NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Jun 8, 8:27 AM EST, 3 hours 3 minutes earlier.
Area forecast discussion, Jun 8, 8:27 AM EST, 3 hours 3 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
927 AM EDT Tue Jun 8 2021
.SYNOPSIS...
High pressure remains offshore through the week with a surface
trough in parts of our area through Wednesday. A cold front moves
into our area Wednesday night into Thursday, before sinking to the
south Thursday night and stalling there into the weekend. Some areas
of low pressure track along this boundary before it moves out to
sea. Another cold front approaches early next week.
&&
.NEAR TERM /THROUGH TONIGHT/...
As of 915 AM, a hot and humid airmass remains in place. The 12z
Dulles RAOB shows a moist environment with already a decent amount
of instability. The 12z Upton RAOB is similar however there is some
drier air centered around 850 mb. Both RAOBs have a freezing level
just above 14,000 feet. The winds through the column are 20 knots or
less. The water vapor imagery shows a mid level low in the vicinity
of Ohio with a weaker short wave from Lake Ontario southward across
central Pennsylvania. A surface analysis placed a trough from
southeastern New York to central Virginia.
The forecast is in good shape overall. Did trim the eastward extent
of the PoPs for a time as convective development should occur first
closer to the surface trough then near a sea breeze. Forcing is on
the weaker side and the surface trough may shift eastward through
the afternoon in combination with the heating of the boundary layer.
This may become the main focus for convective development.
Otherwise, the airmass is unstable once again with a mesoanalysis
already showing 1,000-1,500 J/kg of MLCAPE. The wind field is light
and therefore wind shear is rather weak, which combined with no
organized forcing for ascent should result in pulse type storms with
additional development initiated by convective outflows.
Precipitable water values are nearing 2 inches, via morning RAOBs
and mesoanalysis. Storm motions will also be very slow due to the
weak wind fields. Consequently, biggest concern for today is locally
very heavy rain, but it is conditional on where storms develop and
how long they are able to sustain (weak wind shear should mean they
are relatively short-lived). Given the instability profiles, some
small hail is possible although freezing levels are just above
14,000 feet (warmer temperature profiles limit any severe hail
threat), and potential for wet microbursts/gusty winds especially if
there is some drier air aloft.
With no change in the air mass, expect another hot and humid day.
Temperatures may be slightly lower than Monday thanks to increased
cloud cover. However, much of the guidance may be overestimating the
effect of cloud cover on temperatures today. Thus, have gone on the
higher side of guidance with highs in the upper 80s to lower 90s.
Dewpoints should stay in the 70s. Thus, most of the coastal plains
should see heat index values in the mid to upper 90s. Consequently
the Heat Advisory continues for the urban corridor through early
this evening.
For tonight, expect similar temperature conditions to what we
have seen the last several nights. The one possible exception is
that fog will be more likely anywhere that has a downpour
during the day. The mid level short wave trough doesn't get
fully east of our region until after 06Z, so there is a
potential for pulse showers and storms to continue through the
evening.
&&
.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
Above normal temperatures and unsettled weather will continue
through the short term period.
On Wednesday, a lee-side/thermal trough will be in place across
the Mid Atlantic region, while a couple of short wave/vorticity
impulses are forecast to move across the area as well.
Meanwhile, PW values will remain around 1.75-2.00 inches and
CAPE values increase to 1000- 2000 J/kg. This will allow showers
and thunderstorms to develop across the area, with the
potential for heavy rainfall. Mid level flow is rather weak with
700-500 mb winds of 20-30 knots or less, so there is not much
shear forecast. Therefore, severe weather is not expected,
although any storms that do develop could have locally gust
winds and dangerous lightning.
On Wednesday night, a cold front is expected to sag southward
into the area from the north. This will allow additions showers
and isolated thunderstorms to develop across the area as the
short wave/vorticity impulses finally cross the area overnight
as well. Again, severe weather is not expected, but any showers
or thunderstorms could produce heavy rain.
By Thursday, the cold front is expected to stall across the Mid
Atlantic region, before an area of low pressure develops along
the front to or west overnight Thursday. With the front stalled
near the area, as instability develops during the day, there
will be a chance of showers and thunderstorms during the day. As
the low pressure develops along the front overnight and moves
eastward, along with a couple of short wave/vorticity impulses,
there will remain a chance of showers and thunderstorm overnight
as well. PW values will range from 1.75-2.25 inches, so the
heavy rain will continue to be a concern.
Given the low FFG values because of the all the recent
rainfall, flooding can not be ruled out, and much of SEPA, NJ,
and DelMarVa remain in a Marginal Risk for WPC's Excessive
Rainfall Outlook.
&&
.LONG TERM /FRIDAY THROUGH MONDAY/...
Unsettled weather is expected for much of the long term period,
with a return of below normal temperatures for the end of the
week through the weekend.
On Friday into Friday night, a couple of low pressure systems
will continue to move eastward along the frontal boundary that
will be stalled out near the Mid Atlantic region, which will
finally move the front to our south and offshore Friday night
into Saturday. However, before the front and low pressure
systems move away from the area, there will continue a chance of
showers and thunderstorms for much of the area Friday into
Friday night. PW values remain between 1.75-2.25 inches during
the day Friday, before dropping overnight Friday. Therefore,
there will continue to be a concern for heavy rain during the
day Friday, before rain chances diminish overnight.
A brief drying period could return on Saturday into Saturday
night as high pressure briefly builds across the Mid Atlantic
and Northeast region. However, this dry period may be brief as
another frontal boundary could affect the area around Sunday or
Monday of next week. There is some disagreement in the timing
and strength of the front, so we will stay close to NBM forecast
and keep a slight chance of showers in the forecast Sunday
through Monday.
&&
.AVIATION /14Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Today...VFR overall. Some showers and thunderstorms mostly this
afternoon, which will result in local restrictions at times.
Southwest to west-southwest winds 5-10 knots (more westerly at ABE
and RDG), potentially becoming more southerly at ILG and ACY due to
a sea/bay breeze. Low confidence on the timing details of showers
and storms at each terminal.
Tonight...Mostly VFR, however some fog will be possible especially
for any locations that had earlier downpours. Southwesterly winds
around 5 knots, becoming light and variable. Low confidence on the
extent and timing of any fog.
Outlook...
Wednesday-Wednesday night...Generally VFR conditions expected.
Showers and thunderstorms expected which could temporarily lead
to lower conditions. West to northwest winds 5-10 knots from
around I-95 corridor northward, west to southwest south of I-95
corridor. Wind shift to north to northeast overnight. Moderate
confidence.
Thursday...Mostly VFR conditions expected. Showers and
thunderstorms likely which could temporarily lead to lower
conditions. East to northeast winds 5-10 knots. Moderate
confidence.
Thursday night...Conditions lower to MVFR then IFR overnight.
Showers and thunderstorms likely. East to northeast winds 5-10
knots. Moderate confidence.
Friday-Friday night...IFR conditions during the day, improving
to MVFR overnight. Showers likely, especially during the day.
East to northeast winds 5-10 knots.
Saturday...Conditions improve to VFR, with some MVFR ceilings
possible. A slight chance of showers. Northeast winds 5-10
knots, shifting to east to southeast later in the day.
&&
.MARINE...
Winds and seas should stay below Small Craft Advisory
Conditions today and tonight. However, higher winds and seas
will be possible in the vicinity of thunderstorms this afternoon
and evening.
Patchy fog is possible very late tonight.
Outlook...
Wednesday-Thursday night...Conditions expected to remain below
advisory levels. A chance of showers and thunderstorms which
could lead to locally higher winds and waves.
Friday-Saturday...Winds and waves could reach Small Craft
Advisory levels for some areas.
Rip Currents...
A LOW risk for the development of dangerous rip currents
continues today and Wednesday for the New Jersey and Delaware
beaches, with just modest southerly flow and waves around 2 feet
in the surf zone.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Heat Advisory until 8 PM EDT this evening for PAZ070-071-102-
104-106.
NJ...Heat Advisory until 8 PM EDT this evening for NJZ015-017>019.
DE...Heat Advisory until 8 PM EDT this evening for DEZ001.
MD...None.
MARINE...None.
&&
Synopsis...Gorse/Robertson
Near Term...Gorse/Johnson
Short Term...Robertson
Long Term...Robertson
Aviation...Gorse/Johnson/Robertson
Marine...Johnson/Robertson