Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 272009

Area Forecast Discussion
National Weather Service Mount Holly NJ
409 PM EDT Sat May 27 2017

A weak surface low will move off the coast of Virginia tonight. Weak
high pressure will build into the Northeast on Sunday. A stronger
low pressure system will move into southeast Canada on Monday, which
will bring a cold front through the area. A couple of reinforcing
cold fronts will move through the region during midweek before a
surface high approaches the East Coast by the end of the week.


Weak high pressure across the area tonight with an upper disturbance
crossing through this evening. You can expect a continuation of
mostly cloudy skies with some showers across Delmarva and southern
New Jersey. A few lighter sprinkles may also occur across ern
Pennsylvania and New Jersey this evening. Clouds will tend to
thin out later tonight once the short wave passes the area.
Tonight`s low temperatures will drop into the low/mid 50s up
north and upper 50s across metro Philadelphia and Delmarva
areas. Winds will be light tonight.


The area of weak high pressure will be offshore Sunday morning. Low
pressure will be advancing towards the area from the Ohio Valley.
Overall much of the daytime hours will be dry, but clouds will be
increasing over the area during the day. A couple showers are
possible across the southern Poconos or Berks County by late
afternoon. High temperatures tomorrow will be much like today`s,
with low/mid 70s in most areas. It will be cooler across the
shore areas and over the higher elevations up north. Winds will
be mostly E or SE at 5 to 10 mph.


Primary forecast concern is chances for storms Sunday night into
Memorial Day.

Operational models have come in a little faster with the system
moving through the region to close out the holiday weekend.
Currently, the relevant vorticity maximum is in the Rockies and
adjacent Great Plains. The trough axis is strongly positively
tilted at this time, but with the aid of a kicker vort max in
central Canada, the western side of the central U.S. vort max
will pivot rapidly eastward on Sunday, with the trough axis
becoming more neutrally tilted Sunday night as it approaches the
Great Lakes and Ohio Valley. Attendant surface low will
intensify as it tracks northeastward from the southern plains
to the vicinity of Lake Erie by 00Z Monday, with a broad region
of warm air advection downstream into the Mid-Atlantic.
Isentropic ascent poleward of the associated baroclinic zone
will permit the development of widespread precipitation in the
central Appalachians, the Mid-Atlantic region, and portions of
the Northeast.

A predecessor vorticity maximum Sunday afternoon/evening, in
conjunction with increasingly favorable large-scale dynamics
(including favorable positioning of an upper-level jet streak
and downstream difluent flow), will allow for deep lift and
substantial cooling of the mid and upper levels of the
atmosphere. As a result, elevated instability will increase to
the north of a quasi-stationary boundary positioned in the
southern Mid-Atlantic. Though the strongest ascent and most
widespread precipitation should be to the north and west of the
area Sunday night (in closer proximity to the surface low),
models indicate 35 to 45 kts of 925-850 mb layer flow orthogonal
to the baroclinic zone in the Mid-Atlantic. With such strong
ascent in play, think chances are quite good for a round of
storms Sunday night into at least early Monday. Raised PoPs
quite a bit during this time frame, given the reasonable
agreement among the recent simulations of the NAM/GFS/CMC/ECMWF.

Less obvious is the residual environment within the warm sector
on Monday in advance of a cold front moving through the region.
Latest models swing the front through the region rather early in
the day, indicating that the time window for recovery of
instability will be limited at best. As such, think chances of
convection beyond Monday morning are somewhat more limited,
though given the model variability run-to-run regarding frontal
passage timing...not particularly confident of that assessment.
Nevertheless, felt it wise to reduce PoPs from southwest to
northeast late Monday morning onward, particularly given the dry
looking simulations of most of the operational guidance.

Tuesday looks generally like an "in-between-systems" day as the
main vort max swings northeastward into New England and
southeast Canada. The next perturbation will dig southeastward
into the Midwest on Tuesday and then eastward through the
Northeast on Wednesday as it rounds the semi-permanent longwave
trough in eastern Canada. Associated cold front should generate
another round of showers as it moves through, which looks to be
Wednesday (with timing uncertainty becoming even more
pronounced). Instability looks more limited with this system, so
kept mention of thunder out of the grids for now, though models
may be underplaying this a little bit (i.e., not sure if I`m
buying the pronounced inverted-V boundary layer the GFS is
depicting). The 12Z CMC, e.g., provides some evidence of more
vigorous convective development on Wednesday.

Thereafter, forecast looks a little drier as surface high
pressure becomes established. The 12Z GFS provides a prolonged
break in the active pattern, keeping somewhat drier
northwesterly flow in the region through the following weekend,
while past runs and the most recent 12Z CMC offer a strong low
developing in the Ohio Valley into the Mid-Atlantic by next
weekend. Latest ECMWF has a pretty dissimilar solution to either
camp. Given the large uncertainty during this period, stuck
quite close to the inherited forecast Thursday onward.


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

Weak high will be across the area tonight and Sunday. An upper
disturbance will be over the region through early tonight. A
generally VFR fcst is expected over the area. Clouds will prevail in
many areas the first half of tonight and then they will diminish
overnight. Cloud CIGS will be mostly 040-050. A few light showers
are crossing the southern parts of our area attm and they will
continue into the early evening. It`s possible that a TSTM may
develop across Delmarva later this afternoon. The pops looked low
enough to exclude from KILG`s TAF, however. VFR should continue
into Sunday with clouds increasing again later in the
afternoon. Winds will be mostly 5 to 10 knots thru the period.
Wind directions will be W or SW this afternoon (but S at ACY),
then winds will switch to onshore (E/SE) tonight and remain that
way into Sunday.

Sunday night and Monday: MVFR/IFR CIGs/VSBYs are reasonably likely,
especially Sunday night. Scattered showers and storms expected,
especially Sunday night and Monday morning, with conditions
potentially improving during the afternoon. East to southeast winds
around 10 kts Sunday night transitioning to west by Monday afternoon
after cold frontal passage. Confidence below average.

Monday night and Tuesday: Slight chance of showers/storms north/west
of KPHL. Some chance of low stratus or fog on Monday night.
Primarily VFR on Tuesday. Winds under 10 kts, likely transitioning
from west to north to east during the period. Confidence below

Tuesday night and Wednesday: Generally VFR. Slight chances of
showers, primarily during the day Wednesday. Winds generally west or
southwest 5 to 15 kts, with perhaps some gusts to 20 kts or so
during the afternoon. Confidence below average.

Wednesday night and Thursday: VFR expected. Winds generally west 5
to 15 kts. Confidence average.


Weak high pressure will remain across the waters tonight and Sunday.
An upper air disturbance will bring some showers across Delaware Bay
and the adjacent DE/srn NJ ocean waters through this evening.
Conditions will remain sub-SCA through the period with winds
mostly under 10 knots. Seas will be 2-3 ft on the ocean and 1-2
ft across Del bay.

A low risk for the development of dangerous rip currents exists
for the rest of today. For Sunday, an onshore flow is forecast
to strengthen to around 10 to 15 kts, and this may result in the
rip current risk approaching moderate especially for the coast
of New Jersey.

Sunday night through Thursday: Generally sub-advisory winds and seas
expected. However, winds/seas will be close to advisory thresholds
Sunday night and Monday as a storm system moves through the area. In
addition, there is some risk for visibility restrictions Sunday
night and Monday before a cold front moves through the area later in
the day.

A couple of chances for showers/storms exist through the period,
especially Sunday night and Monday and again on Wednesday.


A Coastal Flood Advisory remains in effect for tonight`s high
tide. It will be in effect for the coastal counties of New
Jersey and Delaware, and for the counties along Delaware Bay and
the lower Delaware River up to near the Commodore Barry Bridge.

Model guidance continues to suggest fairly solid minor coastal
flooding will occur tonight, generally somewhat worse than last
night and somewhat better than the night before. With the
increasing onshore flow through the evening hours, these
projections still appear reasonable. Nevertheless, advisory
thresholds may be somewhat harder to reach on the Delaware
coast and the coast of Delaware Bay, where the past 24 hours of
offshore winds appear to have had a somewhat greater impact in
lowering water levels.

Minor coastal flooding may also occur during Sunday night`s high
tide, as astronomical tides remain high and onshore winds should


NJ...Coastal Flood Advisory from 8 PM this evening to 2 AM EDT
     Sunday for NJZ012>014-020>027.
     Coastal Flood Advisory from 11 PM this evening to 3 AM EDT
     Sunday for NJZ016.
DE...Coastal Flood Advisory from 8 PM this evening to 2 AM EDT
     Sunday for DEZ002>004.
     Coastal Flood Advisory from 11 PM this evening to 3 AM EDT
     Sunday for DEZ001.


Near Term...O`Hara
Short Term...O`Hara
Long Term...CMS
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