Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
955 PM EDT Tuesday may 26 2015
high pressure will remain off the East Coast through the end of
the week. A cold front will approach from the west on Wednesday
and dissipate as it tries to move through the region on Thursday.
Another cold front is forecast to cross the area on Sunday and may
stall in our vicinity on Monday.
Near term /until 6 am Wednesday morning/...
we will remain under the influence of the high over the western
North Atlantic overnight. There will be a general south to
southwest surface flow that will continue to draw low level
moisture into our region.
We are expecting additional cirrus to drift overhead from time to
time during the night. Also, the increase in low level moisture
could result in the development of some low clouds toward morning.
A weak impulse traveling in the middle level flow may touch off
isolated showers in our northwestern counties at that time, as
Minimum temperatures are anticipated to be mainly in the 65 to 70
Short term /6 am Wednesday morning through 6 PM Wednesday/...
yesterday, Thursday looked like the better convective day. Now it
appears to be Wednesday. Not that Wednesday day appears juicier
for the eastern half of our cwa, it has more to do with the demise
of Thursday as a vigorous convective producer.
Short waves emanating from the southern Mississippi Valley
convection are predicted to come around the same ridge. The timing
coinciding with maximum heating of the day looks best in PA where
we have the highest probability of precipitation. Confidence and likelihood of thunderstorms
reaching locations farther to the south and east is lower. Our
timing is closest to modeling consensus, can rgem from a rain
standpoint would be most beneficial as its timing is about three
hours faster than any other model and thus better coincidence of
heating and convection.
As far as strong/severe GOES, model predicted mix layer convective available potential energy has
increased from yesterday and is now averaging 1000-1500j, highest
northwest. Models are now showing a Lee side or susq valley trough forming
which should aid in pooling. Predicted bulk shear is eh, with 25kt
values northwestern part of our County Warning Area and much less southeast. Middle level
lapse rates where thunderstorms are most likely are rather modest
(5.5c/km), we do have warmer temperatures at 500mb arriving. The entrance
region of the 250mb jet is still rather northwest. The one plus is
that middle level dry air is still predicting approximately a 15c
drop in Theta-E values and the Theta East Ridge in itself passes over
our County Warning Area. It looks like a case of non home grown stronger storms
initiating first with nearly a climatology potential of a couple storms
going severe. Best chances northwestern part of our County Warning Area. Precipitable waters are pretty
high, topping around 1.75", but modeling is showing OK progression
and with dry conditions in place, the flooding threat (beyond poor
drainage) is relatively low.
As far as temperatures go, we are going Above All stat guidance (a near
status quo from today) based on predicted 925mb temperatures of 20-21c.
This should get US into the middle to upper 80s (sans elevation and
immediate coast) as modeling idea of mostly cloudy day looks
overdone. Given the possibility of a Lee side trof, we accepted
stat guidance dew points. This has peak heat index values of
around 90f, which is below early season headline thresholds.
Long term /Wednesday night through Tuesday/...
high pressure remain off the East Coast. A cold front will be moving
across the region at the start of the period and wash out on Thursday there
could be some rain showers/thunderstorms and rain lingering Wednesday night. There is some
disagreement in the models as to areal coverage and how long any
precipitation lingers. However, all guidance has trended drier, and sometimes
a persistence forecast is the best approach.
The NAM and GFS are now virtually dry on Thu, with the CMC and
European model (ecmwf) having some precipitation in the afternoon. Again, the trend is drier
and probability of precipitation have been adjusted downward.
On Fri, the European model (ecmwf) has some precip, while the GFS remains dry. The
CMC has some precipitation as well. Confidence remains low. Not many
triggers, except for daytime heating. So continue the trend of
lowering probability of precipitation.
The next front begins to approach over the weekend. As is often
the case, the GFS is a bit faster than the European model (ecmwf). The ec keeps Sat
dry, while the GFS brings precipitation in late. By sun mrng, the GFS has
the front through the area, while the European model (ecmwf) tries to develop a wave
on the front and linger it into early Monday.
While the European model (ecmwf) has been consistently slower with the front and
stalling it in vicinity of our area, the 26/12z European model (ecmwf) has trended markedly
drier with the forecast beyond 12z Monday. It is now in better agreement
with the GFS. Not sure if this is a one model run change or the
start of a trend, but have begun the downward push on probability of precipitation. If the
European model (ecmwf) continues this trend, probability of precipitation may be able to be eliminated for
parts of early next week.
Temperatures will be at or above normal through the period.
Aviation /02z Wednesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
VFR conditions are expected through 0800z.
Low clouds and light fog may develop for the period from 0800z
through 1400z. We have indicated the potential for MVFR conditions
at that time.
VFR conditions should return from 1400z until the end of the taf
period. Scattered thunderstorms are possible on Wednesday
afternoon, mainly to the northwest of kttn, kpne, kphl and kilg.
A light south southwest wind is anticipated overnight. Wind
speeds should increase to 8 to 12 knots for Wednesday with gusts
of 16 to 20 knots, especially in the afternoon.
Wednesday night...lingering rain showers/tsra, especially early. Better chances
across rdg/Abe. MVFR/IFR possible in shra/tsra. MDT confidence.
Thu-sun...early morning fog/stratus followed by scattered afternoon
shra/tsra. Low confidence on which days and areas will see
with at the coast based networks showing wind gusts getting to 25
knots and with modeling showing similar speeds aloft into Wednesday,
we decided to issue a Small Craft Advisory. Better mixing will be right along the
coast, with generally lighter winds and less gustiness farther
Wave watch guidance has come down on seas. Regardless, some 5
foot seas remain possible mainly offshore. With the strongest
winds right along the immediate coast, basically there is a
separation of strongest winds (right along the coast) from the
highest seas (farther offshore).
Wednesday night...Small Craft Advisory flag has been raised for marginal wind but seas
at or above 5 feet.
Thursday through Sat...wind and seas should be below Small Craft Advisory criteria, but
could approach Small Craft Advisory by late Sat.
a top 10 warmest may will occur in large portions of our forecast
The first 25 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
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.
Marine...Small Craft Advisory until 6 am EDT Thursday for anz450>455.