Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
337 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
Near term /until 6 am Tuesday morning/...
high pressure to the east will continue to move further offshore through
tonight. As it does, an area of low pressure will move from the upper
MS valley into southeastern Canada, bringing its associated warm front with it
tonight. This will put our region in the warm sector, coupled with
the S to southwesterly flow from the aforementioned high.
Expect dry conds and mainly high clouds overnight. Moisture will
slowly begin to increase, though tonight should still be relatively
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 today.
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
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
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
Aviation /20z Monday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
VFR conds are expected through the taf pd, with high pressure
off the CST and southwesterly flow today becoming more southerly and then southwesterly on
Tuesday again. Wind could gust to around 20 knots through eve, before
decreasing to less than 10 kts overnight. On Tue, there could be some
diurnal rain showers/thunderstorms and rain mainly for kabe/krdg, but probability of precipitation are too low to
include in individual taf forecasts. Also, timing is uncertain.
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
Normal first date of 90 or higher.
Philadelphia may 29 Allentown may 30 Wilmington June 4 Atlantic City
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 precipitation.
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.
Marine...Small Craft Advisory until 6 am EDT Tuesday for anz450>452.