Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 201524
Area Forecast Discussion
National Weather Service Mount Holly NJ
1124 AM EDT Thu Apr 20 2017
A front settles across portions of our area today and tonight,
and may linger across our northern areas on Friday. Low pressure
in the Great Lakes today into tonight will track across
southeastern Canada during Friday. An associated cold front will
move through our region later Friday. Another low will track
from the Southern Plains to the Southeastern States through the
weekend. High pressure then builds into our region from the
north Tuesday and Wednesday.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The frontal boundary that dropped into our region during the
night has become a bit diffuse and difficult to find.
A mid level short wave trough was passing overhead late this
morning and it should move out to sea this afternoon. Showers
associated with the feature were weakening and moving off the
The cloud cover will continue to develop breaks for the
Marginal instability is expected to develop from the
Susquehanna River Valley of Pennsylvania down into the
Chesapeake Bay region during the afternoon. As a result, there
is a chance that showers and thunderstorms will develop to our
west late this afternoon, possibly advancing into the far
western and southwestern edge of our forecast area around 6:00
The wind is anticipated to remain variable into this afternoon
at less than 10 MPH. The wind direction is forecast to begin
settling into the southeast late in the day. Maximum
temperatures are expected to favor the 70s, with readings not
getting above the 60s in the far north.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
Any convective elements early should weaken ewd across e PA
this evening. A more substantial batch of showers and isolated
tstms should advance ewd from the Ohio Valley into our area late
this evening or overnight...mainly PA/NJ. With PWAT near
1.4"... thunder related elements could produce heavy rain. Light
wind. Abnormally mild tonight with lows 13 to 17 degrees above
FCST basis was a 50 50 blend of the 00z/20 GFS/NAM MOS.
.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Summary...A surge of much warmer air for much of the area
Friday; turning cooler over the weekend and unsettled especially
Sunday; warming then during the first half of next week.
Synoptic Setup...An upper-level trough moves from the Great
Lakes to New England Friday and Saturday, although it weakens. A
secondary short wave sharpens a trough in the Central Plains
Friday night into Saturday, which then moves eastward to the
Southeastern States Sunday night and Monday. This feature exits
into the western Atlantic early next. Some ridging tries to
build into the East during the first half of next week, as a
broad upper-level trough with several embedded short waves is
from the Northern Plains and Midwest to the Great Lakes. This
results a couple of systems moving through, the first in the
form of a cold front later Friday. The second feature is
stronger with plenty of moisture and lift with it Sunday ,
however trends are for this to be farther south. The pattern may
then favor calmer conditions early next week. We used a
model/continuity blend for Friday through Saturday night, then
blended in the 00z WPC Guidance. Some adjustments were then made
following additional collaboration with our neighboring
For Friday...A sharp upper-level trough moving across the Great
Lakes and toward the Northeast is forecast to gradually weaken.
Surface low pressure however will continue to translate
eastward, sliding by to our north through the day and into the
night. Surface high pressure up across the Canadian Maritimes
may still be extended southward toward our coast with a frontal
boundary draped in the vicinity of our northern areas. If this
boundary remains in place, cooler air will be found across our
northern areas. A cold front is then forecast to move through in
the afternoon or evening, and much of the guidance has slowed
this down. The slower passage of the cold front would usually
mean a greater chance for convection as it arrives during peak
heating, however much of the forcing still looks to slide by to
our north with perhaps a split. Much of the guidance however
does show enough convergence with the front and some increase in
the instability to initiate new convection in the afternoon and
early evening mainly from I-95 on south and east. It will be a
race though between developing instability and drying arriving
from the northwest. Looks like any convection would be scattered
at this point and then it shifts offshore during the evening.
Therefore, kept PoPs a bit higher to start with leftover
Thursday night activity, then again in the afternoon and evening
with potential for renewed development. The instability is not
forecast to be all that robust and shear is on the weaker side
(a bit stronger north compared to the south). As a result, any
thunderstorms should be sub-severe.
Some clearing should take place during Friday afternoon as some
drying aloft works in. The main cold air advection is delayed
especially at and below 850 mb and many areas will be into 70s
to lower 80s. Given the prospects of the aforementioned boundary
across the northern areas though, trended high temperatures a
bit cooler here.
For Saturday and Sunday...The cold front from later Friday
looks to stall to our south. As energy slides eastward, a
surface low tracks into the Tennessee Valley Saturday afternoon
then to the Carolinas Sunday morning. A surface trough should
extend north-northeast from this low. There still remains some
differences among the guidance with the northern extent of the
associated rain shield. Some of the farther north models have
shifted southward, and this looks to be due to surface high
pressure building southward quicker and also a weaker upper-
level trough axis. Overall, Saturday should feature more clouds
than sunshine however the rain should hold off until later at
night for the southern areas.
The surface low is forecast to track well to our south Sunday
and Sunday night, however a ribbon of enhanced lift should
result in an areas of rain especially across the southern half
of our area. The PoPs were adjusted to show a slower increase
Saturday into Sunday, and were lowered to chance across the far
north. Given the model differences, did not want to make
significant changes at this time. The prospects of lots of
clouds and areas of rain with an onshore flow will result in a
much cooler day Sunday. The system should shift offshore Sunday
night with some gradual improvement.
For Monday and Tuesday...A trough that may become closed off
for a time across the Southeastern States should slowly shift
into the western Atlantic during this time frame. This will take
surface low pressure with it, however we will have to monitor
if any precipitation can back in from the ocean. We favored some
low PoPs as a result mainly for the eastern zones. Meanwhile,
high pressure is forecast to build down from eastern Canada and
combined with low pressure to our south and east will produce a
northeasterly low- level flow. This will increase at times with
it most noticeable closer to the coast, which is where it will
be cooler and more clouds should be around. High temperatures
each day may be very similar.
For Wednesday...A broad upper-level trough in the Northern
Plains, Midwest to the Great Lakes will result in low pressures
tracking well to our west. The first one lifts into Canada north
of the Great Lakes Wednesday with a trailing cold front. Given
the idea of the trough hanging back to the west longer, this
cold front is currently forecast to remain well to our west with
high pressure building down the East Coast. This will result in
warming, although again cooler closer to the coast with an
.AVIATION /16Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
Today...MVFR Cigs continuing to improve to VFR. Light wind.
Tonight...VFR cigs to start could become IFR in stratus/fog
after 08z/21. scattered showers late with a possible
thunderstorm. Light wind.
Friday...MVFR/IFR ceilings (possible visibilities) in the
morning should improve to VFR ceilings around 5000 feet from
about Midday on. Some showers or a thunderstorm should move
through early, then some isolated to scattered
showers/thunderstorms in the afternoon and evening especially
from TTN, PNE/PHL to ILG on eastward. Brief MVFR/IFR conditions
with any heavier showers or a thunderstorm. Mainly southwest
winds 5-10 knots, becoming northwest 10-15 knots in the
afternoon or early evening as a cold front moves through.
Improving conditions Friday night in the wake of the cold front.
Saturday and Sunday...VFR during the day Saturday with ceilings
gradually lowering. The conditions then lower to MVFR/IFR with
periods of rain and potentially fog late Saturday night and
especially Sunday. The greatest chance for widespread rain
appears to be from near the PHL metro area on southward.
Northwest to north winds 10 knots or less Saturday, becoming
northeast 10-15 knots Saturday night and especially Sunday (some
gustiness near the coast). The conditions should improve later
Monday...VFR overall, however a few showers are possible mainly
in the area of MIV/ACY. Northeasterly winds 5-15 knots with
some gusts near the coast.
No marine headlines anticipated through Tonight. Wind speeds
are forecast to remain below 20 knots and wave heights will
generally remain less than 5 feet.
We do note a persistent easterly swell of 3 to 4 ft and 8
seconds. Seas were raised a foot for tonight in the 6 am update.
The wind direction over our waters will favor the southeast to
Friday...The conditions are anticipated to be below Small Craft
Advisory criteria. The winds will be shifting around as a cold
front moves through in the afternoon or evening. Some showers
and thunderstorms are possible especially in the afternoon and
Saturday and Sunday. The conditions should be below Small Craft
Advisory criteria Saturday, then winds and seas increase Sunday
as a northeasterly flow strengthens.
Monday...While wind gusts may lower to below 25 knots, seas may
build some more as a northeasterly flow continues.
Overall projections today were identical or slightly warmer
than yesterdays. Confidence is above average on the info below.
April will probably be a top 10 warmest in the period of record
for multiple long term climate stations in our forecast area.
Depending on the warmth of this afternoon and Friday and how
cold it is on Sunday or Monday when a chilly rain may occur, we
appear to be on our way for a top 2 warmest April on record.
The usual analysis process first 19 days done, the 20th-26th
per our 330 am fcst, the 27-30th from FTPRHA.
PHL projects #2, 4.9 above normal and 0.5degs from record. POR 1874
ABE projects #2, 6.1 above normal and 0.4degs from record. POR 1922
ACY projects rer 5.0 above normal,0.4F above prev 2010 56.3 POR 1874
So, there could be some slippage but its unlikely that any of
these locations can slip below #10.