NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Feb 9, 3:07 AM EST, 2 hours 53 minutes earlier.
Area forecast discussion, Feb 9, 3:07 AM EST, 2 hours 53 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
307 AM EST Tue Feb 9 2021
.SYNOPSIS...
A weak surface low will move through the northern Mid-Atlantic
today, this low will then progress quickly off to the east and
be located off the New England coast by tonight. High pressure
will build into the Northeast Wednesday. A couple of upper level
systems will affect the region Wednesday night through Friday.
Expansive arctic high pressure will then build to our north and
west early this weekend before another low pressure system may
affect the region by late in the weekend.
&&
.NEAR TERM /THROUGH TONIGHT/...
4 am update: Have expanded the winter weather advisory to include
Berks, upper Bucks, Hunterdon, and Somerset Counties. Though
forecast snow is generally under advisory criteria, high-resolution
guidance indicates potential for a warm nose to move into these
areas to change precipitation type briefly to sleet and/or freezing
rain near or just after the morning rush. The 00z HREF shows a swath
of a light glazing in this general corridor, for example. With
surface temperatures colder than forecast (and well below freezing
early this morning), even a brief window of freezing rain or drizzle
may be impactful, and combined with the potential for light snow,
impacts to travel appear probable. Previous discussion follows...
A quick-hitting system will bring some snow to the northern CWA this
morning, with more scattered coverage of rain or a wintry mix
farther south.
Objective 00z upper-air analysis indicates that the quasi-zonal
upper-level flow across the northern U.S. has become more amplified
in the past 24 hours, as a significant shortwave trough is
progressing through the central Plains. A 250-mb jet streak was
developing in the Great Lakes region and adjacent central
Appalachians. Weak perturbations were embedded in the larger-scale
cyclonic flow at 500 mb, most notably in the Upper Midwest, where an
embedded speed max was positioned. A fairly broad region of low-
level warm advection was present from the Ozarks east-northeast to
southwest Pennsylvania, where a frontogenetically-enforced band of
snow had developed and was moving rapidly east-northeast,
approaching the northern Mid- Atlantic at the current time.
Deterministic models show this snow band moving into the southern
Poconos and Lehigh Valley by 09z, spreading quickly eastward into
northern New Jersey through the morning rush.
Models have been fairly consistent in keeping precipitation shut off
(or much sparser) to the south of a Reading to Morristown line
through 12z. However, model soundings show rapid saturation below
the ice-crystal layer, with at least hints of some freezing drizzle
or even light freezing rain in southeast PA and adjacent areas
developing around the morning rush or just afterward. Given the
brief time window and unclear signal of anything substantive
precipitation-wise, feel an advisory is not warranted for this
potential yet. However, the combined potential of light snow and
perhaps very light icing may force my hand for Berks County, where
confidence is higher that precipitation will occur and impacts will
be more meaningful. Will make the final decision on expansion here
by 4 am. One note: Given the colder than expected temperatures
overnight, models may be too aggressive with the progress northward
of the freezing line during the morning across the area.
As the morning progresses, the first perturbation will zip into
northern Pennsylvania but will then interact with two other
vorticity maxima. The first will approach the West Virginia/Virginia
region, and the second will surge through the southern Great Lakes.
This should provide more widespread lift across the area, and an
expansion of the precipitation shield, particularly to the north of
I-78 and to the east of the Delaware River. Temperatures look warm
enough for mostly rain south of I-76 in Pennsylvania and I-195 in
New Jersey, but there may be a brief period of freezing rain and/or
snow extending from Bucks County east-northeast to
Somerset/Morris/Middlesex Counties. Again, confidence is not high in
this area, so would like to see how the event evolves before
expanding the advisory farther to the south. Aside from the northern
CWA, a second QPF maximum may occur from Delmarva to eastern New
Jersey, with areas near the urban corridor itself in a relative
minimum. This signal is quite noticeable in a majority of guidance,
and is clearly tied to the evolution of the aforementioned vorticity
maxima and their interactions.
North of I-78, a decent round of snow is expected, with 3-6 inches
in the southern Poconos and Sussex County and 2-4 in the Lehigh
Valley and Warren/Morris Counties. A few locations in the higher
terrain of the Poconos will likely exceed 6 inches. Snow ratios will
be quite high for this event, given the very favorable thermodynamic
profiles, so snow rates may be pretty decent for the hours
surrounding morning rush in these areas. Travel may become quite
treacherous should the event materialize as expected.
Precipitation should move quickly off to the east by mid afternoon,
though lower visibilities may linger for a couple of hours given the
light surface winds and near-saturated low levels. Eventually, winds
will become northwesterly, and sky cover will improve. Tonight
should be dry with temperatures a little warmer than early this
morning given the lack of complete decoupling.
&&
.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
Another round of snow is likely for much of the area Wednesday night
through Friday.
The 00z deterministic model suite continues to indicate a lengthy
baroclinic zone positioned roughly from the Ozarks to the Mid-
Atlantic for this period of the forecast. With a midlevel vortex
continuing to meander in south-central Canada, fast quasi-zonal flow
to its south will allow a parade of perturbations to march eastward.
One vort max will surge through the northern Great Lakes and
adjacent Ontario/Quebec Wednesday afternoon and night, with
interaction with a southern-stream low-amplitude trough in the
middle Mississippi Valley. Widespread snow should develop in the
warm-advection pattern downstream, from the Ohio Valley Wednesday
afternoon to the central Appalachians/Mid-Atlantic Wednesday night.
After a dry day Wednesday with slightly below-seasonal temperatures,
snow should move in from the southwest Wednesday night.
Models continue to trend more suppressed and weaker with the
southern-stream trough, owing to continued confluent midlevel flow
from amplifying troughing in far eastern Canada. As a result, the
QPF shield has shifted so that the maximum axis is generally in the
southern half of the CWA Wednesday night and Thursday. Large-scale
lift will be somewhat weak, so snow rates should be light to modest
at best. Nevertheless, temperatures will be sufficiently cold for
snow in virtually all of the CWA. By Thursday, current snow totals
of 1-3 inches are fairly widespread south of I-78, with the highest
amounts generally south of I-76.
A second round of snow may occur as a stronger vort max approaches
the East Coast on Thursday night and Friday. The NAM has trended
well south with this, mostly keeping QPF out of the CWA. The ECMWF
has trended this way as well, though to a much less extreme extent.
The GFS and CMC, meanwhile, are farther north and provide another
round of light snow to much of the area. By Friday, widespread
totals of 2 to 5 inches are possible across the area, with highest
confidence for the southern half of the CWA.
The model trends continue colder for this period as well, given the
lingering influence of the surface high to our north. This suggests
the event will be primarily snow for the CWA, with any mixing likely
to occur in far southern Delmarva and southeast New Jersey. Forecast
temperatures were kept lower than consensus, and mainly weighted
toward colder raw guidance, which tends to perform best in these
regimes.
Though the snow will generally be light, the sufficient cold and
longer potential duration suggest meaningful impacts appear likely
for our forecast area. Continue to stay updated with the latest
forecasts.
&&
.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
Synoptic Overview: The polar vortex will be positioned over
Central Canada to start the extended and will gradually pivot
eastward into early next week while also weakening. The mid-lvl
pattern then shifts to a longwave trough over the central US and
ridging over the east coast/western Atlantic. At the surface,
expansive High Pressure centered over the Upper Midwest will
encompass most of the central tier of the country with frigid
temperatures expected in this region this weekend. The cold
high will attempt to build eastward on Saturday but an area of
low pressure approaching from the south on Sunday will likely
prevent it from becoming established. The High will then shift
east towards the area to start next week, before another low
pressure system from the south likely approaches
Tuesday/Wednesday.
In general the next 10 days or so look fairly active, as the
arctic outbreak (which should reach the deep South this weekend)
will provide plenty of baroclinicity over the southern US which
in turn will foster mid-latitude cyclone development. Given the
synoptic h5 pattern of ridging over the east coast with
troughing over the central US, these cyclones should then track
in the general vicinity of our region. Given the time of year
wintry precipitation in some form would certainly be possible
with these systems, but finer details such as the positioning
and amplitude of the east coast ridge (guiding the low track),
antecedent cold etc. are not easily resolved at this temporal
range, so the generalities offered above will have to suffice
for the present.
Dailies...
Friday night/Saturday: Cold and dry as the High briefly builds
eastward. Highs will largely be in the upper 20s to lower 30s.
Sunday: Quite a bit of variance among guidance over the
evolution of a coastal low, and consequently quite a bit of
variance regarding sensible weather outcomes. Most of this
uncertainty revolves around a southern stream wave which will
not even come onshore the west coast until Friday. The GFS/UKMET
show this wave getting absorbed into the northern stream over
the SE US and the resulting surface is fairly amplified
resulting in a fairly warm (although potentially wintry at
onset)and wet event. The CMC/EC keep this wave well south of
the area, and only develop a weak coastal low in our region with
limited precipitation associated with this feature. Given that
the shortwave in question is still not likely being sampled
well, feel Chc PoPs suffice for now, and will leave FZRA/IP out
of the weather grids until the system is better resolved.
Monday: The strong High will shift eastward on Monday and under
its growing influence a cold and dry day is expected with highs
only in the 20s and lows in the single digits and teens.
&&
.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE,
KRDG, KILG, KMIV, KACY and surrounding areas.
This morning...VFR will gradually transition to MVFR and possibly
even IFR CIGs as the morning progresses. Snow should spread into
RDG/ABE between 09z and 12z, with IFR/LIFR VSBYs possible by 12z.
Precipitation will be lighter, less steady, and may be more of a mix
of rain/snow at the Philly terminals. Freezing drizzle/rain may
occur for a brief period of time, but confidence is not overly high.
Transition to rain is likely after 14z-15z from south to north. At
MIV/ACY, mostly rain is expected. VSBYs at the Philly terminals and
MIV/ACY may become MVFR or IFR, especially after 14z. Winds should
be light and variable. Several amendments to the TAFs should be
expected during this period given the rapidly evolving nature of the
event and the uncertain precipitation at the Philly terminals.
This afternoon...Precipitation should quickly move east of the
terminals; however, lingering restrictions are likely through around
21z, as winds will likely remain light/variable.
Eventually, a steadier northwest wind will develop (5 to 10 kt),
which should scour the lower CIGs/VSBYs. Moderate confidence.
Tonight...Mainly VFR with light northwest winds. High confidence.
Outlook...
Wednesday...Mainly VFR with light north to northwest winds. High
confidence.
Wednesday night through Friday...Restrictions likely often through
the period, with periodic snow. Best chances for highest snow
amounts are near and south of the Philly terminals. Winds primarily
from the northeast at 5 to 15 kt. Moderate confidence.
Saturday...Most VFR although some lingering stratus will be
possible. N-NE winds around 10kts. Moderate confidence.
&&
.MARINE...
Sub-advisory conditions are expected through tonight. Southeast
winds 5 to 15 kt this morning will switch to northwest by late
afternoon and increase to 10 to 20 kt this evening and tonight.
Seas will generally be 2 to 4 feet.
There is a chance of rain this morning, possibly mixed with some
snow off the northern/central New Jersey coast.
Outlook...
Wednesday...Sub-advisory conditions expected.
Wednesday night through Friday...Northeast winds may approach
advisory criteria at times, especially on Thursday and Thursday
night. One or more rounds of snow or a mix of rain and snow are
possible through the period.
Saturday...Generally sub-SCA although seas may run in the 4-5ft
range on the ocean waters.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory until 5 PM EST this afternoon for
PAZ060>062-105.
Winter Weather Advisory until 5 PM EST this afternoon for
PAZ054-055.
NJ...Winter Weather Advisory until 5 PM EST this afternoon for
NJZ007>010.
Winter Weather Advisory until 5 PM EST this afternoon for
NJZ001.
DE...None.
MD...None.
MARINE...None.
&&
Synopsis...Fitzsimmons
Near Term...CMS
Short Term...CMS
Long Term...Carr
Aviation...CMS/Carr
Marine...CMS/Carr