NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Dec 25, 12:52 AM EST, 38 minutes earlier.
Area forecast discussion, Dec 25, 12:52 AM EST, 38 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
1252 AM EST Sat Dec 25 2021
.SYNOPSIS...
Low pressure is forecast to move from the Great Lakes tonight
to the waters off New England on Saturday night. It is expected
to pull a warm front through our region late tonight and
Saturday morning, followed by a cold front on Saturday night.
High pressure is anticipated to arrive in the wake of the cold
front on Sunday. Another frontal boundary is forecast to lift
into our region early in the new week and it should remain in
our vicinity until a wave of low pressure pulls it away to our
southeast on Thursday. High pressure should follow for Friday.
&&
.NEAR TERM /THROUGH TODAY/...
1000 pm update: Made some changes to PoPs, QPF, and ice
accumulation forecasts overnight into Saturday, using the latest
hi-res guidance, radar/satellite trends, and late-evening
surface observations.
The most significant change was a delay in the onset of
precipitation for most of the area, notably north of I-78 and
within the winter weather advisory area. Surface observations
indicate sizable dew point depressions in most of the area,
though they are smaller up north. Overall, think the models are
moistening the low levels too quickly in advance of the system,
a fairly typical bias. Guidance has been trending slower with
precipitation onset for a while, and though radar returns are
gradually increasing, the rate of increase is not very
impressive. Generally slowed onset by about 2-3 hours in most
spots. Temperatures have been verifying well this evening, and
think the distribution of rain versus freezing rain appears
reasonable during the initial bout of precipitation late
tonight. However, given the later onset, think the duration of
any icing will be rather small, so did trim ice accumulations
within the advisory a few hundredths. Currently, think impacts
with this event will be rather limited (and probably tied to
elevation), with most spots freezing-rain-free (too warm) by
12z. The advisory timing/location needed no changing at this
point.
Meanwhile, guidance is a little more bullish with QPF once the
low levels saturate. As a result, increased PoPs farther south a
good bit late tonight into tomorrow morning. Though rain will
be occasional at best (and light), think most spots in PA and
northern/central NJ will see measurable amounts, and chances are
reasonably good into southern NJ and at least northern Delmarva
as well.
Chances are probably better later in the morning into the early
afternoon, as a low-amplitude perturbation zips across the
northern Mid-Atlantic, with stronger attendant lift. Most of
the latest guidance has a round of light rain (probably more
showery in nature) moving across most of the CWA between 15z and
20z. Thus, bumped PoPs considerably during this period of time
as well, especially between I-76 and I-78.
Have a safe and wonderful holiday. Previous discussion
follows...
Warm front is expected to continue to lift north through the
region overnight into Christmas morning. In the mid levels, the
ridge axis will shift east of the region after 06Z tonight. By
tomorrow, mid level flow is expected to be mostly zonal, and the
center of the surface high will pass from west to east just
north of our region. The net result is that we will have a few
rounds of precipitation starting late tonight and continuing
into Christmas Day.
The main period of concern for any impacts will be late tonight
(primarily after midnight) with the first round of
precipitation. While much of the precipitation should be all
rain, periods of freezing rain will be possible for areas in
Carbon (PA), Monroe (PA), Warren (NJ) and Sussex (NJ) Counties,
mostly along and north of I-80 in these counties. Will also be
watching trends closely in the Lehigh Valley and Morris (NJ)
County through this evening, but these areas are starting out
much warmer than the advisory area, so the risk for freezing
rain is considerably lower there.
As for ice amounts within the advisory area, ice amounts should
be light. As is typical with warm air advection regimes, this
looks to be a light precip event overall, so ice amounts should
be less than a tenth of an inch for the most part. Consequently,
the main impacts of concern are icy surfaces, especially for
untreated roads, sidewalks, and bridges.
By tomorrow morning, winds will shift out of the south,
resulting in temperatures climbing above freezing. Consequently
any precipitation during the day tomorrow should be all rain.
Highs tomorrow should range from the mid 40s for the southern
Poconos and NW NJ to lower 60s in portions of Delmarva.
&&
.SHORT TERM /TONIGHT THROUGH MONDAY/...
A strengthening surface low is forecast to be off the New
England coast on Saturday evening, on its way out to sea. The
low is expected to pull a trailing cold front through our region
on Saturday night. We are anticipating some lingering light
rain and clouds until the boundary passes, then a gradual
decrease in cloud cover late on Saturday night. Lows should
favor the 35 to 45 degree range.
A surface ridge is expected to build eastward and into our
region on Sunday and Sunday night, bringing us dry weather
conditions and above normal temperatures.
Low pressure in the Great Lakes is forecast to pull another
warm front toward our area on Monday. Clouds will increase and
lower on Monday in the developing warm advection. The air near
the surface will be slow to moisten. While there is an
increasing chance of light precipitation as the day progresses,
it may not reach our entire forecast area. Presently, the
expected temperature profiles suggest the potential for liquid
precipitation for the Interstate 95 Corridor southeastward. It
could begin as a brief period of freezing rain and/or ice
pellets in areas to the northwest, before changing to light
rain. Highs should range from the middle 30s on the Pocono
Plateau to the middle 40s in southern Delaware.
&&
.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
A frontal boundary is forecast to meander in or near our region
from Monday night through Thursday, resulting in an unsettled
period. We are anticipating mostly cloudy conditions. Weak
impulses are expected to ride along the front, bringing periods
of precipitation. The trend should favor warming temperatures,
so the precipitation will be mostly in the form of rain. It
appears as though the greatest potential for rain will be from
Tuesday night into Wednesday.
Low pressure riding along the front should pass off our coast
early on Thursday. Dry air is forecast to build into our region
in its wake for Thursday night and Friday. Temperatures will
likely remain above normal to end 2021.
&&
.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Tonight... All TAF sites are currently VFR this morning with
weak returns on radar slowly approaching KRDG/ KABE. The low
levels of the atmosphere remain rather dry though, so this
initial batch of precipitation will likely struggle to reach the
ground. Sub- VFR cigs will also likely be delayed a bit, and
should arrive across the western TAF sites in the next couple of
hours. Towards the coastal TAF sites, full saturation doesn't
really occur tonight, and therefore continued VFR conditions
appears likely. A weak area of low pressure, that is currently
over Illinois, will approach from the west with LLWS developing
across all terminals.
Confidence is low with CIGs and very low with VSBYs after 06z
at RDG/ABE/TTN and moderate elsewhere. Confidence is moderate
with winds and LLWS after 06z at all TAF sites.
Saturday...Sub-VFR conditions will likely prevail at ABE and to
some extent RDG/TTN through much of the day (especially the
morning), with VFR likely by late afternoon. Elsewhere, VFR is
expected to prevail through the day. Occasional light
rain/showers are possible at all TAF sites, but impacts will
likely be limited to RDG/ABE/TTN (or at the very least brief in
nature). Surface winds will be light and variable, but southwest
LLWS will likely continue through at least 18z. Surface winds
are forecast to become southwest or west by late afternoon and
increase to around 10 kt (possibly with a few gusts).
Confidence is low on all aspects of the forecast at RDG/ABE/TTN
and moderate elsewhere.
Saturday night...A weak cold front will cross the terminals with
a potential for some transient stratus on the cool side of the
boundary. Both the GFS and NAM actually show this, which makes
sense given the lingering low level moisture. After about 06z
guidance has the drier air making its way in from the west with
gradual clearing. Moderate confidence.
Outlook...
Sunday...VFR. Northwest wind 10 to 15 knots with gusts of 20 to
25 knots. High confidence.
Monday...VFR lowering to MVFR/IFR with a chance of rain.
Northwest wind 5 to 10 knots becoming south. Medium confidence.
Tuesday...MVFR/IFR with a chance of rain. North to northeast
wind 5 to 10 knots. Medium confidence.
Wednesday...MVFR/IFR with rain likely. Variable wind 5 to 10
knots. Medium confidence.
&&
.MARINE...
For the Delaware Bay: winds and waves should stay below Small
Craft Advisory criteria through tomorrow.
For the Delaware and New Jersey Atlantic Coastal waters:
Southwesterly winds will increase late tonight. Gusts at or just
above 25 kt are possible by dawn Saturday morning for areas
generally south of Atlantic City. For all coastal waters, seas
should subsequently increase, exceeding 5 feet by early Saturday
afternoon.
Outlook...
Sunday...A Small Craft Advisory is in effect for our ocean
waters (and will likely be expanded to include Delaware Bay) for
a northwest wind 15 to 20 knots with gusts of 25 to 30 knots.
Sunday night...Northwest wind around 15 knots with gusts near
25 knots.
Monday through Wednesday...No marine headlines are anticipated.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory until 6 AM EST early this morning for
PAZ054-055.
NJ...Winter Weather Advisory until 6 AM EST early this morning for
NJZ001-007.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 4 AM early this morning to 6 PM EST
Sunday for ANZ452>455.
Small Craft Advisory from 1 PM this afternoon to 6 PM EST
Sunday for ANZ450-451.
&&
Synopsis...Iovino
Near Term...CMS/Johnson
Short Term...Iovino
Long Term...Iovino
Aviation...CMS/Haines/Iovino
Marine...Iovino/Johnson