Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 011941
Area Forecast Discussion
National Weather Service Mount Holly NJ
341 PM EDT Sat Oct 1 2016
Persistent low pressure that has been sitting to our west the last
few days will finally start to lift northeast as it weakens. By
Monday, the low will be over New England, and the associated cold
front will slide through our region. A surface high will build and
stay north our our region through much of the remainder of the work
.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
A closed low, which has been stuck in the upper Ohio Valley, is
forecast to slowly move into the eastern Great Lakes region tonight.
Meanwhile, high pressure at the surface up across northern New
England to the Canadian Maritimes has been locked in, however this
feature will gradually weaken and shift eastward tonight.
This will continue with an onshore flow tonight, however it has
weakened. There remains a pronounced low-level inversion with
easterly flow below it which then veers to southerly above it.
This is locking in the low clouds across the area and at times has
produced some drizzle. There is a weak surface low tracking
northward, especially just offshore, however there is an area of
diffluent flow aloft between it and the closed low. This along with
just enough lift up and over the marine layer is result in some
showers lifting northward this afternoon. These are anticipated to
continue to gradually lift north and northwestward through this
evening. Some instability to our south, near and south of a frontal
zone, could result in a few thunderstorms, however we are
anticipating these to weaken as they encounter the deeper marine
layer. Some clearing is approaching southern Delaware late this
afternoon, however overall we are expecting the low clouds to hang
on. Otherwise, some additional drizzle cannot be ruled out through
the night. We kept some POPs in through the night due to the chance
of at least some additional showers.
Given the wealth of low-level moisture in place combined with a
lessening wind field should yield to some fog development. It is not
all that clear if this becomes dense given the stratus that is in
place. The fog should be more widespread in the higher terrain,
however widespread dense fog is not currently expected mainly due to
the expected stratus deck to be maintained.
As for the low temperatures, we used a blend of mostly continuity
and MOS. The hourly temperature and dew point grids were adjusted
based on the latest obs, then the LAMP/LAV guidance was blended into
.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
The pesky closed low is forecast to slowly shift northeastward from
the eastern Great Lakes during the course of Sunday. As additional
short wave energy continues to move around this closed low, some
showers should be around during the day. The day however should
start out with some fog and/or drizzle. The flow should start to
transition during Sunday, therefore allowing some possible drying
working in from the south. As this occurs, the cloud bases should
lift some especially across the southern areas with perhaps even
some breaks of sunshine during the afternoon. If this occurs,
afternoon temperatures are expected to be warmer especially as the
onshore flow lessens a bit more. Overall, some improvement is
expected but we did not rush in the partial clearing. High
temperatures were a blend of MOS and continuity with some local
tweaks made, which resulted in some lowering of the highs for many
.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Overview...The period starts with the occluded/cold front off the NJ
and DE coasts and a stacked low pressure system over the Great
Lakes. On Monday, the mid-level low will continue to weaken and open
up as it makes it`s way somewhere between Albany and Philadelphia.
On Tuesday this feature is forecast to drift off the coast. Come
Wednesday, surface high pressure will be in control from Maine down
to the Gulf Coast with a high amplitude ridge above it. High
pressure is forecast to remain in control through Friday. At the
same time on Friday, well to our south, Hurricane Matthew is
forecast to be over the Bahamas. For Saturday, confidence is higher
that the trof currently off the Pacific Northwest coast will be
somewhere over eastern North America. A cold front will be attached
to it. The lesser known is how strong it will be, where it will
exactly align up, and how it will affect/help steer Matthew.
Temperatures...For reference, normals for Philadelphia for the
period are in the around/low 70s and low/mid 50s. Through the
period, overnight lows are forecast to be above normal. Daytime
highs are forecast to be above normal early in the period and then
at or below normal for the rest of it.
Precipitation...We`ll see a diminishing threat of showers Sunday
night. By Monday, small POPS have been painted for the just the
northern areas. On Tuesday, small POPS were reintroduced CWA-wide as
the mid-level low works off the coast. Uncertainty exists for
Wednesday through Friday with the question being how strong does the
onshore flow become and how much low level moisture works it`s
way onshore. Saturday`s low confidence forecast takes into account
the approaching cold front, mid-level trof, and Matthew.
Winds...Benign is a good adjective with mainly light northwest winds
developing Monday. This will be short-lived as the surface flow will
go more northeasterly on Tuesday and then persist into the weekend.
This looks to be a long duration NE event with SCA or gale force
winds/wind gusts along the coast.
Impacts...nothing much to speak of early in the period, but
depending on how things unfold, there could be some coastal impacts
later in the week if the NE flow increases as forecast.
.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
Rest of This Afternoon...Ceilings mainly ranging between 500-1000
feet. Some showers around will locally reduce the visibility to
MVFR, although some drizzle especially at ACY will lower the
visibility. Northeast winds mostly around 10 knots.
Tonight...IFR/LIFR ceilings as low clouds persist. Some showers or
drizzle around with even some fog developing, which will all reduce
the visibility at times. As of now, widespread fog below 1SM is not
anticipated given the stratus in place. Northeast winds becoming
mainly 5 knots or less.
Sunday...IFR/LIFR conditions to start should improve to MVFR during
the afternoon. If enough drying is able to work in during the
afternoon, ceilings could rise to VFR especially south and east of
PHL. Northeast to east winds 4-8 knots.
Sunday night through Tuesday...mostly VFR conditions expected. Small
chance of showers with MVFR conditions Sunday and Tuesday.
Wednesday and Thursday...conditions lowering to IFR for Delaware
Valley and Coastal Plain terminals as an onshore flow results in low
stratus clouds. These conditions are forecast to persist into
The onshore flow is weakening as high pressure to the north is
tending to shift a bit farther from our area, allowing the pressure
gradient to relax. As a result, the Small Craft Advisory for the
Lower Delaware Bay is cancelled. The advisory continues through 06z
tonight for the Atlantic coastal waters zones as seas remain in the
4-7 foot range, although these gradually subsiding. The conditions
are then expected to be below advisory criteria on Sunday. The
sensible weather features some showers and/or drizzle with perhaps
some fog at times.
Sunday night and Monday...winds and seas should stay below SCA
Tuesday through Thursday...Winds and seas are expected to build
through the day Tuesday, and SCA conditions could develop as early
as midday Tuesday and persist into Thursday. Gale force gusts
possible Wednesday and Thursday.
The high tide tonight looks to fall short of advisory criteria as it
is the lower tide. Despite the departures holding up some, the flow
has weakened along with the seas, therefore the next high tide on
Sunday should also fall short of advisory criteria.
MARINE...Small Craft Advisory until 2 AM EDT Sunday for ANZ450>455.