Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS61 KPHI 160543
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1243 AM EST Sat Feb 16 2019

.SYNOPSIS...
A cold front will move across the waters late tonight, then
will stall well to the south on Saturday. A weak area of low
pressure quickly tracks along the front, however it is expected
to slide to our south during Saturday. The next weak low
pressure system then tracks across our area later Sunday into
Monday. A stronger storm system looks to arrive Wednesday into
Thursday followed by high pressure building down from the
northwest during Friday.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Cold front has moved off shore. The biggest immediate impact is
that dew points behind the front have been dropping
dramatically. Otherwise, the cold air advection has not been as
strong as previously expected, but once winds begin to drop off,
we should see radiational cooling increase, so have not made any
significant changes to the forecast.

Low pressure developing along the front in the lower
Mississippi River Valley and the Tennessee River Valley tonight
should reach the Carolinas by daybreak. The northern edge of the
precipitation shield associated with the low is expected to
reach southern Delaware and the adjacent counties of eastern
Maryland towards daybreak Saturday. Temperatures will be in the
upper 30s in those areas around that time. As a result, the
precipitation type will likely be rain. No precipitation is
anticipated for locations to the north.

Winds shift to the west, then northwest behind the front at 10
mph or so. Low temperatures tonight will range from the middle
and upper 20s in the Poconos and in far northern New Jersey, to
the upper 30s in southern Delaware.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/...
Light rain is forecast to continue into Saturday in southern
Delaware and in the adjacent counties of eastern Maryland, as
the surface low moves off the North Carolina coast and out to
sea. Meanwhile, high pressure is expected to push into eastern
Pennsylvania, far northeastern Maryland, northern Delaware and
New Jersey from the northwest, bringing dry weather and
seasonable temperatures to those areas.

High temperatures should range mainly from the middle 30s to the
lower 40s on Saturday afternoon in our region with a northwest to
north wind 10 MPH or less.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Summary...Colder for the second half of the weekend; A light wintry
precipitation event for some areas later Sunday into Monday; A messy
system probable Wednesday into Thursday with the potential for a
more pronounced cold air damming setup.

Synoptic Overview...An active pattern continues with an expansive
trough in the West with another strong trough in the vicinity of
Hudson Bay eastward. These features will meander about into early
next week, although a closed low near Hudson Bay may reposition
itself some early next week. This places our region under nearly
zonal flow, and with southern stream energy ejecting eastward and
interacting with bouts of colder/drier air filtering southward from
Canadian high pressure, means potential wintry precipitation at
times. The most notable system looks to be in the Wednesday and
Thursday time frame when better synoptic forcing and moisture comes
into play. It is at this time when the guidance is suggesting cold
air damming with high pressure to our north-northeast, and one low
tracking into the eastern Great Lakes and another one redeveloping
near the Mid-Atlantic coast. It is this system especially when
confidence is rather low with the details.

For Saturday night...A zonal flow aloft continues across the East
which results in a weak surface low quickly shifting offshore of the
Mid-Atlantic coast to start Saturday evening. Any precipitation near
far southern Delaware is anticipated to be gone to start Saturday
evening, with some clearing taking place especially across the
northern areas at least for a time. Meanwhile, cold Canadian high
pressure builds to our north however it also wedges down into the
northern Mid-Atlantic region. Given the fast flow, the next system
should already start tossing high cloudiness our way overnight.

For Sunday and Monday...A series of short waves eject northeast
supporting a weak surface low. The flow looks to back some and
therefore this system tracks farther north and right across our
region. While some milder air will get pulled northward ahead of the
system, the thermal profiles support more snow/frozen precipitation
from near the Fall Line and especially northward (I-78 corridor
northward). The precipitation amounts overall look light and with
light intensity rates anticipated, this could have some influence on
the precipitation types especially where the thermal fields are a
little more marginal. We used a model blend to obtain the thermal
fields, then generated the precipitation types from that utilizing
snow ratios. Most is snow or rain, however there could be a fairly
narrow corridor where some sleet or freezing rain occurs. This part
especially carries more uncertainty. As mentioned, any snow
accumulations look to be light (a coating to an inch or two) with
the most along and north of the I-78 corridor. The system looks to
depart the area early Monday, although some light snow may persist a
little longer across the far northern areas.

For Tuesday...A short break in the action is expected for Tuesday as
we get between systems. High pressure once again builds in from the
northwest and this should be the start of a more pronounced cold air
damming setup. The airmass, while on the colder side building in, it
looks rather dry due to high pressure becoming more established to
our north. This may delay the arrival of the precipitation later
Tuesday night, however some guidance is more robust with the warm
air advection and isentropic lift associated with a developing
overrunning setup. As a result, kept some increase in the PoPs from
south to north by later Tuesday night.

For Wednesday and Thursday...This time frame could be quite a mess.
The setup looks similar to the event that took place early this
week, and this involves cold Canadian high pressure anchored to our
north with colder and drier low-level air seeping southward.
Meanwhile, energy arriving from the southwest drives a surface low
into the Great Lakes region by Thursday, however given the
expectation of cold air damming in place a secondary surface low
develops near the Mid-Atlantic coast. This setup typically locks in
the colder low-level air in longer, especially inland from the Fall
Line. Much of the guidance indicates a good moisture feed with this
system, with strengthening isentropic upglide and a ribbon of
frontogenetic forcing. This all translates to varying precipitation
types which are always challenging to forecast but especially at
this time range. We derived our precipitation types from the thermal
fields via a multi-model blend. There very well could be a burst of
snow for parts of the region before a transition to mixed
precipitation or freezing rain occurs. Despite the uncertainty in
the details, the setup suggests that at least portions of our region
could get enough wintry precipitation to result in hazardous travel.
As a result, a mention was added to the Hazardous Weather Outlook,
especially for areas near and west of I-95.

For Friday...The guidance indicates that the system discussed above
should be out to sea as we start Friday. The flow is still zonal
aloft and this results in yet another surface high building into the
area from the west and northwest. The pressure gradient may be
tightened and therefore a gusty northwesterly breeze is
possible.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Rest of Tonight...VFR conditions are expected with decreasing
mid and high level clouds (generally at or above 10000 ft AGL).
Northwest winds around 10 kt. High confidence.

Saturday...VFR conditions are expected with only few to
scattered high clouds. Northwest to north wind 5 to 10 knots.
High confidence.

Saturday night...VFR conditions expected with light (generally
5 kt or less) northerly wind. High confidence.

Outlook...

Sunday...VFR. Light northerly winds becoming locally light and
variable, then becoming light east to southeast.

Sunday night...A period of MVFR/IFR conditions probable with some
snow or a wintry mix. The details are less certain. Winds mainly
light and variable.

Monday and Tuesday...VFR conditions return Monday and continue
through much of Tuesday. MVFR conditions, especially ceilings,
should develop later Tuesday night. Northwesterly winds generally 10
knots or less.

Wednesday...MVFR/IFR conditions probable with snow, rain and a
wintry mix. Northeast or east winds 10 knots or less. Low
confidence, especially regarding the precipitation types.

&&

.MARINE...
Have cancelled the remainder of the Small Craft Advisory as most
observations indicate seas are now below 5 ft.

Winds and seas are expected to remain below SCA criteria
Saturday and Saturday night. Winds will be shifting to
northwesterly shortly and should remain northwesterly or
northerly, but with speeds well below SCA thresholds.

Outlook...

Sunday through Tuesday...The conditions are anticipated to be
below Small Craft Advisory criteria.

Wednesday...Increasing northeasterly winds and building seas,
therefore a Small Craft Advisory looks probable.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Gorse
Near Term...Iovino/Johnson/MPS
Short Term...Iovino
Long Term...Gorse
Aviation...Gorse/Iovino/Johnson/MPS
Marine...Gorse/Iovino/Johnson/MPS


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.