Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 210822
Area Forecast Discussion
National Weather Service Mount Holly NJ
422 AM EDT Fri Oct 21 2016
Low pressure will strengthen across Pennsylvania today and move up
into New England Saturday. A strong cold front will cross the area
tonight. The low will continue to deepen and move into Quebec on
Sunday. A fast moving weak disturbance will cross New York and New
England Sunday night and early Monday. High pressure will build in
for much of next week.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
An area of low pressure was located in the OH River Valley early
this morning. A warm front extends northeastward from the low into
southern NY. Light south-southeasterly flow was noted across the
entire forecast area, indicating we are in warm sector of the
system. It is an unseasonably warm and moist night with both
temperatures and dewpoints in the 60s. Stratus and patchy fog will
hang around through the first part of the morning as low-level
moisture is from the Atlantic Ocean is transported poleward.
The upstream low is forecast to track from the southwestern corner
of PA this morning to the northeastern corner by late this
afternoon. An additional wave of low pressure is forecast to
organize and track northward off the mid-Atlantic coast today.
Rainfall with the low and attendant cold front, which was very heavy
over central PA last night, has remained upstream of the area thus
far. Models are in good agreement that the precip with the cold
front will not make it into our far western zones in eastern PA and
eastern shore of MD until late this afternoon. Meanwhile, we are
starting to see an area of showers appearing off the lower Delmarva
coast early this morning as this activity gets within range of the
U.S. radar network. Models are simulating this band of showers to
stream northward through eastern NJ late this morning and early
afternoon. This convection has origins of tropical moisture from a
disturbance off the Southeast U.S. coast, so it could conceivably
contain locally moderate to heavy rainfall. Some of the high-res
model guidance is showing isolated QPF amounts of 1-2 along and
east of the GS Parkway. More robust convection and greater
instability is forecast to remain well offshore, mitigating the risk
for mesoscale flash flooding.
The latest max temperature forecast for today has trended warmer by
a few degrees, especially from the Philly metro- south and east,
where there appears to be a opportunity for breaks in the low
clouds to develop later today. If partial clearing is able to
coincide w/ peak heating, then highs could reach 80F. We could see
more daily record highs fall today.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
Low pressure will track northeastward across the area this evening.
This system will be accompanied by a strong cold front that will
pass through the area. This low will eventually merge with the
offshore system overnight, resulting in a rapidly deepening cyclone
over New England.
Gusty northwest winds will develop behind the cold front as the low
rapidly deepens upstream and the pressure gradient tightens. Strong
CAA advection will steepen lapse rates in the boundary layer which
will allow stronger winds aloft to mix down to the surface later
into the night. The strongest wind gusts of around 30 mph will
likely first appear in the southwestern portion of the forecast
area overnight. Temperatures will quickly fall into the 40s and 50s
overnight behind the front.
Showers are likely along the front to pivot eastward through the
area tonight. Tried to reflect the best chance for showers in the
PoP/Weather grids based on the forecast timing of the front. A brief
downpour is possible with these showers but do not expect heavy
rainfall totals as the cold front will be progressive. Did not see
enough support from the high-res NCAR ensembles to include thunder
in the forecast.
.LONG TERM /SATURDAY THROUGH THURSDAY/...
Saturday thru Sunday...Deep low pressure will continue to
circulate cool air across the region. Temperatures both days will
be much more like October than of recent days. Sat will be some 5
degrees below normal and by Sunday readings will moderate back to
normal values. Gusty winds, especially Saturday, will give the air
a brisk fall-like feel. Winds may gust 35 to 40 mph Saturday, but
will be a little less on Sunday.
A quick and weak disturbance will cross north of the area Sunday
night and it may bring a few gusty showers to the southern Poconos
and northern NJ. Pops for the present fcst will just be in the
slight chc range, since the air may be too dry for any measurable
precipitation to occur. Temperatures Monday will be like Sunday,
near normal. A cold front will cross the area behind the
Tue thru Wed night...dry weather expected with high pressure
building in. Temperatures will be mostly below normal with the
upper trough settling across the area Tue. The trough will then
move away Wed.
Thu...OP models show the next disturbance, a deep low moving
across the Great Lakes, beginning to affect our weather Thu. We
have indicated increasing clouds and chc pops for Thu attm.
.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
CIGs have been variable between VFR, MVFR and IFR all night as
rounds of low clouds develop and dissipate. Expect this to continue
through the morning. Patchy dense fog may impact the southern
terminals through 12Z or 13Z. The current thinking is the stratus
should eventually lift later this morning or early afternoon but
there is high uncertainty with the exact timing. After a short
improvement to VFR at many sites this afternoon, lower clouds should
return this evening. Showers are likely tonight with a cold front
moving thru. Post-frontal stratocu may prolong MVFR CIGs through the
rest of the night.
An abrupt windshift from south to northwest is expected with the
fropa. Gusty NW winds arrive shortly after the fropa late tonight.
The gustiest winds are expected overnight across southern and
Sat...Mostly VFR with gusty winds. Few showers with restrictions
possible N/W Sat morning.
Sat night/Sun...VFR with gusty winds.
Sun night/Mon morning...Mostly VFR but a few showers far North.
Mon afternoon thru Tue...VFR.
Today...Southeast winds 5-15 kt this morning while seas in the
coastal waters are 3-4 ft. The pressure gradient will weaken this
afternoon, causing the winds to become light and variable as the
waters become situated in between two areas of low pressure (an
offshore low that will drift northward about 150 miles east of our
coastline and another low moving across central PA). There may be
some isolated lightning tied to the offshore system this afternoon
if stronger convection develops but the bulk of the high-res CAM
guidance keeps this offshore. Kept thunderstorms out of the forecast
for our coastal waters for now.
Winds will quickly shift from south to west-northwest this evening
in concert with a strong cold fropa. Post-frontal winds speeds are
forecast to increase to SCA levels, first in the DE Bay and southern
coastal waters during the late evening and then in our northern
coastal waters overnight. Models have trended a bit slower with the
arrival of the steadier pressure rises and strong CAA behind the
front. Therefore, gales may not occur until overnight in the DE
Bay/southern coastal waters and not until early Saturday morning
across the northern waters. The Gale Watch was upgraded to a Warning
but the onset time was adjusted accordingly.
Sat thru Sunday...Gale conditions across the waters, ending first
over Delaware Bay Sat night and then across the southern Atlantic
waters around noon Sunday. Scattered showers over the northern
coastal waters Sat morning, otherwise fair weather expected.
Sun night thru Tuesday...SCA conditions expected.
Tue night thru Wed...Mostly sub-SCA conditions.
MARINE...Gale Warning from midnight tonight to noon EDT Sunday for
Gale Warning from 4 AM Saturday to 6 PM EDT Sunday for ANZ450-
Gale Warning from midnight tonight to 2 AM EDT Sunday for