Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 182337
Area Forecast Discussion
National Weather Service Mount Holly NJ
637 PM EST Sat Feb 18 2017
Low pressure in southeast Canada will send a cold front southward
into and through our area Sunday or Sunday night. Canadian high
pressure will extend south into the Mid Atlantic States Monday,
then move into the western Atlantic late Tuesday. A warm front
passes eastward through our area early Wednesday. A cold front
may sag southward into Pennsylvania and New Jersey Thursday
night. It should move north of our area Friday night as strong
low pressure moves from the Great Lakes into southeast Canada.
Its trailing cold front is expected to move off the East Coast
.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
630 pm update: Surface analysis valid at 23Z indicated a 988-mb
low in southwest Quebec with a trailing (weak) cold front
pushing through the eastern Great Lakes as warm air surges
northeastward downstream of it in the warm sector. An associated
vorticity maximum in southeast Canada will progress
southeastward to the Canadian Maritimes by 12Z Sunday as a
reinforcing shortwave trough digs into southern Quebec.
Meanwhile, a southern stream upper low in vicinity of the Ohio
and Tennessee Valleys will trudge eastward to the Carolinas by
12Z Sunday. Weak isentropic ascent just downstream of this upper
low will aid in the generation of light precipitation, possibly
not much more than sprinkles, in the Mid Atlantic west of
Chesapeake Bay this evening and tonight. The eastward extent of
these sprinkles is the main forecast question overnight, with
high-resolution models consistently showing sprinkles or very
light rain showers progressing northeast into central/southern
New Jersey and Delmarva in the 03Z-15Z time frame. Precipitation
will have a hard time reaching the ground given the relatively
dry low levels (and weak overall ascent). Nevertheless, did
expand the sprinkle mention northward generally to the south and
east of I-95 and increased the duration of this mention through
most of the night.
Tonight will be considerably warmer than last night -- probably
by 15-25 degrees -- given the residual warm air advection in
place downstream of the aforementioned cold front. Current temp
forecast looks OK. However, did lower dew points this evening
given recent obs.
Previous discussion below...
A spring-like afternoon in progress as low-level warm advection
has increased as a warm front continues to lift north of our
area. The 12z Sterling, VA raob showed +14C at the top of a
strong low-level inversion (about 1500 feet). Some cooler
temperatures are noted right along the coast though with a more
southerly wind component off the chilly ocean water. Through
tonight, our region is mostly between systems. One tracks well
to our north as the parent trough slides from Hudson Bay toward
the Canadian Maritimes. This will however push a weak cold front
into our area later tonight. The second system is a closed low
that will open up/weaken as it tracks eastward from the
Tennessee Valley to the southern Mid Atlantic overnight.
It is mostly the southern system that is driving an increase in
mostly high level cloudiness across our area this afternoon and
especially tonight. There is a low-level jet around 40 knots
forecast to develop close to our northern zones for a time
tonight before shifting into coastal New England. This will aid
in some moisture advection; however, forcing with the incoming
cold front is weak and moisture looks rather limited. As a
result, perhaps some sprinkles across mostly our southern zones
overnight. Given the expected cloud cover on the increase
tonight in combination with a southwesterly wind, will result in
milder temperatures. Low temperatures are mostly an even blend
of MOS and continuity.
.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
Surface low pressure will move into the Canadian Maritimes Sunday,
with a weakening upper-level trough sliding offshore of the southern
Mid Atlantic region. A weak cold front associated with the northern
surface low will settle across our area but then tend to stall or
even wash out. There is a secondary cold front that is forecast to
stay to our north during the day Sunday, and colder air is found
behind it. The initial front has little change in temperature with
it with just some weak cold air advection forecast in the
afternoon; however, the boundary layer remains warm. The low-
level flow turns more from the west and northwest during the day
which will add a downslope component for the coastal plain. As
a result, another warm afternoon is expected although the far
north should be a bit cooler given the weak cool air advection
settling in. Some spots, mainly from Philadelphia on south and
east, may be very close to their record high temperature for the
An area of mainly mid level clouds associated with the southern
system will exit to the east in the morning, with an increase in
sunshine expected. High temperatures are a MOS blend, however these
were boosted a bit especially for the coastal plain (I-95 south and
east) given a downsloping component to the flow. It should be warmer
at the coast given enough of an offshore flow.
.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
500 MB: A southeastward moving strengthening short wave in Quebec
Sunday night moves past the Maritimes Tuesday while a strong
ridge in the Great Lakes slides east. A weak short wave cuts
eastward into the ridge across New England Wednesday. In the
wake of the short wave, a ridge dominates over the mid Atlantic
States Thursday into Saturday before a strong short wave moves
into the Great Lakes over the weekend.
Temperatures: As our forecast area proceeds to a top 5 warmest
February on record and a top 12 warmest winter in the period of
record dating back to the late 19th century, the coming week of
Monday through Saturday should average 10 to 15 degrees above
normal. The coolest day is expected to be Tuesday...only several
degrees above normal, but thereafter, some model guidance points
to a three day period of temperatures 15 to 20 degrees above
normal between Thursday and Saturday. BUT... there may be a a
wrinkle developing. Dependent on the strength of the short waves
exiting the western USA late this week and weekend, there is a
chance of a low level cooler-colder flow setting up near and
north of I-78 Friday onward.
Forecast basis: Unless otherwise noted, a 50 50 blend of the
12z/18 GFS/NAM MOS guidance Sunday night-Monday night, thereafter
the 12z/18 GFS MEXMOS Tuesday, and then the 15z/18 WPC guidance
Thursday - Saturday, except used the warmest avbl guidance for
Wed-Thu which was the 12z/GFS MEXMOS which am confident is onto
the warmth...and our forecast may be 5 degrees too cool Wed and
As an example of guidance...todays high temp at phl 67... warmer
by 3 degrees than ydys warmest guidance which was the highres
NMM and WRF and against this mornings 00z/18 guidance was
warmer than the warmest guidance which were the ADJUSTMET and
HIRESWRF. Modeled MOS southwest flow warmth seems to have a
The dailies including any possible significant insight...
Sunday night...Mostly clear. Small chc some strato cu toward dawn
Monday (NAM has a layer of high rh near 950MB). Northwest wind gust
15 mph. Confidence: above average.
Monday...After any low strato cu dissipate, mostly sunny with a
cooler north northwest flow gusting near 20 mph. Still above normal
temps. Confidence: above average.
Tuesday...WAA increasing high clouds limiting max temperature potential.
Chance showers may invade E PA late in the day in the leading
portion of the instability burst. Wind light southeast to
south. Confidence: average on max temps which have favored the
milder of the avbl guidance.
Tuesday night...Chance of light showers ahead of a warm front.
Low probability there could be a touch of freezing rain high
terrain north of I-80. This ice risk isn`t in the HWO or zones
since confidence is below average and guidance favors min temps
during any rain, above freezing. Confidence: average or below
average on the occurrence of any rain.
Wednesday...Becoming Sunny and warmer. Forecast temps may end up
several degrees warmer than now predicted. West wind.
Confidence: above average except confidence on max temps
remaining this cool is below average.
Thursday...mixed clouds and sun and warm. Max temps may end up
5f warmer than now predicted. Southwest wind. Confidence: above
Friday...early morning stratus/fog along a cool front that has
settled into our area. maybe a shower north of the boundary or
maybe the boundary doesn`t make it down into our forecast area.
otherwise variable clouds. Confidence: below average on the
scenario. This ECMWF colder soln will need to be monitored for
future impact in our area. Confidence below average on the temps
and chance of any rain.
Friday night...a good chance of showers ahead of the cold front
moving east into our area or the ECMWF solution of a warm front
moving north from I-78. Shower timing and temperature uncertainty
as well as wind uncertainty, though probably turning southeast to
southwest, depending on latitude in our area.
Saturday...Maintained continuity despite the faster GFS solution.
EC and GGEM operationals are slower and at this time, did not
want to raise hopes for the GFS blustery dry westerly flow
on Saturday. Confidence below average.
.AVIATION /23Z SATURDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
For the 00Z TAFs...VFR through the period. High ceilings this
evening should rapidly lower to around or slightly below 10 kft
by 06Z. BKN CIGs expected to continue through the 12Z-15Z time
frame before dissipating during the day tomorrow. A sprinkle is
possible, especially at KILG/KACY/KMIV, overnight. Light
southwest winds will slowly veer to northwest by morning. Speeds
should increase to around 7-12 kts during the day tomorrow,
perhaps with an occasional gust to 20 kts.
Sunday night...VFR northwest wind. MVFR sc may develop toward dawn.
Overall confidence: above average.
Monday...Possible MVFR sc early otherwise a VFR northwest to
north wind... gusty 15 to 20 kt. Confidence: above average.
Tuesday...VFR lowering high cigs develop during the day. chance
of showers late in the day vcnty KABE KRDG. southeast to south
wind. Confidence: above average.
Tuesday night...Conditions may briefly lower to MVFR overnight
into early Wednesday morning with a chance of showers and a
southwest wind. Confidence: average.
Wednesday...Becoming VFR and a west wind develops. Confidence:
Thursday...VFR. Southwest wind. Confidence: above average.
The conditions are expected to remain below Small Craft Advisory
criteria through Sunday. A southwesterly flow through tonight will
gust to around 20 knots at times, then turn west and northwest late
tonight and Sunday as a weakening cold front moves through.
Sunday night - Monday...Sub Small Craft Advisory conditions expected,
although northwest to north winds may approach advisory levels across
the northern New Jersey coastal waters overnight Sunday into Monday
morning. Confidence: average.
Monday night-Thursday...Sub Small Craft Advisory conditions expected
although winds may gust up to 20 knots at times. Confidence: above
Allentown tied their record high for the date at 67 degrees
(originally set in 2011).
Record max temps Sunday that may be approached (within 2f) or