Area Forecast Discussion
Issued by NWS Mt. Holly, NJ-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KPHI 262026
Area Forecast Discussion
National Weather Service Mount Holly NJ
426 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 frontal passage 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 the area from the west on Sunday.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
-- Changed Discussion --Vorticity maximum is moving northeastward through the Midwest
this afternoon into the Great Lakes region, with attendant
surface low in northern Illinois. Quasi-stationary front extends
east from the surface low into northwest PA before extending
southeastward into the southern Mid-Atlantic region, with a
cold air damming regime in place east of the Appalachians in the
Northeast. Persistent east-northeast flow has brought increased
low-level moisture underneath midlevel subsidence via
downstream ridging relative to the aforementioned vorticity
maximum. As the vorticity maximum moves northeastward tonight,
the backdoor cold front in the southern Mid-Atlantic should lose
its southward progress and move poleward as a warm front.
However, models often overdo the progress of such fronts,
especially at night, and suspect this bias does exist to some
degree with today`s simulations. I generally undercut guidance
temperatures tonight by a few degrees, and this may not be
enough, particularly in the notoriously stubborn southern
Poconos, where temperatures have been running under guidance
today by about 3-5 degrees. With that in mind, temperatures will
remain near freezing early this evening at elevations
around/above 1000 feet, so light glazing may continue on
elevated surfaces via a thermal/moisture profile suggestive of
freezing drizzle. No societal impacts have been noted today with
the frozen precipitation, and with temperatures gradually
warming overnight, am not expecting much if any impact going
forward. By late evening/overnight, precipitation is expected to
be all liquid (including after contact with surfaces), and the
forecast features all rain tonight.
High-resolution models have been consistent in showing large-
scale ascent increasing in the northern Mid-Atlantic downstream
of the Great Lakes vort max late this evening and overnight,
with a batch of rain moving into NY/PA/MD/WV this evening. Given
the northeastward motion of the vort max, our CWA will be on the
southern fringe of the strongest ascent, with good agreement
among hi-res guidance that chances for precipitation drop off
considerably south of the Mason-Dixon Line. As such, have the
highest PoPs in the southern Poconos/northern New Jersey (65-90
percent), moderate chances (30-60 percent) in the I-95 corridor,
with slight or unmentionable PoPs generally south of the Mason-
One other question mark tonight is the extent/severity of fog.
Currently, thinking that winds will be high enough to preclude
more substantial/widespread fog formation (especially with the
approach of the shortwave trough, which tends to also mitigate
widespread fog). However, fog formation may be more substantial
near and off the coast, where dew points will exceed sea surface
temperatures late tonight. Thicker fog, should it develop over
the ocean, may advect onto the New Jersey coast and create local
visibilities below a half mile.
-- End Changed Discussion --
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
-- Changed Discussion --By tomorrow 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 afternoon tomorrow. 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.
-- End Changed Discussion --
.LONG TERM /MONDAY 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 /20Z SUNDAY THROUGH FRIDAY/...
-- Changed Discussion --The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
General MVFR conditions will gradually lower to IFR this
evening. Light rain will move in between 03Z and 06Z at
KRDG/KABE and by 09Z at KTTN/KPNE/KPHL/KILG and continue through
around daybreak. There may be slow improvement to MVFR after
sunrise and even VFR during the afternoon, but scattered
showers remain possible through the day. A lightning strike or
two may occur with showers during the afternoon tomorrow in SE
PA/N NJ, but confidence is too low to include in TAFs. Chances
of precip are lower at KACY/KMIV. Winds generally east 10-15 kts
through early evening with a trend toward S or SSW late tonight
through the morning hours tomorrow.
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.
-- End Changed Discussion --
-- Changed Discussion --Marginal but persistent small craft advisory conditions will
continue through Monday. East to southeast winds 10-20 kts with
higher gusts will occur, especially this evening and again
tomorrow afternoon. Seas will likely remain elevated (above 5
feet) offshore the New Jersey coast through at least tomorrow
afternoon. As a result, extended the small craft advisory
through 7 pm Monday.
Another concern is fog tonight and Monday, with dew points
expected to surge to values above sea surface temperatures late
tonight and tomorrow. Visibilities will likely lower to around a
mile at times late tonight and tomorrow and may become less than
a mile locally. At this time, not confident enough in issuing an
advisory, but will monitor closely tonight for potential need
for statements or advisories.
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.
-- End Changed Discussion --
Onshore flow is expected through Monday morning. Astronomical tides are
also increasing, coincident with the New Moon this Monday, March 27.
The ESTOFS remains most aggressive of the guidance suite for the
sunrise Monday high tide along the DE and NJ oceanfront. Although the
ESTOFS has outperformed the other guidance in recent past events, its
trends for positive departures (surge) appear to be lessening. Both the
SIT multi model review and GFS ETSS are more conservative, and
verifying better as of Sunday afternoon. The wind will also be trending
more parallel (southerly) to the shore by sunrise Monday, so any minor
tidal inundation flood risk remains a low potential with no action at
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...Small Craft Advisory until 7 PM EDT Monday for ANZ450>453.