Area forecast discussion
National Weather Service Mount Holly New Jersey
841 PM EDT Wednesday may 22 2013
Synopsis...
a Bermuda high pressure system well off the East Coast will continue
to pump warm and moist air across the middle Atlantic region tonight
through Thursday. A low pressure system near Chicago this evening
will move northeast across the Great Lakes and then down the Saint
Lawrence valley into Friday. A second low pressure system will
develop off New England Friday and remain across the waters into
Saturday. High pressure will move in Sunday and remain into early
next week.
&&
Near term /until 6 am Thursday morning/...
there have been multiple multicellular storms forming to our west
and southwest tied to an incoming short wave that is teaming up with a
Lee side trough. The storm motion continues to favor our western
and northwestern zones as the surface flow remains southerly. A
few cells are beginning to fire up along the surface cold pool
moving close to Berks County. The main threat tonight will be
heavy rainfall and strong wind gusts. There is still plenty
surface based instability for the storms to tap into so we could
see a continuation of the convection for several more hours.
Otherwise, the warm front should continue to lift northward through
tonight and out of our County Warning Area. Overall, a warm and muggy night is in
store across the entire County Warning Area. This should also lead to additional
stratus to develop overnight along with some fog. For now, added
in patchy fog as we are thinking the moisture should be mostly
focused into the form of clouds.
As for low temperatures, we mainly used an even GFS/NAM MOS
blend. The hourly temperature and dew point grids incorporated
some of the lamp/lav guidance to assist in trends.
&&
Short term /6 am Thursday morning through 6 PM Thursday/...
this looks to be an active day with showers and some thunderstorms
becoming more numerous. This is in response to the large scale
lift increasing as an upper-level trough approaches along with the
arrival of a cold front and weak surface low. There may be some
showers /perhaps some thunder/ ongoing as the day starts as
increasing lift overspreads the area. This should be mainly across
the inland areas. We are anticipating an abundance of clouds to be
present during the day, with areas of stratus and perhaps fog
gradually lifting some. The presence of the clouds adds uncertainty
to the amount of instability that will be present. The forecast
soundings indicate the airmass will be rather buoyant, therefore
showers may tend to develop fairly quickly even with initially
weaker lift.
There is the chance for some stronger convection, especially if pockets
of increased instability can occur. As stronger southwesterly flow
overspreads the area, any robust updrafts that can be sustained will
be capable of producing gusty winds and hail provided stronger
instability can be obtained. Since there is some uncertainty
regarding the strength and organization of convection, no enhanced
wording was included in the forecast. The precipitable water values are forecast to
be about 1.50 to 1.75 inches, therefore locally very heavy rain
could occur. As of now, mainly some localized poor drainage flooding
could result. A mention is in the hazardous weather outlook. The
probability of precipitation increase from west to east during the day, although we did slow
the increase down some across the eastern zones. The activity may be
held inland for awhile as larger scale lift takes longer to get more
organized activity going along the coast.
As for temperatures, mostly a GFS/NAM MOS blend was used.
Temperatures are expected to be lower than today given more clouds
throughout the day coupled with increasing showers and storms.
&&
Long term /Thursday night through Wednesday/...
a period of wet weather at the beginning of the extended period as
the upper flow begins to sharpen and a surface/upper low is shown
developing across the New England region Thursday night into Friday. This
low is shown only moving slowly eastward into Saturday. This
represents a change in the longer range GFS/European model (ecmwf) models over the
past few runs. They were showing more rapid clearing on previous
runs. We have accordingly increased cloud/pop grids for these
periods over previous values. Temperatures will drop below normal
after the cold front passage Thursday night and temperatures will stay
below normal into the weekend. Highs generally in the 60s Friday/Sat
and lows mostly in the 40s/50s Friday and Sat nights.
The remaining part of the extended period looks to feature plenty of
dry weather. The only period when precipitation is in the grids is fro next
Wednesday when a slight chance is in there. The high pressure system to the
west on Sunday will crest over the region Monday night/Tuesday and
then continue eastward through the middle of next week. Temperatures
will be a little below normal Sunday with highs in the upper 60s/low
70s in many areas. Readings will moderate Monday through Wednesday
with highs by Wednesday reaching back into the upper 70s and few low
80s. Humidity levels will be low/moderate Sunday and Monday and then
the humidity will slowly increase into the middle of next week.
&&
Aviation /00z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Tonight...VFR, then IFR/MVFR ceilings and visibilities are
forecast to develop. The timing of this is of low confidence.
There could be a shower or thunderstorm especially in the evening
north and west of kphl, then another chance mainly toward
daybreak. South-southwest winds generally under 10 knots.
Thursday..IFR/MVFR ceilings and/or visibilities. There is a chance
for some improvement to VFR, however given increasing showers we did
not forecast this at this time. Showers and some thunderstorms are
forecast to become more numerous during the day with reduced
visibilities. Locally heavy rain and gusty winds could accompany
some of the activity. South-southwest winds generally 10-15 knots.
Outlook...
Thursday night through Sat morning...lower clouds/showers and lower
visibilities/ceilings possible.
Sat afternoon through Monday...mostly VFR.
&&
Marine...
a southerly flow will continue through Thursday ahead of a cold
front and surface low pressure. This flow is forecast to increase
during this time, however forecast soundings indicate a sharp
low-level inversion is in place due to much warmer air over the
cooler waters. This will tend to keep the stronger winds from aloft
from mixing down to the surface, although the GFS is a bit stronger
within the inversion level. We are thinking that the winds generally
remain under 25 knots, however the seas are forecast to build within
the southerly flow. It appears wavewatch is running about a foot to
high so far, therefore we did undercut the guidance some. As a
result, the Small Craft Advisory for the ocean zones /mainly for
seas/ remains in effect starting early this evening and continues
through Thursday. Some fog should be maintained into Thursday,
especially across our northern waters. Showers and some
thunderstorms are expected Thursday, most numerous in the afternoon,
and some storms could produce locally gusty winds and very heavy
rain.
Outlook...
the Small Craft Advisory flag has been extended into Thursday night for now. The slower
moving front and developing low pressure to the north may result
in the Small Craft Advisory flag being extended further in time. Improving weather
by the second half of the weekend.
&&
Phi watches/warnings/advisories...
PA...none.
New Jersey...none.
Delaware...none.
Maryland...none.
Marine...Small Craft Advisory until 6 am EDT Friday for anz450>455.
&&
$$
Synopsis...O'Hara
near term...gorse/Heavener
short term...gorse
long term...O'Hara
aviation...gorse/Heavener/O'Hara
marine...gorse/O'Hara
