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Area Forecast Discussion (AFD)

Issued by Philadelphia/Mt Holly (PHI)

FXUS61 KPHI 251342

Area Forecast Discussion
National Weather Service Mount Holly NJ
942 AM EDT Sun Aug 25 2019

High pressure located across New England and southeast Canada will 
weaken through Tuesday. Low pressure will move into Ontario and 
cause a cold front to cross our area during the middle of the 
upcoming week. More high pressure will arrive for Thursday and 
Friday. Another front may affect the weather next weekend.


930 am update: Cloud cover is starting to fill in as the onshore
flow increases. Therefore, I increased cloud cover from the
Delaware Valley and further east through the morning. Otherwise,
no significant changes to the forecast.

Previous discussion...

Models have really struggled with the evolution of a midlevel vortex 
atop the Northeast this weekend. The mid/upper low is quite a bit 
farther south than yesterday's simulations were suggesting, and it 
has been slower to move eastward as well. This has had subtle but 
important effects on surface flow and may have noticeable impacts on 
the weather today, especially near the coast. For starters, winds 
have been slow to develop a more easterly component. Most surface 
observations early this morning are between 340 and 30 degrees, 
which is not very conducive for a surge inland of lower clouds and 
drizzle/light showers. Radar indicates some sprinkles or light 
showers off the New Jersey coast early this morning, but these have 
shown little progress inland. As the upper low presumably shifts 
eastward today, the surface high to our north/northeast should as 
well. As this occurs, winds will veer to a more east-northeast 
direction, which should begin to allow lower clouds and possibly 
some light precipitation to move inland. Nevertheless, I am 
suspicious that the models are overdoing the precipitation in 
general, so I have lowered PoPs from the previous forecast.

Regarding cloud cover, the models have also struggled with the 
scattered-broken midlevel clouds across the area early this morning, 
so have bumped sky cover from the previous forecast accordingly. 
These have had some effect on radiational cooling (at least in an 
hour-by-hour sense), but have not had too much impact on the overall 
low temperatures forecast early this morning, except in the far 
northwest CWA (where cloud cover has noticeably restricted a sharper 
temperature drop). With these trends in mind, felt that a continuity 
forecast for today was appropriate. Should see some gradient from 
east to west this afternoon, especially with the onshore flow, but 
the degree to which this occurs is uncertain given the above 
discussion. Statistical consensus did a reasonable job yesterday but 
may perform a little worse today owing to the projected increased 
effects of sky cover and onshore flow. Nevertheless, confidence is 
just not there to make too many deviations from these values owing 
to the model errors observed so far.

All of the above said, it should be a sufficiently nice day to get 
outside and enjoy the cooler conditions!


As discussed yesterday, models are still showing a shift southward 
in the strongest low-level moisture fetch from the ocean during the 
overnight hours. This should occur in conjunction with ageostrophic 
southward acceleration of flow east of the higher terrain, inducing 
higher pressure farther to the south overnight. Have continued to 
depict this in the forecast, though my caveats regarding the overly 
aggressive model generation of precipitation remain. As such, 
lowered PoPs about 5-10 percent for the tonight period, with the 
highest PoPs on the Atlantic coast with negligible PoPs northwest of 
a Dover to Sandy Hook line. Additionally, there may be some 
improvement in sky condition during the evening hours (especially 
the farther north and west you go), with a resurgence of lower clouds 
past midnight.

Effects on temperatures may be more pronounced than what has been 
seen early this morning, with the north/west CWA probably verifying 
colder than model consensus and the south/east CWA probably 
verifying warmer. Have made some adjustments to the low-temperature 
forecast with these effects in mind; however, the forecast is 
reasonably close to continuity.


The maritime airmass across the area, resulting from the onshore
flow behind the retreating high, will continue early this week.
This will produce some low clouds and some light precipitation 
near the coastal areas. Variable cloud cover will be found 
further inland across ern PA and nrn NJ. Temperatures both days 
will be a little below normal for late August. 

Temperatures will return to near normal Wednesday when a cold front
approaches the area. A batch of showers and tstms will accompany
the front with rainfall totals of 1/2 to 1 inch possible. We have
mostly chc pops in the fcst for now, since the models haven't fully
decided if the fropa will be during the day or overnight into Thu
morning. Some fine tuning will occur once the models come into 
better agreement. 

Late this week, high pressure from the W/SW will build back
across the area. A drier and slightly warmer airmass will return
to the Middle Atlantic. Highs will become above normal with some
upper 80s arriving to the area.


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

Through tonight...Mainly VFR, with ceilings generally hovering 
around 4000-8000 feet. Temporary MVFR ceilings are possible from
the Delaware Valley and points east through mid day. Winds 
becoming east- northeast 10 to 15 kt with gusts to 20 kt 

Mon/Tue...Mostly VFR, but some low clouds and light precipitation
  possible near the shore. MVFR/IFR possible there.

Wed...MVFR possible with some showers/tstms. 

Thu...Mostly VFR expected.


Advisory conditions are expected to persist through tonight on the 
waters, with seas already around 5 feet at buoy 44009, and wind 
gusts meeting criteria on the beaches. Expect a slow trend upward 
with both through tonight, with gusts approaching 30 kt at times and 
seas building to 5 to 7 feet on the Atlantic waters. 

Models have been too slow in building the ocean wave heights to this 
point, so have forecast conditions slightly higher than their 
simulations depict. If trends continue, could see seas build to 8 
feet late tonight. 

SCA conditions will continue into Monday and perhaps maybe 
extended into Monday night at some later time. Sub-SCA expected 
after that. Clouds and a chance for light showers Mon/Tue. A 
chance of showers/tstms Wed/Wed night. Mostly fair Thu. 

Rip Currents... 

Northeasterly winds in the 15 to 30 mph range and seas between 4 and 
7 feet will result in a high risk of rip currents for the New Jersey 
Shore on Sunday with a moderate risk of rip currents expected for 
the Delaware beaches.

This elevated rip current risk is expected to persist until at least 
Tuesday as a prolonged period of onshore flow and elevated seas 
looks likely.


A prolonged period of northeast to east flow will continue through 
at least Tuesday. This persistent onshore fetch, combined with 
increasing astronomical tides due to an approaching new moon on 
Thursday, will likely lead to increasing coastal water levels.
Some spotty minor coastal flooding is possible with this
afternoon's/evening's high tide, but better chances for more
widespread minor-/advisory-level flooding appear to be with the
afternoon/evening tides on Monday and Tuesday.


The NWS Radar at Dover, DE (KDOX), is out of service at this 
time. The time when it will return is presently unknown.


NJ...High Rip Current Risk until 8 PM EDT this evening for NJZ014-
MARINE...Small Craft Advisory until 8 PM EDT Monday for ANZ430-431-


Near Term...CMS/Johnson
Short Term...CMS
Long Term...O'Hara
Tides/Coastal Flooding...