NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Feb 13, 5:52 AM EST, 3 hours 38 minutes earlier.
Area forecast discussion, Feb 13, 5:52 AM EST, 3 hours 38 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
552 AM EST Sat Feb 13 2021
.SYNOPSIS...
A few weak low pressure systems will ride along a stationary
front that is just south of our region through Monday night. A
stronger low will approach our region Tuesday. High pressure
briefly returns on Wednesday before another system approaches
Thursday into Friday.
&&
.NEAR TERM /THROUGH TONIGHT/...
Adjusted temperatures to reflect slightly colder surface
temperatures thus far this morning across southeast PA. As has
seemed to be the case so far this winter, models have
underestimated the dew points with dry air ahead of this system,
so made a few tweaks to reflect those observations. This may
bode well for a quick burst of snow for more of us this morning
if wet-bulbing effects occur over a greater stretch of real
estate before the TROWAL forms overhead this morning.
A quiet morning to start the weekend will quickly take a turn for
the worst as a return to wintry weather overtakes the region. At the
surface, a quasi-stationary frontal boundary to our south will pave
the way for a weak wave of low pressure to ride northward through
the weekend. Higher up, a mid-level trough will pivot through the
stalled long wave flow pattern due in part to the breaking of the
Polar Vortex. As this shortwave trough crosses the Great Lakes, low
pressure will deepen on the coast to our south.
Weak ridging ahead of this trough will feature mid-level height
rises across the Mid-Atlantic as warm air advection sets up off the
Atlantic. With cold Canadian high pressure to the north over
Ontario, northerly winds will continue CAA over the Northeast. With
the "warm" moist advection associated with cyclogenesis along the
stationary boundary baroclinic zone, this sets the stage for a
freezing rain event. Hi-res guidance continues to struggle with the
timing and expanse of the precip shield with this system, but
general consensus is for snow and sleet to initially overtake the
region before a transition to sleet and freezing rain across the
Lehigh Valley, I-95 Corridor, and Coastal Plain. It is possible that
precip remains all or mostly snow across the Poconos and Sussex
County, NJ. Surface temps will likely rise enough above freezing for
plain rain to fall later this afternoon across far southern Delaware
and along the southern New Jersey coast. Taking all of this into
consideration, the axis of greatest freezing rain accumulation still
appears to be along to just southeast of the I-95 corridor, where up
to a tenth to 0.15 inches of accretion is possible. This is below
Warning criteria, thus continued with the Winter Weather Advisory,
adding Morris County, NJ to the pot given freezing rain chances
there. One bit of good news is that the timing of the low pressure
appears to have sped just a touch, with most of the precipitation
coming to an end by daybreak Sunday.
Otherwise, a raw and unpleasant day with northeasterly winds up to
15 mph and highs in the mid to upper 20s across most of the forecast
area.
&&
.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
Through this period we should mostly be in between the two
bigger systems (Saturday and Tuesday). However, a secondary low
is likely to develop and propagate along the front through this
time. Most guidance depicts this low being weak enough and far
enough away from our region to have limited impacts for us.
However the GFS and NAM are depicting a more intense low with a
track closer to our region resulting in another round of wintry
precipitation Sunday night. For now, I have a chance of precip
in the forecast through that period, but I don't think it is
likely given that the best upper level support will still be
well to our west. Given that, significant surface cyclogenesis
is unlikely.
The track that the secondary low does take will have
implications for temperatures. However, regardless of the exact
track, expect persistent cloud cover and northerly/northeasterly
winds. Consequently, expect little diurnal range in
temperatures, remaining in the 20s and 30s throughout this
period.
&&
.LONG TERM /TUESDAY THROUGH FRIDAY/...
Overview: Until the front moves further away from our region
(or washes out) and the mid level closed low over Central
Canada/Northern U.S. Plains shifts (possibly by next weekend?),
we are going to be stuck in the same weather pattern, with many
more chances for wintry mix and only one completely tranquil day
(Wednesday)
Details:
Monday night through Tuesday...As a southern stream mid level
short wave trough lifts out of the Big Bend region, surface
cyclogenesis occurs near the NE Gulf coast. No significant
changes with this forecast, although it is interesting that
there is very poor guidance agreement (worse than the 00Z runs
from yesterday) as to the onset of precipitation with this
system. Some guidance shows light precipitation moving in as
early as Monday afternoon, while others show the precip not
starting until late Monday night. It still appears this low will
take a track similar to the weekend system, albeit a bit
further northwest. This will mean that our region will likely
stay just NW of the center of the low, resulting in another
round of wintry mix. However, since it may be closer to our
region, a period of all rain is possible even as far northwest
as the 95 corridor.
Wednesday...At this point, Wednesday looks like the best chance for
our region to see breaks in the clouds and tranquil weather. With
northwesterly winds though, temperatures will remain well below
normal, with highs from the upper 20s to upper 30s.
Thursday into Friday...Guidance is still depicting very high
PoPs for the next system which is surprising given the split
flow pattern (which are notoriously difficult to model so far
out). Several models depict a significantly more inland path
with this system, which could leave much of our region in the
warm sector with mostly rain. Although some operational models
showed a trend southeastward with the track, closer to what they
are depicting with the Tuesday system. The exact track is all
dependent on a southern stream mid level trough phasing with a
northern stream trough early enough so that the surface low
lifts almost due north from the western Carolinas eventually
into western PA. In the best of circumstances, this is a
difficult scenario to model, and given the uncertainty in the
periods leading up to this system, it is far from ideal
circumstances. So for now, have stayed close to the previous
forecast, and gone with mostly a mention of rain or snow.
&&
.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
Today...VFR conditions early this morning will fall through the late
morning to MVFR and IFR from south to north after 15Z as a wintry
mix moves into the region. IFR conditions expected through the
afternoon for most terminals. Snow initially will quickly
transitions to sleet then freezing rain. There is a possibility that
precipitation will remain all snow across the Poconos and far NW NJ.
Northeast winds from 5 to 10 knots with 15 to 20 knots at KACY.
Moderate confidence in trends but low confidence on exact timing of
precip type changes.
Tonight...IFR conditions expected to persist through the night with
scattered freezing rain and sleet showers. North to northeasterly
winds from 5 to 10 knots with 15 to 20 knots at KACY. Moderate
confidence.
Outlook...
Sunday...mainly IFR ceilings initially, with some gradual
improvement to at least MVFR possible in many areas by
afternoon. There is a slight chance a wintry mix of
precipitation will linger on Sunday morning. Winds northerly 5
to 10 kt. Low confidence especially on the timing of improvement
or the potential for precipitation.
Sun night/Monday...Persistent MVFR ceilings are likely, though
there may be some brief improvements to VFR. There is a chance
for periods of wintry mixed precipitation through this period,
though confidence on this is very low. Northeasterly winds 5 to
10 kt. Low confidence especially on the potential for any
improvement to VFR and wintry mixed precipitation.
Monday night/Tuesday...Ceilings will likely lower to widespread
IFR conditions as widespread precipitation moves in. Visibility
restrictions are possible as well. For most of the TAF sites,
expect wintry mixed precipitation. However, KMIV and KACY may
be primarily rain through the event. Winds shifting from
northeasterly to northwesterly through the period at 5 to 15 kt.
Low confidence.
Wednesday...Ceilings should improve to VFR by mid day. Winds
will be varying between northerly and northwesterly with speeds
mostly less than 10 kt. Moderate confidence.
&&
.MARINE...
SCA conditions developing over the coastal waters and the Delaware
Bay as a weak coastal low lifts northward through the region.
Northeasterly winds gusting to 25 knots with seas building to 4 to 6
feet (2 to 4 feet on the Bay). Small Craft Advisory is in effect for
the coastal waters from 7 am through midnight Sunday night and from
7 am through 6 am on the Delaware Bay.
Outlook...
Sunday...SCA conditions continue on the Atlantic coastal waters.
Sunday night and Monday...once seas subside below 5 feet Sunday
night, winds and seas should stay below SCA criteria.
Monday night into Tuesday...Northeasterly winds increase and SCA
conditions are once again possible starting late Monday night
continuing into Tuesday night primarily for the Atlantic Coastal
waters. Winds should shift to northwesterly by late in the day
on Tuesday.
Wednesday...Once seas subside below 5 feet, winds and seas
should stay below SCA criteria.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory from 1 PM this afternoon to 10 AM EST
Sunday for PAZ070-071-101>106.
NJ...Winter Weather Advisory from 1 PM this afternoon to 10 AM EST
Sunday for NJZ008>010-012>015-017>020-022-025>027.
Winter Weather Advisory until 7 AM EST Sunday for NJZ016-021-
023-024.
DE...Winter Weather Advisory until 7 AM EST Sunday for DEZ001>004.
MD...Winter Weather Advisory until 7 AM EST Sunday for MDZ012-015-
019-020.
MARINE...Small Craft Advisory until midnight EST Sunday night for
ANZ450>455.
Small Craft Advisory until 6 AM EST Sunday for ANZ430-431.
&&
Synopsis...Johnson
Near Term...Davis
Short Term...Johnson
Long Term...Johnson
Aviation...Davis/Johnson
Marine...Davis/Johnson