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

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/... 
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 

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

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 

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 /08z Tuesday through Saturday/... 
the following discussion is for kphl, kpne, kttn, kabe, krdg, 
kilg, kmiv, kacy and surrounding areas. 

First round of showers and storms has already exited the area early 
this morning. Another round is moving into ridge at 08z and could 
impact the philly terminals between 09z-11z. A brief period of MVFR 
to locally IFR visibilities is possible with this activity. Clouds should 
limit fog development early this morning. 

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. 


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. 


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). 


Wednesday through Thursday...sub-advisory conditions are 

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... 
New Jersey...none. 
Marine...Small Craft Advisory until 5 am EDT Wednesday for anz450>455. 


near term...Klein 
short term...Klein 
long term...gigi/Klein 
rip currents...iovino/Klein 

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