from near the Philadelphia Metro on southward. The greatest risk
area for severe thunderstorms is across about the southern half
of the area, and convective organization would greatly improve
cold pool development. The forecast soundings from around the
Philadelphia Metro into southern New Jersey and northern
Delaware do show some veering with height in the low levels this
evening, although the overall flow is light. This however could
be enough to get some low-level rotation and pose a non-zero
tornado risk, especially if some stronger storms track near the
stalled surface front. Overall, locally straight-line damaging
wind gusts are the main threat.
The flash flood threat mostly depends on if convection can
train over the same area for awhile. Some guidance continues to
hint at this idea, and given the warm and humid airmass in place
some potential still exists. Therefore, the Flash Flood Watch
remains unchanged from previously.
As for the temperatures, mostly blended the MOS in with
continuity. It will be a muggy night although slightly drier air
may get into our far northern zones through the night.
Short term /6 am Tuesday morning through 6 PM Tuesday/...
a similar synoptic setup for Tuesday as our region remains in a
more zonal flow aloft with a weak surface front oscillating
across our area. Within this flow regime, subtle short waves
will traverse our area and some of these will probably be
convective remnants (mcv) from previous day storms. One of these
may be moving across mostly the northern areas during the
morning, with associated mostly shower activity. Much of the
region should start the day with low clouds, with this most
likely along and north of the aforementioned surface front.
The convective evolution through the day Tuesday could be a
challenge due to morning clouds and showers. Convective
development could be enhanced a little though due to a subtle
weak southern stream short wave trough moving northeastward from
the southern states. It is the southern half of the area that
appears to have the better chance of achieving higher
instability, however the severe thunderstorm threat looks to be
reduced due to poor lapse rates and weak buoyancy. As a result,
the Storm Prediction Center has about the southern half of our area in a marginal
risk for severe thunderstorms.
An abundance of moisture will again be in place especially the
southern half of the area, with precipitable water values around
2 inches. This will result in locally very heavy rain and at
least a localized flash flood threat does exist. However, the
scope of the threat is less certain given uncertainty with the
overall convective evolution. We therefore held off with
extending the current Flash Flood Watch beyond 6 am. This will
be re-evaluated with subsequent shifts. The flow in the mid
levels should be strong enough to promote storm organization
into clusters during the afternoon, and where the boundary layer
heating is maximized is where a locally strong to damaging wind
gust is possible.
Cloud cover and some morning showers will hold temperatures
down again across the northern areas, therefore the high
temperatures are mostly a MOS/continuity blend which lowered the
highs some across the north especially.
Long term /Tuesday night through Monday/...
unsettled weather continues to be expected for the middle to
end of the week, with drier conditions early in the weekend,
with more unsettled weather late in the weekend into early next
A frontal boundary will remain near the area Tuesday night
through Thursday with multiple waves of low pressure moving
along this boundary. Meanwhile, multiple short waves/vorticity
impulses will move across the area within the west to southwest
flow aloft. There will continue to be instability across the
area, there will continue to be a chance of thunderstorms as
well. Precipitable water values will remain elevated with values 1.5-2.0 inches,
which will continue the threat for heavy rainfall. While it
will not rain the entire time, the periods with the greatest
rain chances will be tied to the strongest short wave/vorticity
impulses and surface low passages. Tuesday evening, Wednesday
afternoon/evening, and Thursday afternoon/evening. The late
overnight and early morning periods may have breaks in
As we go later in the day Thursday into Thursday night, an area
of low pressure will be moving our north and strengthen as it
does so, which will drag a cold front across the area. This
front may not fully clear our area and push to our south until
during the day Friday. So this will keep enhanced precipitation
chances through Thursday night and Friday morning. There remains
instability later Thursday into Thursday night, so
thunderstorms will remain in the forecast, and with precipitable water values
approaching 2.0 inches, heavy rainfall could continue to be a
threat. As the front pushes to our south and east during the day
Friday, the showers will come to and end. Winds could be quite
gusty later Friday, possibly reaching 20-30 mph during the
High pressure builds to our west on Friday night, briefly
building over the area Saturday, then shifting southward and
offshore Saturday night. Dry conditions are expected during this
period as the front will be south of the area. However, this
could begin to change by Sunday into Monday of next week as the
front to the south is forecast to begin moving back northward as
a warm front. This could bring a return of shower/thunderstorm
chances late in the weekend into early next week.
Aviation /00z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Tonight...some showers and thunderstorms with MVFR/IFR at
times, with most of the activity before 06z/2am, then MVFR to
even IFR ceilings overnight. Winds light and variable overall.
Tuesday...MVFR to IFR ceilings in the morning should improve to
VFR at most terminals (abe to ttn may remain mvfr). Some rounds
of showers and mainly afternoon thunderstorms are expected,
which will result in times of MVFR/IFR conditions. Light and
variable winds should turn light northerly before becoming
locally light south or southwest. Lowered confidence on the
extent of thunder and wind direction.
Tuesday night-Wednesday night...periods of MVFR ceilings
possible. Showers and thunderstorms possible, which could lead
to lower conditions.
Thursday...conditions may improve to VFR during the day.
However, showers and thunderstorms remain possible which may
lead to lower conditions. Southwest winds could gust 15-20 knots
Thursday night... VFR possible early, lowering to MVFR
overnight. Especially with any showers and thunderstorms.
Friday...MVFR conditions possible early, before improving to
VFR during the day. Winds shift to northwest with gusts 20-25
Friday night-Saturday...VFR conditions expected. Northwest
winds may gust 15-20 knots during the day Saturday.
the conditions are expected to remain below Small Craft
Advisory through Tuesday. Winds should favor a southerly
direction, however then turn easterly across the far northern
Atlantic coastal waters Tuesday afternoon. A few stronger
thunderstorms through this evening and then again Tuesday
afternoon could produce 34 knots or greater wind gusts.
Tuesday night-Thursday...conditions expected to remain below
advisory levels. However, high winds and waves are possible in
and around thunderstorms.
Thursday night-Friday night...Small Craft Advisory conditions
possible with guts 25-30 knots possible.
Saturday...conditions expected to drop below advisory levels by
a low risk for the development of dangerous rip currents is
expected to continue through Tuesday with waves in the surf zone
3 feet or less. A long period swell near 14 seconds could be
more present as we go through Tuesday and this could elevate the
rip risk, however waves are expected to remain on the lower
PA...Flash Flood Watch until 6 am EDT Tuesday for paz060-061-070-
New Jersey...Flash Flood Watch until 6 am EDT Tuesday for njz009-010-
Delaware...Flash Flood Watch until 6 am EDT Tuesday for dez001>004.
Maryland...Flash Flood Watch until 6 am EDT Tuesday for mdz008-012-015-