Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 kphi 161152 
afdphi

Area forecast discussion
National Weather Service Mount Holly New Jersey
652 am EST Mon Dec 16 2019

Synopsis...
a warm front will slowly lift into the area 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 PM this evening/...
645 am update: precipitation has spread into the area generally
south of I-76 early this morning, with several surface
observations and mping reports of light snow and/or sleet. So
far, precipitation rates are quite light and temperatures are
above freezing, so traffic impacts are likely minimal. However,
as wet-bulbing continues and surface temperatures subtly drop,
impacts may start to increase, especially if precipitation rates
increase. For now, expecting this first burst of precipitation
to stay in this general area. 06z model guidance indicates a
second round entering the area later this morning (and farther
north). This may be when the biggest traffic impacts occur, but
confidence is still rather low given the fairly light
precipitation in general and marginal surface temperatures.

Main change to the forecast was to tweak dew points down a
little bit for the next couple of hours (models are still biased
too high). However, think temperatures will cool to model
guidance as wet-bulb effects continue, so made little
modification to these values for now.

Previous discussion...

Phase one of the winter-weather event will begin this morning as
a weak midlevel perturbation progresses quickly east through the
Great Lakes and northeast downstream of a central-U.S.
Positively-tilted trough and attendant developing surface low in
the mid-south. A northeastward-extending baroclinic zone will
aid in sustained isentropic ascent in the northern mid-Atlantic
today. As a relative maximum in frontogenetic lift approaches
the area this morning, should see precipitation become more
widespread near and south of the I-76 corridor during and
especially after the morning commute. However, this
precipitation has its work cut out for it, as the low levels are
dry (generally several degrees drier than model guidance),
thanks to the residual effects of a nearby surface high. The CMC
seems to be handling this best, and its timing appears to be
slowest with precipitation onset. However, the drier air also
suggests that wet-bulbing will be a dominant player in surface
temperatures and low-level thermal profiles this morning,
allowing for most precipitation to fall as snow (once the
cooling occurs). The question will be how quickly precipitation
develops and becomes snow. The extent of dry air near the
surface would suggest this process may be slow, but
frontogenetic lift may compensate for this fairly quickly. With
consensus of high-resolution guidance indicating snow may get
going by the end of the morning commute, did not want to change
current Winter Weather Advisory along the I-76 corridor this
morning yet and will monitor trends. Regarding snow amounts,
expecting generally sub-advisory amounts, but should snow rates
be fairly substantial or snow mix with sleet (as some of the
high-res guidance indicates), slippery conditions could readily
develop.

Temperatures are expected to warm slowly from south to north
today, so transition to rain is expected as the day wears on.
Additionally, the frontogenetic lift should wane as large-scale
ascent associated with the aforementioned perturbation moves
away from the area. Expecting to see a lull in
intensity/coverage of precipitation this afternoon, as a result.
However, a light wintry mix may continue near and north of the
I-76 corridor, with generally rain to the south. Farther north,
the close proximity of the surface high should really inhibit
northward progress of the precipitation. Areas near/north of
I-80 may be dry through most of the afternoon.

&&

Short term /6 PM this evening through Tuesday night/...
phase two of the winter-weather event should begin in earnest
this evening as the developing surface low in the lower
Mississippi Valley lifts northeast through the Tennessee Valley
into the Appalachians by Tuesday morning. Meanwhile, a strong
upper-level jet streak will acquire anticyclonic curvature,
allowing for enhanced lift in much of the northern mid-Atlantic
in the favored right entrance region. With the continued
presence of the low-level baroclinic zone, sustained isentropic
ascent will be enhanced as a synoptically-induced low-level jet
impinges upon the thermal gradient overnight. This will produce
a large area of precipitation, likely becoming moderate to
locally heavy during the overnight hours into Tuesday morning.

The low-level jet and associated warm advection will
successfully generate a warm nose aloft as near-surface air
struggles to warm above freezing (generally north/west of the
I-95 corridor). As a result, much of the precipitation that
develops during the evening will be a mixture of rain, freezing
rain, and sleet. A fairly lengthy period of icing is expected,
especially northwest of the fall line, through the overnight
hours. However, precipitation type is likely to vary with time,
as enhanced precipitation will act to cool the column
sufficiently for changeover to sleet/snow at times. High-
resolution model guidance is rather consistent in showing this
around 06z Tuesday, coinciding with the nocturnal maximum in the
low-level jet and associated frontogenetic lift in the County Warning Area.
Such a switchover would likely lead to a burst of heavier sleet/snow,
which may make travel quickly hazardous. With periods of icing
on either side of this transition, travel impacts may become
quite substantial during the overnight hours. With time, near-
surface air will warm to or slightly above freezing from
southeast to northwest in the advisory area, but this process is
likely to be slow owing to the strength of the retreating
surface high and ageostrophic effects via cold-air damming.

Forecast surface temperatures were based largely on the colder
guidance, owing to the above considerations. As a result,
decided to extend much of the advisory through the Tuesday
morning commute for southeast PA and northern/central New Jersey. The
advisory was extended through early afternoon in the Lehigh
Valley and adjacent Warren/Morris counties in New Jersey and through the
afternoon in the southern Poconos and northwest NJ, where sub-
freezing temperatures are likely to persist through the day. Ice
accumulations up to a quarter inch are expected in much of these
areas, though confidence in amounts is rather low owing to
several anticipated precipitation transitions. Given the signal
for a burst of snow during the overnight hours when
frontogenetic lift is maximized, snow amounts were increased
northwest of the fall line as well, though I am expecting these
totals to be rather variable, elevation-dependent, and transient
given the near-freezing surface temperatures.

Southeast of the I-95 corridor, precipitation will likely be
predominantly rain, with any winter-related travel impacts
likely over by or after the evening commute. Rain may become
somewhat showery on Tuesday, and there are even indications that
some marginal instability reaches Delmarva and far southern New
Jersey by Tuesday afternoon. Did not include mention of thunder
at this point, but certainly cannot rule it out. As a matter of
fact, some hi-res guidance indicates a convective line may
develop near/south of the southern County Warning Area during the afternoon as a
cold front moves through.

One thing to watch will be rainfall totals near/southeast of the
urban corridor. Consensus model quantitative precipitation forecast is generally 1-2 inches in a
swath encompassing southeast PA, central/southern NJ, and
northern Delmarva. These amounts may be enough for some
localized/poor-drainage flooding. Such potential would be
exacerbated by convectively-enhanced rainfall.

As the cold front advances through the area by Tuesday evening,
precipitation will quickly shut off, with snow showers remaining
over the Poconos Tuesday night. Expecting most of the
precipitation to be done before a switchover to snow in the rest
of the cwa, but cannot rule it out entirely Tuesday evening.

Finally, winds behind the front will begin to gust up to 25 mph
or so, especially in the higher terrain of the northern County Warning Area.
Should ice accumulations be significant in these areas, some
tree damage and power outages are possible.

&&

Long term /Wednesday through Sunday/...
main forecast problems for the medium-range period are passage
of an Arctic cold front on Wednesday and poor model continuity
this weekend.

As the early-week system moves offshore Tuesday night and
Wednesday, a strong/digging shortwave trough will move into the
northeast on Wednesday. An attendant (reinforcing) cold front
will move through the County Warning Area during the day, which may be
accompanied by some snow showers/squalls. Model soundings
certainly look favorable for snow squalls, with a deep mixed
layer developing during the afternoon near and upstream of the
front. Some marginal surface-based instability may develop in
this regime, with winds atop the mixed layer of 40+ kt. The
question remains the organization of the precipitation that
develops, with the stronger large-scale lift generally north of
the area. Most models depict precipitation waning by the time it
reaches the cwa, which is also fairly late in the day. Thus,
expect the best chances for snow showers/squalls to be in the
far northern cwa, where I have increased pops a little bit.
Cannot rule out this potential farther southeast (as in the 00z
ecmwf), but would like to see a little more model agreement
before getting more aggressive with this potential.

The bigger story will likely be the winds and cold, with wind
gusts 25 to 40 mph by Wednesday afternoon/evening and very cold
air moving into the region. Wind chills will fall to the single
digits by Wednesday night (sub-zero in the Poconos and northwest
new jersey), with only muted recovery on Thursday. Highs on
Thursday will likely be below freezing everywhere except
central/southern Delmarva. However, winds should begin to
decrease Thursday and Thursday night as a surface ridge
approaches the area. A slow warming trend is expected Friday and
Saturday.

The forecast becomes quite uncertain this weekend, with poor
run-to-run model continuity and even worse model-to-model
agreement. There are still indications of a low developing near
the East Coast, but the loose trend the past 24 hours is for a
more suppressed and farther offshore system. Stuck close to
forecast continuity combined with a consensus blend for the
weekend, which keeps low chances for precipitation on Saturday
and Saturday night, for now.

&&

Aviation /12z Monday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.

Today...conditions will deteriorate to MVFR through the morning
and eventually IFR for much of the region by the afternoon as
precipitation moves in. Precipitation is expected to start as
mostly light snow but should mix with rain and possibly some
sleet between 15 and 20z, before changing over to rain or
freezing rain 21z or later. The taf sites from the I-95 corridor
and points south should become all rain by the evening. Light
and variable winds early will become northeast to east 5-10
knots. Moderate confidence in the overall trend, but low
confidence in the specific details especially with the timing
of the transitions.

Tonight...IFR or lower will predominate with rain from phl/pne
and points south and a mix of snow, sleet and freezing rain
farther north. Low-level wind shear probable as surface winds
mainly east/NE 5-10 knots veer sharply to south/southwest aloft.

Outlook...

Tuesday...sub-VFR in rain, except freezing rain possible at
kabe. NE winds become northwest around 10 knots.

Tuesday night...improving conditions through the night. Northwest winds
5-10 knots with some higher gusts of 15-20 knots possible.

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 should remain below Small Craft Advisory conditions through
Monday and Monday night. There will, however be some rain around
for Monday and Monday night and this could lead to visibility
restrictions.

Outlook...

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.

Friday...sub Small Craft Advisory conditions expected.

&&

Phi watches/warnings/advisories...
PA...Winter Weather Advisory from 1 PM this afternoon to 1 PM EST
Tuesday for paz061-062.
Winter Weather Advisory from 1 PM this afternoon to 10 am EST
Tuesday for paz060-103>106.
Winter Weather Advisory from 1 PM this afternoon to 6 PM EST
Tuesday for paz054-055.
Winter Weather Advisory until 1 am EST Tuesday for paz070-071.
Winter Weather Advisory until 10 am EST Tuesday for paz101-102.
New Jersey...Winter Weather Advisory from 1 PM this afternoon to 1 PM EST
Tuesday for njz007-008.
Winter Weather Advisory from 1 PM this afternoon to 10 am EST
Tuesday for njz009-010-012-015.
Winter Weather Advisory from 1 PM this afternoon to 6 PM EST
Tuesday for njz001.
Winter Weather Advisory until 1 am EST Tuesday for njz016>019.
Delaware...Winter Weather Advisory until 10 PM EST this evening for
dez001.
Maryland...Winter Weather Advisory until 10 PM EST this evening for
mdz008.
Marine...none.

&&

$$
Synopsis...fitzsimmons
near term...cms

National Weather Service Glossary of Abbreviations