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Area forecast discussion 
National Weather Service Mount Holly New Jersey
938 am EDT Tuesday Aug 4 2015

Synopsis...
a cold front over western Pennsylvania will approach from the west
this morning. The front will move through our region this
afternoon and tonight before stalling just to our south Wednesday
through the end of the week. Waves of low pressure will ride
along the boundary with the final wave of low pressure tracking
just off the middle-Atlantic coast on Saturday. High pressure builds
in from from eastern Canada Sunday and Monday.

&&

Near term /until 6 PM this evening/...
adjusted a few parameters in the grids this morning for the
update. Quite a bit of sun right now as the overnight convection
has moved offshore and we await the arrival of the cold front.
Dewpoints are up from yesterday as planned, especially across the
coastal plain. This combined with plenty of solar heating this
morning will see this airmass hit convective temperatures. The cumulus will pop
this afternoon...leading to showers and thunderstorms.

For precipitation placement later today, I eliminated probability of precipitation from our
northwestern zones as the focus will be further east. Strong to
severe thunderstorms are possible later today along the coast.

With 850 temperatures slated to hit about 18 degrees c, I lowered this
afternoon's high temperatures by 1 degree cwa-wide. Heat indices will
hit the lower to middle 90s across our southeastern zones where
the best moisture is pooling.

Previous discussion...overnight surface analysis shows a
cold front positioned over western New York and the Ohio Valley.
Meanwhile, a surface trough was located just east of I-95 in the
middle-Atlantic region. Winds are out of the S-SW (nw) and dewpoints
in the low 70s (low 60s)to the east (west) of the trough.

Upper-level divergence in the right entrance region of an upper-
level jet streak over the northeast states and DPVA from a subtle
middle-level shortwave trough has been the source of lift for nocturnal
convection that was moving across eastern PA and the northern half
of New Jersey. Hi-res cam guidance had a poor handle on the overnight
convection, so forecast through early this morning based more off of an
extrapolation of current satellite/radar and from mesoanalysis
trends. There is enough elevated instability to keep the showers and
storms going early morning as they move eastward this morning. We
are particularly keeping an eye on a cluster of storms near the Mason-
Dixon line just to our west as they currently have severe
thunderstorm warnings with it. At 07z, there was 2000+ j/kg of
elevated cape downstream of these storms.

The aforementioned cold front will advance eastward through the Lehigh
Valley and northwest New Jersey early this afternoon and I-95 corridor late in the
day. Once the debris clouds from the current convection moves off
the coast, strong heating will allow the atmosphere to destabilize
this afternoon. A hot (highs in the lower 90s) and humid (dewpoints
in the upper 60s/low 70s) airmass will be present ahead of the
front. Forecast soundings from the 00z NAM and GFS are vastly
different but after accounting for moisture biases from both models,
MLCAPE will range from around 1500 j/kg near the I-95 corridor to
3000 j/kg in the lower Delaware-Maryland-Virginia during peak heating. The 00z hi-res
ncar ensembles support these numbers as well. Bulk shear will also
be moderately strong as westerly winds in the middle-levels increase to
40-50 knots. The one limiting factor will be the absence of organized
lift aloft. However, surface convergence near the frontal boundary
should provide just enough lift to initiate convection this
afternoon. Storm Prediction Center has placed areas east of I-95 in a slight risk for
severe storms. The environment will support damaging straight line-
winds from wet microbursts and large hail with the strongest cores.
The threat appears to be greatest from approximately 2-8 PM.

&&

Short term /6 PM this evening through 6 am Wednesday/...
the cold front will slowly move eastward through the coastal plain this
evening and eventually off the coast overnight. Accordingly, the
highest probability of precipitation reside this evening over southern New Jersey and the Delaware-Maryland-Virginia.
Put a little more weight in the hi-res guidance for this evening's
round of convection than the ongoing round early this morning as the
latter round of convection is mainly focused/forced along the well-
defined synoptic cold front.

Post-frontal northwest winds will be light, but there appears to be enough
dry air advection to limit the fog potential overnight. If the front
slows down enough and skies are able to clear overnight, then fog
may develop near the coast, especially in areas that receive rain.

&&

Long term /Wednesday through Monday/...
the cold front will move off the coast Wednesday while high pressure
builds in the Midwest. A relatively tight pressure gradient for this
time of year is forecast over our area, leading to a breezy northwest wind.
Dry air advection will be more prominent that cold air advection
behind the front. Maximum temperatures range from the upper 70s in the Poconos
to near 90f in the more urban areas along and east of I-95. However,
it will feel comfortable with dewpoints only expected to be in the
50s.

The extended part of the forecast (thursday night and beyond)
remains untouched. Previous discussion from the day shift
follows...

In a model initialization flip from the weekend, the WRF/NAM
initialized the 500mb pattern better than the GFS or European model (ecmwf) in the
western conus, while in eastern noam the GFS looked better. The GFS
looked closer at 850 mb and 925mb. Deep/dting the initialization
over the past 24 hours, the ridging in Montana and Idaho is
stronger and down the Road this is leading to a more bifurcated
solution to how the energy rounds this ridge, thus there is no
decrease in uncertainty with this sounding run. The GFS was not
used because of its weaker western Continental U.S. Initialization and poor
run to run continuity. Gefs mean with the wave of low pressure is
farther south and then a faster exit on Saturday. Our forecast is
closest to a European model (ecmwf)/can ggem/mf-arpege blend. It should be noted
that the UKMET has trended southward with a much weaker low while
the NOGAPS and jma have gone back to not having any precipitation reach our
County Warning Area until Saturday.

So the active middle stretch of the long term continues with
temperatures closer to the normal. The potential remains for a
soaking rain centered around Friday, although there remains
uncertainties as to the impact within our County Warning Area. Last night's naefs
ensemble mean continue to show an anomalously strong surface low
pressure system for early August with a five year return period
and South Dakota of between 2 and 3 as it crosses the central Appalachians.

Thursday, probability of precipitation were moved back approximately 12 hours. Too late
in its forecast cycle to have confidence in the WRF/NAM timing,
but many of the models do not have any measurable at all Thursday
day based on the more roundabout approach for the short waves.
Because of the drier trending, we went above stat guidance for maximum
temperatures.

Thursday night through Friday night there unfortunately remains
uncertainty as to the evolution of the system, but the synoptic
scale set-up still points to heavy rain, even if it does not
necessarily occur or affect our County Warning Area the most. The heavy rain will
be tied to convection and thus how unstable aloft it becomes
within our County Warning Area will determine the final outcome. Highest probability of precipitation are
Thursday night and Friday with thunder emphasized more for the
southern part of our County Warning Area. Come Friday night the predicted
instability (really lack thereof) aloft does not substantiate the
ecmwf's precipitation/quantitative precipitation forecast solution and was not used. This does not mean
heavier rain could not happen earlier. Maximum temperatures may very well be
overstated on Friday if this timing holds.

Because of timing discrepancies we kept in a low chance for
precipitation on Saturday. After above, the end of the long term
appears quiet as high pressure noses its way southward from Canada
and the frontal boundary and low pressure systems move offshore.

&&

Aviation /14z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.

VFR this morning and early afternoon. Scattered rain showers and thunderstorms and rain expected to
develop again during the middle and late afternoon. Based on timing of
a cold front, the I-95 (ttn/pne/phl/ilg) and southern New Jersey (miv/acy)
terminals have the best chance to see storms. Included a prob30
group in the 06z tafs for these terminals. Later taf cycles will
likely add a tempo and then prevailing group for thunderstorms and rain when
confidence in timing and locations increases.

Outlook...

Wednesday and Wednesday night...mainly VFR conditions.

Thursday through Friday night...MVFR conditions during showers/
tstorms, with some IFR conditions possible. While confidence of
occurrence is increasing, timing confidence remains low.

Saturday...predominately VFR conditions expected to return as
shower frequency diminishes.

&&

Marine...
with some upwelling enhancing the inversion over the water, I
eliminated the mention of 30 knots over the ocean waters.

Previous discussion...extended the Small Craft Advisory that was already in effect
for today into tonight. S-SW winds will gust to 25 knots today and
early this evening. The seas will be elevated to 4-6 feet during
this time. Winds are expected to weaken after sunset but seas will
take several more hours to subside to below Small Craft Advisory criteria.

In Delaware Bay, we'll top the wind gusts out at 20 kts (sub
small craft).

Outlook...

Wednesday through Thursday...sub-advisory conditions are
expected.

Thursday night through Friday night...mostly sub-Small Craft Advisory conditions;
however, a stalled front and a wave of low pressure moving along
it could enhance seas and/or winds for the coastal waters, making
Small Craft Advisory conditions possible.

Saturday...sub Small Craft Advisory conditions expected to
return. This is based on timing of waves of low pressure close,
confidence at this juncture is less than average.

&&

Rip currents...
there is a moderate risk for the formation of dangerous rip
currents today along the New Jersey shore and at the Delaware
beaches. Seas have continued to build in the stiff southerly flow.
Wave heights in the surf zone will be up to 4 or 5 feet.

&&

Phi watches/warnings/advisories...
PA...none.
New Jersey...none.
Delaware...none.
Maryland...none.
Marine...Small Craft Advisory until 5 am EDT Wednesday for anz450>455.

&&

$$
Synopsis...Klein
near term...Klein/kruzdlo
short term...Klein
long term...gigi/Klein
aviation...gigi/Klein
marine...gigi/Klein/kruzdlo
rip currents...

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