Area forecast discussion
National Weather Service Mount Holly New Jersey
359 am EDT Thursday may 23 2013
Synopsis...
a Bermuda high pressure system well off the East Coast will slowly
recede today and tonight. A low pressure system moving through the
Saint Lawrence valley will slowly drag a cold front into and through
the region late tonight into Friday. A second low pressure system
will develop on the front off the New Jersey coast Friday and
slowly trundle northeast reaching the Canadian Maritimes on
Monday. A high pressure system will build east in its wake early
next week. A warm front is forecast to approach and move through
the forecast area on Tuesday into Wednesday.
&&
Near term /until 6 PM this evening/...
a rainy period in store today as showers and scattered thunderstorms
are expected to increase in coverage through the day. Surface low
pressure will move out of the Great Lakes and into New England
today, which will push a cold front through central Pennsylvania and
toward our area later today. Showers and thunderstorms are expected
to continue to develop ahead of the frontal boundary. A pre-frontal,
Lee-side trough is expected to develop during the day while a short
wave/vorticity maximum moves across the area as well. This will likely be the
focus for a line of showers and thunderstorms late in the day. The
Storm Prediction Center has placed a portion of the area under a slight risk for severe
weather this afternoon. One limiting factor will be the expected
cloud cover across the area which may limit the amount of
instability across the area. But we could still have cape values
around 500-1000 j/kg, and with shear values around 30-40 knots, we
could have a few storms become organized enough to produce some
strong winds.
Another concern today will be heavy rainfall. Precipitable water values are forecast
to be around 1.50-1.75 inches. Therefore, some showers/thunderstorms
could be rather efficient rain producers. Poor drainage and
urban/small stream flooding is possible this afternoon, especially
if any training occurs.
Mav/met MOS guidance were fairly close, although the mav was
slightly higher. We used a blend, with majority of the mav MOS. This
will give highs generally in the middle-upper 70s, and if the sun comes
out at all for an extended period of time, a few areas could reach
80.
&&
Short term /6 PM this evening through 6 am Friday/...
showers and thunderstorms are expected to be ongoing across the area
by Wednesday evening as the pre-frontal trough makes its way across
our area and the initial short wave moves across the area.
Instability will likely decrease some after sunset, but with more
positive vorticity advection expected to take place overnight in the cyclonic flow aloft,
along with the passage of the frontal boundary, showers and
thunderstorms could continue to develop through the night. The front
will not clear the entire area overnight, may be near the coast by
sunrise. With precipitable water values still around 1.50-1.75 inches ahead of the
front, some heavy rain remains possible.
&&
Long term /Friday through Wednesday/...
neither the GFS nor WRF-nmmb looked stellar off their
initialization. In fact a model compromise seemed to look the best.
Overall though the deep/dt with the upcoming system was slower and
this forecast package did go that way with a wetter start to the Holiday
weekend. This package is close to the gefs mean which is slightly
more progressive than the op GFS and closest to a modeling
consensus. The model 500mb pattern forecast is anomalous for late
may, so we did edge slightly more progressive than the slower
solutions.
The slowing front will be moving through our County Warning Area as we start the
long term. With the trough sharpening, it becomes an underrunner with
instability still present aloft even with the boundary moving
offshore. We included the chance of thunder in the morning, but
theoretically thunder could linger into the afternoon (fri night off
the previous ecmwf). Not confident about temperature forecast as daytime maximum
temperature may be driven by how far temperatures fall by 12z Friday. Kind of
rare to have temperatures go nowhere on a May Day (and also have the
calendar day high possibly occur at 101 am) and we showed a slight
bump up for now.
As the trough aloft closes, model diags are showing some rather robust
middle level qvec convergence and fgen forcing forecast and a
developing middle level trowal over our County Warning Area with the entrance region
of the 250mb jet to boot. Its no wonder that quantitative precipitation forecast have become
wetter Friday night and Saturday. It still depends upon how
quickly and far south the 500mb low tracks. For now we remain most
confident for the northeastern part of our County Warning Area. While probability of precipitation were
increased Friday night and Saturday across the board, the highest
remain in the northeastern part of our County Warning Area. With only the European model (ecmwf)
supporting thunder for Friday night (none of the models do on
saturday), we precluded them. Conditions are expected to improve
as Saturday moves along, but with our County Warning Area remaining under the
closed low, its going to be tough to get much sunshine before we self
destruct. Min and maximum temperatures were edged lower and close to a stat
guidance compromise.
We kept in precipitation chances for Saturday night northeast and kept Sunday
dry. We will still be in the cyclonic flow of the closed low, so
more self destruct sunshine is possible, especially farther
northeast one is within our County Warning Area. We showed a temperature improvement
vs Saturday, confidence though is not high until the model trend
settles.
We did not change much in the remainder of the long term. Monday
looks to be the best day on the Holiday weekend with temperatures
returning to near normal by Tuesday and above normal on Wednesday.
We kept low chance probability of precipitation in associated with the next approaching
warm front.
&&
Aviation /08z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
A cloudy, rainy period expected today as showers, and possible
thunderstorms, are expected to increase trough the day ahead of an
approaching cold front. The low clouds/fog have not formed as of
this issuance, but still may develop in the hours leading to
sunrise. So for now, we will leave them in the forecast. Even if
they do not form, we still expect cloud cover to continue to
increase and ceilings to lower through the morning, leading to a period
of MVFR through the day. As mentioned, showers are expected to
increase in coverage through the day and could reduce visibilities if heavy
enough. There is also the potential for thunderstorms, mainly this
afternoon into this evening, so we will keep them in the forecast
for now.
The cold front is not expected to cross the area until overnight
Thursday into Friday morning. Conditions could lower more ahead of
the frontal boundary Thursday night, and showers will likely
continue at least until the front moves into the region.
Outlook...
Friday through Saturday...MVFR to possibly IFR conditions due to
clouds and showers. Worst conditions expected at northeastern
terminals and airports. Conditions could improve as Saturday
continues. North to northwest wind gusts could exceed 20 kts.
Saturday night through Monday...mostly VFR. Some gusty northwest winds
possible on Sunday.
&&
Marine...
Small Craft Advisory remains in effect for today and tonight as
winds are expected to continue to gust near 25 knots in the
south-southwesterly flow ahead of an approaching cold front. Seas
are expected to continue to increase to near 5-6 feet today into
tonight as well.
Outlook...
the Small Craft Advisory flag has been extended into Friday day for now. Winds may
not be a factor, but swells should remain. The increasing
northwest flow around the developing low should result in more
Small Craft Advisory conditions on all our area waters later
Friday into Saturday evening. There is a small chance that even
gale force wind gusts might occur for a time on Saturday. Small
Craft Advisory wind gusts might linger on the ocean into Sunday
morning. After that sub Small Craft Advisory conditions are
expected for the rest of the Holiday weekend.
&&
Phi watches/warnings/advisories...
PA...none.
New Jersey...none.
Delaware...none.
Maryland...none.
Marine...Small Craft Advisory until 6 PM EDT Friday for anz450>455.
&&
$$
Synopsis...o'gigi/hara
near term...Robertson
short term...Robertson
long term...gigi
aviation...o'gigi/Robertson/hara
marine...o'gigi/Robertson/hara
