Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 151929
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
229 PM EST Fri Dec 15 2017

.SYNOPSIS...
A surface low will move from the Carolinas early this morning to
offshore the Mid-Atlantic tonight. High pressure will build into
southeast Canada, allowing a backdoor cold front to move into or
through the area Saturday night and Sunday. This front will then
stall before lifting northward as a warm front Sunday night and
Monday. A cold front will progress through the region around the
middle of next week. Another system may affect the area near the
close of next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

2 PM Update...Expanded the Winter Weather Advisory slightly
westward to include eastern Chester, eastern Montgomery and
Lower Bucks County where radar is showing a band of heavier snow
early this afternoon. The heaviest snow should shift east of
this area by 4 or 5 PM but with 15:1 snow ratios, a quick two
inches of snow can be easily attainable. Additionaly the
advisory was expanded on the south side to Talbot County for a
brief period of freezing rain this afternoon. We have already
recieved reports of a freezing rain/sleet/snow mix along the
eastern shore of MD.

12 PM Update...Winter Weather Advisory was expanded northward
to include central NJ (Monmouth, Mercer and Middlesex Counties)
with 2-3" of snow expected. The axis of highest snowfall amounts
were shifted slightly northeast into southern and east-central
NJ (though locations just to the west, including the I-95
corridor, are still at risk of getting under the heavier snow
band) with max storm-total amounts increased ever so slightly.
Snowfall graphics updated on our winter webpage. Given upstream
trends (already snowing just south of the Mason-Dixon line in
north-central MD), onset was pushed up an hour (onset timing
graphic was just updated, but unfortunately did not make it into
our briefing package).

930 AM ESTF...Focus at the beginning of the shift has been on
timing (both onset and ending) of snow and messaging impact for
this quick hitting event this afternoon and evening. Based on
upstream observations and the latest available guidance (12Z
guidance is starting to come in as we speak), snowfall is
expected to develop right over the area (Delmarva/SE PA/SW NJ)
between 1 and 3 PM, then quickly expand north/eastward toward
the I-78 and possibly I-80 corridor between 3 and 4 PM. The
experimental wintry precipitation onset graphic has been updated
and sent to our winter weather webpage. The back end of the
snow looks to progress rather quickly eastward between around 4
PM (near Reading, PA) and 8 PM (coastal NJ).

Here is the key message- The ingredients are there for the snow
to have a high impact on travel regardless of snowfall amounts
(1-3" in the Winter Weather Advisory area; less to the north and
west):

(1) The period of steadiest/heaviest snow is expected to occur
during the 3-6 PM time frame, aligning with the afternoon/
evening rush hour.

(2) With cold air in place (both road and air temperatures will
be at or below freezing across most of the area except southern
DE), snow will stick on untreated roads and possibly even
treated highways in a few spots where heavier snowfall rates
occur.

(3) There is a potential for locally higher snowfall rates of 1"
per hour and poor visibility to occur underneath a narrow band
of heavier snow. There is high confidence that a heavy snow
band (brief in duration) does develop given the pattern but
there is still uncertainty on the exact location. The greatest
signal for mesoscale banding is across the southern half of NJ
between the NJ Turnpike and the Garden State Parkway but it
could develop as far west as the I-95 corridor and as far east
as the NJ/DE coast).


A Special Weather Statement was issued for most of the forecast
area that is outside of the Winter Advisory area since some
impacts on the commute home this afternoon/evening will likely
occur.

The next focus with the forecast late this morning will be on
snowfall accumulations and updates to the Winter Weather
Advisory headlines. Locally higher amounts of 3-4" are not out
of the question underneath the mesoscale band (again, most
likely to occur in southern/central NJ). Even though Winter Weather
Advisory criteria is higher, there is also a potential need to
expand the advisory northward into central NJ (especially
Monmouth County).

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
The previously mentioned surface low quickly lifts away from our
region, with snow ending from southwest to northeast. A surface high
is expected to build over the southeastern U.S. As a result
over our region, low and mid level northwesterly flow will
increase. However, through the overnight hours, it does not look
like a favorable fetch for lake effect snow to reach the
southern Poconos, so have kept the region dry after midnight.
Temperatures should be higher than what we saw this morning,
thanks to lingering cloud cover. Lows are expected to be in the
teens and 20s across the region.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
The large-scale pattern in the long term will change somewhat
with fairly zonal flow across much of the eastern U.S. next
week. However, the flow will remain progressive, with numerous
systems passing near or through the region during the period.

This weekend`s forecast is a little tricky in a few respects.
Though two surface highs will build into eastern North America
(one in southeast Canada and the other in the southeastern
U.S.), a deformation axis will form in between, with onshore
flow equatorward of the Canada high allowing for a backdoor cold
front to move into the Mid-Atlantic Saturday and Saturday night.
The differences among the models are rather pronounced, with
statistical guidance varying between the warmer Saturday/cooler
Sunday (ECS) and vice versa (MET/MAV). The warm look of the ECS
on Saturday looks questionable given the antecedent cold near-
surface air and potential for additional snow cover in portions
of the area. Furthermore, the low levels will feature warm-air
advection as a weakening midlevel perturbation rapidly
progresses to the Mid-Atlantic by Sunday night. Nevertheless,
the source region of the surface flow on Sunday will likely be
onshore, and this makes me hesitant to go with considerably
warmer values this day. I went on the low side of consensus
both days, especially Saturday (with residual effects of snow
cover and heights that do not rise significantly until dark).

With backdoor cold fronts, there is always a worry of
fog/stratus, which is not readily apparent in much of the
guidance. The pattern is not overly favorable, with the surface
high a bit west of where it needs to be for more favorable (and
stronger) onshore flow.

Then there is the matter of a northern-stream perturbation
moving through New England on Saturday, which may generate
enough lift (with favorable northwesterly low-level fetch) to
bring some snow showers to the Poconos. The 00Z ECMWF even
suggests this could spread east-southeast into the Lehigh Valley
and northern New Jersey. For now, kept the PoPs confined to
mainly Carbon, Monroe, and Sussex (NJ) Counties with the caveat
that these may need expansion southeastward should the ECMWF be
on to something.

The aforementioned weak perturbation moving into the region
Sunday night may generate some light rain or snow in the area,
especially north of the Mason-Dixon Line. The models are
uniformly unimpressed with the system, which is unsurprising
given its weakening phase. One thing to watch will be its track,
though, which is a little uncertain given the influence of a
northern-stream kicker and the always unknown position of the
backdoor front (typically farther south than progged this far
out). With a slightly southward track (one reason the ECMWF is
colder on Sunday), the isentropically-generated lift/precip on
Sunday night spreads over more of the area. Meanwhile, the GFS
looks rather sparse save for areas north of I-78. Plenty of
uncertainty here, so kept fairly low/broad PoPs. The 00Z CMC,
notably, is more in agreement with the ECMWF.

A surface low develops and moves through southeast Canada Monday
night and Tuesday, but models generally keep the strongest lift
north of the region. The 00Z GFS looks suspiciously wet so far
south of the system, though it is noticeably deeper with the
attendant trough. There also appears to be more phasing with a
southern-stream perturbation near the Gulf Coast. There is a lot
of uncertainty here, given that the southern-stream perturbation
stems from a trailing upper low in the Southwest, and the
evolution of these systems is generally of low predictability.
At 18Z Tuesday, the 00Z ECMWF has two distinct southern-stream
vorticity maxima in association with the Southwest upper low,
whereas the GFS has a single (stronger) entity (with typical
long-range timing errors, too), with the disparities between
the two giving me little confidence forecasts will remain like
they are now. With all of this said, I do not have a lot of
confidence to change the forecast from what I inherited until
model agreement improves. Main changes were to bring in a
distinct dry period on Monday (after the first perturbation
passes) and keep low PoPs in Tuesday with this second system.
QPF/impacts from both of these systems look low/minor at this
time.

There should be a distinct warming trend Monday and Tuesday
given the midlevel ridging that develops in advance of the
Canada surface low. Temperatures on Tuesday could be around ten
degrees above seasonal averages.

Another deep trough enters the western U.S. near the end of next
week, which should regenerate zonal flow or even slight ridging
east of the Rockies during this period. This will bring a
period of dry and seasonable weather (cooling after frontal
passage midweek, with warm advection returning thereafter). A
potentially strong surface low looks to develop in the central
plains during this period and will move east or northeast via a
strong southwesterly jet streak. The models are all over the
place with effects to the Mid-Atlantic, with potential for
plenty of warm-sector precip near the end of the week (GFS) or
frontolysis limiting the precipitation generated in our area
(ECMWF) or a frontal wave developing and generating a low that
crosses much closer to the area (CMC).

At this point, kept Wednesday and Thursday fairly dry, but
there may be another chance for some precip near the end of the
week.

&&

.AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Today...Starting VFR through much of the morning, but MVFR ceilings
are expected to move in around 18Z. In addition, snow is expected,
primarily along and east of the Delaware Valley which will result in
MVFR, and localized IFR, visibility. Confidence is moderate on both
flight categories and timing.

Tonight...Conditions should improve back to VFR between 00 and 03Z.
Northwesterly wind gusts up to 20 kt will be possible at KACY. For
the rest of the TAF sites, northwesterly winds will generally be
near or below 10 kt.

Outlook...
Saturday: Generally VFR, with snow showers possible in the Poconos
and vicinity. West winds 10 to 15 kts with gusts around 20 kts or
so. High confidence.

Saturday night: Generally VFR with light and variable winds. There
is a low chance of sub-VFR CIGs/VSBYs if stronger onshore flow
develops. Medium confidence.

Sunday: Generally VFR, with increasing cloudiness late. Light east
or southeast winds. Medium confidence.

Sunday night and Monday: Generally VFR, though brief sub-VFR
conditions are possible with light rain or snow, especially north of
PHL. Light winds generally transitioning from southeast to southwest
during the period. Low confidence.

Monday night and Tuesday: Sub-VFR conditions possible, with
scattered showers during the period, especially on Tuesday. Winds
primarily between south and west at 5 to 15 kts. Medium
confidence.

&&

.MARINE...
Today...winds and seas should stay below small craft advisory
criteria.

Tonight...northwesterly winds will increase, primarily after
midnight. On the Atlantic Coastal waters, wind gusts right around
gale force are possible, but still uncertain. Therefore, have issued
a gale watch for these locations. On the Delaware Bay, small craft
advisory conditions are likely.

Outlook...
Saturday: Westerly gales possible in the morning, with advisory-
level winds likely during the afternoon. Fair weather.

Saturday night: Residual advisory-level northwest winds in the
evening will diminish overnight. Seas below criteria. Fair weather.

Sunday and Sunday night: No headlines anticipated. There may be some
light rain off the New Jersey coast, with potential for some
visibility restrictions.

Monday and Monday night: No headlines anticipated. Mainly fair
weather, but there could be some showers off the New Jersey coast at
times during the period.

Tuesday: Southwest winds increasing to near or above advisory levels
by afternoon, with seas also building. A chance of showers and
attendant visibility restrictions.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory until 9 PM EST this evening for PAZ070-
     071.
     Winter Weather Advisory until 7 PM EST this evening for PAZ102-
     104-106.
NJ...Winter Weather Advisory until 9 PM EST this evening for
     NJZ012>027.
DE...Winter Weather Advisory until 9 PM EST this evening for
     DEZ001>003.
MD...Winter Weather Advisory until 9 PM EST this evening for MDZ008-
     012-015-019-020.
MARINE...Gale Watch from 1 AM EST Saturday through Saturday morning for
     ANZ450>455.
     Small Craft Advisory from 1 AM to 6 PM EST Saturday for ANZ430-
     431.

&&

$$
Synopsis...CMS
Near Term...Klein
Short Term...Johnson
Long Term...CMS
Aviation...CMS/Johnson
Marine...CMS/Johnson



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