Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 241544

Area Forecast Discussion
National Weather Service Mount Holly NJ
1144 AM EDT Wed May 24 2017

Low pressure will move out to sea today, meanwhile another low
pressure system will track up the Ohio Valley before moving into the
eastern Great Lakes tonight and early Thursday. A warm front will
lift northward into portions of our area Thursday, with another low
pressure system developing along the cold front. As the low
strengthens it moves away from our area during Friday. Weak high
pressure may briefly build in for Saturday, then low pressure and
its associated fronts move through Sunday night and Monday with the
main cold front scheduled to shift offshore Tuesday morning.


9 AM surface analysis shows a frontal boundary positioned south
of the OH Valley, stretching eastward across NC and then
northeastward off the east coast. Two waves of low pressure
were noted along the boundary: 1) A 1000 mb low about 200 miles
off the Delmarva coast and 2) a 1009 mb low near the KY-IN
border. In between these two low pressure systems, weak high
pressure was ridging down along the New England and northern
Mid-Atlantic coastline.

Only a few light showers associated with the offshore low were
lingering over the forecast area as of mid morning, but the
last of these showers should end late this morning. The drying
trend can be linked to mid-level shortwave ridging that builds
in this afternoon.

Although there will likely be partial breaks in the cloud cover
today (one such break already occurring in C and SE PA mid
morning) but do not foresee the sun peaking out for an extended
period of time today. Soundings show quite a bit of moisture
trapped underneath the mid-level subsidence inversion throughout
the day, so pockets of stronger heating from the breaks in the
clouds should quickly lead to the development of stratocu as
boundary-layer mixing deepens.

Below normal temperatures can be expected today owing to the
clouds and onshore flow around the surface high. Only minor
adjustments were made to the high temperature forecast with the
930 AM ESTF update, mainly to highlight low 70s for inland
(e.g., E PA) and northern (e.g., I-70 and I-80 corridors) areas
except at higher elevations. Highs closer to the coast should
generally be in the mid to upper 60s.


930 AM ESTF...Updated hourly PoPs/Weather grids for tonight to
slow down the onset of the steadier precipitation from south to
north tonight. Although showers will likely remain upstream of
our area this evening, will keep chance PoPs for Delmarva late
this evening to account for the possibility of precip to move
in a bit earlier than guidance is currently indicating (a
common model bias in overrunning setups).

Previous Discussion...
The warm front associated with a low pressure center over the Ohio
Valley will propagate north into our region late tonight. At the
same time, a mid level vorticity maxima will develop over eastern
PA in advance of the mid level short wave trough. The vort max may
develop before the front arrives, but still, expect enough lift with
either feature to result in widespread rain across the region,
especially after midnight. There remains some signal for heavy rain
(precipitable water values well above normal and a deep warm cloud
layer). However, the window for very high pwat values is rather
narrow, and storm motions aren`t particularly slow. Thus,
expect the flooding threat to be limited.

There is also some meager elevated instability depicted in the model
soundings, generally for locations south of a PHL to ACY line, and
generally after 09Z. Thus, kept mention of slight chance of
thunderstorms across Delmarva very late tonight, but expect
mostly rain showers.


Summary...Unsettled at times however some warmer temperatures are

Synoptic Setup...A significant upper-level through/closed low is
forecast to be shifting east Thursday from the Ohio Valley and
southeastern states. A potent short wave on the south and east
side along with a speed maximum should allow for the trough to
take on a negative tilt later Thursday, then the entire system
shifts up into New England Friday. Some brief weak ridging may
slide into the east Saturday, however the next trough amplifies
from the Midwest and Great Lakes later Saturday and Sunday. This
trough looks to close off and be slow to shift eastward through
Tuesday, sending a slow moving low pressure system to our north
while its fronts cross our area. Timing and therefore the details
are less certain given an amplified pattern with the idea of
some closed lows involved. We used a model/continuity blend for
Thursday through Friday night, then blended in the 00z WPC
Guidance thereafter. Some adjustments were then made following
additional collaboration with our neighboring offices.

For Thursday and Friday...An amplified flow will maintain a
significant upper-level trough/closed low as it migrates across
the East. The model guidance is in good agreement with a potent
short wave rotating around the base of this trough during
Thursday, then swinging to our east in the evening. This energy
is accompanied by a strong 500 mb jet streak, which assists in
making the trough take on a negative tilt. The stronger flow
however is forecast to slide to our south and east, although
Thursday starts with a southeasterly low-level jet shifting
northward. There is some diffluence to the flow aloft over our
region, which will enhance the ascent.

At the surface, low pressure is forecast to gradually move up
the Ohio Valley and into the eastern Great Lakes. Meanwhile, a
warm front lifts north across Delmarva while a new surface low
develops at a triple point. It is this new low that tracks
northeastward and strengthens while moving to near Cape Cod
early Friday morning. This development typically keeps the warm
front from clearing much of our area, therefore overunning
showers with some elevated instability may allow for some
thunder. Despite the trough taking on a negative tilt, enough
stable air across the northern half of the area may result in
less thunder. This though will depend on the location of the
warm front and thus the warm sector, as some guidance has
stronger instability in the afternoon across parts of Delmarva.
A burst of warm air advection in the morning may result in a
band of heavier showers/convection with some thunder near and
north of the warm front. The most concentrated showers may be
focused prior to midday Thursday. As of now, any severe
thunderstorm threat appears to be just to our south given the
placement of the true warm sector.

Some showers should continue Thursday night as the closed low
moves overhead, then improving conditions Friday as strengthening
low pressure moves away. However, cyclonic flow combined with
another short wave with even a surface trough reflection should
result in some showers. A gusty west-northwest wind should
develop for a time Friday as the pressure gradient tightens and
cooling occurs aloft allowing for better mixing.

For Saturday and Sunday...The flow turns more zonal for much of
this timeframe with perhaps a weak but brief ridge in the east
Saturday. The model guidance overall shows a short wave within
this flow although the timing is a bit different and therefore
it is not real clear what affect this may have on our weather.
It may enhance a frontal zone to our southwest, which gradually
lifts northward Sunday. Low pressure tied to another trough from
the Midwest to the Great Lakes slides eastward, and this will
push a cold front our way Sunday night. Given the idea of an
incoming frontal zone from the southwest and nearby short wave
energy, cannot rule out some convection mainly Sunday.

For Monday and Tuesday...A closed low is forecast to expand across
the Great Lakes and Midwest to the Ohio Valley during this time
frame. This will drive surface low pressure eastward mainly north
of the Great Lakes. The cold front is forecast to move across our
area Monday and shift offshore early Tuesday. The second half of
Monday may be convectively active, however this will depend on
short wave timing and convective development to our west. As a
result, kept chance PoPs for Monday then slight chance to chance
PoPs on Tuesday. We are anticipating warmer temperatures ahead
of the cold front.


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

Currently, 1 kft AGL CIGs are borderline MVFR-IFR at KMIV and
KACY. Expect these CIGs to increase throughout the day but
likely remain MVFR. A brief period of MVFR CIGs possible at
other TAF sites thru about 17Z but predominately VFR expected.

After 00Z, lower clouds will begin to expand further across the
region, eventually followed by rain showers. Widespread MVFR
ceilings and visibilities are expected no later than 06Z. Many
locations may see IFR conditions after 06Z.

NEly winds this morning will veer slightly out of the east this
afternoon. Wind speeds are expected to be 5-9kt this morning
and 9-13 kt this afternoon and tonight. There may be a brief
period of gusts near 20 kt mainly this afternoon.

Thursday...MVFR to IFR conditions, with showers especially during
the day. Some thunderstorms are possible mainly during the day. Some
fog possible especially at night, however stratus may dominate.

Friday...MVFR/IFR conditions should start the day, then improving to
VFR. A few showers are expected. West-northwest winds may gust up to
25 knots, then diminishing at night.

Saturday...Generally VFR, however some showers possible at night
with lowering ceilings.

Sunday...MVFR/IFR conditions possible with some showers and
afternoon thunderstorms.


Winds and seas are expected to build through the day time hours.
Seas above 5 feet are expected beginning this afternoon and
continuing at least through the overnight hours. Winds gusts
near or above 25 kt are possible this evening and overnight. On
the Delaware and southern New Jersey coastal waters, seas may
subside late tonight.

Thursday...A southeasterly low-level jet should maintain gusts of 25-
30 knots for a time Thursday before weakening across the northern
ocean zones. The seas will therefore be enhanced especially across
the northern ocean zones. Therefore, the Small Craft Advisory for
our two northern ocean zones has been extended through 22z Thursday.
For the remaining ocean zones, seas may remain elevated enough
however confidence is lower at this time.

Friday...West-northwest winds may gust to 25 knots in the afternoon
and seas may still be near 5 feet on the ocean zones. The conditions
should diminish at night. Therefore, a Small Craft Advisory may be

Saturday and Sunday...The conditions are anticipated to be below
advisory criteria, however seas may build some during Sunday.

There is a moderate risk for the formation of dangerous rip
currents along the New Jersey Shore through this evening.


Some of the highest astronomical tides of the year are occuring
through the end of the week. As a result of that and onshore flow,
minor coastal flooding is expected with the high tide this evening
and Thursday evening. One source of guidance shows water levels
along the northern NJ shore approaching moderate flooding thresholds
with the Thursday evening high tide, but this seems unlikely at this
time. Minor coastal flooding may again be possible with the Friday
evening high tide.

The Coastal Flood Advisory that was already in effect for the
New Jersey and Delaware coastline as well as most of the
Delaware Bay shoreline was expanded to include the far upper
part of the Delaware Bay and the tidal Delaware River portion
up to the Commodore Barry Bridge area (Salem County, NJ and New
Castle County, DE) this evening. Both the ESTOFS and ETSS
guidance are predicting RDYD1 (Reedy Point) to just reach local
advisory criteria (i.e., 0.3 ft above minor) at high tide this
evening. See no reason to discredit either sources guidance as
both models have verified well with this morning`s high tide.
Also, the latest tidal departures at this morning`s high was 0.7
ft above astronomical prediction. With continues onshore flow,
these positive tidal departures would only need to increase by
0.10 ft to reach advisory criteria for the next high tide cycle
this evening.


NJ...Coastal Flood Advisory from 6 PM this evening to midnight EDT
     tonight for NJZ012>014-020>027.
     Coastal Flood Advisory from 8 PM this evening to 1 AM EDT
     Thursday for NJZ016.
DE...Coastal Flood Advisory from 6 PM this evening to midnight EDT
     tonight for DEZ002>004.
     Coastal Flood Advisory from 8 PM this evening to 1 AM EDT
     Thursday for DEZ001.
MARINE...Small Craft Advisory until 6 AM EDT Thursday for ANZ452>455.
     Small Craft Advisory until 6 PM EDT Thursday for ANZ450-451.


Near Term...Klein
Short Term...Johnson/Klein
Long Term...Gorse
Tides/Coastal Flooding...Johnson/Klein is the U.S. government's official web portal to all federal, state and local government web resources and services.