NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Apr 2, 12:23 AM EST, 37 minutes earlier.
Area forecast discussion, Apr 2, 12:23 AM EST, 37 minutes before
Area Forecast Discussion
National Weather Service Mount Holly NJ
123 AM EDT Mon Apr 2 2018
.SYNOPSIS...
A weak front stalls just to our south and east through tonight, then
low pressure tracks along it late tonight and Monday morning. High
pressure then builds in later Monday afternoon before shifting
offshore Monday night. Low pressure tracks across the eastern Great
Lakes region Tuesday and Tuesday night, pushing a warm front to our
north. An associated strong cold front then moves through Wednesday,
followed by high pressure building to our south Thursday and Friday.
The next cold front moves through Friday night, however low pressure
may develop along it into Saturday before high pressure might build
in from the northwest on Sunday.
&&
.NEAR TERM /UNTIL 6 AM THIS MORNING/...
1215 am update: Concerns are many when looking at the latest
hi-res model simulations, radar/satellite trends, and current
observations. Made several adjustments to hourly
temperatures/dew points the next few hours, with considerable
weight given to hi-res 2-m values for each. Current obs are
highly heterogeneous spatially and will likely remain so until
the precipitation begins, as local effects are dominating
owing to gradually increasing cloud cover and light winds.
Models once again are too high with dew points (especially north
of the Mason-Dixon Line), so I suspect once the precipitation
gets going, wet-bulb effects will be stronger than progged. I
lowered temperatures, in some places substantially, after 08Z as
the precipitation increases rapidly.
Latest radar shows elevated precipitation beginning in south-
central/southeast Pennsylvania, though dew points are too dry
for the moment to allow for this precipitation to reach the
ground. Nevertheless, I suspect the precipitation will begin a
little bit faster than our current forecast suggests. May make
some last-minute changes to PoPs/Wx grids if radar trends
continue, though refrained from doing so at the moment.
Hi-res models continue to show mesoscale banding to the north
(generally along and north of an Allentown to Morristown line),
and the 00Z NAM has followed this trend northward. The HRRR and
NAM Nest are suggesting a band of 5+ inches in the northern
tier of counties, which may warrant a last-minute upgrade to a
winter storm warning. Would like to see more support for this
(especially regarding placement) before making the jump. For
what it is worth, the 00Z RGEM is less aggressive with the
northward banding and the overall QPF/snow totals, lowering
confidence somewhat. BUFKIT soundings, however, show a 1-3 hour
period of very strong lift in the dendritic growth zone in this
region, so even if warning accumulation criteria are not met,
the snow rates and lowered visibilities may become high impact,
especially given the timing. There are also indications of a
second QPF maximum to the southeast of this main band, with the
consensus in central/eastern New Jersey. Regarding the urban
corridor, thinking has not changed much, with pavement probably
mostly wet (with exceptions being where/when snow rates are much
heavier) but with grassy surfaces likely seeing some
accumulations.
Will continue to review the latest models/guidance and make
final decisions at 3:30 am.
Previous discussion...
Snow continues to spread east into the Ohio Valley as of late
this evening. While we have raised temperatures slightly early
tonight, they will fall quickly once precipitation begins, due
to the wet bulb effect. The snowfall forecast between I-78 and
the PA Turnpike to I-195 corridors and points south has been
adjusted downward slightly, but winter weather headlines are
intact, especially due to the main impacts centered in the
morning rush hour.
Previous Discussion...
The cold front is continuing to make slow progress south
through our area and should stall over or just south of southern
Delaware late tonight. A mid and upper level low is expected to
propagate east over the region late tonight. In response to
that, a surface low is forecast to deepen and intensify as it
moves along the front and off shore.
Trended close to the previous forecast and the 12Z NAM and HRRR.
Some of the coarser scale models, including the GFS, do not
appear to be picking up on the intense frontogenesis that will
be possible north of the center of the low. If this develops,
heavy snow bands could develop, which could result in fast
accumulating snow, despite marginal surface temperatures
(especially near and east of the fall line). As mentioned by the
previous forecast, this may be the type of event where there is
significantly more accumulation on grassy surfaces than paved
surfaces. None the less, the timing will be lousy for the
morning commute, so even if there are little accumulations on
the paved surfaces, slippery conditions are possible for the
morning commute. Only southern Delaware and the adjacent
Maryland counties look to stay warm enough to limit any change
over to snow.
As far as temperatures, continued the trend of favoring the
operational model, specifically the NAM, over any MOS guidance.
In these scenarios of widespread, potentially heavy precip, MOS
guidance tends to underestimate wet bulb effects, and is too
warm.
&&
.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TODAY/...
1215 am update: Lowered temperatures a couple degrees from
previous forecast, especially north of the I-78 corridor, with
the thinking that remaining clouds and fresh snow may limit
diabatic warming more than the statistical guidance is
suggesting. Incorporated heavy use of hi-res 2-m temp/dew values
for the forecast through the day. Expect the snow to rapidly
move out in the 14Z to 16Z time frame.
Previous discussion...
If there is a silver lining, it is that this is a fast moving
system and the precipitation should be out of the area by mid
day. We could even see breaks in the clouds by early afternoon.
Consequently, all of the area should see highs in the 40s and
considerable melting of the snow.
&&
.LONG TERM /TONIGHT THROUGH SUNDAY/...
Summary...Temperature roller coaster as the pattern remains active
with a series of systems moving eastward.
Synoptic Overview...A closed low near the Hudson Bay Canada area
will meander some through much of next week. Short wave energy will
slide around its southern periphery, with a significant one later
Tuesday and Wednesday resulting a sharpening trough in the Great
Lakes and Northeast. This will drive low pressure northeastward with
a trailing strong cold front crossing the East Wednesday. The
pattern looks to be such to maintain a trough from about the Midwest
to New England and adjacent Canada although there is some
uncertainty with the overall amplification of it. This may have an
affect on southern energy along a frontal zone and how a few surface
waves of low pressure evolve eastward Friday through Sunday.
For Monday night and Tuesday...The flow aloft backs some as a strong
short wave moves across the Northern Plains and Midwest Monday night
into Tuesday. As this occurs, surface high pressure will be offshore
and this will allow for a return flow to become established. The
backing flow combined with surface low pressure initially in the
Central Plains will result in downstream warm air advection. This
will enhance the development of a surface warm front which will lift
our way on Tuesday. Some short wave energy along with increasing
flow will enhance isentropic lift north of the warm front, and
therefore an area of rain should occur late Monday night and
Tuesday. This should end from the south for much of the area during
Tuesday as the warm front and best forcing shifts northward.
Temperatures could very well be challenging on Tuesday as it will
depend on the northward movement of the warm front and the timing of
the rain. The southern areas look to have a better shot at getting
into the warm sector, while the far northern areas may have to wait
until Tuesday night. We continued with a chance of rain later Monday
night then an area of likely PoPs for awhile on Tuesday.
For Wednesday and Thursday...As strong short wave energy along with
a robust wind field shifts eastward from the Midwest, an upper-level
trough is forecast to sharpen across the Great Lakes region then
quickly lift into eastern Canada. The trough may start to try and
take on a negative tilt, meanwhile low pressure tracks across the
eastern Great Lakes and into adjacent Canada during Wednesday. This
will drive a strong cold front eastward, which is forecast to cross
our area during Wednesday. There looks to be good forcing (aided by
a 50-60 knot 850 mb jet) with the front with warm air advection
occurring ahead of it. This will result in a band of showers with
the front. Forecast soundings show weaker instability, however if
parts of the area can partially clear ahead of it then temperatures
will be even warmer allowing for an increase in instability. Based
on this, maintained a slight chance of thunder on Wednesday with the
front. Some of the forecast soundings suggest the potential for a
low-topped convective line just ahead of the front given ample shear
but lower instability, with perhaps a strong wind threat. This will
be fine tuned as we get closer.
The guidance shows a sharp thermal gradient aloft just behind the
front on Wednesday, with ample cold air advection occurring
Wednesday afternoon and evening. This may tend to result in
temperatures peaking earlier in the day then falling with showers
and then cold air advection. A chilly airmass is in place Thursday
although aloft some warming is forecast to occur throughout the day.
Surface high pressure builds to our south Thursday with a northwest
flow occurring across our region. Most places may only be in the 40s
during the day Thursday, then 20s and 30s at night.
For Friday through Sunday...An active pattern looks to continue,
however the overall evolution of it is less certain. An upper-level
trough is forecast to remain in place across eastern Canada into the
Great Lakes and New England, however the degree of amplification
will have affects along with southern stream energy. A cold front
looks to move through later Friday then stall to our south. As
additional energy ejects into the Plains late Friday, surface low
pressure should develop along the leftover boundary. This may end up
tracking due eastward Saturday and Sunday, however this track will
depend on the northern stream trough orientation and also the
sharpness of the short wave trough sliding underneath it across the
mid-Mississippi Valley. This will have to be watched especially
given the potential for an overrunning precipitation event with a
chilly airmass in place. For now, we kept the forecast close to a
continuity/WPC blend overall.
&&
.AVIATION /05Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Through noon...Conditions will quickly deteriorate to sub-VFR
as precipitation overspreads the area from west to east from 06Z
to 09Z. Any rain will rapidly change to snow near/north of PHL
but may only occasionally switch to sleet and/or snow at
ILG/MIV/ACY for the event's duration. Heaviest snow will occur
between 09Z and 13Z, with runway accumulations likely at RDG/ABE
but far less probable at the Philly terminals. Precipitation
quickly ends 14Z to 16Z, but sub-VFR conditions will likely
linger for the rest of the morning. Winds generally between
north and east around 10 kts, gusting to 20 kts on occasion,
especially south/east of PHL. Moderate confidence.
Monday afternoon...MVFR to VFR conditions are expected, with
CIGs 2000-4000 feet (though they may scatter out, at least
occasionally). Winds switching to north or northwest around 10
kts, possibly becoming variable by late in the day. Low
confidence.
Monday night...Predominantly VFR during the evening, but will
likely become sub-VFR after midnight as rain moves in late.
Winds generally light and variable. Moderate confidence.
Outlook...
Tuesday...MVFR/IFR in the morning with rain likely. The rain
should taper off in the afternoon, and ceilings and visibilities
are anticipated to improve. Southeast winds shift to south and
southwest and may become gusty at times during Tuesday afternoon
and evening. Moderate confidence.
Wednesday..A period of MVFR/IFR conditions as a band of showers
(possibly a few thunderstorms) move through with a strong cold
front. Southwest winds 10-20 knots and gusty quickly shifts to the
northwest with the passage of the cold front. Low confidence
regarding timing of the lower conditions and thunder potential.
Thursday and Friday...VFR Thursday, then potentially areas of MVFR
Friday as a cold front arrives late. Lower confidence for Friday.
&&
.MARINE...
Small craft advisory conditions are expected to develop on the
coastal waters and lower Delaware Bay late tonight. Wind gusts
to 30 kt will be possible especially on the coastal waters. A
brief period of northeast gale force gusts is possible Monday
between sunrise and late morning, mainly across the Atlantic
water of Southern NJ. This has been mentioned in the MWW and
HWO. As of the late Sunday evening update, also increased seas
over the Atlantic Waters, as guidance is biased too low.
On the upper Delaware Bay, winds and seas should stay below SCA
criteria tonight and tomorrow.
Outlook...
Monday night...The conditions are anticipated to be below Small
Craft Advisory criteria.
Tuesday and Wednesday...Southerly flow increases Tuesday night and
Wednesday ahead of a strong cold front, and a Small Craft Advisory
looks likely especially for the Atlantic coastal waters. Winds turn
northwesterly by later Wednesday with the passage of the strong cold
front. Warmer air moving over the colder waters ahead of the cold
front should limit the mixing to some extent especially away from
the immediate coast, therefore the chance for gale force gusts looks
low at this time.
Thursday and Friday...Small Craft Advisory looks probable into
Thursday, then again Friday afternoon.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory until 1 PM EDT this afternoon for
PAZ054-055-060>062-070-071-101>106.
NJ...Winter Weather Advisory until 1 PM EDT this afternoon for
NJZ001-007>010-012>020-026-027.
DE...Winter Weather Advisory until 1 PM EDT this afternoon for
DEZ001.
MD...Winter Weather Advisory until 1 PM EDT this afternoon for
MDZ008.
MARINE...Small Craft Advisory until 3 PM EDT this afternoon for
ANZ450>455.
Small Craft Advisory from 5 AM to 11 AM EDT this morning for
ANZ431.
&&
Synopsis...Gorse
Near Term...CMS/Franck
Short Term...CMS/Franck/Johnson
Long Term...Gorse
Aviation...CMS/Gorse
Marine...Franck/Gorse/Johnson