Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 230545
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1245 AM EST Tue Jan 23 2018

.SYNOPSIS...
Low pressure will track northeastward to the Saint Lawrence
River Valley and northern New England late Tuesday. An
associated strong cold front will move through our region
Tuesday afternoon. High pressure is expected to build into our
region for the remainder of the week before moving out to sea
Saturday. A cold front is then forecast to arrive on Sunday.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
1230 am update: Surface temperatures remain quite variable
across the CWA, with the northern CWA socked in with low clouds
and fog with temperatures in the upper 30s and low 40s, the
Delmarva and far southeast New Jersey around or even above 50,
the urban metropolis around 50, and the rural areas outside of
the metro in the lower 40s. To be frank, hi-res models and
near-term stat guidance have basically no clue regarding any of
this, so the updated hourly temperature forecast is based
primarily on extrapolation, at least for the next few hours.

Slowed the precip onset even more, reducing PoPs in the western
CWA by 10-20 percent through 09Z and about 10 percent
thereafter. Thinking most of the showers will hold off through
08Z or 09Z in our CWA, and the trend continues to be slower.

Recent (00Z) model guidance shows some healthy precip (QPF)
northwest of the Fall Line once the showers move in from the
west/southwest, with the 04Z HRRR showing totals exceeding an
inch by 12Z in the Lehigh Valley and southern Poconos. Cell
motions will be fast, but such precip may cause some localized
hydro issues. Notably, the WRF-NMM is lower (generally 0.5-1.0
inch(es) during this period), but the coarser guidance (e.g.,
the 00Z NAM) is picking up on higher precipitation totals with
the incoming batch of showers during the late-night period.

The general evolution of the precipitation is in reasonable
agreement amongst the hi-res guidance. The convection in
Ohio/eastern Kentucky will either progress northeast or
dissipate before reaching our area; however, a large area of
showers (perhaps with embedded storms) moves in from the
southwest. This precipitation is already developing in the
western Carolinas and southwest Virginia, so there is fairly high
confidence the models have the correct idea.

Still think the lightning threat is too small for inclusion the
rest of the night, but elevated instability will be increasing
late tonight, so cannot rule it out completely.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TODAY/...
1230 am update: A quick look at the 00Z model output for this
period suggests that general thinking remains unchanged from the
discussion below. Several batches of showers/storms look to move
through the area through at least 18Z. Marginal instability will
exist, but there remain suggestions of some surface-based
instability in the immediate vicinity of the bands of
convection. With very strong winds just off the surface,
isolated damaging winds are possible. Hodographs are basically
off the charts as well, with decent veering in the surface to
700-mb layer. Forecast SRH of 300+ J/kg is certainly favorable
for an isolated tornado, with of course the main question being
will there be sufficient instability for any such occurrence.
With the widespread nature of the showers expected during the
morning hours, I am awfully skeptical, but unfortunately, this
will not become clear until the initial batch(es) of showers
reach the area.

I am becoming more concerned about heavy rain potential,
primarily north of the Mason-Dixon Line. The hi-res model output
is showing storm totals 1-2 inches in general (highest with the
HRRR), especially north/west of Philly. I bumped up QPF with
this update for most spots, though the drier look of the GFS did
not prompt me to increase amounts considerably (yet).

Previous discussion...

Low pressure will track from Michigan across Ontario into Quebec by
late in the day Tuesday. This will drag a strong cold front across
our area with numerous showers. Showers are expected to quickly
overspread the area in the morning and persist into the afternoon.

The forecast models, especially the NAM, are depicting enough
elevated CAPE for at least a few thunderstorms. Also, worth noting,
the mid level lapse rates will be fairly steep (near 7 C/km) with a
very strong low-level jet ahead of the cold front (around 60 knots
at 925 mb). If convective elements are able to become surface-based,
there is more of a concern for locally damaging winds mixing down to
the surface. This is especially for southeast PA, southern NJ, and
Delmarva where the instability will be the greatest. The Storm
Prediction Center has these areas in a marginal risk for severe
storms. The limiting factor will be that the low levels will be
fairly stable so confidence of severe weather is not high. The most
likely window for this to occur looks to be mid morning through
early afternoon after which time the cold front will clear the area
with showers ending. Also, PWATs will be up to 1.25 inches so
locally heavy rain will occur which can lead to local poor drainage
flooding.

Highs Tuesday range from near 50 across the southern Poconos to the
upper 50s to mid/upper 60s across SE PA, southern NJ, and
Delmarva.

&&

.LONG TERM /TONIGHT THROUGH MONDAY/...
Surface low pressure is forecast to be located in Maine on Tuesday
evening. The system is expected to strengthen as it moves across
Atlantic Canada on Tuesday night and Wednesday. The gradient between
the departing low and high pressure building into our region from
the west should result in brisk conditions developing for Wednesday.
A west northwest wind around 10 to 20 MPH is anticipated at that
time.

Surface high pressure should move from the south central states on
Wednesday to the Carolinas and Virginia late on Thursday. The center
of the high is anticipated to slide over the western North Atlantic
on Friday and Saturday. The air mass is forecast to bring dry
weather conditions for the period from Wednesday into Saturday.
Temperatures are expected to be seasonable on Wednesday and Thursday
with a warming trend as we head toward and into the weekend.

Low pressure and a cold front are forecast to approach from the west
over the weekend. There is a spread in the guidance solutions
regarding the timing of the arrival of the cold front in our region.
We will carry a chance of showers for Sunday and Monday.

&&

.AVIATION /05Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Rest of tonight...Some patchy fog will occur at ABE/TTN with
occasional low stratus at ACY through 09Z or so. Showers will
move in from the southwest after 09Z, with CIGs/VSBYs becoming
sub-VFR promptly during this period. Moderate to heavy showers
may occur at times and with little warning, causing quick/brief
reductions to IFR/LIFR. Low-level wind shear will continue
through the night (generally southwest 35-50+ kts at 2000 feet).
Winds generally east or northeast at ABE/TTN and more
southeast/south elsewhere, with speeds below 10 kts. Overall
confidence is slightly above average.

Tuesday...Generally sub-VFR through 20Z with rapid improvement
west to east thereafter. Showers likely through the morning, and
at least a slight chance of thunder. For now, no mention in the
TAFs as coverage/timing are very uncertain. Winds will become
southwest during the day, possibly becoming gusty during the
afternoon. Showers are capable of producing strong/erratic gusts
and localized convective turbulence. Overall confidence is
average.

Outlook...
Tuesday night through Saturday...Mainly VFR.

&&

.MARINE...
For tonight, south winds should begin to increase, especially
after midnight, with speeds reaching small-craft advisory criteria a
couple of hours before daybreak. Also, areas of fog may develop over
the waters overnight with showers arriving west to east near
daybreak.

Tuesday...Strong Small Craft Advisory conditions over the waters
with a few gusts to Gale force possible around midday, especially in
any heavier showers/thunder. Since this looks to be brief, we opted
for SCA, not Gale Warning. Marine Weather Statements and possibly a
Special Marine Warning may be needed for these locally stronger
winds. SCA conditions expected for DE Bay Tuesday. South winds in
the morning shift to the west in the afternoon behind the passage of
a cold front.

Outlook...
Tuesday night...A Small Craft Advisory is in effect on our ocean waters
for westerly wind gusts around 25 knots.

Wednesday through Thursday...A west northwest wind could gust around
25 knots and the Small Craft Advisory may be extended in time.

Thursday night through Saturday...No marine headlines are
anticipated.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 4 AM early this morning to 6 AM EST
     Wednesday for ANZ450>455.
     Small Craft Advisory from 4 AM early this morning to 6 PM EST
     this evening for ANZ430-431.

&&

$$
Synopsis...Gorse/Iovino
Near Term...CMS
Short Term...CMS/Fitzsimmons/Gorse
Long Term...Iovino
Aviation...CMS/Iovino
Marine...Fitzsimmons/Iovino


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