Area Forecast Discussion
Issued by NWS Mt. Holly, NJ-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KPHI 250947
Area Forecast Discussion
National Weather Service Mount Holly NJ
547 AM EDT THU AUG 25 2016
-- Changed Discussion --High pressure will move out to sea today. A weak cold front will
move across the area Friday and stall to our south over the
weekend. High pressure builds to our north over the weekend, then
offshore Monday. An area of low pressure may pass to our north
Monday, bringing a front across the area Monday into Tuesday.
-- End Changed Discussion --
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A weakening short wave trough lifts well to our north early this
morning. As this occur, weak surface low pressure also lifts into
Canada. A weak surface cold front is then forecast to be draped from
Ontario Canada southwestward down the Ohio Valley. This weak front
will ever so slowly move eastward today. Meanwhile, a lee side
trough should develop from about south-central Pennsylvania
southward to central Virginia. As we look aloft, a persistent ridge
remains centered across the southern states with our area mainly on
the northern periphery of it.
A southerly flow will be maintained along with some downstream warm
air and moisture advection well ahead of the aforementioned weak
cold front. This will allow for a warmer afternoon along with some
added humidity. A plume of high PW (near 2 inches) is forecast to
shift eastward by late in the day from the Ohio Valley into our
region. As this occurs, some convection is expected well to our west
ahead of the weak cold front. This activity could organize into a
line as it shifts eastward along with the high PW plume. Early this
morning, convection was entering western Pennsylvania and this may
be tied to a weakening MCV. There is enough instability forecast to
potentially allow some convection to arrive in our western zones
this afternoon. Overall the model guidance, including several
convection-allowing, tends to dissipate the convection as it
approaches the I-95 corridor by early this evening. This is mostly
due to the influence of the upper-level ridge farther south and east
along with any convection outrunning the main forcing. The forcing
overall is forecast to remain farther to the west from the eastern
Great Lakes to the Ohio Valley. As a result, we continued with
slight chance to low chance pops across the western areas this
afternoon. The slight chance pops then edge into the I-95 corridor
toward evening however it is less certain if thunder occurs here.
Otherwise, more cloudiness is expected today in part due to some
incoming convective cloud debris. A south to southwest breeze should
increase this afternoon and may become a bit gusty at times
especially across the coastal plain.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
The upper-level ridge anchored to our southwest is forecast to build
a bit northeastward, however much of our area remains on its
northern periphery. As the main upper-level energy remains near and
north of the Great Lakes, a weak cold front is forecast to slowly
approach our area from the west tonight. There appears to be a pre-
frontal trough in our vicinity, although the weak cold front may try
and jump into this weakness toward morning.
Backing up a bit, any lingering convection should dissipate mostly
early as it nears the I-95 corridor and it encounters more influence
from the southern ridge and shifts east of the main forcing. We
therefore lower the pops through the evening.
This will leave behind some cloudiness especially the first half of
the night, however more importantly it will be a much warmer and
more humid night. This could lead to patchy fog, however there may
be enough mid to high level clouds around especially across the
northern areas to prevent any development.
.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
A return flow will continue into Friday ahead of an approaching
frontal boundary. There will again be a chance for isolated showers
or thunderstorms to develop during the day, but overall chances are
small. 925/850 mb temperatures and thicknesses will continue to
increase, making Friday the warmest and most humid day of the week.
Heat index values could approach 100 degrees across for portions
of the Delmarva, southern New Jersey, and the Philly metro area.
The frontal boundary will sag south of the area Friday night, and
stall just to our south over the weekend while high pressure builds
to our north. This high will keep dry weather in place across the
area through the weekend. Temperatures will remain warm over the
weekend, but humidity levels will be much lower with dewpoints only
in the 60s.
The high pressure will build offshore of New England Sunday night
into Monday, and a cold front will begin approaching the area Monday
into Tuesday as an area of low pressure passes to our north. There
are a couple of short wave/vorticity impulses forecast early next
week, so there will continue to be a chance for isolated showers or
thunderstorms Monday into Tuesday.
Another frontal boundary could follow behind the Monday/Tuesday
frontal boundary around Wednesday into Thursday. Additional showers
or thunderstorms could develop as a couple of short wave/vorticity
impulses continue to move across the area each day.
Temperatures through next week will continue to be several degrees
above normal, but not as warm as Friday.
.AVIATION /10Z THURSDAY THROUGH MONDAY/...
-- Changed Discussion --The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
Today...VFR, with a ceiling developing between 5000 and 15000 feet
at most terminals. Isolated to scattered showers/thunderstorms are
possible from the west, however these should be mainly into the ABE
and RDG areas after 16-18z. Given low coverage expected, no mention
included in the TAF`s. South to southwest winds increasing to around
10 knots, with some local gusts up to 20 knots possible this
Tonight...VFR overall. Isolated to scattered showers or
thunderstorms possible mostly early, mainly west of a TTN to PHL to
ILG line. Local MVFR visibility may develop late due to fog/haze.
South-southwest wind diminishing to near 5 knots.
Friday...Generally VFR conditions expected. A chance of isolated
showers and thunderstorms which may lead to temporary lower
Friday night through Sunday night...VFR conditions expected.
Monday...Generally VFR expected. A chance of isolated showers and
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-- Changed Discussion --High pressure to our east will maintain a southerly flow across the
region through tonight. Meanwhile, a weak cold front approaches from
the west late tonight. The southerly flow is a bit stronger, with
some enhancement taking place especially this afternoon and evening.
This should be more notable across the northern coastal waters.
While some gusts may reach 25 knots mainly later this afternoon and
evening, confidence is not all that high for how long and widespread
it can get. As a result, we carried 15-20 knots but capped the gusts
before 25 knots and held off on a Small Craft Advisory.
Friday through Monday...Sub-Small Craft Advisory conditions
expected to continue across the waters.
For today...The risk for the formation of dangerous rip currents is
moderate for the New Jersey beaches, especially toward later this
afternoon when the southerly wind increases to 20 mph and the tide
is moving into the lower half of the tide cycle. The risk is
currently low for the Delaware beaches.
Friday and Saturday are outlooked as a low risk.
Sunday through Wednesday...model ensemble guidance on every
successive cycle continues to send 2-4 ft long period (15 to 17
second) east southeast swell into our waters from the central
Atlantic - Gaston. It could be even higher but we`re taking the
conservative approach at this time from this distant offshore
hurricane. This is all dependent on size, strength and track but
there is little doubt that swells will build here...just how much
is the question. Swimming and wading dangers will probably
increase. For now we`re anticipating a moderate or greater risk
for Monday through Wednesday and possibly starting Sunday.
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