Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 260146

Area Forecast Discussion
National Weather Service Mount Holly NJ
946 PM EDT Sat Mar 25 2017

A backdoor front settles to our south tonight, allowing
Canadian high pressure to briefly move into our area on Sunday.
Low pressure moving from the Mississippi Valley into the Great
Lakes will allow a warm front to move northward through the
region on Monday, followed by a cold front Tuesday night.
Canadian high pressure will build into our area Wednesday
through Friday. By next weekend, low pressure moving into the
Ohio Valley may send a warm front toward the region.


As of 0100z, the backdoor front has just about cleared our forecast
area. This boundary will continue to settle southwestward into
the overnight hours. Immediately in the wake of the front, an
hour or so of northeast winds gusting to around 30 mph occurred
at several locations however this seems to be settling down now
from north to south as the warm boundary layer has cooled quite
a bit. There will still be a cool breeze however through the
night. Strong high pressure centered just southeast of James Bay
Canada this evening will continue to build down into the northern
Mid Atlantic region. This is resulting in much cooler air along
with the onshore flow.

There is not much lift left at this time which is evident with
the showers ending. Some sprinkles are possible across the
northern areas this evening, with some showers potentially
redeveloping late across about the northern half of the area.
These may be in response to a subtle impulse moving through and
providing just enough lift within an ongoing overrunning setup.
We scaled back the PoPs for awhile, then blended mostly into
continuity toward morning. Toward daybreak as the moisture
deepens, there could be some drizzle when the lift is rather
weak. In addition, some fog is expected mainly over the higher
terrain of the northwestern zones.

Temperatures in the higher elevations of the Poconos and adjacent
far northwest New Jersey may reach freezing for a couple of
hours near daybreak, which may result in a little light freezing
rain/drizzle. Since this appears to be more localized and the
occurrence is less certain, no advisory is being issued.


For Sunday the forecast area will remain in the cool moist air north
of the front. A widespread overcast and winds off the ocean will
limit heating and temps will struggle to rise. Daytime max temps
will generally be about 20 deg colder than today. The overrunning
pattern will continue along with chances for some showers or
drizzle, again with the chances increasing from Delmarva northward
thru NJ and eastern PA. Any rain amounts should be limited to a few
hundredths of an inch. The 12z GFS is quite a bit more bullish on
QPF for the next 24 hours or so but it seems to be overdone.


Unsettled weather will continue through Tuesday night with a
return to fair weather Wednesday and Thursday. Some uncertainty
remains with regard to freezing rain chances on Sunday night.
Then, considerable uncertainty with the next system Friday into
Saturday. Temperature-wise, around 10 to 15 degrees above normal
from Monday thru Wednesday, falling to around 5 degrees above
normal Thursday into Saturday.

In terms hydrology, given the above normal temperatures through the
long term, the snow pack north of I-78 will continue to melt. The
model consensus brings additional rainfall amounts of up to 1.5
inches across this area through Tuesday night with most of this
spread out between more substantial precip Sunday night into
Monday morning and Tuesday night. We expect rises on area
streams and rivers, but no flooding concerns at this time.

There remains the potential for spotty minor coastal flooding along
the ocean front for the high tides centered on Sunday evening and
again near sunrise on Monday. See: Tides/Coastal Flooding section.

As low pressure moves through the Ohio Valley into the Great Lakes
from Sunday night into Monday, overrunning precip is likely. For the
most part, the models remain consistently too warm for freezing rain
Sunday night into Monday morning. In addition, boundary layer winds
are sufficiently strong enough to advect warm air northward through
the low-level column, despite somewhat of an in-situ cold air
damming signature. In addition, the lack of low-level dry air will
limit evaporational cooling. Given the proximity of the cold air, we
cannot rule out freezing rain, but the probability of it occurring
is low. Nevertheless, we maintain the wording from the previous
shift with regard to freezing rain potential in the HWO. Please note
that the most likely ice glaze map on our website only reflects
amounts tonight into early Sunday afternoon.

Expect showers to continue Monday afternoon into Wednesday,
with another round of more substantial precip likely centered on
Tuesday night, associated with a cold frontal passage. We do
not anticipate any p-type issues during this period. There
remains uncertainty with the timing of this front, particularly
given the time range, with the GFS being the most progressive.
For now, we expect a gradual return to fair weather on
Wednesday, continuing into Thursday.

There is considerable model uncertainty beyond day five, as the GFS
and ECMWF diverge in their handling of low pressure exiting the
Mississippi Valley. Initially a southern stream system, the
ECMWF phases it with a northern stream system, bringing a period
of precip to the entire area Friday night into Saturday. The
GFS keeps both streams separate, and takes a southern stream
system to our south, but close enough for precip across Delmarva
and southern NJ. The GFS does not have support from the UKMET
or Canadian, both of which are more similar to the ECMWF. For
now, the forecast brings in chance PoPs Friday into Saturday.
Also, given the orientation of the surface high, cold air
damming is possible. Thickness value indicate the cold air is
right on our door step, so p-type could be an issue.


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

Rest of Tonight...VFR ceilings at 6000 feet or higher, then
lowering to MVFR and IFR as an onshore flow continues to become
more established. Some showers or drizzle at times are possible
mainly after 06z. Confidence in the timing of the ceilings
lowering to MVFR/IFR is less than average. Northeast winds
around 10 knots, with some lingering gusts up to 20 knots
possible this evening.

Sunday...IFR/MVFR conditions, mainly ceilings. Some showers or
drizzle possible at times. East-northeast winds around 10 knots,
with some local gusts to 20 knots at times.

Sunday night thru Monday night...Periods of MVFR/IFR possible
in low clouds and fog. Restrictions will be more widespread
Sunday and Monday night, while VFR develops on Monday from
around I-95 and points southeast.

Tuesday thru Thursday...MVFR possible Tuesday morning northwest
of I-95, otherwise, VFR thru thru the period.

There is the potential for wind gusts up to around 25 knots,
out of the south-southwest on Monday, then northwest on


A short duration northeasterly surge with gusts 25-30 knots
occurred in the wake of a backdoor front, especially the
southern waters into Lower Delaware Bay. This appears to be
settling down some now. Some decrease in the winds are noted
across the northern coastal waters, however some increase is
expected again as the pressure gradient tightens through the
overnight. Therefore a Small Craft Advisory remains up for the
New Jersey coastal waters with some delay farther south. Wind
gusts to about 25 knots are anticipated, however there is higher
confidence in seas building to 5-6 feet over the SCA area.
Elsewhere, the conditions should remain below criteria outside
of the aforementioned northeasterly surge.

Sunday night...Small Craft Advisory in effect for the NJ waters
for combined seas increasing to 5 or 6 feet and scattered gusts
25 kt.

Monday thru Wednesday...Seas may remain elevated Monday into
Tuesday, and the SCA may need to extended into this period.
Quite a bit of uncertainty with regards to winds and seas on
Wednesday, which may be near SCA thresholds.



In the wake of a backdoor frontal passage, a 36 hour period of
onshore flow is expected through Monday morning, with easterly
wind gusts up to 25 MPH at times. Astronomical tides are also
increasing, coincident with the New Moon this Monday, March 27.
The ESTOFS remains most aggressive out of the guidance suite,
with near minor flooding for the sunrise Monday high tide along
the DE and NJ oceanfront, and it has has outperformed the other
guidance recently. Both the NOS and ETSS are more conservative.
The wind will also be trending more parallel (southerly) to the
shore by sunrise Monday, so there remains uncertainty with
regard to water levels reaching the minor flooding threshold.


A daily record high temperatures was set at Atlantic City (KACY
Airport) today when the temperature reached 81 degrees at 323 PM
this afternoon. The old record was 79 degrees from 1963.
Temperatures have since dropped precipitously once a sea breeze
front moved through (it was 60 degrees at 440 PM EDT, marking a
20 degrees drop in one hour).

March monthly average temp: While the negative departures will
diminish considerably the next few days...the month as a whole
for PHl is currently expected to end up about one half to 1
degree below normal.


MARINE...Small Craft Advisory from 6 AM Sunday to 2 AM EDT Monday for
     Small Craft Advisory until 2 AM EDT Monday for ANZ450-451.


Near Term...AMC/Gorse
Short Term...AMC
Long Term...Franck
Tides/Coastal Flooding...
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