Area forecast discussion
National Weather Service Mount Holly New Jersey
348 am EDT sun may 19 2013
a frontal boundary extended from the north central states, across
the Ohio River valley to the waters off North Carolina early this
morning. The boundary is expected to weaken and dissipate as it
meanders over the same general area through Monday. Meanwhile,
high pressure should remain to our northeast and east. Another
frontal boundary from the north is forecast to drop into our
region late on Tuesday and it should lift back to the north on
Wednesday and Thursday. A cold front from the west is expected to
arrive on Friday, followed by high pressure for Saturday.
Near term /until 6 PM this evening/...
a 06z surface analysis placed the center of high pressure from
just north of New England and then well to our east. A frontal
boundary remained draped from North Carolina northwestward to the
upper Ohio Valley then back into the Midwest. A weak surface
trough was noted across the Delaware-Maryland-Virginia. An upper air analysis placed
a trough in the west to the plains, with a ridge to its east. A
trough was near northern New England, and a middle level feature was
near the central Appalachians. There were several areas of focused
850 mb from the Southern Plains toward the Midwest, with a small
area from Long Island to western New York.
Our weather will be dominated by a middle level feature that nears
the middle Atlantic toward evening. At the surface, high pressure
will remain to our northeast and east which maintains a
southeasterly flow. As we go upstairs however, the flow is
forecast to be more southerly today and this will assist in the
moisture advection. There continues to be low-level warm air advection forecast
through the day. This will help to maintain at least some
isentropic lift as the more stable marine influenced air mass
resides below some warming. The model guidance continues to show
decent 850 mb and 700 mb Theta-E advection. The earlier northward
push of showers appeared to be aligned along the leading edge of
the 850 mb Theta-E surge. This is forecast to spread northward,
and the showers are following this. The weaker lift is also
resulting in some drizzle early this morning given the rather
moist low levels. Overall, we will carry chance to low likely probability of precipitation
for showers through the day as confidence is on the lower side
regarding any real organized activity.
Overall, today will feature a wealth of low-level moisture
combined with the onshore flow resulting in an abundance of
clouds. We can hope for some breaks of sunshine, but the majority
of our County Warning Area should maintain a cloudy sky. There is the potential
that our far southern zones get a few breaks of sunshine as warmer
air arrives here compared to elsewhere. The forecast soundings are
showing some instability developing mainly from about midday on
across portions of the Delaware-Maryland-Virginia. This could result in some
thunder, however as of now we opted to leave out a mention as it
would probably be rather isolated and confidence is not all that
As for high temperatures, we opted to go with the cooler GFS MOS
with even some tweaks downward a bit. This is based on a solid
cloud deck remaining with mostly a southeast flow continuing. The
warmest temperatures were placed across portions of Delaware and
Maryland as the winds may turn more southerly here sooner. We used
some of the lamp/lav guidance in the hourly temperature grids,
especially for awhile this morning, to help with the trends
including steadier values for most places.
Short term /6 PM this evening through 6 am Monday/...
the aforementioned middle level feature looks to linger near the middle
Atlantic tonight, as a ridge aloft gradually slides eastward. High
pressure at the surface as a result is forecast to slide farther
to our east, which will allow the low-level flow to veer from
southeast to more south or southwest overnight. This maintains
Theta-E advection through the night with at least some warm air advection as
well. There does not appear to be much in the way of organized
lift moving through, therefore the showers during the day should
decrease in areal coverage.
However, as the flow veers and moisture increases in the lower
levels some model guidance indicates that this moisture generally
becomes locked in the lowest levels. If enough drying can occur
above this, then some drizzle may be the main precipitation type
during the course of the night. This could be mainly for our
central and northern zones, as farther south enough warming may
result in the cloud bases rising some and a less conducive setup
for drizzle. We noticed that some of the high resolution guidance
indicates little to no precipitation occurring during the night,
with perhaps some showers developing toward daybreak as some
positive vorticity advection/lift arrives from the southwest with the middle level feature. As
a result, we lowered probability of precipitation into the chance range and carried showers
far south with light rain/drizzle farther north. We also added in
patchy fog, first starting in the higher terrain then to other
As for low temperatures, we mainly used an even GFS/NAM MOS
blend. Temperatures should be on the mild side given the clouds
and continued warm air advection. There is a chance that especially the far southern
areas have some increase in the temperatures overnight.
Long term /Monday through Saturday/...
the middle level pattern is expected to feature the polar vortex well
to our northeast with a closed low over the middle Missouri River
valley on Monday. There should be a ridge extending from the
southeastern states to the Great Lakes. The polar vortex will
remain the dominant feature over eastern Canada during the new
week. Meanwhile, the closed low to our west and its associated
trough will progress eastward, gradually squeezing the ridge and
pushing it out over the Atlantic. As the trough begins to move
toward the eastern states, it should start to interact with the
polar vortex around Wednesday as that feature's influence begins
to expand to the west and south. The resulting middle level trough
will likely move eastward across the Great Lakes and the
northeastern states in the Thursday through Saturday time frame.
The surface flow is forecast to veer to the south and southwest
for Monday and Monday night causing the clouds to lift and
temperatures to warm, resulting in the development of unstable
conditions, especially away from the ocean. Surface based cape
values are anticipated to rise near 1000 j/kg in our western
counties during the afternoon. A fair amount of low level moisture
will remain in our region. As a result, we will continue to
mention a chance of showers for Monday and we will also include
the potential for afternoon thunderstorms. The guidance is
indicating a decrease in precipitation for Monday night with the
loss of daytime heating and with the lack of any focusing
Temperatures are forecast to rise near +15c at 850 hpa for
Tuesday. A south to southwest surface flow will continue and
temperatures are expected to rise into the lower and middle 80s in
much of our forecast area. Lingering low level moisture will help
enhance the instability and we are anticipating scattered showers
and thunderstorms, especially in the afternoon and evening.
Surface based cape values in the 1500 j/kg neighborhood are
possible in our southwestern counties. The most unstable areas
will likely have precipitable water values in excess of 1.5 inches
with there being the potential for localized downpours.
A frontal boundary is forecast to drop into our region from the
north late on Tuesday. There is a fair amount of uncertainty as to
how far south the boundary will progress. It should continue to
have a least a little momentum into Wednesday. However, as the middle
level ridge axis passes overhead on Wednesday or Wednesday night,
the boundary is expected to begin lifting back to the north.
Regardless, unsettled conditions will likely continue.
As the middle level trough begins to influence our region late in
the week it should move a cold front across the northeastern
states on Friday. As a result, we will carry a chance of showers
and afternoon thunderstorms for Friday.
While there will be a chance of precipitation in the forecast for
each day from Monday through Friday, the week should not be a
total washout. Actually, most of the time precipitation will not
be falling. Overall, temperatures for the week should average
above normal for this time of the year.
After the cold front passes, dry air is anticipated to begin
building into our region from the northwest and north for Friday
night and Saturday.
Aviation /08z Sunday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Today...MVFR/IFR ceilings prevail through the day, and there is
lower confidence regarding any improvements. It is possible that
terminals with IFR ceilings improve to MVFR in the afternoon, and
the visibilities should be mostly dependent on the showers.
Southeast winds generally near 10 knots.
Tonight...IFR ceilings, with the potential for some improvement
to MVFR especially south and east of kilg. Overall, some showers
should be around with even this perhaps transitioning to some
drizzle, therefore times of local visibility restrictions. Some
fog is anticipated which will also lower the visibility, although
we are currently not anticipating fog to reduce the visibility to
under 1sm. Southeast winds 4-8 knots, should turn more south-
Monday through Thursday...mainly VFR. However, MVFR and IFR
conditions are possible especially during the late night and early
morning hours each day. Also, there will be a chance of showers
and thunderstorms through the period which could lower ceilings
and reduce the visibility at times.
high pressure remaining to our northeast today will maintain a
southeasterly flow, although this should start to turn more from
the south later today across the southern waters. This shift to
southerly should then overspread all the waters tonight. This flow
regime, which is anticipated to remain under 25 knots, will allow
the seas to build some more though. The wavewatch guidance has
been running 1-2 feet to high, however it is catching up as seas
reached 5 feet earlier at buoy 44009. These higher seas are
anticipated to spread northward today, however confidence is not
all that high they get above 6 feet. Therefore, the Small Craft
Advisory remains in effect for our ocean zones through tonight.
The conditions are anticipated to remain below advisory criteria
on Delaware Bay. Some additional increase in the dew points
tonight could result in some fog development.
high pressure to our northeast and east will continue to
influence the coastal waters of New Jersey and Delaware into
Tuesday. A frontal boundary from the north is forecast to drop
into our region on Tuesday night before retreating back to the
north on Wednesday and Thursday.
Wind speeds are forecast to remain less than 25 knots from Monday
through Thursday. Wave heights on our ocean waters could remain
around 5 feet on Monday into Monday night and the current Small
Craft Advisory may need to be extended to cover that period. A
lull in wave heights is possible for Tuesday and Wednesday with 5
foot seas forecast to return to our ocean waters for Thursday as a
long southerly fetch becomes established along the East Coast.
Marine...Small Craft Advisory until 6 am EDT Monday for anz450>452.
Small Craft Advisory until 6 am EDT Monday for anz453>455.