Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS61 KPHI 282003

Area Forecast Discussion
National Weather Service Mount Holly NJ
403 PM EDT Tue Mar 28 2017

Surface low pressure will move east from the Mid-Atlantic region
into the open Atlantic tomorrow. Canadian high pressure builds into
the Northeast Wednesday and Thursday. Low pressure develops in the
central U.S. on Thursday, moves east into the Mid-Atlantic on
Friday, and progresses offshore Saturday. High pressure returns to
the area by late in the weekend. Another surface low may affect the
region by the middle of next week.


Weak low pressure lies along a stationary front just over the
Delmarva. Meanwhile, a cold front is pushing through central NY/PA.
Low pressure will lift to the northeast this evening, followed by
the cold front later tonight.

The first area of rain is pushing northeast through northern NJ and
the Poconos, and patchy fog is developing in its wake. There will be
some locally dense fog, but it is not widespread enough to warrant a
Dense Fog Advisory.

Latest HRRR indicates a secondary area of showers and possible
thunderstorms near the Delmarva. This is also shown in the 12Z NAM
and 12Z GFS. This area then lifts to the north and east through this
evening as low pressure moves off the Atlantic coast. E-NE flow on
the northern side of the stationary front is keeping NJ/PA stable,
and just south of the front is a warmer and more unstable airmass,
with 250-500 J/kg SB CAPE across DE/MD. Much higher SB CAPE values
can be found across VA (1000-2000 J/kg) where storms are beginning
to develop. Would expect storms to move into the Delmarva, but do
not think they will last long as they track to the north and east.
Will keep chances for thunderstorms capped at isolated.

Wave of low pressure departs early this evening, but it will take
some time for the cold front behind it to pass through the region.
It will then take even more time for northerly winds to increase
enough for drier air to filter into the region. Patchy fog will
develop this evening, and will persist until ending prior to
daybreak Wednesday.

Lows tonight will range from the mid and upper 30s across the
Poconos and northern NJ to the lower 40s for the rest of NJ and SE
PA, and in the mid to upper 40s in MD/DE.


High pressure builds in from the north, bringing plenty of sunshine.
A slight downsloping flow will help moderate the temperatures, and
highs will top off in the mid 40s in the Poconos, upper 50s across
northern NJ and the Lehigh Valley, low 60s across central and
southern NJ and SE PA, and in the mid to upper 60s in the


The long term period starts off quiet as a surface ridge axis
passes overhead Thursday morning. Surface flow becomes
northeasterly with time, and the cool/maritime origins of this
flow suggest Thursday may be a bit cooler than Wednesday,
especially if increasing cloudiness occurs early in the day.
This increased cloudiness would commence downstream of another
potent southern-stream vorticity maximum progressing eastward
from the central plains. Broad southerly low-level flow
amplifies downstream ridging in the eastern U.S., with
substantial warm advection in much of the Midwest.

However, with the retreating surface high in eastern Canada,
this is a favorable setup for cold air damming east of the
Appalachians, particularly with the more southerly track of the
aforementioned vort max. As with the systems affecting the area
early this week, I think the models are overly aggressive
sweeping the baroclinic zone/associated warm front north through
the region Thursday night and Friday. Though the 12Z GFS is a
bit of an outlier with the southern/slower track, the
disagreement in the midlevels between the 12Z CMC/ECMWF do not
provide confidence enough to discard the GFS solution, which
would favor a somewhat colder scenario for our area (though the
GFS is itself switching winds to south way too fast in such a
setup). For temperatures, generally went somewhat below guidance
Thursday night and Friday, and my suspicion is that I am still
too warm during both periods.

The implications of this are most important for the southern
Poconos and Sussex County, NJ, where precipitation will likely
begin Thursday night and temperatures will once again flirt with
the freezing mark. Model guidance is at least somewhat
suggestive of a wintry mix of precipitation in this region, and
thermal profiles suggest the presence of a warm nose, which may
mean that freezing rain and/or sleet would be possible. For now,
kept things simple with a mix of rain and snow in this area
(particularly with surface temperatures forecast slightly above
freezing), but later shifts may need to include mention of sleet
or freezing rain in this region. By some time on Friday,
temperatures are expected to warm here enough for precipitation
to be all rain, but given lackluster performance with models
this winter scouring out this cold air in an accurate manner,
current forecast may be overly optimistic (at least Friday

A sustained southerly low-level fetch downstream of the surface
low moving through the region on Friday brings PWATs > 1.0 inch
(approaching 1.25 inches) to central/southern portions of the
CWA. With aid of upper-level jet coupling, substantial
differential cyclonic vorticity advection, and considerable low-
level isentropic ascent, widespread rain should develop on
Friday and continue through Friday night as the low approaches
the Mid-Atlantic coast. QPF looks quite decent, with widespread
0.5-1.5 inch totals possible (and perhaps more, if some
simulations verify). The potential for storms seems limited at
this time, given the CWA`s position generally along/north of the
surface low track. However, if the warm sector is able to move
more poleward than currently forecast, convection may also be a
consideration (particularly for Delmarva).

The surface low is expected to move offshore by Saturday, but
wraparound showers may still occur, particularly north of the
Mason-Dixon Line. Only gradually lowered PoPs late Friday night
and Saturday given this possibility. Winds will switch to the
north, but temperatures may actually be a little warmer than
Friday`s readings given greater potential for sunshine, subtle
downsloping, and boundary-layer mixing. However, not sold on the
very warm MEXMOS readings at this point given the origins of the
upstream surface high.

Sunday and Monday should generally be dry as upstream ridging
moves through the region. Temperatures will be near to slightly
above seasonal averages.

Another in a series of southern-stream systems is projected to
move into the Ohio/Tennessee Valleys by early next week. With
increased large-scale ascent downstream of the attendant surface
low, precipitation will probably break out Monday night and
Tuesday across the area. This low`s track would be farther to
the west, which suggests this system will be warmer, giving our
region greater potential for convection. Did not add thunder to
the grids at this point given track/timing uncertainties, but
think this is a good bet given relatively decent agreement among
the model suite. Models suggest a reinforcing system may
approach the area midweek, keeping the QPF train chugging.


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

Tonight...MVFR/IFR conditions possible in showers and fog early.
Isolated thunderstorms possible early, but confidence not high
enough to warrant inclusion in the TAF. Sub-VFR CIGs/VSBYs will
continue for much of tonight in fog after showers depart.
Improvement to VFR not until prior to daybreak Wednesday. LGT/VRB
winds this evening, becoming N 5-10 late tonight.


Wednesday night through Thursday night...VFR with generally
light winds. Confidence well above average.

Friday through Saturday...Sub-VFR conditions likely, with
lowered CIGs and VSBYs and periods of rain. East or southeast
winds Friday around 10 kts becoming northerly and potentially
gusty on Saturday. Confidence average.

Saturday night and Sunday...VFR with north or northwest winds
around 10 kts with at least some potential for gusts to 20 kts.
Confidence average.


VSBY reductions possible in showers and fog tonight, with
improvements not until well after midnight. Unlimited VSBYs expected
on Wednesday. Sub-SCA conditions tonight through Wednesday, but some
gusts to 20 KT possible Wednesday morning/early afternoon, mainly on
the ocean.


Wednesday night...Marginal small craft advisory conditions
possible, especially off the New Jersey coast.

Thursday and Thursday night...Sub-advisory conditions expected.

Friday through Saturday...Marginal advisory conditions possible.
Rain likely.

Saturday night and Sunday...Sub-advisory conditions expected.


Though astronomical tides will be gradually diminishing through the
week now that we are past the new moon, the threat of minor tidal
flooding along the NJ and DE Atlantic coasts increases late in the
week. This is a result of a low pressure system bringing a prolonged
period of on shore flow. The tide of most concern at this point is
the high tide on Friday evening/late Friday night. By this tide
cycle, it will take a surge of 0.8 to 1.0 feet to reach minor
flooding thresholds, which is possible but still uncertain (it
will be dependent on how quickly the on shore flow develops and
how strong it will be by then). At least one source of guidance
shows water levels reaching minor tidal flooding thresholds with
the Thursday evening/night high tide, but that seems unlikely
as the latest forecast depicts onshore flow either developing
right around or just after the time of that high tide.


KNEL appears to be reading 10F too warm the past several days.




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