Scientific Forecaster Discussion
fxus61 kphi 270958
Area forecast discussion
National Weather Service Mount Holly New Jersey
558 am EDT Thu Jul 27 2017
a weak front will move across the area later today and tonight. Low
pressure will develop along the front across the Ohio Valley and
then move towards our area Friday through Saturday. The low will
strengthen as it slowly moves away later Saturday and Sunday. High
pressure will begin to build over the area next week.
Near term /until 6 PM this evening/...
a cold front that is currently over the upper Midwest will approach
from the northwest late in the day while a warm front approaches
from the southern mid-Atlantic region. Models show a weak wave of
low pressure (area of enhanced pressure falls) forming along the
intersection of a Lee surface trough to east of the Appalachians and
the 850 mb warm front late this afternoon.
There appears to a couple of features to support convective
development this afternoon. The first source of lift will be ahead
of an upper shortwave trough that will be digging in from the upper
Midwest this afternoon. The bulk of the lift from this disturbance
will pass through our northern zones (north of I-78) during the
early to mid afternoon. Expect this first round of showers to be
light across the north since morning cloud cover will limit
destabilization in the boundary layer beforehand. Farther south,
scattered showers and storms may develop late in the day (after 4
pm) owing to isentropic lift near the low-level warm front.
Consensus among hi-res cams points to convective initiation occuring
in east PA and S/c New Jersey between I-78 and the Mason-Dixon line). The
official forecast reflects this scenario though we extended the axis
of higher pops southward toward the axis of higher instability near
the Chesapeake Bay.
D1 convective outlook from Storm Prediction Center includes the entire region in a
marginal risk for severe storms. The environment could become
supportive of marginally severe wet-microbursts, especially across
the far southeast PA/S New Jersey/east Maryland/de, where 70-degree dewpoints return,
low-level lapse rates steepen and MLCAPE values are forecast to
reach 1500 j/kg (possibly higher near the Chesapeake and
Delaware bays) late this afternoon.
High temperatures range from the mid to upper 70s north of I-80
to the upper 80s in eastern Maryland.
Short term /6 PM this evening through 6 am Friday/...
showers and storms will likely continue into tonight and possibly
expand in coverage this evening as another shortwave disturbance/
remnant mesoscale convective system (currently over the Midwest early this morning) arrives.
Even though the majority of hi-res guidance (nam nest, hrrr, NCEP
WRF arw/nmm, and ncar ensembles) focus the bulk of the convection
over east PA and S/c New Jersey (between the I-78 corridor and the PA
turnpike), there is still some skepticism that it may be too far
north. Accordingly, kept chance pops in the forecast for Delmarva as
A somewhat slow west-northwest storm motion (15-20 kt) that is oriented
parallel to the 800 mb warm front could potentially provide a training
setup of storms. Accordingly, there is a concern for locally heavy
rainfall and even localized flash flooding tonight. After
coordination with surrounding offices, held off on a Flash Flood
Watch since heavy rainfall will likely be very isolated. However, we
will mention this conditional threat in the severe weather potential statement.
Drier air behind the cold front should reach NE PA and northwest New Jersey late
Forecast low temperatures range from the low 60s in the Poconos to
the mid 70s in the downtown Philadelphia, at the shore and in
Long term /Friday through Wednesday/...
a prolonged period of wet weather will continue Friday and into
the weekend as a front and low pressure moving along it will be
across the middle Atlantic region. The models seem to be slower
in moving the low away Saturday, so pops have been increased for
the weekend. Overall, total rainfall when all is done will be
around 1-2 inches over nrn New Jersey and the Lehigh Valley, 2-3 inches
over the Delaware valley and srn New Jersey and 3 - 3.5 inches over Delmarva.
Thunderstorms may bring locally higher totals to some areas. Eventually,
flood watches may be issued for parts of the region, so pay attention
to future forecasts. Temperatures will be mostly below normal Friday
and into the weekend.
Improved conditions are expected Monday which should last into
the middle of next week. It's possible that future fcsts may
begin to show some possibility for some showers Monday, with the
trend of the low pulling away slower beginning to show up.
Confid in this is still limited, so we will keep the dry fcst
for now. Temperatures will be near normal Mon/Tue and then rise
to a little above normal for Wed.
Aviation /10z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
MVFR cigs have been persistent over ridge and Abe for most of the
night and intermittent at ilg. Low clouds (stratocu) will
likely expand into the I-95 terminals as mixing deepens later
this morning. Cigs should eventually increase to above 3 kft
above ground level between 18z and 21z this afternoon.
A prob30 group for thunderstorms and rain was added for most taf sites after 21z
today. The threat of storms will continue through the evening and
possibly overnight. Uncertainty in location and coverage of storms
precludes any mention of showers or storms in a prevailing group.
VFR cigs are favored for tonight (mvfr in storms) though there is
some potential for low clouds to redevelop overnight. Fog could
develop if there are breaks in the cloud cover.
A S-SW wind under 10 kt can be expected today and tonight.
Friday thru Saturday...widespread lower ceilings/visibilities with frequent
showers and tstms.
Sat night thru Monday...mostly VFR. Scattered showers Sat night
and near the shore Sunday with lower conditions possible.
a wly wind around 5 kt early this morning will become swly 5-10 kt
after sunrise, then sly 10-20 kt this afternoon and tonight. Wind
gusts could briefly approach 25 kt this evening but most areas
should remain below Small Craft Advisory owing to weak mixing profiles. Seas on the
ocean will be in the 2-3 ft range.
Fri...mostly sub-Small Craft Advisory conditions. Showers and tstms.
Fri night...winds and seas building to near Small Craft Advisory. Showers and
Sat thru Monday...Small Craft Advisory conditions expected. Showers Saturday.
After reviewing the latest trends in observations and guidance,
it was decided to raise the risk for the development of dangerous
rip currents to moderate for today. The beach day should start
off as a low risk through the morning, but the combination of
an underlying long (10 second) period southeasterly swell,
increasing southerly winds to around 15 mph and building seas
to around 3 ft this afternoon should enhance the risk to a