Area Forecast Discussion
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FXUS61 KPHI 260821
Area Forecast Discussion
National Weather Service Mount Holly NJ
421 AM EDT Mon Sep 26 2016
High pressure centered over the New England this morning will shift
offshore this afternoon. A cold front crossing the Appalachians
later today will progress through the forecast area tonight. The
front will stall offshore on Tuesday. An upper low is expected
meander over the eastern U.S. during the second half of the week
before eventually lifting northeastward sometime during the weekend.
Meanwhile, several waves of low pressure may develop along the
stalled boundary and move up the eastern seaboard.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
High pressure will continue to move eastward away from the region.
This will place us on the backside of the high. As a result, we will
see increasing southerly winds today. Bufkit analysis shows the
potential for a few gusts as high as 20 mph today in the afternoon
hours. A cold frontal boundary with an area of low pressure across
the Great Lakes will continue to move eastward toward our region.
Rain should off till after sunset but clouds will be on the increase
from west to east.
With the southerly flow, areas that stay sunny east of the I-95
corridor should see highs in the mid 70`s, likely a few degrees
warmer than MET/MAV. On the other hand, further west highs will
likely be kept back in the 60`s due to afternoon cloudcover.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
The previously mentioned cold frontal boundary moves through the
region tonight. Enough lift and moisture will be present for a
period of showers ahead of the front. Model agreement is fairly good
in terms of QPF and timing of the rain. We are looking at a mid to
late evening start in eastern Pennsylvania and Maryland with a
midnight-ish start across New Jersey and Delaware. QPF totals
have lowered slightly compared to the last few model cycles. This
is something that typically happens when models are trying to
account for drier than normal conditions. Still a 1/4-1/3 inch of
rain is expected to fall regionwide. An isolated thunderstorm is
possibile with LI`s approaching 0 across Delmarva and areas
slightly further north. Lows should be around 60 or so in most
.LONG TERM /TUESDAY THROUGH SUNDAY/...
The cold front will move off the coast early Tuesday morning. In
response to strong downstream ridge blocking over the western
Atlantic Ocean, the southern tail of the front is expected to slow
down or stall nearby to our east off the NJ-Delmarva coast and to
our south near the VA-NC border on Tuesday. The vertically-stacked
closed low still be located well upstream of us over the northern
Great Lakes. A potent 125 kt upper-level jet streak positioned on
the eastern flank of the trough will continue to provide synoptic
lift near the front. Accordingly, we anticipate the back edge of the
showers to not make much progress eastward past the I-95 corridor on
Tuesday. Precipitation should be mainly light with the deeper
moisture and instability shifting offshore, but it looks to persist
through most of the day in the coastal plain and perhaps linger into
the evening near Cape May, NJ and in southern DE.
Lower than normal forecast confidence continues to plague the
forecast for the second half of the week and upcoming weekend with a
potential for a cutoff low to meander over the eastern CONUS and
several waves of low pressure moving slowly up the eastern seaboard
(somewhere between the lee side of the Appalachians and the coast).
Models are having a difficult time resolving the details in this
complex setup as evident by the large spread between the guidance
and considerable run-to-run shifts seen with almost all of the
global models. The high degree of uncertainty seems to be related to
how much energy from the primary surface low (associated with the
cutoff low over the Midwest) transfers to secondary low pressure and
how quickly the transfer occurs. This will all determine if there is
one organized secondary low or several disorganized waves as well as
the strength and track of each disturbance. The good news is that
the spread seems to be gradually diminishing with each run, allowing
us to take some useful clues from these trends.
We may be able to briefly squeeze out a dry stint Tuesday night into
part of Wednesday with the closed low still over the Great
Lakes/Midwest and before secondary cyclogenesis to the lee of the
Appalachians. After that, it is looking more likely that we`ll see a
prolonged period of unsettled conditions later Wednesday through the
end of the work week and possibly extending into the weekend. This
is when the upper low becomes cutoff to our west. Meanwhile, high
pressure anchored over southeastern Canada will create a CAD setup
with cool air draining down the eastern side of the Appalachians.
This high anchored to our north would provide a blocking setup.
Therefore, any secondary low that develops to our south would likely
stall somewhere in the mid-Atlantic region. The end result would
produce cloudy, cool and potentially rainy conditions for several
days. Still too early to say how much precipitation we will get
from this event but it can range from mainly light rain and
drizzle to periods of moderate or even locally heavy rain.
.AVIATION /08Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
VFR conditions through late afternoon. Ceilings will build and lower
starting late this afternoon with MVFR conditions for the overnight
hours. Rain showers will also lower vsbys some as well to MVFR
levels. Timing looks to be around 03z for KRDG on the 27th and more
toward 07z at KACY (plus or minus a couple of hours). Southerly
winds increasing throughout the day to around 10 knots with isolated
gusts up to 20 knots in the afternoon. Winds decreasing and becoming
more westerly tonight.
Tuesday...Quickly improving to VFR in the western terminals
(ABE/RDG) early morning and then I-95 terminals either late
morning or early afternoon. MVFR/IFR conditions and showers may
linger near the coast (MIV/ACY) through the afternoon or evening.
Forecast confidence: moderate.
Tuesday night and Wednesday morning...Mainly VFR. Forecast
Wednesday afternoon through Friday...Sub-VFR conditions in
showers likely. Best chances for more widespread rain appear to be
on Thursday and Friday. Forecast confidence: low.
Waveheights will build this afternoon, potentially reaching five
feet for a period this evening ahead of a cold front. Southerly
winds will also ramp up as well tonight with gusts from 25-30 knots
looking likely based on sounding and model data. With an increased
confidence for SCA gusts, we have issued an SCA for tonight into
Tuesday morning...SCA continues thru 15Z for both the ocean and
the bay. SW winds ahead of a cold front will gust to around 25 kt
early while seas in the coastal waters near 5 ft may persist later
in the morning.
Tuesday afternoon through Wednesday night...Winds and seas below
Thursday through Friday...NE winds 10-20 kt. Seas may build to 4-6
ft and thus a SCA may be needed. Low confidence in forecast for
late in the week.
Even though waveheights will likely be a bit lower today compared
to yesterday, increased southerly winds by this afternoon will
still likely contribute to a moderate risk for the formation of
dangerous rip currents today. This moderate risk may continue into
the first part of Tuesday before diminishing toward afternoon as
winds and seas subside behind a cold front.
MARINE...Small Craft Advisory from 6 PM this evening to 11 AM EDT
Tuesday for ANZ430-431-450>455.