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Area forecast discussion 
National Weather Service Mount Holly New Jersey
848 PM EDT Monday may 25 2015

a high pressure system will be anchored off the East Coast
through the end of this week. A cold front will approach from the
west Wednesday and eventually dissipate as it tries to move
through the forecast area Thursday. Another cold front is forecast
to approach the area on Sunday and stall in our vicinity on


Near term /until 6 am Tuesday morning/...
high pressure located over the western North Atlantic will
continue to influence our region overnight. A general south to
southwest surface flow is expected. Some locations will maintain
a 5 to 10 miles per hour wind while others will go light and variable.

Low temperatures are forecast to be mainly in the 60s. While low
level moisture should continue to increase slowly, we are not
anticipating any widespread fog or haze overnight. The Gap between
temperature and dew point readings should remain greater than 4f
at most locations.


Short term /6 am Tuesday morning through 6 PM Tuesday/...
on Tue, the weak cold front associated with the low in southeastern Canada will move
mainly north of the area during the afternoon hours. This front and
increasing moisture with daytime heating will bring the potential
for showers/tstms, mainly over the northwestern areas and higher terrain.
The GFS is a bit more bullish on the precipitation than the ECMWF,
NAM/WRF and CMC. So while its possible for some precipitation further S and
E, confidence is not high enough to make changes at this time.

With temperatures forecast to be in the middle to upper 80s dew points still look
to be in the lower 60s, so it looks to be hot but comfortable.


Long term /Tuesday night through Monday/...
the GFS and WRF-nmmb/NAM initialization looked pretty good with
no clear cut Winner. However, the deep/dt on the GFS run showed a
slower evolution of the pattern which we took into consideration.
We basically are going to end may with typical July weather,
warmer and more humid with shower and thunderstorm chances. While
there are model differences (nam/ec prefer Wednesday over Thu, GFS
prefers Sat over sun) as to the most widespread convective days,
we are going to take a combination of consensus and bias adjusting
to key on Thursday and sun as the most convectively active days in the
long term and Saturday the relative lull. Regardless after not
being able to buy a thunderstorm, its tough to find a day to not
include a thunderstorm.

There is active convection in the Gulf coasts states and the mean
flow is going to take it toward the western part of our County Warning Area on
Tuesday night. We carry some low probability of precipitation on Tuesday night. It will be
a more humid night with min temperatures not as low.

On Wednesday, a cold front is expected to come closer to our County Warning Area.
Coupled with higher dew points and the relative core of warmest
air aloft (925mb analysis has it over PA this morning, but more
south grown heat will likely keep a status quo in 850/925mb temperatures
through wed) passing over our County Warning Area would be more conducive for thunderstorms
to develop with convective temperatures within reach. The latest European model (ecmwf)
takes a slightly faster timing than most other models. This Bucks
the trend of an overall slower evolution that the models have through

Looking at modeling similarities vs differences, all of them have
the most conducive for convection thermodynamic features in the
northwestern part of our County Warning Area. The GFS and WRF-nmm agree about middle level
dry air making storms pulsier. But from there kinematically there
are too many differences Wednesday vs Thursday to resolve at this time as to
more organizational/stronger storms. Given the northwestern consensus we
upped late probability of precipitation to likely, but added no enhanced wording. While
thunderstorms will be strong, high precipitable waters will also produce locally
heavy downpours.

Temperatures, given decent winds aloft plus no indication of a
dew point pooling in our cwa, with breezy conditions, stat
guidance (and model guidance more confidently) is likely too high
with afternoon dew points, or any surprise wont be on the high
side. This brings heat index values close to air temperatures and
likely top in the lower 90s. We are going with the higher NAM stat
guidance for maximum temperatures given ongoing GFS stat guidance cool bias
with maximum temperatures.

On Wednesday night, convection will try to spread across our County Warning Area.
There are predictions of short waves, so we will keep a chance
(not as high as the afternoon though), going through the night.

The cold front should be entering our County Warning Area on Thursday. With the
weakening boundary, the models are offering differing opinions as
to how active the front can remain. The GFS is most bullish as it
has deeper qvec convergence, bulk shear and entrance region of the
250 mb jet. The other models are a bit more diffuse. Regardless,
the relatively higher probability of precipitation along the boundary will be centered in
our County Warning Area. As the models are differing as to how active, we kept
probability of precipitation as chance. Combination of more upwind debris as frontal
proximity, air mass is slightly cooler on Thursday. Maximum temperatures nudged
down slightly. Dew points though should pool more efficiently so
zero sum gain with apparent temperatures.

The frontal boundary should linger and dissipate over our County Warning Area on
Thursday night and Friday. It could act as a focus for more
convection. But given, its dissipating, overall probability of precipitation are lower
than Thursday.

We will key on Sunday vs Saturday as the convectively more active
day given the better model consensus and overall tendency for a
slower timing. Probability of precipitation hit a relative low on Saturday before ramping
up again on Sunday. Also given the slower frontal timing, maximum
temperatures are much closer to wpc guidance and ec MOS guidance than GFS
MOS. Initial look at Monday has the front stalled just south of
our cwa, this should be relatively cooler with still some showers
and thunderstorms around.


Aviation /01z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.

VFR conditions are expected through the taf period.

Isolated showers and thunderstorms are possible on Tuesday
afternoon, generally to the northwest of krdg and kabe.

A south to southwest wind around 5 to 10 knots overnight may
become light and variable in spots. A south to southwest wind is
anticipated for Tuesday at speeds about 8 to 12 knots with gusts
around 16 to 20 knots, especially in the afternoon.

Tuesday night...patchy radiational fog is possible late.

Wednesday through Thursday...while predominately VFR, scattered
showers and thunderstorms will produce some MVFR to briefly IFR conditions
at times.

Thursday night through Saturday...overall coverage of showers and
thunderstorms (thus MVFR to brief IFR conditions) should be less. There
could still be some patchy radiational fog each morning.


a Small Craft Advisory remains in effect for the northern waters through tonight. Wind could
gust to around 25 knots and seas could reach around 5 ft, though right
now both wind and seas are below criteria. Based on latest
guidance, will not be extending ending time.

Tuesday night through Wednesday night...a strong southerly flow
might produce Small Craft Advisory gusts and seas reaching 5 feet
on the ocean waters. Usually in these scenarios, the models tend
to be too bullish. We will continue to monitor, but not confident
to extend current advisory.

Thursday through Saturday...winds and waves should be below Small Craft Advisory
threshold as a weaker pressure pattern should prevail.


a top 10 warmest may will occur in large portions of our forecast

The first 24 days of may in our forecast area were averaging
about 5 to 6 degrees above normal.

Running the numbers from the forecast through the end of the month we
find the following.

For Philadelphia...period of record (por) back to 1874.
Projecting a monthly average temperature of 69 to 70 rising from
the current value of 67.8, at least 5 to 6 degrees above normal.
Heading for most likely the third warmest may on record. Warmest
average temperature in may on record was 70.8 in 1991. Then 69.2
2004 and 68.2 in 2012.

For Allentown...period of record (por) back to 1922. The monthly
average temperature is currently 63.9 degrees. It is likely to
increase by a degree or two later this week, leading to a top 10
warmest may this year at Allentown. The warmest average
temperature in may was 67.2 back in 1991.

For Wilmington.... the month of may is likely to end up in the
top 5- 10 warmest. The current monthly temperature is 65.8 degrees
and should rise a degree or two by months end. The warmest may was
in 1896 where the monthly temperature was 70.1 degrees. In 1991
the monthly temperature was 69.1 degrees.

For Atlantic City... the month is likely to end up in the top
three warmest Mays. Currently, the average temperature for the
month is 64.5 degrees and is likely to rise a degree tornados two by
months end. The warmest may was back in 2004 where the average
temperature reached 66.9 degrees. In 1991 the monthly average
temperature was 66 degrees. Both 2011 and 2012 saw a monthly
average temperature of 65 degrees.

Normal first date of 90 or higher.

Philadelphia may 29 Allentown may 30 Wilmington June 4 Atlantic
City June 11.


Here is our annual look as to how hot, hot, hot the Summer will
be based on past performances.

A full fledged El Nino is underway in the tropical Pacific. All
dynamical and statistical models keep it going, the differences
that exist are to its strength through this upcoming Summer. The
dynamical and statistical models are averaging a moderate El Nino
for the combined Summer months. Some models, including the cfs2
and European model (ecmwf) are predicting a strong El Nino. In the CPC monitoring
El Nino era starting since 1950, the highest (warmest) Summer
anomaly recorded was +1.5c (near or at the threshold for strong el
ninos) during the Summer of 1997.

This upcoming week will really seal the deal that this may will
be unseasonably warm in Philadelphia. It "may" even be a top three
warmest. This follows on the heels of an unseasonably warm April.
Since 1872, the combination of both warm (top third of all years)
aprils and Mays has only occurred twenty-three times. But it has
occurred eleven times since 1990 and six of the last Seven

So we scoured the climate records to find budding or in progress
el ninos that coincided with both warm aprils and Mays locally in
the CPC monitoring era. We found three past Summers that met the
criteria. The one sign is that all three analog Summers were
wetter than normal, hopefully keeping drought conditions from
intensifying further.

The North American multi model ensemble predictions for this
upcoming Summer has warmer and drier than normal weather expected
in our area. The international multi model ensemble forecasts are
slightly more optimistic predicting near normal temperatures and

The analogs for Philadelphia are:

Year June average July average August average Summer average Summer precipitation

1969 73.4 75.1 75.2 74.6 18.30
1977 68.6 77.8 76.2 74.2 15.50
1991 75.7 79.0 79.0 77.9 12.01
average 72.6 77.3 76.8 75.6 15.27

2010 73.3 78.1 76.6 76.0 11.28

The official CPC outlook for our forecast area is for equal
chances of above and below normal temperatures as well as equal
chances of above and below normal precipitation.


Phi watches/warnings/advisories...
New Jersey...none.
Marine...Small Craft Advisory until 6 am EDT Tuesday for anz450>452.



near term...iovino
short term...nierenberg
long term...gigi

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