Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
510 am EDT Sat Apr 18 2015
low pressure will quickly move across New England this afternoon,
pulling a dry cold front across our area this evening. High
pressure will then build in Saturday night into Sunday before
shifting offshore. Low pressure will move from the Great Lakes to
near Hudson Bay Monday and Tuesday then stall. An associated warm
front will lift northward into portions of our region during
Monday, followed by a cold front Monday night. The next cold front
arrives Wednesday night with potentially another cold front moving
through Thursday night into Friday.
Near term /until 6 PM this evening/...
patchy shallow fog developed in parts of our region during the
night. It should dissipate rather quickly after daybreak.
High pressure centered over the Ohio River valley early this
morning will slide eastward today. Meanwhile, a robust middle level
short wave trough is forecast to move from eastern Ontario and
southwestern Quebec, across northeastern New York and New England
during the afternoon and evening hours. The short wave is expected
to push a cold front toward our region late in the day.
We are anticipating a sunny day with some wisps of cirrus
overhead. A developing westerly surface flow will draw warm air
into our region. Maximum temperatures are forecast to be around 80
degrees. Readings are expected to get no higher than the lower and
middle 70s in the elevated terrain and no higher than the 60s
along the immediate coast.
Philadelphia's last 80 plus degree day was on September 28 when
the temperature rose to 86 degrees.
Short term /6 PM this evening through 6 am Sunday/...
a cold front is expected to begin moving into the northern part
of our forecast area this evening. It should reach the Lehigh
Valley and the Interstate 78 corridor in New Jersey about 800 PM.
The boundary will continue to push southward, reaching
Philadelphia around 1000 PM and Cape May shortly after midnight.
There will be little moisture associated with the cold front and
the sky should remain mostly clear.
The wind is forecast to shift from the west to the north with the
frontal passage. The wind is then anticipated to veer toward the
northeast late tonight. Temperatures are expected to drop to
around 40 degrees in the elevated terrain up north and into the
middle 40s to lower 50s range elsewhere.
Long term /Sunday through Friday/...
the synoptic setup is comprised of a closed low that ejects
across the plains into Sunday. This feature builds ridging
downstream, which results in dry weather for much of Sunday. An
air mass change though will take place for Sunday as high
pressure builds down from the north. The initial closed low looks
to get absorbed into a large scale trough from the Midwest as it
undergoes amplification eastward through early next week. This
should then result in another closed low across the Great Lakes
region Tuesday through Thursday, before shifting closer to New
England Friday. The main precipitation event looks to occur Monday
although some quicker arrival time with some of the guidance will
result in rain developing sooner. Given the surge of moisture, the
rain could be heavy at times. The pattern thereafter results in
cyclonic flow across the east along with cooling and a series of
cold fronts. We used a model blend for Sunday into Tuesday, then
blended in the 00z wpc guidance. Some adjustments were then made
following additional collaboration with our neighboring offices.
For Sunday...high pressure building down from the north to start
Sunday then it will shift offshore during the course of the day.
This will allow for the wind to be onshore along with a cooler
air mass. The clouds increase mainly late Sunday as the next
system begins to approach from the southwest. A strengthening low-
level jet and warm air advection will help push a warm front toward our area
overnight Sunday. The strength of the low-level jet moving in may
allow for stronger ascent to arrive sooner /deeper isentropic lift
over the departing surface high/, therefore we increased the probability of precipitation
quicker by a few hours. Some of the rain may become heavy at times
toward daybreak Monday.
For Monday and Tuesday...as energy ejects eastward from a closed
low and also from the Midwest, low pressure organizes over the
Great Lakes before shifting northward toward Hudson Bay. A robust
low- level jet at least during the morning combined with warm air advection, deep
moisture advection and large scale ascent will result in an area
of rain. There are some hints of some elevated instability within
this, however held off with a thunder mention as the initial deep
saturation tends to not be conducive for thunder. An area of precipitable water
air of near 1.5 inches is forecast to surge across the cwa,
therefore the rain could be heavy at times especially in the
morning. Other than some typical ponding of water on the roadways
and poor drainage flooding during heavier rain, no other flooding
is anticipated at this time.
The warm sector may get into our southern areas and perhaps to
about the Philadelphia Metro, and therefore kept a mention of some
thunder Monday afternoon and early evening but did expand this
northward a bit. The instability looks weak enough to keep the
intensity of any thunderstorm on the lower side, however isolated
strong wind gusts and small hail cannot be ruled out if the warm
sector can get farther north /enough heating occurs/. The
steadiest rain should taper off to showers during the late morning
and early afternoon from southwest to northeast as a weak surface
low shifts northeast near a triple point. A robust low-level jet
sliding across the area Monday morning should allow for an
increase in the surface winds Sunday night and Monday morning,
with the strongest gusts closer to the coast due to an initial
Low pressure and a cold/occluding front should be exiting to our
east Tuesday morning, with any showers ending. A developing closed
low though is forecast to be around the Great Lakes with cyclonic
flow resulting. A few showers may persist Tuesday mainly across
the northern zones.
For Wednesday through Friday...a closed low is forecast to be in
place from the Great Lakes to eventually New England. This will
result in cyclonic flow across our area, although it is not real
certain how short wave energy within the flow results in some
stronger forcing as the main system starts to shift to New England
with time. These short waves should send a couple of cold fronts
our way, with one generally on Wednesday and another probably
Thursday night into Friday. There may be enough lift within the
cyclonic flow combined with daytime heating to promote a few
showers mainly across the north. Overall, we favored the wpc
guidance/continuity during this time frame which also advertises
Aviation /09z Saturday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy, and surrounding areas.
Patchy fog at some of our taf sites will persist until about
1200z or 1300z. We have included its mention at krdg, kabe, kmiv
and kacy. It appears as though the fog is rather shallow but it is
having an impact on the visibility values reported by the ASOS
units. Otherwise, VFR conditions are expected through the taf
period with a mostly clear sky.
A light and variable wind early this morning should settle into
the north to west quadrant at 6 knots or less for the middle to late
morning hours. The wind is forecast to become westerly for the
afternoon at speeds around 6 to 12 knots. Gusts near 20 knots are
possible at our northern taf sites, krdg, kabe and kttn. A cold
front is expected to drop through our region from north to south
early tonight and the wind direction is forecast to shift to the
Sunday...VFR into the evening with increasing cloudiness. The
conditions then lower to MVFR/IFR at night as rain develops from
southwest to northeast. East-southeast winds 10-15 knots, with
some gusts to about 20 knots especially closer to the coast and at
night. Low-level wind shear may start to develop at night in
advance of a warm front and as a robust low-level jet moves in.
Monday...MVFR/IFR with periods of rain in the morning tapering to
some showers, then some improvement may occur in the afternoon.
There is a chance for some thunderstorms in the afternoon and
early evening, especially from about the kphl metropolitan to south and
east. Low- level wind shear mainly in the morning. East or
southeast winds around 10 knots, becoming west-southwest.
Tuesday...MVFR possible early, otherwise VFR. A few showers
remain possible mainly from kttn to kabe northward. Westerly winds
mainly 10 knots or less.
Wednesday...mainly VFR. Some afternoon showers are possible,
mainly from kphl and to the north and west. Westerly winds
generally 10 knots or less.
high pressure is forecast to influence the coastal waters of New
Jersey and Delaware for today. A cold front from the north is
expected to pass through the region tonight.
The wind should be light and variable for today as warm air from
the west rides over the cold water. The wind direction should
settle into the north and northeast tonight after the cold front
passes. Speeds are anticipated to pick up to around 15 knots at
that time with waves on our ocean waters building to 3 to 4 feet.
Sunday...a strengthening low-level jet arrives at night,
therefore a northeast wind will shift to easterly at night and
increase. This may be strong enough for gusts to around 25 knots
later at night. The onshore flow will also start to build the
Monday and Tuesday...winds will increase as a result of a
strengthening low-level jet moving through mostly Monday morning.
While the stability near the surface should keep much of the wind
above the ground, it does appear there will be a period of time
where the onshore flow gusts to around 30 knots Monday before
shifting to the south and diminishing some. As a frontal system
moves in the flow is expected to turn more southerly later Monday
then westerly into Tuesday. Seas will build to and above 5 feet on
the ocean and may be slow to subside on Tuesday. A Small Craft
Advisory looks to be needed.
Wednesday...the conditions are expected to be below Small Craft
we have upgraded the Fire Weather Watch to a red flag warning for
the counties of Berks, upper Montgomery, Bucks, Mercer, inland
Monmouth and points northward. It is in effect from 300 PM until
800 PM. Relative humidity values are forecast to drop into the 20
to 25 percent range there this afternoon and fine fuels will
continue to dry. As a cold front approaches from the north, a
westerly wind is anticipated to gust into the 20 to 25 miles per hour range
increasing the threat for wildfire spread.
The relative humidity levels are forecast to drop into the 20 to
30 percent range in southeastern Pennsylvania, extreme
northeastern Maryland, northern Delaware and southern New Jersey
and fine fuels will continue to dry there, as well. However, wind
speeds are expected to be lower than those in areas to the north,
limiting the potential for wildfire spread and the need for a red
Relative humidity levels should be higher and wind speeds lower
in central and southern Delaware and in the adjacent counties of
northeastern Maryland than at locations to the north. As a result,
there are no significant fire weather concerns for today in those
PA...red flag warning from 3 PM this afternoon to 8 PM EDT this
evening for paz054-055-060>062-103-105-106.
New Jersey...red flag warning from 3 PM this afternoon to 8 PM EDT this
evening for njz001-007>010-012-013-015.