Area Forecast Discussion
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FXUS61 KPHI 231039
Area Forecast Discussion
National Weather Service Mount Holly NJ
639 AM EDT THU JUN 23 2016
Low pressure in western Pennsylvania will move southeastward and
off the Atlantic coast later today. High pressure will then build
in from the north for Friday into early next week. A slow moving
cold front is forecast to approach from the northwest on Tuesday
and move through the region on Wednesday.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Made some minor edits to the PoP and weather grids with the 630 AM
ESTF update. Took into account a slightly faster exit of the
widespread showers and embedded thunderstorms late this morning.
Also, ramped down the threat of strong and heavy rainfall/flooding
with this morning`s round as the greater instability and deeper
convection in on track to stay suppressed to our south.
Overnight surface analysis shows an area of low pressure located
along over the Midwest (near the IA-IL border). This low was
located along a stalled frontal boundary that extends eastward
along the OH Valley and then arcs southeastward across the
southern mid-Atlantic (near the VA-NC border).
An MCS developed last night just downstream of the surface low.
Based on the orientation of 1000-500 mb thickness contours and the
flow in the mean cloud layer, this MCS should move to the east-
southeast and through the Chesapeake Bay/Delmarva region this
morning. The timing of the precip from this MCS closely follows the
HRRR, WRF ARW/NMM as well as the latest radar trends upstream.
Expect showers to arrive across our far western zones in southeast
PA as well as eastern MD shortly after sunrise (6 or 7 AM) and then
progress eastward during the mid morning. The back edge of the
widespread showers is forecast to advance quickly eastward, moving
off the coast around midday. There will be a sharp northern cutoff
of precip somewhere near the PA Turnpike/I-195 corridor. Note, there
were considerable changes to the PoP and weather grids compared to
the previous shift.
The stronger convection associated with severe storms (tornadoes,
damaging winds and flash flooding) were present on the southern
flank of the MCS and along or just south of the frontal boundary.
Therefore, we expect these stronger storms to track to our south and
west this morning. However, numerous lightning strikes were noted
farther north on the cool side of the boundary. Kept a chance for
thunderstorms from about RDG-PHL-ACY southward later this morning,
where model soundings indicate a 2-3 hour period of 250-750 J/kg
(highest south) of elevated instability. SPC Day 1 outlook still has
the southern third of the CWA in a slight risk of severe storms.
There will be a strong wind field aloft ahead of the MCS but the
threat of damaging winds looks to be limited assuming these storms
remain elevated in nature by the time they reach our area.
Greater uncertainty in the forecast revolves around how much will
the atmosphere be able to rebound/destabilize this afternoon in wake
of this morning`s MCS. Based on the latest guidance, there will
likely be scattered convection developing during peak heating across
eastern PA and central/northwest NJ along the residual outflow
boundary and/or differential heating boundary to the north of where
MCS tracked earlier. Majority of the model forecast soundings show
the atmosphere capped, which would hinder convective updrafts and
our thunderstorm potential. Still kept a slight chance of thunder in
the weather grids for the mid to late afternoon.
High temperatures should generally in the in the upper 70s and lower
80s. The warmest conditions are expected across the northern half of
the CWA as well as the lower Delmarva, where the greatest potential
to see extended breaks in the clouds reside.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
Some of the hires convective-allowing model guidance advertises a
second MCS developing along or just south of the Ohio River. If it
develops, it should track mainly to our southwest tonight given
the distant position of the frontal boundary to our south and with
weak high pressure building in from the north. Kept low chance
PoPs in the forecast for our Delmarva/Delaware Bay zones tonight
to account for the possibility of the northern periphery that
these showers and storms clip us. There is still a limited risk
that a storm or two becomes strong to severe this evening mainly
across the Delmarva. The threat would transition from winds to
hail late in the evening as convection becomes increasingly
Patchy fog may also develop late this evening and especially
overnight. There greatest potential will reside across the southern
portion of the area, where the ground remains wet after today`s
precip. However, convective debris clouds may linger across the
lower Delmarva, which should inhibit the formation of radiational fog.
.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
High pressure will be the dominant wx feature for the extendd
period. The high will build down from Canada on Fri and influence
our wx into early next week. THe current fcst is for dry and
pleasant wx durg this time.
There is one potential fly in the ointment though. A mid level
trof is fcst to knock back the ridge a bit on Fri. The majority of
the guid keeps any precip assocd with this trof to our w and s.
However, the ecmwf is more robust and brings precip acrs our area
and then develops a sfc low which moves out to sea on Sat. For
now, will maintain a dry fcst, since the ecmwf is the outlier and
go with the majority of the mdls and wpc.
By Mon, an area of low pres will be movg acrs ern Canada. Its
assocd cdfnt will approach from the w late Mon and cross the area
on Tue, with a secondary fropa Tue night into erly Wed. Both cfp
will bring the chc of shwrs/tstms. High pressure will then build in
for the remainder of Wed.
Temps look to be near seasonal values with comfortable dew points.
.AVIATION /12Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
VFR thru 11Z. Then a complex of showers and embedded thunderstorms
will move across SE PA, S NJ, E MD and DE between 11Z and 15Z. MVFR
were included in the 06Z TAFs from RDG to PNE, southward. A 2 hr
tempo group from TSRA was added across these southern terminals.
MVFR cigs possible in wake of this morning`s precip before
improvement to VFR occurs between 17-20Z. More scattered showers
could develop after 18Z before ending around 02Z. Added a 2-3 hour
prob30 group for most of the terminals for late in the day. While an
isolated thunderstorm is not out of the question this afternoon,
confidence too low to include in TAFs.
Patchy fog could develop late this evening and overnight.
Winds will be light (under 10 kt) and variable thruout the period.
Friday through Monday...VFR under high pres. Mdt confidence
Friday. High COnfidence for the rest of the period.
Winds and seas below SCA today and tonight. A sly swell around 2-4
ft will continue in our coastal Atlantic waters thru the period.
One potential impact on our southern waters could be locally gusty
winds in thunderstorms this morning as well as this evening.
Coverage of storms will be low.
Friday through Monday...No marine headlines are anticipated.