Area Forecast Discussion
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FXUS61 KPHI 201047
Area Forecast Discussion
National Weather Service Mount Holly NJ
647 AM EDT WED JUL 20 2016
A front will remain stalled over the Delmarva region and southern
New Jersey today before finally pushing south of the area
tonight. Meanwhile, high pressure will build into the area. The
high is expected to move offshore Thursday and Friday. A cold
front may move into the area and dissipate Friday night into
Saturday. Strong high pressure then re-establishes itself into
next week, but another cold front will slowly approach from the
northwest late in the weekend.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
High pressure was located over the Great Lakes region early this
morning. Meanwhile, a stationary front was analyzed over the
Washington DC area, extending eastward into the Delmarva and
southern NJ. The airmass south of the boundary is moist (dewpoints
in the upper 60s-lower 70s) and unstable (MUCAPE 1500-2000 J/KG)
while much drier air (dewpoints in the upper 50s to low 60s)
resides north of the boundary. Strong to severe thunderstorms
develop in the unstable warm sector during the night as an upper
shortwave trough moved southeastward toward the Chesapeake and
Delaware Bays. Additional showers and isolated storms will
continue to develop early this morning in the northern Delmarva
and southern NJ, where low-level convergence along the front will
provide enough lift for parcels to their LFC. The environment is
slightly more stable and thus not as favorable for severe storms
this far north as it is over the lower Delmarva.
There is quite a bit of model disagreement regarding how the front
moves over the course of the day. There will be a brief wind surge
out of the northeast this morning as high pressure builds in from
the north, so this may cause the front to drift southward a bit
through midday. However, the front may briefly waver back to the
north during the first half of the afternoon as light southerly
flow develops in response to a portion of the elongated surface
high moving offshore. We are confident that areas north of the PA
turnpike and I-195 should stay dry and see plenty of sunshine
today. Locations farther south into far southern NJ and the
Delmarva are at risk of seeing additional showers and storms that
form during peak heating this afternoon. Confidence for afternoon
storms is highest across the southern half of DE and adjacent
locations in eastern MD, since these areas will more than likely
reside in the warm sector south of the front. We think the 00/06Z
NAM is a little too bullish on low-level moisture and instability
(NAM forecast profiles show 2-3 kJ/kg of SBCAPE for this afternoon)
across our far southern areas. However, even if it`s overdone by a
factor of two (which is more in line with the RAP/HRRR), there
will be a significant dry layer present above the moist boundary
layer. Accordingly, there is a conditional threat (dependent on
thunderstorms developing) for pulse severe storms in this thermodynamic
environment that is conducive for wet-microbursts. PoPs mostly
follow a blend of HRRR/RAP/SREF with slightly modifications to
remove the noise caused by the hi-res model forecast grids.
It will feel very comfortable out today for a middle of summer day
in the mid-Atlantic region. Temperatures will be in the mid 80s
across most of the area; cooler along the shore and across the
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
The stalled front will finally push south of the area tonight as
high pressure moves overhead. Shower or storms that develop this
afternoon across the Delmarva and far southern NJ should dissipate
toward sunset, although cannot rule out an isolated shower or
storm in our southern most zones after dark where elevated
instability is forecast to reside. The setup will be favorable for
radiational fog with clear skies and light winds under the high.
Not expecting fog to be widespread across the entire area though
given how short the night is right now.
.LONG TERM /THURSDAY THROUGH TUESDAY/...
Hot and humid conditions rebuilding across the area late this week
into this weekend and early next week, with dangerously hot
conditions likely. The warming trend begins Thursday, though
Thursday looks to be the least warm day. Troughing across the
northeast begins to move out of the area Thursday, while ridging
begins to build in from the west. This will allow thicknesses and
925/850 mb temperatures to climb through early next week. With
the building thicknesses and 925/850 mb temperatures, daytime
highs are expected to climb into the upper 80s/low 90s Thursday,
then the low to mid 90s Friday, followed by mid to upper 90s
Saturday, Sunday, and Monday, back to low 90s again by Tuesday.
Heat index values will begin approaching 100 degrees by Friday,
but the dangerously high heat index values over 100 are expected
through the weekend into Monday.
Rain chances through the extended will be centered around a couple
of frontal boundary passages. The first is late Friday into Friday
night, and the second period being Monday into Monday night.
Behind the frontal passage, conditions will still likely be quite
warm Tuesday, but not as hot as the weekend or Monday.
.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Isolated showers and storms in VC of MIV early this morning.
These should stay south of ACY and ILG. VFR expected for the TAF
sites today with most of the showers and storms remaining south of
the terminals. The convection could be close to ILG- MIV- ACY
though but confidence not high enough to mention with 06Z TAFs.
Light N-NE winds early this morning will veer more out of the NE
or E-N after sunrise. A brief wind surge near 10 kt could accompany
this wind shift. Winds then become light and variable around
midday before the direction favors more out of the S or SE late
in the day.
There is a chance for fog late tonight into the early morning
hours on Thursday. Winds will be light and variable tonight.
Friday-Friday night...Generally VFR. A chance of showers and
thunderstorms which may briefly lead to lower conditions. Gusty
southwest winds 15-20 knots.
Storms are moving into the coastal waters off DE. These storms
may pose a threat to mariners (locally strong winds and higher
seas as well as lightning) early morning as they move eastward.
Otherwise, winds and seas will remain below Small Craft Advisory
criteria. Nly winds will veer out of the NE early this morning.
The could gust 15 to 20 kt this morning. Winds will slacken a bit
early this afternoon and then shift out of the S-SE late in the
Thursday-Thursday night...Sub-Small Craft Advisory conditions
Friday-Friday night...Small Craft Advisory conditions possible with
increasing winds ahead of an approaching cold front.
Saturday-Sunday...Sub-Small Craft Advisory conditions expected.
RIP CURRENTS... The risk for the formation of dangerous rip
currents today will be low at the DE and NJ beaches, but very
localized moderate rips are possible, especially along the NJ
coast where onshore flow will be strongest this morning.
We have had some reports of upwelling along the coast, where
temperatures have dropped into the low 60s.