Area forecast discussion
National Weather Service Mount Holly New Jersey
406 PM EDT sun may 19 2013
Synopsis...
a frontal boundary to our south is expected to weaken and dissipate
as it remains nearly stationary through Monday. High pressure
centered east of New Jersey will sink south and east slowly through
mid-week. Another high will build toward US from eastern Canada. An
associated frontal boundary from the north is forecast to move into
or near our area late on Tuesday and then lift back to the north
on Wednesday and Thursday. A cold front from the west is expected
to arrive on Thursday night or early Friday, followed by high
pressure for Saturday.
&&
Near term /until 6 am Monday morning/...
the onshore flow in front of the approaching warm front will
continue tonight...the front will cross through overnight. Low
clouds and drizzle along with some fog will precede the front and a
few showers are possible behind it. Probability of precipitation overall will slowly
decrease through the night...arriving at slight chance/low chance by
morning. Winds will mostly be Erly/southeasterly this evening and then veer
to southwesterly by Monday morning. Lows will not too low tonight...with lows
60s across the southern areas and upper 50s across the north.
&&
Short term /6 am Monday morning through 6 PM Monday/...
there will be some improvement in the weather Monday. Upper heights
behind the departing warm front will be rising a little and winds
through the lowest layers will veer to a more wrly direction. These
factors should allow some drier air to work its way into the area.
There will still be some higher dew points during the morning...and
any sunshine will probably create some instability showers during
the afternoon. Probability of precipitation will be slight chance in the morning...rising to chance
by afternoon. Cape values point to the possibility for a few
thunderstorms...so they were left in there from before. Winds will be mostly
southwesterly at around 10 miles per hour.
&&
Long term /Monday night through Sunday/...
there is better model agreement through much of the long term.
Whether that continues is a question, but for the time being the
models agree at middle levels that the remnants of a middle level low that
has been moving slowly east will move to near the middle-Atlantic coast
on Tuesday and then weaken as an eastern U.S./Western Atlantic ridge
begins to build. We carry chance to slight chance probability of precipitation with this.
The ridge then builds and establishes a confluent zone off New
England, and that will carry an associated back door cold front as
surface high pressure ridges down from eastern Canada. Whether the
front makes it into our area is another question. Neither the latest
run of the GFS nor the last three runs of the European model (ecmwf) are bringing it
this far south, but it may get close enough to trigger some activity
in the Tuesday to Tuesday night time frame. We carry chance to
slight chance probability of precipitation that are highest north.
By Wednesday, the building middle level ridge axis will be to our east,
and that opens US up to any energy that might be kicking out an
advancing and weakening middle level low. That low will be part of a
Rex block over the northern plains and Canada into early Wednesday,
but the models agree to open it and move it into the western Great
Lakes by late Wednesday. HPC is hitting Wednesday probability of precipitation pretty hard;
we're not quite so sure about when energy from the opening low makes
it here and so we carry more chance probability of precipitation that are better northwest
and focused around Wednesday into Wednesday night.
Through the above time period, there always is a lot of moisture
available and instability generally waxes during the afternoon
hours. That increasingly becomes due to a south to southwest flow
around surface high pressure that is associated with the middle level
ridge and that resembles a Bermuda high, and it means that what the
previous forecaster noted still applies: there is the potential for
localized downpours.
The low from the west weakens into a neutral trough rotating around
an eastern Canadian vortex as it approaches our area on Thursday. By
the end of the day, there is an upper jet that has US in the right
exit region, and without going into the specifics of the forcing and
the wind fields this far out, at least some of that come together
with still-copious moisture over the forecast area in a time period
centered around Thursday evening or Thursday night. Of course, the
amount of instability available is to some degree dependent on when
the front arrives. This system seems to be the best opportunity for
organized convection with heavy downpours and strong winds, but we
have lots of time and lots of model runs to assess that.
We note that the timing of this has sped up from previous runs
despite there being a Rex block upstream earlier in the period, but
this forecaster bit the bait thanks to model agreement. We presently
carry lingering chance probability of precipitation for the first part of Friday and then go
dry for the rest of the forecast period. The latest European model (ecmwf) pushes
energy moving around the eastern Canadian vortex late in the weekend
further south. The GFS is slower with said energy. The middle level
flow looks fairly meridional, so either solution is a plausible
excuse to go optimistic.
Until that late week front arrives, nighttime mins will be well
above normal. Daytime maxes also will be above normal, but subject
to any convection. It also will be humid until the front arrives.
Temperatures will return to normal or a bit below normal after the
front.
&&
Aviation /20z Sunday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
A continuation of the mostly IFR conditions is expected at the
terminals this evening into Monday morning. The surface/low-level flow
will remain mostly onshore until the surface warm front crosses the
area. Once this happens...low level winds will begin to veer to S
then SW Monday. This should allow drier air to arrive and an
eventual scouring out of the lowest clouds. The mav/met guidance
shows the trend tomorrow, the exact timing may be a few hours off.
Monday...SW winds will increase to around 10 knots by afternoon and
ceilings will improve back to VFR around the noon hour. The VFR
conditions should last into the afternoon...but there may be a few
scattered showers with temporarily lower conditions later Monday afternoon.
Outlook...
Monday night through Thursday...MVFR and IFR conditions are possible
from late at night to early in the morning through this period.
Mainly VFR conditions are expected otherwise, although they
would deteriorate in any convection. Confidence is moderate.
Thursday night...possible convection with associated MVFR or IFR
conditions. Confidence is low especially on the timing.
Friday...VFR if convection clears the area by then. Worse if not.
Confidence is low.
&&
Marine...
winds and seas continue to slowly build across the ocean waters and
over del Bay. The Small Craft Advisory conditions that have been on the southern
waters already appear to be heading north...so the ongoing flag will
continue across the ocean waters. Guidance suggests that the
conditions will continue into Monday...so we will extend the flag
for now...although it will be a low-end advisory at best. Occasional
showers and some fog on the waters tonight and scat showers or a
thunderstorm Monday. Winds will veer to srly late tonight and then become
southwesterly on Monday as the warm front crosses the region.
Outlook...
wind speeds are forecast to remain sub-advisory into Wednesday
night, then begin to increase some in advance of the late week cold
front. Wave heights are forecast to be sub-advisory over the Bay
through Friday, but they are forecast to start at advisory levels,
subside to sub-advisory levels on Tuesday but then rise again in
advance of the cold front.
&&
Phi watches/warnings/advisories...
PA...none.
New Jersey...none.
Delaware...none.
Maryland...none.
Marine...Small Craft Advisory until 6 PM EDT Monday for anz450>455.
&&
$$
Synopsis...delisi
near term...O'Hara
short term...O'Hara
long term...delisi
aviation...O'Hara/delisi
marine...O'Hara/delisi
