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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Wednesday
• 
• High: 75 °
• Low: 61 °
• Chance of T-Storms

• Thursday
• 
• High: 75 °
• Low: 63 °
• T-Storms

• Friday
• 
• High: 82 °
• Low: 61 °
• Chance of T-Storms

• Saturday
• 
• High: 84 °
• Low: 61 °
• Partly Cloudy

• Sunday
• 
• High: 84 °
• Low: 68 °
• Chance of T-Storms

Area forecast discussion 
National Weather Service Mount Holly New Jersey 
908 PM EDT Tuesday may 22 2012 


Synopsis... 
a low pressure system southeast of Cape Cod will drift northeast 
tonight and Wednesday. A high pressure system will then build in 
behind it over the Canadian Maritimes on Thursday and slowly sink 
southwest over the Memorial Day weekend and become established off 
the southeast United States coast. A backdoor cold front will try 
to sag south into the area about Sunday before retreating north on 
Memorial Day. A front door cold front will approach the area from 
the Great Lakes on Tuesday. 


&& 


Near term /until 6 am Wednesday morning/... 
the main focus early this evening has shifted to the potential for 
locally heavy rain. The environmental winds are fairly light...and 
this is resulting in a slow southeast trundle to the showers. The 
airmass is also fairly moist...with an axis of precipitable water 
near 1.50 inches running through southeast Pennsylvania into 
northern New Jersey. 


The combination of slow movement and high precipitable water 
raises the specter of localized heavy rain. One hour gridded flash 
flood guidance shows that 1.75 to about 2.00 inches of rain would 
be needed to initiate flash flooding. As long as the cells keep 
moving...the threat of flash flooding should remain low. 
However...we will continue to monitor for the potential for the 
rest of the evening. 


Otherwise...despite breaks in the clouds this afternoon... 
the scattered shower activity has not shown any lightning as of 
early evening. In fact...tops have not gotten to minus 20 celsius 
yet...and this would seem to explain the lack of lightning data to 
this point. The loss of daytime heating soon probably means the 
window of opportunity for thunder with the scattered showers 
developing is closing. 


The latest hrrr run is trying to develop a broken line of 
thunderstorms in the best instability. This lies across west central 
Pennsylvania (where the sunshine had a better chance to 
destabilize the airmass. The in situ bulk shear profile is not 
that impressive...and does not appear to get any better into the 
later evening hours. While there is some activity now in the axis 
of best instability...it is unclear just how much of this activity 
survives to the Lehigh Valley after 800 PM. 


For now...will leave the mention of thunder in deference to the 
activity to the west. As the activity continues to drop top the 
south along the line out west...a mention of isolated thunder was 
retained in early morning for the Delaware River west. 


The next forecast problem for tonight is the low clouds and fog. 
The low clouds did break this afternoon...and patches are trying 
to reform early this evening. NAM model soundings show that the 
redevelopment of stratus could take some time...especially west of 
the Delaware River. Based on the current cloud cover and the NAM 
model soundings...the timing of the stratus reappearance has been 
pushed back a bit. 


Statistical guidance suggests that tonight could end up being a 
a better setup for areas of fog (moist low levels...high pressure 
to the northeast). However...NAM model soundings suggest the the 
low level hydrolapse rates do not become favorable for until later 
tonight...and for a shorter time that the MOS guidance would 
indicate. Areas of fog was left in the forecast...and trends will 
be watched to look for potential for dense fog later tonight. 


For the most part...temperatures and winds appear to be on track 
for the overnight. 


&& 


Short term /6 am Wednesday morning through 6 PM Wednesday/... 
expect periods of showers and possibly thunderstorms during the afternoon 
and they may be more widespread than today especially if we get more 
breaks in the clouds, but don't see any significant forcing 
mechanisms. Guidance temperatures look reasonable once again. 


&& 


Long term /Wednesday night through Tuesday/... 
the WRF-nmmb initialization was better than the GFS off of the 12z 
soundings and the former was given more weight than average today. 
By the way today is the first day of the new GFS. In the big picture, no 
changes to our thinking with mainly diurnally driven localized heavy 
rain convection into Friday. Then model uncertainties as to the 
backdoor on the weekend with the trend toward warmer and more 
humid weather incorporated. 


There should be a diurnal downtrend in the convection on Wednesday 
evening as forecast convective available potential energy and li(s) decrease. We did show an uptick 
in probability of precipitation later at night as the short wave associated with the closing 
southeastern Continental U.S. Low comes close to the area. Both the GFS and WRF-nmmb do 
show some weak surface based instability as this feature slides by and 
we mention some thunder late. Given the continued uptick in dew 
points, we maintained the mention of patchy fog late. A stat 
guidance compromise was used for mins. 


On Thursday, we leaned more toward the WRF-nmmb solution of giving 
the Canadian Maritimes high a bit more of a bite and sheltering of 
our County Warning Area from convection. In reality GFS MOS supports the WRF-nmmb 
surface features more so than the GFS itself. Both would favor more 
activity west vs east. Conceivably being right for the wrong 
reason, the GFS brings in dry air aloft in the east during the 
day on Thursday which would make it harder for convection to 
occur. Without any organized trigger with the aforementioned short 
wave moving farther offshore in the morning, kept probability of precipitation as chance. 
Given more of an onshore influence from the surface high and how long 
will it take to clear low clouds, we leaned maximum temperatures much closer to 
the lower NAM MOS than GFS MOS. 


Wash, rinse, repeat on Thursday night. But, without a late short 
wave, we dropped probability of precipitation as the night progresses and removed the 
mention of thunder. The WRF-nmmb solution supports stratus at the 
least and a suggestion of drizzle at the most as the .01 precipitation 
field booms late at night. For now we went the patchy fog Route 
and accepted the stat guidance mins. 


On Friday, the models agree on a slightly more east push in the 
instability axis, maybe into the Delaware valley. Other than 
mesoscale features, I.E. Differential heating, orographic lifting, 
maybe sea or Bay breezes, not much organizational activity expected 
again. Probability of precipitation were again kept below likely. We still prefer more of a 
modified marine influence from our surface high to the east than the 
GFS showed and we thus kept maximum temperatures below mex MOS. 


Both the GFS and WRF-nmmb did show an axis of higher instability 
working its way into western PA during the day on Friday. If the 
convection could maintain itself, it would get into our County Warning Area before 
running out of gas on Friday night. We did up probability of precipitation slightly. 


There remains some uncertainty as to how south will the backdoor 
front get over the weekend. Right now it appears to be a difference 
between warm weather and some typical summertime convection around 
because of the lingering front, or a couple of warm sector days with 
high temperatures reaching the 90s on the coastal plain away from 
the ocean. The models continue to go back and forth with this 
scenario. More consensus oddly about hot weather on Monday and 
Tuesday. We nudged maximum temperatures upward with this package and 
maintained low probability of precipitation because of the uncertainty of the frontal 
position. 


&& 


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


Conditions remain VFR early this evening. Scattered showers dot 
portions of the region...and terminal locations north and west of 
kphl stand the best chance of seeing local MVFR conditions in 
showers through about 0300 UTC. There is also a line of 
thunderstorms over west central Pennsylvania. Near term models try 
to bring this activity to the Lehigh Valley between 0300 UTC and 
0400 UTC...but this looks overdone. In any event...thunder was not 
carried in any of the new terminal forecasts for this evening. 


The lower levels are not as most as this time Monday...so the return 
of the stratus should be delayed by a couple of hours. IFR 
conditions are expected to return between 0300 UTC and 0600 UTC from 
east to west. A period LIFR conditions is expected between 1000 UTC 
and 1300 UTC at all locations. There is a chance visibilities could 
drop to near 1/2sm fog between 1000 UTC and 1300 UTC...but confidence 
in this occurring at kphl is low at this time. 


Conditions should rise back through IFR into MVFR between 1400 UTC 
and 1600 UTC...and after 1600 UTC...VFR conditions are expected. 
Scattered thunderstorms are possible after 1800 UTC Wednesday...but 
due to timing and placement questions...they were not included in 
the updated terminal forecasts at this time. 


Outlook... 
Wednesday night through Friday...IFR conds each night and early 
each day in stratus and/or fog becoming VFR in the afternoon. Local IFR 
conds in showers or thunderstorms. Outlooking less coverage and duration 
of IFR and MVFR conditions on Friday than Thursday. 


Saturday and Sunday...possibly some MVFR or IFR stratus or fog early 
otherwise VFR. Isolated thunderstorms possible mainly late each day 
with MVFR conditions. 


&& 


Marine... 
the long period southeasterly swells are starting to come down. 
Based on extrapolation and the latest wnawave output...seas could 
drop below 5 feet across southern waters before 0600 UTC (44009 is 
getting close). If this occurs...the current Small Craft Advisory 
for seas could be dropped across the southern ocean waters early. 


Further north...44065 is still showing seas near 6 feet...and it may 
take until the end of the current Small Craft Advisory (1000 utc) 
for seas to drop below small craft levels there. So...for the time 
being...no significant changes were made to the wave height for this 
evening. 


Model soundings suggest that fog could develop across the ocean 
waters after midnight. There is not much hint of this yet on surface 
observations or low level cloud product. Since the low levels are 
moist...boundary layer winds are light...and dew points are just 
about at the sea surface temperature...fog is still expected. 


Outlook... 
we are not expecting Small Craft Advisory conditions to occur 
throughout the outlook period as the onshore flow from the Canadian 
Maritimes should not be that strong and even if we get a backdoor 
front, winds are not expected to be strong either. The greater 
marine concerns will be fog as dew points should rise above the 
water temperatures. This may the case every morning into Sunday. Also 
there may be stronger winds near thunderstorms, although the 
greatest instability is expected to be west of the coastal waters. 


&& 


Hydrology... 
there will probably be narrow swaths of 72 hour total 1-3 inch rfall 
between tonight and Friday...mainly in east PA and possibly nnj. Precipitable waters  
this week around 1.4 inches increase to at least 1.75 inches this 
weekend. 


&& 


Rip currents... 
the southeast swell should be dropping tonight into Wednesday. 
While the period remains fairly long (near 9 seconds)...the surf 
height should come down as well. Based on the above...our local 
study shows the New Jersey coast coming in at moderate...and the 
Delaware coast coming in with a low risk of rip currents 
Wednesday. The above will be reflected in the evening surf zone 
forecast for Wednesday. 


&& 


Phi watches/warnings/advisories... 
PA...none. 
New Jersey...none. 
Delaware...none. 
Maryland...none. 
Marine...Small Craft Advisory for hazardous seas until 6 am EDT 
Wednesday for anz450>455. 


&& 


$$ 


Synopsis...gigi 
near term...Hayes 
short term...nierenberg 
long term...gigi/99 
aviation...gigi/Hayes 
marine...gigi/Hayes 
hydrology...drag 
rip currents...Hayes