Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
648 am EDT Thursday Jul 31 2014
several weak surface troughs will move into the area through
Friday, before high pressure over the western Atlantic pushes an
offshore warm front northwestward across the East Coast this
weekend. Several disturbances will move northeast along this
boundary through the weekend. Another cold front will then move
across the area late Tuesday into Wednesday.
Near term /until 6 PM this evening/...
middle and upper level short wave trough lifts northeast over the
region today. Instability is somewhat limited (ml cape values are
generally 500 j/kg or less and convective temperatures are near or just
below forecast highs), but not negligible. However, peak heating
and, thus peak instability looks to occur after the shortwave
begins to lift away from the region. Highest chance looks to be
further north closer to the short wave trough, and thus slightly
higher effective shear. Even so, given the limiting factors,
expect isolated to scattered coverage at most.
With the broad trough still remaining northwest of the region, and some
cloud cover through the day, expect temperatures to once again be
Short term /6 PM this evening through 6 am Friday/...
the previously mentioned short wave trough should be well northeast
of the region by 00z. In its wake, mean flow transitions to
southwesterly. With warm waters, and still relatively cool air, this
could be enough for showers to develop over the bays and move into
Delaware-Maryland-Virginia and far southern New Jersey. Even if any thing develops, coverage
should be very limited given the meager instability in place.
Overnight cloud cover is expected to be a bit more expansive, and
increasing low level moisture will limit cooling, so should see lows
near normal tonight. If the cloud cover is not as persistent as
forecast, a very weak up slope and on shore flow in addition to the
increasing moisture could lead to patchy valley and near shore fog.
Long term /Friday through Wednesday/...
medium range guidance still showing decent consistency over the past
several days, through at least the weekend, with the evolution and
movement of the western Atlantic ridge and offshore surface
baroclinic zone. Temperatures look to be several degrees below
normal through the weekend with a moderating trend expected to occur
early next week.
Friday...deeper moisture advects into the region with ridging
offshore building and heights across the East Coast beginning to
increase. Better instability over the western sections of the County Warning Area
with more middle-level impulse activity should lead to even more
scattered convection during the peak heating hours. Weak surface
convergence under a predominant south-southeast flow will aide in
better lift for shower activity along the higher terrain.
Temperatures should again end up cooler than normal.
Saturday...another 24 hours later and there is still a glaring
difference along the New Jersey coastline with the track of a
surface wave early this weekend. The NCEP models continue to waver
between an offshore solution and one closer to the coast, while the
non-NCEP suite has been pretty consistent bringing the wave right
across the Delaware-Maryland-Virginia into eastern New Jersey. The timing of the
stronger surface waves is also suspect with NCEP models showing
better forcing on Sunday while the non-NCEP group have it on
Saturday. With such an anomalous trough in place the numerical
guidance is not handling the timing of the embedded shortwaves well,
even within 24 hours of the event. This remains the biggest fly in
the ointment for saturday's potential but thinking that a faster
progression is likely. Upper level jet dynamics with the right
entrance region sitting across the middle-Atlantic and ample
sub-tropical moisture available to tap into, we could be looking at
a heavy rain potential...greater chances east of the river closer to
the frontal boundary. Still a great deal of uncertainty at this
point, though it could be more to do with a smaller Sample size of
synoptic patterns similar to the one we are in.
Sunday...there looks to be a dry period, at least the first half of
the day, before more scattered showers with embedded thunder
commences. NCEP guidance pegs Sunday as the main action day with the
non-NCEP members keeping the front nearby but honing in on a
stronger middle-level shortwave, cold pool, sweeping through during the
afternoon hours. Even with a more predominant easterly flow, the
colder air aloft, with higher than normal pwats, should be able to
overcome a slightly more stable surface to ignite isolated to
Monday - Wednesday...slight drying trend expected on Monday though not
entirely void of any showers with cold air still aloft with a few
more shortwaves shearing across overhead. High pressure should be
nosing in by Tuesday to allow for better drying. More moisture and
heat advects in and could push temperatures back to normal.
Aviation /12z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
For the 12z tafs...mostly VFR conditions should continue through
at least 06z. There is a slight risk of rain showers and thunderstorms and rain across the
region. First this afternoon, with the highest risk at kabe and
kttn. Then overnight tonight, kilg may be on the northern edge of
overnight rain showers. In both cases, coverage should be very limited, so
have only included a vcsh mention for now. There is also a risk
of br developing after 00z, leading to MVFR visibilities. At this
time, br looks likely at kmiv and kacy given weak on shore flow
and increasing low level moisture. For the rest of the terminals a
combination of middle level clouds and slightly higher dew point
depressions means a much lower risk of br.
Friday - Sunday...mostly VFR with scattered showers and thunderstorms
possible each afternoon. Local sub-VFR conditions in showery
activity. Medium confidence.
Monday - Tuesday...VFR. Isolated showers possible on Monday.
expect winds and seas to remain below Small Craft Advisory today and
tonight with winds staying near or below 15 knots and seas between 2
and 3 feet.
Friday - Monday...sub-sca conditions are expected across our
waters this period.