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fxus61 kphi 160255 
afdphi

Area forecast discussion
National Weather Service Mount Holly New Jersey
955 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/...
a frontogenesis band of precipitation to our southwest has been
persistent through the evening hours. Despite the fact that the
surface high will be near/over our region overnight, I am
concerned that the mesoscale forcing will be enough with this
band that we will see snow arrive in far northern Delmarva as
early as 4 to 6 am. Consequently, we have expanded the Winter
Weather Advisory to include Cecil County Maryland and New Castle
County de, as anything that comes in that early will be all
snow.

Have also expanded the advisory into portions of southwest New Jersey
(more on that in the short term section below).

In addition to the changes in the Winter Weather Advisory, also
adjusted pops to start earlier in northern Delmarva, and
adjusted temperatures and dew points to reflect current trends.
For most of the region from philly northward, fair weather
should continue for most, if not all of the night.

&&

Short term /6 am Monday morning through Tuesday night/...
with the late evening update, expanded the mention of sleet and
snow across southwestern New Jersey. Consequently, Salem up to
northwestern Burlington County was added in to the advisory.
Though these areas will only have very light snow accumulations,
my concern is that sleet on top of snow will create very
slippery conditions, which could coincide with the tail end of
the morning commute. Consequently, am concerned that even with
light snow amounts, there will be impacts to travel.

Models show that it will take time for the heavier frontogenesis
band to lift north (and should be weakening as it does so.
Consequently, the highest quantitative precipitation forecast should be over Delmarva and south
Jersey, and results in less quantitative precipitation forecast 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 South Dakota 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 New Jersey 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 /03z 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. East-
NE winds less than 10 kt.

Tuesday...sub-VFR in rain, except freezing rain possible at kabe. NE
winds become northwest less than 10 kt.

Tuesday night...improving conditions through the night. Northwest 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 Small Craft Advisory conditions tonight and Monday.

Outlook...

Monday night...sub-sca winds/seas expected. A chance of rain
and visibility restrictions.

Tuesday...Small Craft Advisory winds/seas probable with visibility restrictions.

Tuesday night...Small Craft Advisory conditions with strong northwest winds.

Wednesday-Wednesday night... Small Craft Advisory 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... Small Craft Advisory 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>062-103>106.
Winter Weather Advisory from 1 PM Monday to 1 PM EST Tuesday
for paz054-055.
Winter Weather Advisory from 7 am Monday to 1 am EST Tuesday
for paz070-071.
Winter Weather Advisory from 4 am Monday to 7 am EST Tuesday
for paz101-102.
New Jersey...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.
Winter Weather Advisory from 7 am Monday to 1 am EST Tuesday
for njz016>019.
Delaware...Winter Weather Advisory from 4 am to 10 PM EST Monday for
dez001.
Maryland...Winter Weather Advisory from 4 am to 10 PM EST Monday for
mdz008.
Marine...Small Craft Advisory until 1 am EST Monday for anz450-451.

&&

$$
Synopsis...O'Brien
near term...Johnson/Robertson

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