Scientific Forecaster Discussion
fxus61 kphi 151534
Area forecast discussion
National Weather Service Mount Holly New Jersey
1034 am EST sun Dec 15 2019
deep low pressure will continue to move north over eastern
Canada through today while high pressure builds in to our south
and west. This high will move over the region tonight before
moving off to the north and east Monday. A warm front slowly
lifts northward into our area by later Monday, as low pressure
tracks eastward along it and crosses our region during Tuesday.
High pressure builds in later Wednesday through Friday, then
weakens into Saturday as the next low pressure system may
approach by this time.
Near term /through tonight/...
a tight pressure gradient remains across the area in between a
strong low pressure over eastern Canada and high pressure
across the upper Mississippi Valley. This is keeping a steady
west to northwest flow across the area. Now that daytime mixing
has begun, stronger winds from aloft will again begin mixing
down to the surface. Wind gusts of 25-35 mph, with occasional
gusty of 35-45 mph will be possible across portions of the
Embedded within this west to northwest flow a weak surface
trough will move across the area through today. Meanwhile, a
short wave/vorticity impulse will move across the area as well.
There is some moisture being trapped underneath the inversion
which is keeping partly to mostly cloudy skies across much of
the area. There could be enough lift with the short
wave/vorticity impulse to interact moisture and squeeze out a
few sprinkles, or flurries in the Poconos, into this afternoon.
Any precipitation would be light and not amount to much.
For tonight, winds will diminish quickly early this evening
both due to the diurnal cycle as well as a weakening pressure
gradient as high pressure moves in from the west. Skies will
clear for a time as the high builds over the area but by later
tonight mid and high clouds will increase ahead of the next
approaching system. Nevertheless, still expect that with a
period of mainly clear skies and diminishing winds this evening
along with low dew points that lows should dip into the 20s to
low 30s setting the stage for our next system. Most of the
precip with this system will hold off until the day Monday but
by late tonight there could be some light mixed precip
developing over the Delmarva into southern New Jersey with the leading
edge of light snow nearing southeast PA.
Short term /Monday through Tuesday/...
complex winter-weather event will unfold during this period, which
may come in two fairly distinct phases. The first will occur on
Monday as a surface low organizes in the southern U.S. With a
strengthening baroclinic zone stretching east-northeast to the mid-
Atlantic. As the low intensifies in the right-rear quadrant of a
140+ kt 250-mb jet streak positioned from the Ohio Valley to
southern New England, sustained warm advection will occur on the
upstream side of a broad surface high encompassing much of the
western Atlantic into New England. Precipitation will spread
eastward into the northern mid-Atlantic through the day as the
strength of the isentropic lift increases, but models are
disagreeing with the exact placement of the baroclinic zone and the
speed with which the precipitation saturates the antecedent low-
level (dry) air. The trend has been for a more southern position of
the baroclinic zone (which also agrees with model errors with the
system so far well to our west), so have spread higher pops into our
area farther to the south on Monday (and have lowered them a bit
farther to the north). I also am suspicious the higher-resolution
output is too quick to saturate the low levels. Precipitation may
not become substantive until after the Monday morning rush.
Additionally, the precip will have a difficult time moving northward
given the closer proximity to the surface high and farther distance
from the strongest isentropic lift, suggesting areas north of the I-
78 corridor may struggle to see precipitation for a lengthy portion
of the day.
The low-level thermal profiles will be favorable for snow north of
the baroclinic zone at onset. However, the higher quantitative precipitation forecast may be just
south of the freezing line, which will likely limit snow totals to
some degree (as well as the residual effects of the retreating
surface high). Finally, the timing of the event is not particularly
favorable for higher snow totals (as temperatures will diurnally
warm, aided by continued warm advection). Most model guidance is
suggesting snow will generally be a couple inches or less in most of
One of the most difficult aspects to this first portion of the event
will be surface temperatures. The models are in their usual camps
(nam on the cold side; GFS/CMC warmer). The European model (ecmwf) is siding with the
NAM, which tends to perform better in these types of events. I
weighted the forecast on the cold side, with a much slower
transition to liquid precipitation for much of the area. Could see
some sleet and freezing rain mix in with the rain and snow, as the
low-level thermal profile will be hovering near the freezing mark as
surface temperatures slowly warm. This precipitation transition will
creep northward through the day, perhaps near or just north of
Philadelphia by late in the day.
Phase two of the event takes over Monday night and Tuesday, perhaps
separated by a relative lull in precipitation (during the
afternoon/early evening). During this phase of the event, a warm
nose will develop around 800 mb (aided by a synoptically enhanced
low-level jet). As isentropic lift continues to increase in advance
of the surface low lifting northeast through the Tennessee Valley
and Appalachians overnight, precipitation will increase once again.
The near-surface air will warm more slowly than aloft, likely
allowing for a period of sleet and freezing rain generally north and
west of the I-276/I-195 corridors. Any remaining snow in the
Poconos and northern New Jersey will slowly transition to freezing
rain and sleet as well. Models are quite varied in timing the
transition from south to north (and the speed of the warming in
general), but the setup appears conducive for a period of icing in
this general region Monday night, perhaps lingering into Tuesday
morning north of the I-78 corridor. Some of the European model (ecmwf) and sref
ensemble members have substantial ice accumulations in the northern
third to half of the County Warning Area. However, other members have little or
nothing, with the warmer guidance suggesting the near-surface air is
scoured relatively quickly after the warm nose develops. A tough
forecast, to be sure.
Currently, expect ice totals of a trace to a few hundredths near the
I-76 corridor and a tenth or two north of the I-195/I-276 corridors.
Uncertainty is high, as freezing rain will likely mix with
snow/sleet during the transition period, and surface temperatures
may warm more quickly than forecast (i.E., The window for icing
would be cut short). Totals may not be much higher in the southern
Poconos and northwest New Jersey, as more of the precipitation may
occur as snow/sleet, and the highest precipitation totals may occur
farther to the south.
Temperatures will continue to warm on Tuesday, with the transition
to plain rain creeping northward to the higher elevations of the
Poconos by afternoon. However, as the surface low moves offshore,
colder air will filter into the area from the north and west, with
precipitation type likely returning to snow late in the day in the
northern County Warning Area. That is, if the precipitation does not taper off first
(a distinct possibility). For the rest of the area, precipitation
should have ended well in advance of sub-freezing temperatures.
One thing to watch Tuesday afternoon into the long term is the
winds. If icing is substantial in the northern cwa, the stronger
northwest winds in the wake of the system may be effective in
Downing some tree limbs and power lines.
Long term /Tuesday night through Saturday/...
precipitation should be to the North/East of the area by Tuesday
night as the surface low progresses to the northwest Atlantic.
However, a strong/digging trough will quickly move into the
northeast on Wednesday, allowing for some snow showers in the
Poconos and vicinity. The ambient environment is favorable for snow
squalls in much of the cwa, as model soundings show a well-mixed
boundary layer with nearly saturated parcels at the capping
inversion with non-zero cape. However, not sold on this potential
yet outside of the far north/west (given more consistent model
output to our north, in closer proximity to the strongest large-
scale lift). For now, have left mention of this potential out of the
forecast, pending more consistent model support.
Main concern for the end of the week will be the much colder
temperatures. With strong winds looking likely Wednesday and
lingering into Wednesday night, wind chills will fall into teens and
twenties during the day and in the single digits and teens Wednesday
night and Thursday morning (likely sub-zero in the poconos).
High pressure will build into the region to close the week, with
continued cold temperatures but dry conditions. Models are
suggesting the development of a coastal low next weekend, but this
is approaching la-la-land in model world. Obviously, we will monitor
this potential in the coming days.
Aviation /15z Sunday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Today... mainly VFR. Westerly winds of 10 to 15 kt with gusts
to 30 kt possible through the latter part of the morning into
the early afternoon before winds diminish late in the afternoon
and evening. High confidence.
Tonight...VFR. Light west winds becoming light and variable
Monday...restrictions likely with a mix of rain and snow. Light east
to northeast winds. High confidence in sub-VFR and winds; low
confidence in precipitation type.
Monday night...restrictions likely with rain near/south of philly
and a wintry mix to the north. Light northeast winds at the philly
terminals and north; variable winds at ilg/miv/Acy. Moderate
Tuesday...restrictions likely with mainly rain (possibly freezing
rain/sleet at abe). Winds becoming northwest and increasing to 5 to
15 kt. Moderate confidence.
Tuesday night...becoming mainly VFR with northwest winds 10 to 15 kt
with higher gusts. Moderate confidence.
Wednesday...mainly VFR, though MVFR cigs may develop (especially
northwest of phl). Cannot rule out a snow shower at rdg/Abe.
Northwest winds 10 to 20 kt with higher gusts. Low confidence.
Wednesday night and Thursday...mainly VFR with northwest winds 5 to
15 kt. Higher gusts possible Wednesday evening and again on
Thursday. Moderate confidence.
the Small Craft Advisory remains in place for Delaware Bay and
the Delaware Atlantic coast. The Gale Warning remains in effect
for the New Jersey Atlantic coast. Ocean waters with seas
generally 5 to 8 feet.
Winds and waves diminish for all waters this evening.
Monday...sub-advisory winds/seas expected. A chance of snow and
Monday night...sub-advisory winds/seas expected. A chance of rain
and visibility restrictions.
Tuesday...advisory winds/seas probable with a chance of
Tuesday night through Wednesday night...at least advisory conditions
probable, and gales possible, as winds become northwesterly.
Thursday...lingering advisory conditions possible, but winds and
seas should be lowering during the day.
Marine...Gale Warning until 7 PM EST this evening for anz450>453.
Small Craft Advisory until 9 PM EST this evening for anz454-
Small Craft Advisory until 6 PM EST this evening for anz430-