Scientific Forecaster Discussion
fxus61 kphi 160731
Area forecast discussion
National Weather Service Mount Holly New Jersey
331 am EDT sun Jun 16 2019
high pressure will continue moving farther offshore today as a cold
front approaches from the north. This front will stall across the
northern mid-Atlantic and stick around through much of the upcoming
work week. Several waves of low pressure are expected to develop
along this boundary and pass across the region leading to several
rounds of showers and thunderstorms through late this week.
Near term /until 6 PM this evening/...
several weak perturbations are moving into/through the northeast
early this morning. One is moving through New York/PA at this time, with
an area of light rain/showers generally affecting areas north of I-
78 early this morning. In general, models are handling the southern
extent of the precipitation poorly, and made earlier adjustments to
the forecast to increase pops southward to the urban corridor.
Precipitation amounts with this first batch of precipitation should
be a tenth of an inch or less (highest amounts in northern new
A second/stronger convectively-maintained perturbation is in Ohio,
with heavier and more extensive precipitation near/downstream of it.
This precipitation has moved into northern/central Pennsylvania,
perhaps a little south of where most convection-allowing models had
placed it, with the WRF simulations seemingly handling it best and
the hrrr handling it worst. The track of this perturbation will be
east-northeast, which should allow the heaviest precipitation to
move just north of the area this morning. However, expect some
precipitation northwest of the fall line at least and possibly a
little farther southeast given radar trends. As a result, increased
pops this morning for about the northwest half of the County Warning Area and
trended it a little faster given what has transpired so far
A third perturbation arrives late today, and this is the one that
should produce stronger convection. There are two main forecast
questions associated with this perturbation. Where will it track,
and how will the downstream environment destabilize? Regarding the
first question, the 00z NAM tracks it along the Mason-Dixon line.
The 00z GFS tracks it across northern Virginia eastward to Delmarva.
The 00z European model (ecmwf) is much farther north (generally through PA/nj). These
differences are critical because convection will likely develop in
close proximity to this vort Max. Hi-res models are similarly
variable, with hrrr runs developing strong convection anywhere from
eastern PA/central New Jersey to central/southern Maryland eastward through
Additionally, extent of destabilization is questionable given the
showers occurring in the northern half of the area this morning.
This will undoubtedly mitigate diabatic heating to some degree
today, but it is unclear how far to the south this will occur. My
suspicion is that it is a little farther south than consensus given
this morning's trends, but this will ultimately depend on the
precipitation moving into/through Pennsylvania this morning. Where
the environment remains relatively unperturbed by the predecessor
precipitation/clouds, BUFKIT soundings show MLCAPE approaching 1000-
1500 j/kg this afternoon in an adequately sheared (30-35 kts deep-
layer bulk wind difference) environment. Low-level helicity will
also improve through the day, with hodographs exhibiting modest
curvature (0-3 km srh approaching 150 j/kg by evening). Where
convection develops, mixed convective modes may occur initially with
all types of severe possible, though convection-allowing guidance is
tending to develop a mesoscale convective system rather quickly.
Timing of the stronger storms looks to be after 3 PM and may peak
during the evening hours based on the latest hrrr/NAM nest
Currently, think the best chances for severe storms this afternoon
are generally south of a reading to Trenton to Asbury Park corridor,
and have included enhanced wording in the public forecasts with this
update. Heavy rainfall is certainly possible with stronger storms,
but today's convection looks fairly progressive as well, so not
expecting a widespread flooding threat.
Used a consensus blend for the temperature forecast today, given the
uncertainty with cloud cover/precipitation. Generally, this lowered
temperatures about 1-2 degrees from the previous forecast.
Short term /6 PM this evening through 6 PM Monday/...
main forecast concern tonight is the ultimate evolution of strong
convection developing this afternoon/evening...
Generally, models (coarser and hi-res) are indicating storms
congealing into a mesoscale convective system that moves through
portions of the County Warning Area during the evening before progressing offshore
overnight. Where this occurs is a big question mark (see the near-
term forecast discussion), but another concern is the tendency for
some models to regenerate precipitation upstream of the primary mesoscale convective system
overnight. The NAM nest and WRF simulations are both depicting a
second round of convection developing near and somewhat north of
where the first round of storms moves through. This appears to be
tied to low-level warm advection downstream of a fourth (yes,
fourth) perturbation approaching the area late. This prompted ME to
keep pops rather high through about 06z before trending them to
slight or low-chance values thereafter.
Should this second round of storms occur, heavy rainfall becomes a
bigger concern. I am not biting on this potential yet, since models
are not exactly agreeable on the first round of convection (much
less the second). However, will need to monitor convection-allowing
model trends closely today to determine if this is a plausible
Lows tonight should be in the 60s to around 70.
Long term /Monday night through Saturday/...
overview: generally and unsettled pattern this week and into next
weekend with a decent westerly flow aloft between a persistent
low/trough across eastern Canada and high pressure across the Gulf of
Mexico and the southeast states. Disturbances from the plains will continue
east across the Ohio Valley and then across the middle Atlantic.
This will bring rounds of showers and tstms with the possibility of
both heavy rains and some severe weather also. The timing of the
systems vary from model to model and from run to run, so we will
update each day with the latest pop/qpf trends and not get too
specific at the greater time ranges, since the models struggle to
remain consistent at these time ranges.
Precipitation: overnight, we have reduced probability of precipitation for the first part of
Monday and increased then for later Monday and Monday night. This is
shaping up to be a period of excessive rains across some part of our
County Warning Area. The NAM/GFS seem to be favoring the Delmarva region while the
latest ec and CMC show the greater totals across PA and into nrn New Jersey.
The Storm Prediction Center also placed parts of the County Warning Area in a slight risk for severe
weather for Monday also.
Another period for possible showers/tstms is Tue night into Wed
morning, so we kept the high chc/low likely pops for this period.
Beyond that, few changes were made, except to improve the look of T
he maps between the offices. Mostly chc pops with the wavy front
Temperatures: generally at or slightly above normal thru the period.
Monday will probably be the warmest day of the week, but right now
the differences between the days is only 1 or 2 degrees. Highs
mostly in the low/mid 80s for most areas. A trend towards
higher readings will begin towards next weekend. Overnight lows
will be generally above normal with frequent clouds expected.
Aviation /08z Sunday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Today...generally VFR with at least a slight chance of showers this
morning near/north of the philly terminals (with negligible impacts
expected). However, chances for stronger storms (and sub-VFR
conditions as well as strong/erratic winds) increase this afternoon,
though location/timing is a very low-confidence forecast at this
point. Think the threat is high enough for thunder mention at the
terminals late this afternoon, with fine-tuning of the tafs to be
expected once the convective evolution becomes clearer. Winds
generally southwesterly 7 to 15 kts with stronger gusts in the
Tonight...chances for storms continue during the evening hours but
are expected to diminish after midnight. However, there is a chance
storms could linger for a while longer, especially at ilg/miv/Acy.
Generally VFR outside of storms. Southwest winds near/below 10 kts.
Monday/Monday night...VFR during the morning, then showers and
tstms with lower ceilings/visibilities overnight.
Tue...early showers/tstms then improving conditions.
Wed/Thu...mostly VFR but sct tstms with lower conditions at
seas will remain near/above 5 feet today on the Atlantic waters,
with southwest winds likely gusting to 20 to 25 kts this afternoon.
Models have trended a little longer with elevated seas, so have
extended the advisory for the New Jersey waters through 10 PM. The
advisory for the Delaware Atlantic waters continues until 6 PM. Sub-
advisory conditions are expected on Delaware Bay through tonight.
A chance of storms exists on the waters this afternoon and tonight.
Some of the storms could be strong/severe, with erratic/strong wind
gusts and locally higher waves possible in their proximity.
mostly sub-Small Craft Advisory conditions. The main hazard will be sct tstms
with higher winds and seas. Mon night and Tue night/Wed. Are
periods that have better chc for tstms.
Marine...Small Craft Advisory until 10 PM EDT this evening for
Small Craft Advisory until 6 PM EDT this evening for anz454-