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FXUS61 KPHI 141938

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

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.


It is a beautiful day out there with temperatures in the low 70s, 
relative humidities generally in the 30-40% range, and abundant 
sunshine. Although a weak cold front will move through the area 
later this evening, it will be a dry frontal passage as the mid-lvl 
wave and associated dynamics will pass well north of the area. 
Overnight mins will run several degrees colder than last night, due 
to the lack of low clouds, with mid-upper 30s possible in the 
Poconos/NW NJ and 40s elsewhere. Although some fog in the prone 
river valleys of eastern PA and NW NJ can't be ruled out, the 
relatively dry airmass should prevent anything widespread from 


Another pleasant day is expected tomorrow as high pressure shifts 
overhead before moving offshore tomorrow 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.


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 

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 

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.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, 
KMIV, KACY and surrounding areas.

This afternoon/tonight...VFR conditions will prevail overnight. 
Westerly winds this afternoon will shift to the northwest/north 
behind a cold front this evening, however speeds will largely 
decrease to 5 kts or less. High confidence. 

Tuesday...VFR conditions, with variable (perhaps beginning to favor 
a SE direction Tuesday afternoon) winds 5 kts or less. High 


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.


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 
tomorrow afternoon.


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.




Near Term...Carr

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