Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 290351
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1151 PM EDT Tue Mar 28 2017

.SYNOPSIS...
Low pressure off the Mid Atlantic tonight will drift into the
open Atlantic tomorrow. Canadian high pressure builds into the
Northeast late 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.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
Rain/drizzle is coming to an end along the NJ coast and gradual
clearing will be developing overnight along a somewhat
increasing north wind.

Today...Partly to mostly sunny with a gusty north to Northwest
wind 10-25 mph.

Forecast basis: 50 50 blended 00z/29 GFS/NAM MOS guidance.

&&

.SHORT TERM /WEDNESDAY NIGHT/...
Fair skies (some cirrus) but also some radiational cooling.
Light north wind.

Forecast basis: 50 50 blended 00z/29 GFS/NAM MOS guidance. May
apply some cooler than guidance adjustments in the 330 AM
Wednesday fcst for Wednesday night.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
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
morning).

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.

&&

.AVIATION /04Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Through 12z Wed...VFR vsby and still some leftover IFR/MVFR
cigs which will be improving to VFR sct clouds by 12z. North
wind 5 to 15 kt.

After 12z Wed...VFR sct-bkn near 4000 ft will become VFR clr by
21z. North to northwest wind gusty 20-25 kt.

Wednesday night...VFR with some cirrus.  Light north wind.

OUTLOOK...

Thursday 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.

&&

.MARINE...
No headlines through tonight. Northerly flow becomes northwest
during the daylight hours with gusts 20 kt. A few gusts 25 kt
possible this evening in the NNJ waters when we may need to
issue a short fuse SCA, then northerly winds diminishing late
Wednesday night.

OUTLOOK...

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.

&&

.TIDES/COASTAL FLOODING...
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.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...CMS 1150
Near Term...Drag 1150
Short Term...Drag 1150
Long Term...CMS
Aviation...CMS/Drag 1150
Marine...CMS/Drag 1150
Tides/Coastal Flooding...1150



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