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000 
FXUS61 KPHI 221151
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
651 AM EST Wed Nov 22 2017

.SYNOPSIS...
A cold front will move across the area early this morning while
an area of low pressure moves northeastward offshore the Mid 
Atlantic coast. High pressure will build toward the area 
tonight, and control the weather Thursday and Friday. A cold 
front is forecast to move across the area on Saturday while an 
area of low pressure moves northeastward offshore the Mid 
Atlantic coast. High pressure builds across the area Monday, 
then offshore Tuesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
At 630 am, a band of showers was moving through the urban
corridor, extending southward through Delmarva. This will be the
main show for the metropolitan area, with much higher rainfall
totals farther to the southeast, where showers have occurred for
much of the night. Current PoPs/Wx grids look good and made no
changes with this update. Stronger convective cores exist within
the larger band of showers, so keeping mention of slight chance
of thunder remains a good idea. Several lightning strikes have
occurred overnight, especially off the coast, but also for areas
of southern Maryland/Delaware and eastern New Jersey.

A few sites in Delmarva and southeast New Jersey will exceed
0.75 inches, with some localized 1"+ totals already recorded.
Most hi-res guidance ended up doing reasonably well with the
event, though generally biased westward with the QPE maximum.

Some light showers may occur in the southern Poconos and
vicinity this morning in association with the upstream vort max
moving through the Northeast today. Temperatures are generally
too warm for snow, but some of the colder spots in
Carbon/Monroe/Sussex (NJ) Counties may see a few flakes before 
all is said and done. Impacts will be negligible given the very 
light/very brief duration of the precip.

Latest hi-res guidance keeps showers lingering along the coast
through late morning, so clouds may stick around longer in these
locations. Otherwise, drying/some clearing should be expected
farther to the west, with northwest winds becoming gusty this
afternoon as a cold front moves through the area. Temperatures
will remain fairly steady/mild this morning as cold-air
advection lags the front but should trend downward (slowly) this
afternoon. Made no changes to temperature forecast with this
update.

Still a little uncertain about cloud cover later this morning
and this afternoon. The model forecasts look overly optimistic,
but given the fairly strong agreement, I am hesitant to deviate
in the negative direction. Still, a longer lasting
scattered/broken stratocumulus deck would not surprise me,
especially if winds/mixing are stronger than forecast (with the
residual moisture from this morning's rain contributing to
generation/maintenance of the clouds underneath midlevel
drying/descent).

Safe travels today.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
Upstream of the potent vort max moving through the region 
today, a surface high will build into the Appalachians 
overnight. With clear skies and a decreasing surface pressure 
gradient, temperatures will be much colder tonight. Forecast 
lows are several degrees below seasonal averages, ranging from 
around 20 in the southern Poconos to around 30 in the urban 
corridor and at the coast. Northwest winds will be decreasing 
during the period, possibly becoming light and variable late.

The main uncertainty with the forecast is how quickly the winds
will die down, with the potential for some temperature error if
winds diminish faster than progged (i.e., it would be colder). 
For now, I used a model blend, as I do not have much of a feel 
for how quickly the winds decouple tonight. With the trend for 
the past day being subtly slower with the incoming high (and 
departing vort max), it may take a bit longer for the winds to 
diminish than the models suggest.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Fairly quiet weather is expected for most of the extended 
period through next Tuesday, with a chance of isolated showers 
this weekend.

High pressure is forecast to build across the area Thursday 
into Friday and bring dry weather to the region. Temperatures 
are forecast to be near or slightly below normal Thursday, 
before warming back to near or slightly above normal Friday. 

The high builds offshore Friday night in advance of an 
approaching cold front coming through the area Saturday. Also, 
an area of low pressure is forecast to be moving northeastward 
offshore of the Mid- Atlantic coast on Saturday. This low is 
currently expected to remain far enough offshore to not bring 
major effects to the area Saturday. However, as the front moves 
through Saturday there is a chance of isolated/scattered showers
across the area. For Saturday night into Sunday, the low moves 
farther to our northeast with high pressure to our west. This 
will keep a strong northwest flow across the area Saturday night
and Sunday. It is possible that some isolated lake effect 
showers could make their way across our area Saturday night into
Sunday in the northwest flow. Temperatures remain above normal 
Saturday before the front moves through the area, but falls back
to near normal or slightly below for Sunday.

By Sunday night, high pressure begins to approach from the west
and the flow turns more northerly, cutting off the lake effect 
showers potential across our area. The high pressure fully 
builds across the area Monday, then pushes offshore by Tuesday. 
This will bring dry weather back to the area early next week. 
Temperatures are expected to be near normal or slightly below 
for Monday, then warm back to above normal Tuesday as return 
flow develops.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas.

Today...From KPHL northwestward, a few showers possible through
14Z, with brief sub-VFR CIGs/VSBYs with the heavier precip. For
MIV/ACY, more prolonged sub-VFR conditions are likely in 
heavier showers through the morning. Thereafter, A SCT-BKN deck 
around 3500-5000 feet may continue through 18Z or so across the 
area. Light southwest winds will veer quickly to northwest and 
become gusty this morning (speeds 10-15 kts with gusts to 25 kts
during the afternoon). Confidence low to medium this morning; 
high this afternoon. Frequent updates to the TAFs are likely 
during from 12Z to 15Z as precipitation moves through.

Tonight...VFR with northwest winds diminishing. Confidence 
high.

OUTLOOK... 

Thursday-Friday night...VFR expected. Westerly winds Thursday-
early Friday. Becoming southwest later Friday into Friday night.
High confidence.

Saturday-Sunday...Generally VFR. Isolated showers possible 
which may occasionally lower conditions. Southwest winds early 
Saturday, becoming northwest and gusting 20-25 knots Saturday 
night into Sunday. Moderate confidence on showers and sub-VFR 
conditions, high confidence on winds.

&&

.MARINE...
No changes to the small craft advisories this morning as 
northwest winds will pick up late this morning and exceed 
criteria easily this afternoon and evening. Seas will likely 
hover around 5 feet through tonight, as well. Showers, possibly 
with a couple lightning strikes, are expected through the 
morning hours, before improvement this afternoon.

OUTLOOK... 

Thursday-Saturday...Sub Small Craft Advisory conditions 
expected, although seas may be around 4 feet Thursday before 
subsiding.

Saturday night-Sunday...Small Craft Advisory conditions likely 
behind cold frontal passage.

&&

.EQUIPMENT...
The KDOX radar remains out of service. Replacement parts are 
expected to arrive on Wednesday, November 22nd.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 AM EST Thursday for ANZ450>455.
     Small Craft Advisory from noon today to midnight EST tonight 
     for ANZ430-431.

&&

$$
Synopsis...Robertson
Near Term...CMS

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