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000 
FXUS61 KPHI 160002
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
702 PM EST Sun Dec 15 2019

.SYNOPSIS...
High pressure will move from overhead of the region tonight to 
offshore on Monday morning. A warm front will slowly lift through 
part of the area on Monday. A wave of low pressure tracking along 
the warm front will move across the area on Tuesday before departing 
Tuesday night. An Arctic cold front will move through the region on 
Wednesday. High pressure is expected to build in for the end of the 
week.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Winds have diminished, so only minor changes with the early
evening update to reduce the wind speeds through the evening
hours.

Most of the night will remain precipitation free until very late in 
the night. Any clouds from the daytime will dissipate by the early 
evening, before more clouds fill in overnight. If clouds remain 
cleared out long enough and winds become light enough, temperatures 
may actually cool down more than forecast which could have an effect 
on precipitation types as we go into Monday. There is a chance that 
some precipitation could begin during the early morning hours, 
just before sunrise. It looks like once precipitation moves in, 
it would begin as mostly snow everywhere, except across southern
Delaware and Maryland where a mix of rain, sleet, and snow 
could occur at the onset. Little to no accumulation is expected 
through 7 am Monday, but that will change during the day as 
discussed below.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY NIGHT/...
Some of the details of the upcoming winter weather event are 
beginning to resolve themselves. 150 kt 300 mb jet streak will lift 
from south to north through the region Monday morning, and behind 
it, a warm front will extend out ahead of developing low pressure 
over the Gulf Coast states. Meanwhile, surface high pressure over 
New Jersey Monday morning will move out to sea in the afternoon.

Light overrunning precip will develop out ahead of the warm front 
and will move into Delmarva and the Delaware Valley Monday by late 
Monday morning. Initially, temperatures will be cold enough to 
support snow throughout, but as the warm front lifts north, 
temperatures will warm above freezing, and the rain/snow line should 
lift to around Philadelphia by lunchtime. Snow, or a mix of rain and 
snow, will continue across the Lehigh valley, and most of northern 
New Jersey, while snow continues north of I-80.

The change with this forecast compared with the previous model runs 
is that the low is taking a bit more of a southern track. This 
brings the heaviest QPF a bit farther south, and results in less QPF 
for the southern Poconos, lehigh Valley, and far northern New 
Jersey, and slightly higher amounts for southern New Jersey and 
Delmarva. For the southern Poconos, this also results in slightly 
lower forecast snow amounts, now generally on the order of 1-2 
inches for Monday and Monday night.

Precip ramps up Monday night and Tuesday as the low lifts to the 
north, and passes over Delmarva and southern New Jersey by Tuesday 
afternoon. 

In terms of sensible weather/p-type, precip should change to all 
rain across southern New Jersey, portions of southeast Pennsylvania, 
and Delmarva by late Monday afternoon, and should continue as rain 
through the duration of the event into Tuesday evening as 1000-500 
mb thicknesses rise to above 550 dam and 850 mb temps will be above 
+6C. Moderate to locally heavy rain is possible with the passage of 
the low late Monday night through Tuesday afternoon. Over an inch of 
liquid equivalent is possible during the duration of the event.

For areas mainly along and north of the Fall Line, 850 mb temps 
gradually warm to above freezing, but it will take some time for 
surface temps to rise above freezing. As a result, a prolonged 
period of sleet and freezing rain, eventually changing to rain and 
freezing rain should persist across those areas Monday night through 
Tuesday morning, then precip changes to plain rain.

For the southern Poconos, the wintry weather continues through 
Tuesday afternoon.

Will go ahead and issue a Winter Weather Advisory for areas mainly 
along and north and west of I-95 from Monday afternoon through 
Tuesday morning, and for areas north of I-80 until Tuesday 
afternoon. 

Low pressure departs Tuesday evening and moves out to sea Tuesday 
night. Behind the departing low, a surge of colder air filters into 
the region as a cold front works its way to the south and east. 
There may be some wintry precip, especially up north, with the cold 
frontal passage, but conditions should dry out after midnight 
Tuesday night.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Overview...

A fairly progressive pattern for the middle and latter part of the 
week. At upper levels, a sharp shortwave trough will bring a shot of 
cold air to the region for the midweek period. This could be 
accompanied by scattered snow squalls in parts of the area. Heading 
later into the week, there will be a gradual trend for the building 
of extensive ridging over the central US, likely leading to 
significant warm anomalies there over the next week or two. However, 
current indications are that at least through this period, the 
prevailing ridge axis location will be too far west for those large 
warm anomalies to work into the Northeast and mid-Atlantic. A 
continued tendency for cold surface high pressure to be centered to 
our north will also make it difficult for us to see more than a 
tempered and gradual warm-up even as ridging to our west expands 
further towards the weekend. There are also hints of a coastal storm 
developing next weekend, but this remains highly uncertain.

Dailies...

Wednesday-Wednesday night... An Arctic shortwave and associated 
surface low and cold front will dive southeast into New England on 
Wednesday. Very robust shortwave with 500mb height anomalies in the 
range of -2 to -2.5 SD in the base of the trough. 850mb temperature 
anomalies as low as -20C in the heart of this trough. So this is 
going to bring quite a cold blast to the Northeast and mid-Atlantic 
for midweek. The main question for Wednesday is whether this Arctic 
blast will be preceded by snow squalls. This will largely depend on 
trough positioning. Overall, looks to be a favorable, potentially 
very favorable, setup for snow squalls in areas that experience the 
strongest forcing and lift from the trough and front. The concern 
locally has been whether we may be too far south to experience the 
more favorable dynamics. In those dynamically favored areas, most 
guidance indicates an environment of steep low level lapse rates, 
non-zero CAPE (~25-75J/kg), and high relative humidities (>80%) in 
the boundary layer and snow growth zone. These are all the things 
one would look for to see snow squalls, and most model Snow Squall 
Parameter values are also quite high ahead of the front. I fully 
expect intense snow squalls in parts of the Northeast on Wednesday, 
and currently think northern NJ and the Poconos are probably fair 
game for this potential, but am less confident further south. I 
bumped PoPs up to chance across the far northwest and expanded 
slight chance PoPs a little further southeast to account for this. 
We will continue to watch this over the next couple days. Overnight, 
behind the front, plunging temperatures as Arctic air pours into the 
region. Teens and low 20s will be common for lows with wind chill 
values in the single digits. Did not want to take temps quite a low 
as some of the colder guidance due to the wind allowing for 
continued mixing, but the wind chills will make it feel colder.

Thursday-Friday... High pressure in control behind the Arctic front, 
so it should be dry, but cold. In particular, the daytime period on 
Thursday looks very cold. Many areas will fail to get out of the 20s 
during the day, with some teens for highs in the far north. Thursday 
night looks like an excellent radiational cooling setup as high 
pressure crests overhead, so even with some warming temperatures 
aloft I expect it to be about as cold as Wednesday night, though 
without much wind it will not feel quite as cold. Some moderation by 
Friday as heights rise and 850mb temperatures warm considerably. 
Most areas should add about 10 degrees to their Thursday highs, 
though that still leaves us below average.

Next weekend... Lots of uncertainty as usual this far in advance. 
Global model guidance, especially the GFS and EC, had been showing 
potential for the development of a phased coastal storm over the 
weekend. They have backed off that idea on their latest runs due to 
keeping the northern and southern streams separate. We have many 
days to watch this and there will probably be more large swings in 
the guidance in the days ahead. This period should start to come 
into better focus towards the middle of the week as the storm from 
the early part of this week moves away.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, 
KMIV, KACY and surrounding areas.

Tonight...VFR conditions are expected overnight, with increasing
mid level clouds after 06Z. Initial bands of precipitation 
could get as far north as Wilmington between 09 and 12Z, and if
they do, then reductions to MVFR conditions are possible at that
terminal. Winds will become light and variable overnight.
Moderate confidence.

Monday...VFR conditions will deteriorate to MVFR and eventually
IFR for much of the region through the day as precipitation
moves in. Precipitation is expected to start as mostly light
snow, but could mix with sleet between 15 and 21Z, before
changing over to rain or freezing rain 21Z or later. Light and 
variable winds early will become northeast to east 5-10 knots.
Moderate confidence in the overall pattern, but low confidence
in the details especially with the timing of the transition.

Outlook... 

Monday night...Sub-VFR in rain at I-95 corridor terminals and for 
KMIV/KACY. Sub-VFR in snow, sleet, and freezing rain at KRDG/KABE. E-
NE winds less than 10 kt. 

Tuesday...Sub-VFR in rain, except freezing rain possible at KABE. NE 
winds become NW less than 10 kt.

Tuesday night...Improving conditions through the night. NW winds 10-
15 kt with gusts up to 20 kt.

Wednesday-Wednesday night... Mainly VFR. Slight chance of an 
afternoon or evening snow shower or squall with brief sub-VFR 
conditions especially north of TTN. West-northwest wind of 10 to 15 
kt with gusts up to 30 kt possible, diminishing overnight. Moderate 
confidence.

Thursday-Friday... VFR. Light northwest winds becoming light and 
variable on Friday. High confidence.

&&

.MARINE...
Winds and seas continue to diminish, so the gale warning was
cancelled. For the northern coastal waters, small craft advisory
conditions could linger through the evening. Otherwise, winds
and seas should remain below SCA conditions tonight and Monday.

Outlook... 

Monday night...Sub-SCA winds/seas expected. A chance of rain 
and VSBY restrictions.

Tuesday...SCA winds/seas probable with VSBY restrictions.

Tuesday night...SCA conditions with strong NW winds.

Wednesday-Wednesday night... SCA conditions expected with a period 
of gale force conditions possible due to northwest winds gusting up 
to 35 kt. Seas 3 to 5 feet.

Thursday-Thursday night... SCA conditions may linger into Thursday 
morning, but are expected to diminish below advisory levels for the 
afternoon through the overnight.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory from 1 PM Monday to 7 AM EST Tuesday 
     for PAZ060>062-070-071-101>106.
     Winter Weather Advisory from 1 PM Monday to 1 PM EST Tuesday 
     for PAZ054-055.
NJ...Winter Weather Advisory from 1 PM Monday to 7 AM EST Tuesday 
     for NJZ007>010-012-015.
     Winter Weather Advisory from 1 PM Monday to 1 PM EST Tuesday 
     for NJZ001.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 1 AM EST Monday for ANZ450-451.

&&

$$
Synopsis...O'Brien

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