FXUS61 KPHI 191441
Area Forecast Discussion
National Weather Service Mount Holly NJ
1041 AM EDT Tue Jun 19 2018
A cold front settles south of our area by later today and stalls,
then weak high pressure slides to our north through tonight. A weak
area of low pressure tracks along the front and crosses our region
Wednesday night into early Thursday. High pressure builds in
Thursday night, then shifts offshore Friday. A warm front lifts
across the region later Saturday, followed by a cold front during
Sunday. High pressure should then start to build in during Monday.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The cold front has made it into our area as evident of the
decreasing dewpoints advecting into the area. Dewpoints in the
upper 50s to lower 60s moving in and will continue spread
southward as the front advances across the area. The shower
activity ahead of the front has weakened through the
morning hours. However, as daytime heating continues through the
afternoon, instability will develop south of the front into this
afternoon across extreme southern New Jersey, central/southern
Delaware, and eastern shore of Maryland. This will allow for a
few isolated/scattered showers and thunderstorms to develop
through the afternoon hours as a short wave approaches from the
west. Any showers/thunderstorms that do develop will be capable
of producing brief heavy downpours and locally gusty winds.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
High pressure builds in from the Great Lakes-upstate NY area
tonight. North-northeast winds around the surface high will sustain
the transport of cooler and drier air into the region.
Low temperatures range from the mid 50s in northeast PA and
northwest NJ to the mid and upper 60s in the I-95 urban areas and in
the coastal plain. High clouds (convective debris from Midwest
convection) may spread into the area late tonight.
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Summary...Unsettled late Wednesday into early Thursday; turning more
humid and potentially unsettled over the weekend; some cooling early
Synoptic Overview...An upper-level trough is forecast to amplify
across southeastern Canada and the Northeast later Wednesday and
Thursday. This trough lifts out for the end of the week with some
ridging arriving ahead of an upper-level trough. This trough looks
to arrive over the weekend as it may eventually merge into one main
upper-level trough once again over southeastern Canada and the
Northeast. This may then tend to keep our region within cyclonic
flow through early next week. Overall, the pattern looks progressive
with no sustained high heat and humidity.
For Wednesday and Thursday...As an upper-level trough amplifies
across the Northeast from southeastern Canada, some short wave
energy results in weak low pressure developing along a stalled west
to east frontal zone which is to our south. These features track
eastward and move across our region late Wednesday before shifting
out to sea Thursday morning. Most of the shower activity looks to
occur Wednesday night into early Thursday morning, coinciding with
an area of ascent and convergence that moves across the forecast
area. A plume of precipitable water values to around 2 inches is
forecast to advect northward across about the southern half of our
area Wednesday night. This may result in locally heavy rain,
especially where stronger convective elements occur. There is mostly
elevated instability forecast to be present, therefore will maintain
a thunder mention for a time. The PoPs were raised to likely for
a good portion of our area Wednesday night.
As the weak surface low shifts out to sea during Thursday, high
pressure from near and north of the Great Lakes builds eastward.
This allows for drier air to advect across our area, however a north
to northeast low-level flow may delay the clearing longer especially
for the southern areas. The presence of a nearby upper-level jet may
also keep some mid to high level clouds around for awhile. The upper-
level trough is forecast to lift out Thursday night.
For Friday...As the upper-level trough lifts farther away from our
region, a short wave ridge builds east on Friday. This allows
surface high pressure to build across New England and also down into
the northern Mid-Atlantic region. A frontal zone will be positioned
well to our south and an onshore flow looks to result. This will
likely keep the coastal areas cooler. As the next upper-level
trough, which may be a closed low, moves into the Ohio and Tennessee
Valley's, warm air advection is forecast to increase toward our
region late in the day and especially at night. This will help to
strengthen the frontal zone as a warm front. This warm front should
be approaching our far southern areas late Friday night. The best
lift, forcing and instability looks to be west and southwest of our
area, therefore only carried some slight chance PoPs across the
western and southern areas at night.
For Saturday and Sunday...As an upper-level trough or closed low
lifts across the eastern Great Lakes Saturday, it should weaken and
become absorbed into a larger upper-level trough that starts
amplifying across southeastern Canada. This drives surface low
pressure across the eastern Great Lakes later Saturday, pulling a
warm front northward across our region. Some showers and
thunderstorms may accompany the warm frontal passage during the day
Saturday followed by a possible pre-frontal trough Saturday night. A
cold front is currently forecast to move through our area during
Sunday. The main upper-level trough amplifying into the Northeast
Sunday may hold back the stronger forcing behind the actual cold
front. Therefore, the coverage and intensity of the convection is
less certain as the more organized convection may focus across
Delmarva southward. The incoming trough looks decent, therefore will
have to monitor for the potential for a more robust convective setup.
For Monday...The amount of clearing and drying that takes place will
depend on the timing and sharpness of an upper-level trough in the
Northeast as it shifts eastward. A cold front is forecast to be
settled to our south, and as of now it is far enough south to
probably result in a dry day. It is possible the boundary hangs up a
bit farther north as a wave of low pressure develops on it, however
this is less certain. We opted to carry a slight chance PoP for now
in the gridded forecast.
.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Today...Predominately VFR. A brief period of MVFR CIGS has
developed as a cold front continues to sag through the area and
will affect southeastern Pennsylvania, southern New Jersey,
Delaware and Maryland through the late morning into the
afternoon. The lower CIGS should only last an hour or two at
each location. Showers have dissipated, and any showers this
afternoon are expected to develop south of the TAF sites.
Winds have shifted to the northwest this morning and will remain
out of the northwest through this afternoon. Wind speeds are
generally around 8-10 knots, with some gusts in the 15-20 knot
range. Winds will veer out of the N late this afternoon.
However, a sea- breeze will lead to a much more abrupt wind
shift out of the E-NE this afternoon at ACY and possibly at MIV
toward the early evening.
Tonight...VFR. N-NE winds under 10 kt.
Wednesday and Thursday...VFR during the day with increasing clouds,
then MVFR/IFR conditions should develop at night as some showers and
possible thunderstorms move through. The conditions should improve
to VFR during Thursday morning.
Saturday...Developing MVFR conditions are probable as a warm front
lifts across the area with a chance of showers and thunderstorms.
Southerly winds may gust up to 20 knots during the day.
Winds and seas below SCA criteria today. W-SW winds 5-10 kt with
gusts to 15 kt early this morning will become NW by late this
morning and then NE this afternoon. Seas in our coastal waters of 3-
4 ft early this morning should gradually diminish to around 2 ft
Wednesday...The conditions are expected to be below Small Craft
Advisory criteria. Some increase in the winds may occur for a time
especially in the afternoon and evening however gusts should be
at or below 20 knots.
Thursday and Friday...The conditions are anticipated to be below
Small Craft Advisory criteria.
Saturday...A warm front is forecast to lift north of the area late.
Southeast to south winds may gust to around 20 knots at times. There
is a chance of showers and thunderstorms.
A low risk for the development of dangerous rip currents is expected
at the NJ and DE beaches today with waves in the surf zone of 2-3
ft and a light medium-period (6-9 sec) south to southeasterly swell.
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