Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 270134
Area Forecast Discussion
National Weather Service Mount Holly NJ
934 PM EDT Sun Mar 26 2017
Low pressure moving from the Mississippi Valley into the Great
Lakes will lift a warm front northward through our region overnight
into Monday morning, followed by a cold front Tuesday night.
Canadian high pressure will build into the area Wednesday through
Friday. Low pressure and its associated fronts in the Ohio Valley on
Friday will move through our region on Saturday. A weak area of high
pressure is expected to move into our region from the west on Sunday.
.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
The marine influence, courtesy of a cold air damming setup and
onshore, will maintain stratus and some fog across the area.
The extent of the fog may worsen for a time overnight as warmer
air starts to move northward, and the forecast soundings
indicate the inversion lowers some. The extent of any dense fog
is less certain, however the higher terrain of the northwest
zones and closer to the coast may have the highest chance of
occurrence. We went with areas of fog with the drizzle until
PoPs increase enough to include a shower mention.
Temperatures remain around freezing at elevations mainly at and
above 1400 feet in the southern Poconos over to northwestern
Sussex County in New Jersey, so light glazing is probably
continuing on elevated surfaces. This is mainly from freezing
fog/drizzle so far. There may be a little freezing rain as the
showers arrive later on, however temperatures should warm ever
so slowly overnight. No societal impacts have been noted with
the frozen precipitation and since it is confined to the higher
elevations we are highlighting this with a Special Weather
A short wave and associated surface low will track into the
eastern Great Lakes overnight. The associated ascent will mostly
affect our western zones, and therefore showers are expected to
increase and become more numerous across these areas. The
highest PoPs therefore remain for these areas, with these
decreasing with an eastern and southern extent. It appears that
there should not be enough elevated instability, therefore no
thunder was included at this time.
The hourly temperature and dew point grids were adjusted based
on the latest observations, then some high-res guidance was
blended in to try and better capture the colder air in the
Poconos. Low temperatures were lowered a little for several
areas given a slight downward adjustment to the current
.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
By Monday morning, the shortwave trough in the Great Lakes is
expected to continue northeastward into the St. Lawrence Valley,
with the organized large-scale ascent moving into New England
and adjacent southeast Canada. Precipitation will likely become
more showery by late morning, with perhaps even hints of blue
sky in portions of Delmarva during the afternoon. The warm front
will sweep northward well into New England by this point, and
with the warm southerly fetch, temperatures will warm
substantially above today`s values. Forecast highs are 15-25
degrees above today`s values. Forecast temperatures may be on
the low side if partial sunshine occurs, especially considering
the general negative bias of guidance in warm sector regimes
There is some question if localized lift can generate more
convective showers during the Monday afternoon. Forecast
soundings are at least marginally supportive of this,
particularly northwest of I-95, where residual colder air at
midlevels combined with a well-mixed boundary layer may permit
development of isolated/scattered showers. Felt compelled to
include a mention of isolated thunder during the afternoon given
the indications of positive buoyancy during peak heating. This
is conditional, however, as transient ridging upstream of the
departing shortwave trough may preclude sufficient lift
necessary for the development of any convection.
.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
A split flow regime will continue across the conus during this period.
While the northern stream remains displaced to the north, our primary
weather makers will be in the southern stream. A closed low along the
lee of the Rockies will gradually open and eject northeastward. Its
associated weak surface reflection and cold front will traverse the
middle Atlantic, impacting our sensible weather Monday night into
Tuesday night. A cyclonic flow aloft on Wednesday will gradually give
way to ridging on Thursday. The next southern stream system in the
pipeline will impact our region Friday into Saturday.
With good run to run model consistency in terms of the timing of the
cold frontal passage Tuesday night, the next challenge will be how the
next southern stream system is handled during the Friday and Saturday
period, including the extent of phasing with the northern stream. In
particular, the track of this system and the extent of the cold air to
the north will have implications on any potential p-type issues.
Temperature-wise, around 10 to 15 degrees above normal on Tuesday, then
generally normal to several degrees above normal from Wednesday through
Another round of showers is expected mainly after midnight Monday night
into Tuesday night across the entire region. With the cold frontal passage
Tuesday night, showers should move offshore prior to Wednesday morning. We
do not anticipate any p-type issues during this period. We expect patchy fog
Monday night, which may linger into Tuesday afternoon north of a PA Turnpike
to I-195 line. Within the warm sector in advance of the cold front, we expect
sunshine to break out, especially south of the aforementioned line with
temperatures reaching the upper 60s to lower 70s. A low-level moist
tongue will also be in place across this area, with Dew Points well
into the 50s. Models indicate some weak ML Cape, along with negative
Lifted and Showalter Indices, within an environment characterized by
poor lapse rates and weak shear. We have included a chance of thunder
in the forecast, and there may be some localized heavier downpours as
well. Given low Precipitable Water values around one inch, urban and
small stream flooding is not a concern.
In the wake of the cold front, expect a return to fair weather for
Wednesday and Thursday.
Beyond day four, the models have come into better agreement on the
Friday-Saturday system, but there still remains uncertainty regarding
the degree of phasing between the northern and southern stream,
including p-type implications. The GFS, ECMWF, and Canadian take low
pressure from the MS Valley into the Great Lakes, with varying degrees
of secondary development over the middle Atlantic. The UKMET is an
outlier, maintaining a purely southern stream system, with the primary
low moving through the middle Atlantic. In fact, the UKMET represents
yesterdays GFS solution, as it was previously in the former camp. The
forecast brings in chance PoPs Friday into Saturday. Also, given the
orientation of the surface high, cold air damming to some extent is
likely. Thickness values indicate the cold air is right on our door
step, so p-type could be an issue, especially across the northern half
of our CWA.
.AVIATION /01Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
Overnight...IFR ceilings, lowering to LIFR at times, along with
areas of MVFR/IFR visibility due to fog and some drizzle. The
extent of the visibilities due to fog is less certain. Some
showers are expected to develop between through 06Z at KRDG and
KABE and toward 08Z at KTTN, KPNE, KPHL and KILG. East-northeast
winds mainly 10 knots or less, becoming southeast late.
Monday...IFR conditions (ceilings and fog) should improve to
MVFR during the morning, then VFR during the afternoon. Some
showers should continue through the day (most coverage in the
morning north and west of KPHL). A thunderstorm may occur during
the afternoon mainly north and west of KPHL, however coverage
should be isolated. The chance of showers is lower at KACY amd
KMIV, although there may be some drizzle in the morning.
Southeast winds up to 10 knots, turning south during the morning
then southwest in the afternoon.
Monday night...Periods of MVFR/IFR likely in low clouds and fog
across all TAF sites.
Tuesday...MVFR likely Tuesday morning at all TAF sites, with improvement
to VFR by afternoon at all but ABE, RDG, and TTN.
Tuesday night...MVFR possible at ABE, RDG, and TTN early, otherwise,
Wednesday thru Thursday...Predominantly VFR conditions expected.
Friday...VFR conditions may deteriorate to MVFR with the arrival of
the next weather system.
There is the potential for northwest wind gusts up to around 25 knots
Tuesday night into Wednesday.
A Dense Fog Advisory (visibility 1 NM or less) remains in effect for
all zones through 8 AM Monday. Coastal observations along with
some web cams indicate fog development continues especially
across the southern areas, and this should thicken and expand
northward through the night as warming occurs aloft and dew
points increase some.
Marginal but persistent small craft advisory conditions will
continue through Monday. East to southeast winds 10-20 kts with
higher gusts will occur. Seas will likely remain elevated
(at or above 5 feet) offshore the New Jersey coast through at
least Monday afternoon. As a result, the small craft advisory
goes through 7 PM Monday.
Seas may remain elevated into Monday night, and the SCA may
need to extended into this period. A period of northwesterly
wind gusts may reach SCA criteria on Wednesday. Sub-SCA
conditions are expected on Thursday. The approach of the next
system may lead to a return to SCA conditions by Friday.
March as a whole for PHL, is still on track to average one half
to 1 degree below normal, despite the warmth of ydy through
MARINE...Dense Fog Advisory until 8 AM EDT Monday for ANZ430-431-
Small Craft Advisory until 7 PM EDT Monday for ANZ450>453.