Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 271349
Area Forecast Discussion
National Weather Service Mount Holly NJ
949 AM EDT MON JUN 27 2016
High pressure will remain anchored off the East Coast today. A cold
front will slowly approach from the northwest early in the week
while low pressure tracks eastward across Canada. The front is
expected to stall over the area late Tuesday and Wednesday before
finally moving offshore by Wednesday evening. High pressure builds
into the area Thursday. Another cold front will move through the
region Friday night or Saturday and then stall to our south later in
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
No significant changes so far today. We did raise temps a degree
or two for most places, and made some adjustments to the cloud
cover forecast. The fog across the south has begun to lift and
dissipate, but low clouds do remain across southern New Jersey and
the Delmarva. These clouds will continue through the morning and
into the afternoon, but continue to break apart some.
Outside of the low clouds this morning, we should see an increase
in high clouds followed by altocu and stratocu later today from
west to east. Isolated to scattered showers are expected to move
into the area later this afternoon into this evening. Although
CAPE is on the lower end of the spectrum later today, we could
still see an isolated thunderstorm; SPC has us in the General
category. The actual synoptic cold front will remain well to our
west today. The main cause of the showers will be a combination of
a weak shortwave, pre-frontal trof, and/or terrain.
Normal temps for KPHL is 85/67. We`ll end up about normal for the
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
Besides isolated to scattered showers and thunderstorms, the models
are in good agreement regarding low clouds and fog. As mentioned
what`s just beginning to form across Delmarva and far southern NJ
could be more widespread in 24 hours in the increasing southerly
Lows will be in the 60s to maybe 70 in Center City with clouds
overhead...about 5 to 10 degrees warmer from where we are right now.
Dewpoints will be up the same.
.LONG TERM /TUESDAY THROUGH SUNDAY/...
An upper shortwave trough is forecast to dig southeastward over the
Great Lakes and Ohio Valley regions on Tuesday. At the surface, a
pre-frontal trough will be positioned just to our west on the lee
side of the Appalachians Tuesday morning while a cold front is
near the eastern Great Lakes. The cold front will likely jump the
Appalachians and reorganize along the lee trough on Tuesday. NWP
guidance has come into better agreement with the latest 00Z run
regarding the timing of the cold front in the forecast area. This
slow-moving front should arrive in our western zones Tuesday
evening and then likely stall near the Delaware Valley/I-95
corridor late Tuesday night into Wednesday as a result of
downstream blocking offshore.
Despite a clearer picture on the synoptic scale, there is still
considerable forecast uncertainty regarding the timing, coverage and
intensity of showers and thunderstorms for this next event. Model
differences seem to be partially tied to how quickly the primary
cold front transfers its energy downstream to the lee trough and
how much instability to develop to fuel the convection in the
pre-frontal warm sector. Until the details in the mesoscale
become clearer, we`ll have be somewhat conservative with PoPs and
broaden the range of possibilities for potential impacts from
convection for Tuesday and Wednesday.
The latest thinking (although subject to change) is the bulk of the
leftover showers from tonight should move off the coast sometime
during the morning on Tuesday. Then another round of showers and
storms will likely form during peak heating Tuesday afternoon
near the surface trough/cold front either to our west or in our
far western zones of eastern PA. Forecast soundings show the
potential for a moderate amount of instability (MLCAPE above 1000
J/KG) to develop in the warm sector during the afternoon in areas
that see more persistent breaks in the cloud cover. A few locally
strong to severe thunderstorms may develop in this scenario.
Thermodynamic profiles off the 00Z GFS and NAM are supportive of
wet microbursts that could contain damaging winds with a
significant mid-level dry layer evident above a moist boundary
layer. This convection will eventually progress eastward toward
the I-95 corridor late in the day (perhaps after sunset) and then
our coastal communities Tuesday night. If the front remains
stalled over the area through much of the day Wednesday, then
another round of showers and storms could certainly develop during
peak heating. PoPs on Wednesday increase as you get closer to the
coast due to the proximity of the front.
Although there is not much support in the model QPF fields, it`s
difficult to dismiss a potential of at least locally heavy
rainfall and poor drainage flooding or even urban flash flooding
whenever you have a front stalled over the area this time of
year. The dry antecedent conditions would lessen the risk of
flooding. Additionally, the best forcing for ascent from this
system looks to stay to our north as the upper low lifts into
northern New England/southeastern Canada.
High pressure will build in from the Midwest Wednesday night. The
high moves overhead on Thursday and then starts to move offshore
Thursday night. Expect dry conditions during this part of the period.
The upper flow pattern re-amplifies by the weekend as additional
shortwave energy digs around the backside of the trough the Great
Lakes and Canada. Low pressure will track well to our north but
it will drag a cold front through the area sometime Friday into
Saturday. Synoptic lift with this system looks to be weak this
far south, so coverage of showers and storms may be limited. The
front may stall nearby to our south later in the weekend as the
boundary becomes increasingly parallel to the westerly steering
flow at the base of the upper trough. The proximity of the front
to our area will determine if we are able to dry out for Sunday
or continue this unsettled weather pattern through the remainder
of the weekend. The forecast is much more optimistic for the
northern half of the area vs the southern half.
Near normal temperatures are expected for our region over the next
week. In general, this equates to highs in the 80s and lows in the
60s. The core of the heat will remain over the western and southern
CONUS due to the persistent troughing over the northeastern
quadrant of the CONUS through the duration of the long term.
.AVIATION /14Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Today...Mainly VFR outside of ACY/MIV. Conditions have improved
from IFR, to now MVFR at ACY/MIV. It should take several more
hours before a return to VFR there.
A few showers or a thunderstorm are possible toward mid to late
afternoon, especially west of the I95 terminals resulting in
MVFR. Light and variable winds this morning will become southerly
and increase to 10 knots this afternoon.
Tonight...Outside of any showers or thunderstorms, VFR conditions
are expected this evening. After midnight, multiple models indicate
low clouds and fog will work into the area. We started with this
idea and introduced IFR.
Tuesday through Wednesday afternoon...Scattered showers and
thunderstorms may produce temporary/localized cig/vsby
restrictions. A few thunderstorms could also contain locally gusty
Wednesday night through Thursday night...VFR.
Friday...A chance for showers and thunderstorms, particularly west
of I-95 and during the afternoon-early evening hours.
Today...Sub-Small Craft Advisory conditions. South to southeast
winds may gust to around 20 knots.
Tonight...Sub-Small Craft Advisory conditions. South to southeast
winds will gust to around 15 knots. There`s a chance of showers and
Tuesday through Friday... Winds and seas expected to remain below
SCA criteria. A chance for thunderstorms with a slow-moving cold
front Tuesday-Wednesday and then again on Friday. Storms,
especially on Tuesday afternoon and evening, may contain locally
RIP CURRENTS...For today, looks like a low risk this morning
increasing to a moderate risk this afternoon once again due to a
strengthening southerly flow that may turn southeast. This will be
combined with 3 foot seas.