Area Forecast Discussion
Issued by NWS Mt. Holly, NJ-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KPHI 181610
Area Forecast Discussion
National Weather Service Mount Holly NJ
1210 PM EDT Sat Mar 18 2017
Low pressure across the Great Lakes region will redevelop just off
the Mid Atlantic coast later today, then strengthen as it moves
slowly out to sea south of Cape Cod Sunday morning. High pressure
builds in briefly Monday, then a cold front moves through Monday
night with a secondary cold front arriving Tuesday night. Strong
high pressure is forecast to build in later Wednesday and Thursday,
before shifting offshore Thursday night. A warm front will approach
our area from the southwest later Friday.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
-- Changed Discussion --1130 AM update...The Winter Weather Advisory for portions of NE
PA and NW NJ has been cancelled with the threat of heavier/steadier
snow bands on the northern side of a closed upper low shifted
to the southwest the advisory area by almost 200 miles compared
to yesterday`s 12Z model guidance. Minor snowfall accumulations
(generally under an inch) is possible along the ridges with
occasional light snow this afternoon and evening. Spotty freezing
drizzle or light freezing rain may develop this afternoon and
tonight but icing will mainly be confined to the highest ridges
above 1000 ft in elevation as temperatures are above freezing
in the valleys and precip shield is very organized.
With the 700 mb closed low over Lake Erie now expected to dig
southeastward toward the northern Piedmont region of VA late
tonight, the potential for a convective band of moderate showers
on the northern/western side of the circulation has shifted into
south-central PA and the Chesapeake Bay region tonight. This
band could shift into far southeastern PA (southwest of Phila)
and Delmarva tonight. Forecast soundings indicate the effects of
dynamical cooling allowing for wet snow to mix in all the way
down into northeastern MD and possibly changeover to all snow
along and north of the Mason-Dixon line. There is always a
concern that the models are underdoing the strength of the
dynamical cooling underneath the upper low. Accordingly, cannot
rule out the potential for a Winter Weather Advisory (2+ inches
of snow) for Chester-Cecil Counties and nearby locations but
there is no guidance supporting that scenario at this time.
A mid level low located near Lake Michigan early this morning
will progress eastward reaching northwestern Pennsylvania and
far western New York late in the day. A surface warm front is
expected to remain across Virginia and the Delmarva today with
an area of low pressure developing over Virginia during the
We are anticipating a cloudy day with an east to southeast wind
around 5 to 10 MPH. The wind may begin to back toward the northeast
late in the day. The onshore flow is forecast to bring low clouds
into our region this morning.
We continue to expect light precipitation for today, mainly in
eastern Pennsylvania and New Jersey. Precipitation rates should
increase a bit during the afternoon.
-- End Changed Discussion --
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
The mid level low is forecast to drop over central Maryland and
eastern Virginia late tonight. Deepening surface low pressure is
expected to pass off the Virginia coast this evening before moving
out to sea.
The surface wind in our region is anticipated to become
northeasterly and it should increase to 10 to 15 MPH, especially on
the coastal plain. Cold air will begin to be drawn into our region
and rain should begin to change to snow from north to south. An
additional inch or two is possible in our northern counties with a
light dusting possible as far south as the Philadelphia metropolitan
area and central New Jersey.
Low temperatures are expected to favor the 20s up north and the 30s
elsewhere in our forecast area.
.LONG TERM /SUNDAY THROUGH FRIDAY/...
Synoptic Setup...A closed low is forecast to be in the East Sunday,
however this shifts offshore later in the day and at night. Some
ridging arrives during Monday, then another trough amplifies across
eastern Canada Tuesday with this also extending across the Northeast
through Wednesday. A shift in the pattern may then allow stronger
ridging in the East Friday as a trough amplifies across the Plains
and Midwest. The pattern will offer some warming, but also some
shots of colder air once again. We used a model/continuity blend for
Sunday through Monday night, then blended in the 00z WPC Guidance
thereafter. Some adjustments were then made following additional
collaboration with our neighboring offices.
For Sunday...A potent closed low is forecast to be sliding across
the Mid Atlantic region in the morning, with a deepening surface low
well offshore. While there are some differences in the details among
the model guidance, the presence of the closed low overhead may
assist in enough lingering lift in the morning especially closer to
the coast. As a result, we kept some chance PoPs through mid
morning. The system is forecast to move away during the course of
the day with a tight pressure gradient in its wake. This should
result in breezy conditions especially closer to the coast. As the
entire system shifts east, clouds are anticipated to give way to
increasing sunshine through the afternoon.
For Monday and Tuesday...High pressure builds in briefly Monday
before quickly shifting offshore. A cold front is forecast to arrive
Monday night, however it looks like moisture is becoming more
limited with this. There could be a better chance of some
precipitation Tuesday night as a secondary cold front moves through
with possible low pressure developing along it. This however may be
shoved to our south as strong high pressure builds across the Great
Lakes. We are expecting a milder airmass to be in place during this
time frame, with the main cooling arriving Tuesday night in the
wake of the secondary cold front.
For Wednesday and Thursday...A potential start of a pattern change,
at least for a time, looks to offer a trough developing in the
Plains with downstream ridging. This ridging allows for strong high
pressure near the Upper Great Lakes to build down into our area
during the course of Wednesday, then it shifts offshore later
Thursday. This setup allows for colder air to be in place with an
initial northerly flow. However, we eventually get into a developing
return flow as low pressure organizes across the Plains later
Thursday with high pressure slipping offshore. We maintained a dry
forecast and cooler temperatures.
For Friday....A pattern shift looks to result in a strong trough
across the plains which sends organizing low pressure toward the
Great Lakes region. This, in combination with high pressure off the
East Coast, will send warm air advection with a developing warm
front approaching our area. This will result in the start of a warm
up. While the main initial push of warm air advection and
overrunning showers may be focused to our west, some showers may
still arrive at some point Friday. Given timing and details
uncertainty, any PoPs were kept on the lower side.
.AVIATION /16Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
VFR conditions early this morning are forecast to lower into the
MVFR, then into the IFR range between about 1000Z and 1500Z in
response to a developing wind off the ocean. IFR conditions are then
anticipated through the remainder of the TAF period.
Light rain is expected for KTTN, KPNE, KPHL, KILG, KMIV and KACY. A
mix of snow and rain is forecast to KRDG and KABE.
A southeast to east wind around 5 to 10 knots is expected to develop
for today. The wind should become northeasterly for tonight at speeds
of 10 to 15 knots with gusts around 20 knots from
Sunday...MVFR ceilings possible to start, then VFR with thinning
cloud cover through the day. Some light snow is possible in the
morning south and east of PHL. Northeast winds around 15 knots with
gusts up to 25 knots becoming north-northwest, then diminishing to
10 knots or less at night.
Monday...Sub-VFR conditions possible at times mainly at night with a
chance of showers as a weak cold front moves through.
Tuesday and Wednesday...VFR overall. Northwest wind gusts up to 25
An east to northeast wind is forecast to be on the increase from
north to south on our coastal waters from this afternoon into
tonight. As a result, we will start a Small Craft Advisory at 2:00
PM for our waters from Sandy Hook down to Little Egg Inlet. We will
begin a Small Craft Advisory at 11:00 PM for our waters from Little
Egg Inlet south to Fenwick Island. We will start the Small Craft
Advisory for Delaware Bay at 2:00 AM Sunday.
Sunday...Strengthening low pressure offshore will result in a tight
pressure gradient across our area. This combined with pressure rises
and efficient mixing is expected to result in gale force gusts on
the ocean (northeast to north flow). As a result, a Gale Warning has
been issued through 18z. The winds should be a bit lower on Delaware
Bay and therefore we opted to go with a Small Craft Advisory. The
conditions however are expected to subside at night.
Monday...The conditions should be below Small Craft Advisory
criteria as high pressure briefly builds across the area.
Tuesday and Wednesday...Small Craft Advisory criteria possible later
Tuesday night and Wednesday, especially wind gusts, in the wake of a
-- Changed Discussion --PA...None.
MARINE...Small Craft Advisory from 11 PM this evening to 6 AM EDT
Sunday for ANZ452>455.
Gale Warning from 6 AM to 2 PM EDT Sunday for ANZ450>455.
Small Craft Advisory until 6 AM EDT Sunday for ANZ450-451.
Small Craft Advisory from 2 AM to 2 PM EDT Sunday for ANZ430-
-- End Changed Discussion --