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Area Forecast Discussion (AFD)

Issued by Philadelphia/Mt Holly (PHI)

FXUS61 KPHI 260145

Area Forecast Discussion
National Weather Service Mount Holly NJ
945 PM EDT Tue Jun 25 2019

High pressure will remain to our southwest Wednesday and Thursday. 
The high will remain there into Friday as well, but by Friday a mid-
level disturbance will also pass just north of the region. An upper 
level trough and surface frontal system will slowly approach from 
the north over the weekend, with a cold front likely moving through 
on Sunday. High pressure is expected to build back in from the 
northwest early next week.


Weak cold front continues to work its way offshore. A few 
sprinkles and even a light shower passed across Sussex County, 
NJ, but has since dissipated this evening. Drier air will 
continue to infiltrate the region, providing for a refreshing 
night with a break from the higher humidity. Lows will be in the
60s for the most part across the area. Winds will decrease back
to around 5 knots overnight. Some patchy fog is possible in the
river valleys and near the coast as radiational cooling takes 
place. This should burn off shortly after sunrise.


Some patchy fog is possible in the river valleys and near/along
the coast in the morning, but this should burn off by 8 am. Fair
weather will continue Wednesday across our region. Weak high 
pressure will build south to north while upper heights also 
build overhead. Overall a dry day is expected. Some of the 
models are depicting some showers/tstms across NW PA and NY that
may track SE towards the southern Poconos during the late 
afternoon. Held off of adding these to the grids at this moment,
but there is a chance for a quick shower and/or thundershower
across the northern counties tomorrow afternoon. Most showers
and thunderstorms will have a difficult time progressing east of
the Appalachian Plateau with northwesterly downslope flow



A typical summertime pattern takes hold over the CONUS as we close 
out the month of June. An upper level flow that is initially fairly 
zonal will become more amplified as a trough digging into the West 
helps force the building of a downstream ridge over the central US. 
This ridge will initially extend to the East Coast on its eastern 
flank, with high pressure centered south of the mid-Atlantic. This 
will lead to hot weather and a potential heatwave for the end of the 
work week. Heading into the weekend, changes occur as a trough 
dropping south out of Canada will act to erode the eastern reaches 
of the ridge. This will bring some unsettled conditions for the 
weekend. By early next week, the trough pulls away, and high 
pressure should build in from the northwest. This should keep Monday 
and most of Tuesday dry with a frontal system potentially 
approaching beyond then.


Wednesday night-Thursday night... Dry conditions and lows generally 
a little above average Wednesday night. High pressure centered just 
to our southwest and rising mid-level heights will yield a warm day 
Thursday, with highs in the upper 80s and low 90s. 25.12z NAM 
indicated thunderstorm development over southeast PA and Delmarva in 
the afternoon, but other guidance is far less bullish. Given the 
synoptic pattern just described, believe the NAM is in error and 
expecting dry conditions to prevail. A dry night as well with 
overnight lows also a little on the warm side of average, very 
similar to the prior night.

Friday-Friday night... A shortwave will pass to our north Friday 
afternoon into Friday night. Nothing too impressive, and in fact it 
looks to be deamplifying and shearing out as it approaches, not a 
surprise considering the ridge will be continuing to build Friday 
with high pressure remaining to our south. Friday looks to be the 
warmest day of the week, with many locations likely to crack 90 and 
some mid 90s possible in the urban I95 corridor. Dew points look to 
stay mainly in the mid 60s, so a bit sticky but not oppressively 
humid. The very warm and marginally humid air combined with modest 
lift from the shortwave will lead to the possibility of showers and 
thunderstorms developing late in the day and overnight. Currently 
does not look like anything too well organized, but scattered 
showers and storms will be possible for the late afternoon, evening, 
and overnight especially in the northwestern half of the area.

Saturday-Sunday... As mentioned, a trough will be dropping down from 
out of Quebec and into New England over the weekend. Weak low 
pressure and an attendant cold front will exist at the surface, and 
this system will begin to approach the area, moving almost due 
south. On Saturday, with temperatures remaining warm ahead of the 
front and dew points climbing to near 70 in developing southwest 
flow, it appears a pre-frontal trough could serve as a focus for 
scattered showers and thunderstorms in the afternoon and evening. By 
Sunday, the actual cold front approaches, and in fact that may pass 
through as early as Sunday morning. If the timing of the front is 
slower and we heat up again on Sunday, we would see another round of 
showers and thunderstorms. Cannot rule out the potential for some 
stronger storms and locally heavy rain over the weekend, though the 
signals for either aren't overly strong. A good deal of fine tuning 
will be needed here, as the biggest uncertainty right now relates to 
the timing of the frontal system. Models struggling a bit with an 
anticyclonic wave break of the ridge over the central US and how it 
will impact the Canadian trough to its east, which will have impacts 
on the surface low moving offshore and intensifying. Not expecting a 
weekend washout, but scattered showers and storms will be possible 
both days at least in parts of the area.

Monday-Tuesday... The trough to our northeast will slide away early 
next week, and weak high pressure looks to build in. Temperatures 
should be seasonably warm Monday as the trough and associated cold 
pool pull away, and am confident in dry conditions with the high 
building in. Fairly wide spread in guidance by Tuesday, though warm 
and dry conditions will probably continue for at least most of the 
day. However, it appears we will have a frontal system approaching 
by later Tuesday or Wednesday as the ridge to the west will have 
flattened out from the wave breaking evolution over the weekend, 
opening the door for that system to track a little closer to us.


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

Tonight...VFR conditions expected with only some high level 
cloudiness expected. West to northwest winds will decrease to less 
than 5 knots in most areas. 

Wednesday...VFR expected with mostly sunny skies during the morning 
and some daytime Cu during the afternoon. Winds will be mostly NW to 
W at 5 to 10 knots. There may be a sea breeze affecting KACY during 
the afternoon. 


Wednesday night-Thursday night... VFR. Winds very light 
but mainly from the west.

Friday-Friday night... Mainly VFR, but a shower or thunderstorm is 
possible from late afternoon through the overnight especially 
north of PHL. 

Saturday-Sunday... VFR overall, however, showers and thunderstorms 
are possible both days. West-southwest winds of 5 to 10 kt Saturday 
becoming northwest Sunday with gusts to 20 kt possible.


A cold front has crossed the waters as of late afternoon. Winds will 
remain W to NW overnight at 5 to 10 knots. Fair weather is expected. 
Wednesday, a continuation of fair weather with northwest winds 
around 10 knots during the morning and then light onshore winds for 
the afternoon.


Wednesday night-Friday... Sub-SCA conditions with light 
winds and seas 1 to 2 ft.

Friday night-Sunday... Sub-SCA conditions expected to continue with 
seas 2 to 3 ft. A risk of scattered showers and thunderstorms exists 
for much of this period. Southwest winds of 10 to 15 kt Saturday. 
Northwesterly winds develop Sunday with gusts to 20 kt possible.

Rip Currents...

A low risk of rip current development will continue into Wednesday
as winds turn very light and waves in the surf zone diminish to
only around one foot.




Near Term...Davis/O'Hara
Short Term...Davis/O'Hara
Long Term...O'Brien