Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 142314
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
714 PM EDT Mon Oct 14 2019

.SYNOPSIS...
A weak cold front will move offshore tonight, with high pressure
moving through the region on Tuesday. A strong low and attendant
cold front will progress through the eastern U.S. on Wednesday. As
the low intensifies in New England on Thursday, strong northwest
flow will become established across the Mid-Atlantic. High pressure
returns to the region for the weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
A weak cold front moving through Pennsylvania will pass through
the region around midnight tonight. It will pass through dry, so
not even expecting much cloud cover with its passage. Winds will
be light, turning north late, but becoming nearly calm in the
pre-dawn hours.

Overnight mins will run several degrees colder than last night,
due to the lack of low clouds. Mid-upper 30s are possible in
the Poconos/NW NJ and 40s elsewhere. Although some fog in the
prone river valleys of eastern PA and NW NJ cannot be ruled
out, the relatively dry airmass should prevent anything
widespread from developing.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
Another pleasant day is expected Tuesday as high pressure
shifts overhead before moving offshore Tuesday afternoon. Highs
will be slightly cooler than today due to tonight`s cold front,
with most locations seeing highs around seasonal norms
(generally in the mid 60s). Expect abundant sunshine (apart from
some passing cirrus) with generally low humidity values.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
All eyes are on the soaking rain event for Wednesday, followed
by the potential for windy conditions on Thursday.

The 12z operational model runs continue to advertise a potent
midlevel vortex moving from the Great Lakes region Tuesday night
to the Northeast on Wednesday night. A small-scale vort max will
allow the larger-scale vortex to pivot during the day Wednesday,
quickly re-orienting the trough to a neutral tilt on Wednesday
morning and a negative tilt by Wednesday evening. During this
evolution, a predecessor surface low will migrate from the Great
Lakes Tuesday night to the Saint Lawrence Valley by Wednesday
afternoon. A cold front will extend south of the surface low,
with widespread rain near and downstream of the front in a
region of considerable differential cyclonic vorticity advection
and isentropic ascent. As sustained/deep ascent continues along
the front, triple-point low development will ensue near the nose
of a 500-mb jet streak (during the evolution from neutral to
negative tilt of the upper-level trough). Models continue to
show some discrepancies on where this new low will develop and
ultimately track. Nevertheless, some convergence was noted among
the CMC, NAM, GFS, and ECMWF today. This provides increased
confidence in using a consensus approach to most sensible
weather fields during this period.

The biggest uncertainty (as usual) is with QPF, with the GFS
much drier than the ECMWF/NAM (roughly middle of the road), and
the CMC much wetter. This is clearly tied to how close the newly
developing low tracks to the coast, with the CMC nearest the
CWA and the GFS/NAM much farther offshore. The ECMWF is well
between the two extremes and agrees with ensemble means as well.
Given these factors, used a blend of model QPF for the
Wednesday/Wednesday night forecast as well, but with slightly
more weight to the ECMWF and NAM. The result is widespread 1-2
inch totals (highest amounts north and east). Notably, the GFS
features totals about 50-75 percent of these values, with the
CMC in the range of 2-4+ inches. (In other words, large
uncertainty remains.) I also increased PoPs a little bit
Wednesday night, with indications from some of the guidance of a
slower progression of the front/precipitation than previous
simulations.

Given improved consensus on the evolution of the low and
universal indications of soaking rain, increased PoPs to
categorical on Wednesday into Wednesday evening for virtually
the entire CWA. I also did indicate slight chances for thunder
during the day as well, which is most likely if the low tracks
closer to the coast (a la the CMC/ECMWF). This agrees with
neighboring offices as well as the SPC Day-3 outlook.

Once the triple-point low lifts northeastward and stalls as the
system begins to fill on Thursday, the region will become
entrenched in strong northwest flow on the upstream side. BUFKIT
soundings indicate mixing to 800 mb, with gusts of 30 to 40 mph
likely in such an environment. Bumped winds up some more with
the forecast Wednesday night through Thursday night. Forecast is
shy of wind advisory criteria, but we may see some issues with
trees and power lines given (1) the antecedent rains weakening
soils and (2) leaves remaining on the trees.

Temperatures are expected to be much colder Thursday as strong
cold advection encompasses the region. However, this may be
tempered to some degree by strong mixing and improving sky
cover. Farther north, wraparound moisture may linger long enough
for some residual showers (generally north of I-80). The ECMWF
continues to trend this direction (with hints of this from other
models too). Increased PoPs and lowered temperatures in this
region.

High pressure builds into the region to close the week into the
weekend. Should see dry conditions and warming temperatures
through this period. The next system looks to affect the region
early next week, with another decent rain event possible.

&&

.AVIATION /23Z MONDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight...VFR/SKC. Light west winds will become north by
midnight. Wind speeds will generally be 5 kt or less.

Tuesday...VFR/SKC. LGT/VRB winds, becoming S 5 kt or less in the
afternoon. High confidence.

Outlook...

Tuesday night...VFR with light southeast winds. High
confidence.

Wednesday and Wednesday night...Quickly deteriorating
conditions during the day with CIG/VSBY restrictions and rain
likely. South to southeast winds 5 to 15 kt becoming west to
northwest 10 to 20 kt with higher gusts possible (especially
overnight). Moderate confidence.

Thursday...Mainly VFR (though some sub-VFR could linger north
of PHL) with strong northwest winds 15 to 25 kt with gusts to 35
kt. Moderate confidence.

Thursday night and Friday...Mainly VFR with northwest winds 5
to 15 kt (possibly gusty Thursday evening). Moderate confidence.

Friday night and Saturday...Mainly VFR with light and variable
winds Friday night becoming light west or southwest on Saturday.
Moderate confidence.

&&

.MARINE...
Sub-SCA conditions will prevail through Tuesday with fair weather.
Seas will run around 2-3 ft, with initially SW winds shifting to the
North around 10-15 kts tonight before becoming light and variable
Tuesday afternoon.

Outlook...

Tuesday night...Sub-advisory winds/seas expected.

Wednesday...Quickly deteriorating conditions with southeast
winds increasing to 10 to 20 kt with higher gusts and seas
building to near 5 feet. Rain likely.

Wednesday night...Rain likely, especially in the evening. Winds
switching to northwest and increasing to 15 to 30 kt with higher
gusts. Gales possible, especially late.

Thursday...Gale conditions likely.

Thursday night...At least advisory-level northwest winds likely,
with lingering gales possible in the evening.

Friday...Lingering advisory conditions possible, but northwest
winds are forecast to diminish below criteria by afternoon.

Friday night and Saturday...Sub-advisory conditions expected.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...CMS
Near Term...Carr/MPS
Short Term...Carr
Long Term...CMS
Aviation...CMS/Carr/MPS
Marine...CMS/Carr/MPS


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