Area forecast discussion
National Weather Service Mount Holly New Jersey
1113 PM EDT Sat may 18 2013
Synopsis...
a stationary front to our southwest is forecast to slowly dissipate
through Monday. High pressure to our northeast will drop south and
take on the characteristics of a Bermuda high. A frontal boundary
from the north will approach our area on Tuesday and stall nearby.
The front may lift back north later in the week. A cold front
approaching from the west may cross our area late in the week.
&&
Near term /until 6 am Sunday morning/...
the probability of precipitation were increased into the likely range faster across
portions of southern New Jersey down into Delaware given radar
trends. These showers appear to be focused along a convergence
zone within some low-level warm air advection, along with perhaps some enhancement
from an upper-level jet. These may gradually decrease in coverage
for a time before ramping up again late, based on some short term
model guidance. Quantitative precipitation forecast amounts were also raised some more. The
northern zones may end up being rainfree for awhile, but we will
still keep some lower probability of precipitation going.
Otherwise, plenty of clouds along with a southeasterly flow should
keep temperatures nearly steady for the overnight.
&&
Short term /6 am Sunday morning through 6 PM Sunday/...
the front will be slow to cross the area. A weak upper low will also
be interacting with the front...so another day of abundant
cloudiness and scattered showers is expected. We will keep the low
likely probability of precipitation across the area early...then lower then after that. This
is more like the NAM stat guidance...but confid in the amount/quantitative precipitation forecast
for Sunday is rather low at this time. High temperatures will mostly be in
the middle to upper 60s far north...low 70s central areas and perhaps
into the middle or upper 70s far south...especially if any breaks
develop in the clouds late. Quantitative precipitation forecast forecast...mostly 1/10 to 1/4 inch with
the higher totals across the Delaware-Maryland-Virginia.
&&
Long term /Sunday night through Saturday/...
the middle levels still start with a vortex over eastern Canada, a
ridge building from the western Atlantic and a low crossing The
Rockies. There also are the remains of the low that caused the split
flow earlier this week, and they are forecast to move slowly toward
and then across our area into Tuesday. It looks those remains and
the vortex to the north will combine to keep the ridge from building
too much up the eastern Seaboard through the first part of the long
term, and then the low approaching from the west will begin to push
the ridge to the east. That low is expected to open during the
middle-week period and eventually cross our longitude. Whether is does
so before the end of the long term is up in the air. The GFS keeps
something of a Rex block over the center of the nation going into
Thursday, and is therefore not as progressive as the European model (ecmwf). The 12z
gefs support the operational GFS, and the 00z European model (ecmwf) ensembles
support the operational 00z and 12z European model (ecmwf). We therefore have sided
with the forecast from HPC, which leans toward the latter suite and
maintains decent continuity.
The models agree on gradually moving the center of surface high
pressure from north of 40n to south of it, and that transitions US
from a maritime flow with an easterly component to a more southerly
and humid flow and a more (especially away from the ocean) unstable
flow. The vortex to the north still tries to push a cold front down
into our area around Tuesday night, and it may stall nearby. All this
will send US from a period of showers to a period when thunderstorms
are possible. At this point, we really can't carve out a time period
when we aren't carrying a chance of thunderstorms until the system
from the west moves through with a surface cold front. Since we have
sided with the more progressive solution, our probability of precipitation do drop below
chance by Saturday. As noted by the previous long term forecaster,
thunderstorm activity will not be constant. In the absence of any
focused energy kicking out of the system to the west, activity would
be most likely during the afternoon and evening hours.
Nights are forecast to be warmer than normal and rather humid until
the frontal passage toward the weekend. Days are also forecast to be
humid, with temperatures starting around normal but rising to above
normal for much of the week. If the remnants of the middle level Low
Pass US in time on Tuesday, then that may be the warmest day of the
long term. We presently are forecasting drier and cooler air for the
end of the period.
&&
Aviation /03z Sunday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
VFR will gradually lower to MVFR and ultimately poorer conditions
as we go through the overnight. The persistent onshore flow will bring
deeper moisture with some showers tonight into Sunday. We have
sped up the timing of the lower conditions by a few hours when
compared to the previous tafs. Model guidance showing the
deteriorating ceilings/vsbys, so confidence in them occurring is
good, but the timing is still somewhat uncertain. Looks like the
low ceilings/visibilities will carry on into Sunday, so we did not really indicate
much improvement after 12z anywhere. Winds should remain mostly
Erly/southeasterly at 5 to 10 kts in most areas.
Outlook...
Sunday night...mainly MVFR ceilings with possible IFR conditions at
times. A chance of showers. Confidence is moderate.
Monday through Thursday...mainly VFR conditions from late morning
through the evening. There will be a chance of showers and
thunderstorms which could lower ceilings and reduce the visibility,
and they are somewhat more likely in the afternoon and evening.
Also, MVFR and IFR conditions are possible especially during the
late night and early morning hours. Confidence is moderate.
&&
Marine...
the onshore flow between the high well off to the northeast and the
approaching front to the west will cause winds (and then seas) to
slowly increase tonight into Sunday. It appears that seas will reach
Small Craft Advisory criteria (5 ft) across the southern waters first (tonight) and then
across the northern coastal waters Sunday. We will issue Small Craft Advisory flags to
cover this expected increase in seas. Small Craft Advisory flags will extend through
Sunday for now. Sprinkles this evening then scattered/occasional showers
overnight and Sunday.
Outlook...
sustained winds are forecast to remain below 25 knots through
Wednesday night, although there may be some occasions when 25 knot
gusts occur mainly on Monday. Winds may begin to increase again on
Thursday. Seas on our ocean waters are forecast to remain at around
5 feet through Monday night. They may remain there longer in the
south to southwest flow, although that flow is forecast to relax some
for the middle-week period.
&&
Rip currents...
there is a moderate risk for the formation of dangerous rip
currents along the New Jersey beaches Sunday.
&&
Phi watches/warnings/advisories...
PA...none.
New Jersey...none.
Delaware...none.
Maryland...none.
Marine...Small Craft Advisory from 6 am Sunday to 6 am EDT Monday for
anz450>452.
Small Craft Advisory until 6 am EDT Monday for anz453>455.
&&
$$
Synopsis...delisi
near term...gorse/Kline
short term...O'Hara
long term...delisi
aviation...delisi/Kline/O'Hara
marine...delisi/O'Hara
rip currents...Kline/drag
