Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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573
FXUS61 KPHI 201431
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
931 AM EST Mon Feb 20 2017

.SYNOPSIS...
High pressure will build over the Mid Atlantic today and tonight
before shifting offshore. A warm front will slide through the region
Tuesday night. We may then see a cold front nudge into eastern
Pennsylvania and northern New Jersey Thursday night, but it
should quickly lift back north as a warm front. As low pressure
lifts into eastern Canada this weekend, the associated cold
front will sweep through our region on Saturday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The 930 AM update was to adjust the hourly temperatures upward
faster as plenty of heating is ongoing, resulting in a quicker
temperature rise so far for much of the region. Some low clouds
trapped beneath an inversion within the north-northwest flow is
edging into the southern Poconos, however this should dissipate
through midday due to dry air entrainment and more subsidence.

Otherwise, high amplitude mid-level ridge axis over the Great
Lakes will continue to build Canadian high pressure south into
our region today. Upper level jet axis will be positioned just
northeast, with our area within the confluent region, promoting
large scale subsidence through the column. Expect a sunny day
with some cirrus, given the proximity of the aforementioned jet.
We went above guidance, given the expectation of greater
boundary layer mixing than depicted in model soundings, as well
as a recent cold bias. High temperatures will average around 10
degrees above normal. With mixing within the diurnal boundary
layer, northwest winds will gust up to 20 mph at times
especially through early afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
The mid-level ridge axis moves to the Appalachians tonight, with
the surface ridge situated across our area. Winds will veer
from the northwest this evening to the east-northeast overnight,
becoming light and variable. High cloudiness will be
increasing, especially after midnight, and to the west of the NJ
turnpike/I-95. The model trend is to bring this upper level
moisture in faster, which will inhibit to some extent ideal long
wave (radiational) cooling. At this point, given the dry air
mass in place and initially clear sky until at least midnight
(especially the Pine Barrens), 2-meter temperatures will fall
quickly. Some of the MOS guidance that was initially bullish on
low temperatures has backed off some. For now, we have increased
low temperatures slightly across the typically colder locations
by a couple of degrees to account for this, but they will still
average around 5 degrees above normal.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
For a relatively quiet pattern, the long term is shaping up to
have quite a bit to discuss.

Tuesday and Tuesday night...main question through this period
will be how quickly will the surface high and upper level ridge
shift off shore. As once that happens, the warm front and
rain showers will move into our region. For now, this looks to
occur primarily Tuesday night. Thus, it still looks like Tuesday
will be the coldest day of this week, with highs generally in
the 40s and 50s (this is still 5 to 10 degrees above normal).

Wednesday and Thursday...Big story through this period will be
the warmth! Significant warm air advection pattern is expected
to develop. Stayed close to the previous forecast with highs
generally in the 60s on Wednesday and in the 60s and 70s on
Thursday. This is several degrees higher than any guidance, but
with our most recent warm air advection event (Saturday and
yesterday), all guidance showed a cold bias anywhere from 5 to
10 degrees for the MOS guidance and up to 15 degrees for the
operational runs of the GFS. It looks likely that some more
records will be broken, especially on Thursday (climate section
below has the record highs for both dates for our climate
sites). As for precip through this period, GFS continues to show
some QPF over the region on Thursday, but this seems unlikely
as we will be at or near the saddle point between three surface
lows (one in eastern Canada, one off the coast of FL, and one
over the high plains). For now, just have a slight chance across
Delmarva.

Friday...There remains poor agreement between models and poor
run to run consistency with how far south the cold front will
get on Thursday night/early Friday. With the 00Z runs, the GFS
shows the front briefly sinking into E Central PA and NW NJ
Thursday night, before lifting north of the region by mid day
Friday. ECMWF shows the front staying well north of the region.
This looks to be primarily related to differences in the tracks
of the previously mentioned surface low which will be lifting
from the High Plains to the Great Lakes region through this
time. ECMWF keeps the low on a more progressive and northerly
track versus the GFS. Hesitate to make many changes to the
forecast given the uncertainty, but if models trend closer to
the ECMWF solution, this could mean that the current forecast is
too cool (temps could be close to Thursday highs with the ECMWF
solution) and too wet (with a further north track, our region
would stay in the dry slot through the day on Friday and Friday
evening). Will have to continue to watch this as we get closer.

Saturday...Regardless of the previously mentioned differences
between the models, it looks like the prime period for precip
with this low will be Saturday morning and afternoon in the warm
sector just ahead of the cold front. The cold front looks to
sweep through our region later in the day (meaning we should
have warm air advection through much of the day time hours). In
the upper levels, it looks like we will have a jet over our
region through this period. Looking at the GFS model soundings
through this period, instability is lacking, but there are large
bulk shear values. As mentioned previously, the operational runs
of the GFS have had a very strong cold bias, so accounting for
that, we could see modest instability during the day on
Saturday. If all of this timing holds, and if we do warm more
than what the GFS is depicting, then it could set the stage for
strong thunderstorms. There are still too many what ifs to
include in the hazardous weather outlook at this time, but we
will continue to monitor as we get closer.

Sunday...strong cold air advection behind the cold front could
mean the first day in over a week of near normal temperatures.
Could see lake effect snow showers propagate into the Poconos
while we have strong northwesterly flow, but that doesn`t look
to last too long as a surface high should build over the region
late in the day resulting in light boundary layer winds.

&&

.AVIATION /15Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Today...VFR. North-northwest winds around 10 knots, with some
local gusts occasionally up to 20 knots.

Tonight...VFR. North to northeast winds 5 knots or less.

Outlook...
Tuesday...VFR during the day with an increase in clouds mainly
at or above 15000 feet. Ceilings could lower to MVFR with some
rain showers at night. Winds becoming southeasterly 10 knots or
less.

Wednesday and Thursday...Mostly VFR conditions are expected.

Friday...Starting VFR, however could lower to MVFR if rain
showers move in earlier than forecast.

&&

.MARINE...
Today and tonight...Canadian high pressure continues to build south
over the waters. Northwest winds today will veer to the north-
northeast this evening, then east-northeast overnight. Sub-SCA
conditions forecast.

Outlook...
Tuesday through Friday...Winds and seas should stay below SCA
criteria. A gradual shift from southeasterly to southwesterly is
expected on Tuesday. By Thursday night, winds could be light
and variable shifting to southeasterly on Friday.

&&

.CLIMATE...
Another significant warm-up expected later this week. The record
high temperatures for Wednesday and Thursday are listed below.

Wednesday 2/22    Thursday 2/23

ACY 68-1991       72-1985

PHL 68-1997/1974  75-1874

ILG 69-1997/1974  72-1985

ABE 68-1974       71-1985

TTN 66-1997       74-1874

GED 72-1997       no data

RDG 70-1974       72/1932/1922

MPO 56-1997       60-1977

&&

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

&&

$$
Synopsis...Gorse/Johnson
Near Term...Franck/Gorse
Short Term...Franck
Long Term...Johnson
Aviation...Franck/Gorse/Johnson
Marine...Franck/Johnson
Climate...



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