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Area Forecast Discussion (AFD)

Issued by Philadelphia/Mt Holly (PHI)

FXUS61 KPHI 181950

Area Forecast Discussion
National Weather Service Mount Holly NJ
350 PM EDT Wed Sep 18 2019

A ridge of high pressure will gradually build southward into the mid-
Atlantic through the end of the workweek as Hurricane Humberto 
continues to move to the northeast, away from our region. The high 
will push offshore this weekend as an area of low pressure and 
attendant cold front approach the region.


A good amount of shallow cumulus is dotting our area this afternoon, 
as peak heating combines with weak lift from a shortwave sliding 
east of our area. This cloud cover should pivot south and thin out 
tonight in the wake of the aforementioned wave. Consequently expect 
mostly clear skies (particularly over the northern 2/3rds of the 
area) overnight and this combined with diminishing winds should 
provide a good setup for radiational cooling. Lows will likely 
remain in the lower 50s in the urban corridor, Delmarva, and the 
immediate coast, while most other areas will bottom out in the 40s 
(some 30s will even be possible in the Poconos/NW NJ). 


The surface high pressure that has been centered to our north will 
move overhead by Thursday afternoon. Large-scale subsidence 
associated with the high & the upstream H5 ridge should make for a 
very dry day tomorrow (forecast PWATS are only around 0.3 inches). 
Think that even shallow cumulus will have a hard time developing in 
this environment (except maybe over the immediate coast) so would 
expect mostly sunny skies. Low-lvl flow will remain from the 
northeast for much of the day so highs should remain seasonable 
(generally in the low 70s).


Tranquil weather is expected through the remainder of the week with 
only a few marine concerns early into the forecast. High pressure 
will largely dominate our weather pattern as Hurricane Humberto 
moves to the north and east across the open Atlantic, further 
offshore. Upper level ridging will keep the high overhead for the 
most part until a shortwave trough progresses eastward across the 
Great Lakes and into the Northeast by Monday. 

Surface high pressure will dominate our weather Friday through 
Sunday with gradual warming all three days with a shift from onshore 
northeasterly winds to more of a southwesterly flow. This, combined 
with adiabatic compression will lead to significant warming. By 
Saturday, highs will likely sit some 5 to 15 degrees above average 
across the region. Sunday will likely be the warmest of the two, and 
the warmest of the next week at that with upper 80s most everywhere 
expect the higher elevations of the southern Poconos. A few 90s are 
not out of the question at this point either. 

By Monday, a shortwave trough progresses toward the eastern U.S., 
though there remains some uncertainty in its timing and 
organization. The GFS tends to keep the trough more neutrally 
tilted, while the ECMWF portrays a positive tilt with a southern 
detachment off the main trough over Canada into the central U.S. 
This significantly slows the progression of the surface low and cold 
front with this simulation, thus the timing of precipitation remains 
somewhat unresolved. The variance rides into the middle of next week 
with the placement of a deep trough and "Four Corner's" low by 
Wednesday, with the ECMWF retaining its slower approach. 


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

Rest of Today/Tonight...VFR conditions with 4-6k ft. clouds thinning 
after sunset. Winds generally northeasterly around 10 kts this 
afternoon decreasing to around 5 kts overnight.  

Thursday...VFR conditions with northeasterly winds 5-10 kts 
initially, eventually becoming light and variable in the


Friday through Sunday...Mostly VFR conditions are expected through 
this time with light winds from a prevailing westerly direction.

Monday...Generally VFR, though restriction may ensue as showers and 
thunderstorms will begin to work their way into the mid-Atlantic 
ahead of a cold front. The timing remains somewhat uncertain, 
however. Southwesterly winds from 5 to 10 knots with gusts to 15 


Seas above SCA criteria (generally 6-8 ft) on the ocean waters are 
expected to persist through the day on Thursday. Wind gusts in the 
25kt range are expected to peak tonight and then decrease through 
the day on Thursday. Conditions should remain sub-SCA for Delaware 
Bay although winds could gust up to 20 kts through the overnight 


Friday...SCA conditions likely with elevated seas 
from 5 to 8 feet. Westerly winds from 10 to 15 knots with gusts 
up to 20 knots possible in the morning hours.

Saturday...SCA conditions as seas remain above 5 feet. Westerly
winds continue from 10 to 15 knots with gusts to 20 knots at 
times during the morning hours. Winds slacken during the 
afternoon to below 10 knots. 

Sunday...sub-SCA conditions expected with southerly winds from 10 to 
15 knots and gusts to 20 knots. Seas from 1 to 3 feet.

Monday...sub-SCA conditions expected with southwesterly winds from 
10 to 15 knots and gusts to 20 knots. Seas from 2 to 4 feet.

Rip Currents...  

A high risk for the development of dangerous rip currents is 
expected today. Enhanced conditions (either moderate or high
risk) will likely remain around for several days with Hurricane
Humberto well offshore sending swells back across our waters.


NJ...High Rip Current Risk until 11 PM EDT this evening for NJZ014-
DE...High Rip Current Risk until 11 PM EDT this evening for DEZ004.
MARINE...Small Craft Advisory until 8 AM EDT Saturday for ANZ450>455.


Near Term...Carr
Short Term...Carr
Long Term...Davis