Scientific Forecaster Discussion
fxus61 kphi 211435
Area forecast discussion
National Weather Service Mount Holly New Jersey
1035 am EDT Thu Jun 21 2018
low pressure and its associated cold front will move offshore and
south of the region today. A secondary cold front weakens as it
crosses the area tonight, followed by brief high pressure Friday.
Meanwhile, a low pressure system organizing over the Mississippi
Valley will lead to a warm frontal passage on Saturday and a cold
frontal passage on Sunday. Canadian high pressure builds into the
region on Monday and moves off the eastern Seaboard by mid-week.
Near term /until 6 PM this evening/...
the cold front is across the srn areas attm. Scattered showers
continue across southeast PA, srn New Jersey and nrn Delmarva as of the late
morning. Drying is seen crossing southeast over the srn Poconos and nrn
New Jersey. This improving sky trend will continue southeast through the
afternoon. Low chc pops for most areas thru 16z-18z.
Behind the cold front, expect only a modest (<5 degrees) cool down
as compared to yesterday. However, the bigger impact will be with
dew point temperatures, as behind the front dew points are expected
to be in the 50s across the region (compare with dew points in the
60s and 70s early this morning). Consequently, there won't be much
difference between the heat index and air temperature today.
Short term /6 PM this evening through 6 am Friday/...
a secondary cold front is expected to sink south tonight. However,
it will be rather diffuse as it does so. Expecting mostly dry
conditions across the region, except in Delmarva where the first
front could continue to provide a focus for shower development if it
stalls over northern Delmarva.
Onshore flow developing late tonight may promote low clouds to
spread over the coastal plains late tonight.
For areas west of the fall line, lower dew points and clearing skies
will mean low temperatures in the 50s and lower 60s.
Long term /Friday through Wednesday/...
highlights include widespread precipitation late Friday into the
first half of the weekend, with a chance of strong thunderstorms on
Saturday. Additional rounds of showers with embedded thunder are
likely Sunday. A drying trend commences Sunday night with fair
weather expected through Wednesday. While an onshore at times will
make for cooler temperatures down the shore, for the remainder of
the area, temperatures at or slightly above normal through the
Synoptically, a closed low over the middle Mississippi Valley will
eject northeastward into the eastern Great Lakes by Friday. This
sets the stage for a deep layer southwest flow Friday night into
Saturday, until the shortwave associated with the aforementioned low
moves across the region. The longwave trough will finally traverse
the area on Monday, and will be followed by broad mid-level ridging
Friday...onshore flow develops between high pressure south of Nova
Scotia and a northward advancing warm front to our south. This will
promote an overunning regime, but with high pressure providing a
source of dry low-level air, precipitation will initially struggle
to enter the region. The GFS is the most progressive in bringing
precip across the region during the day, while the NAM is too dry,
and the European model (ecmwf) and rgem are toward the middle of the envelope. The
synoptic setup supports the latter two solutions, so continued to
confine chance pops southwest of the I-42 to I-476 corridors. With the
proximity of the warm front and increasing instability, continued a
slight chance of thunder across the southern Maryland and Delaware during the
afternoon and evening.
Friday night through Saturday night...above the surface, a deep
southwesterly flow out ahead of the next shortwave will advect
precipitable water values up to 2 inches (1 to 2 Standard dev above
normal) into the region. Expect strong isentropic ascent just north
of the warm front to produce widespread rainfall Friday night, which
will be heavy at times, leading to the potential for poor drainage
flooding. By Saturday, the warm front will struggle to move north of
the area bounded by the I-276 to I-95 and I-78 corridors. Near and
especially to the south of this area, there will be the potential
for strong to possibly severe thunderstorms. This is supported by
surface based cape values up to 2000 j/kg and bulk shear around 40
kts. In addition, 0-3 km lapse rates will steepen, with soundings
exhibiting an inverted-v profile, albeit weak. Therefore, storms
will have the potential to produce strong winds. Thunderstorms near
the warm front where LCLs are lower will also have to be watched for
rotation, given moderate 0-1 km shear.
Sunday...a chance of showers and isolated thunderstorms with the
cold front off to the west.
Sunday night through Wednesday...
Showers and storms with the cold front discussed above will
sweep through the area with the front Sunday night into early
Monday followed by clearing skies by late Monday along with
falling dew points and temperatures near seasonal. Beyond this
time, a sprawling area of high pressure looks to build in from
the north bringing mainly clear skies with seasonable
temperatures and comfortable humidity levels through mid week.
Aviation /15z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Today...showers lingering across srn New Jersey/southeast PA and nrn Delmarva
this morning. Improving sky conditions northwest. These improving
conditions will continue to build southeast and arrive late this
morning and early afternoon. At most taf sites, winds should be
light and north-northeast through the day. However, a sea breeze is likely
to develop this afternoon and could affect kacy and kmiv,
turning winds to southeasterly mid to late afternoon.
Tonight...conditions should start as VFR. However, there is a chance
for MVFR ceilings especially at kacy and kmiv after 06z. Winds may
start out light and variable this evening, but settle out of the
east after 06z. High confidence for the forecast at most taf sites,
moderate confidence in the forecast for kmiv and kacy.
Friday...predominantly VFR. Cannot rule out brief MVFR ceilings in
rain showers. Easterly wind gusts up to 20 mph, especially Acy and phl.
Friday night...low clouds and potential fog will lead MVFR, with
IFR possible, especially late. Easterly wind gusts up to 20 mph.
Saturday...conditions gradually improving to MVFR, with VFR possible
by afternoon, especially I-95 corridor terminals. But rain showers and thunderstorms and rain
will lead to brief ceiling/visibility restrictions. Southwest winds.
Saturday night...a return to low clouds and potential fog with
MVFR, and even IFR possible. Winds shifting to the northwest.
Sunday...any low clouds and fog will dissipate with a return to
mainly VFR until a late day risk of storms could bring brief
restrictions. Winds west-southwest. Medium confidence.
Sunday night...showers and thunderstorms possible along with
brief ceiling/visibility restrictions. Low confidence.
Monday...becoming mainly VFR. Medium confidence.
winds and seas should remain below Small Craft Advisory criteria
today and tonight. However, wind gusts near or just above 20 kt will
be possible very late tonight.
low risk expected on Thursday.
A moderate risk for the formation of dangerous rip currents is
outlooked Friday and Saturday due to easterly wind gusts up to 25
mph and wave heights building up to around 5 ft in the surf zone.
a Small Craft Advisory was issued from noon Friday through
Friday night for the Atlantic coastal waters south of Manasquan
Inlet New Jersey to Fenwick Island Delaware. During this time frame, the
pressure gradient between high pressure to the north and an
approaching warm front to the south will lead to easterly wind
gusts around 25 kt and seas 4-6 ft.
North of Manasquan Inlet to Sandy Hook NJ, Small Craft Advisory criteria is not
expected to be reached until late Friday afternoon/evening, so
this area will likely need to be added in subsequent forecast.
As the warm front moves across the waters, a brisk southwest
flow may lead to continued Small Craft Advisory level seas through Saturday.
A return to more tranquil, sub-Small Craft Advisory boating conditions is
expected Sunday into mid next week.
with a persistent onshore flow developing Thursday night and
continuing through early Saturday, increasingly positive
departures from astronomical tides are expected. The guidance
has also trended upward over the past day. The high tide of most
concern is Friday night, when minor flooding is possible along
the oceanfront of Delaware and New Jersey.
Marine...Small Craft Advisory from noon Friday to 6 am EDT Saturday for