Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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854
FXUS61 KPHI 201123
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
623 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 see a cold front then nudge into eastern
Pennsylvania and northern New Jersey Thursday night, but it should
quickly lift back north as a cold front. As a low pressure system
lifts into eastern Canada this weekend, the associated cold front
will sweep through our region on Saturday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
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 continued surface pressure rises and mixing within the diurnal
boundary layer, northwest winds will gust 15 to perhaps 20 mph at
times.

&&

.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 /11Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Today...VFR. Northwest wind gusts 14 to 18 knots late this morning
into the afternoon.

Tonight...VFR. Northwest winds will gradually veer to the east, as
they become light and variable after midnight.

Outlook...

Tuesday...Ceilings could lower to MVFR with rain showers continuing
into Tuesday night.

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

Friday...starting VFR 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 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.

&&

.FIRE WEATHER...
Relative Humidity values will fall in to the low to mid 30s this
afternoon across portions of northern DE, NJ, northeast MD, and PA.
In addition, fuel moistures are forecast to range from 7 to 10 pct
outside of the Poconos. But at this point, we expect winds to
largely remain below 20 mph, in terms of sustained winds and
gusts. This will limit the potential for spread of any wildfires
that may develop.

&&

.CLIMATE...
New records for this date have been established at PHL ILG ABE
TTN and RDG.

Here are/were the previous record highs for Sunday:

PHL 68-1948 now 70-2017
ILG 67-1997 now 71-2017
ABE 63-1997 now 68-2017
TTN 68-1948 now 69-2017
GED 74-1976
ACY 73-1961
RDG 67-1997 now 68-2017
MPO 58-1981

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...Johnson
Near Term...Franck
Short Term...Franck
Long Term...Johnson
Aviation...Franck/Johnson
Marine...Franck/Johnson
Fire Weather...Franck
Climate...Staff



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