Area forecast discussion 
National Weather Service Mount Holly New Jersey 
348 am EDT sun may 19 2013 


Synopsis... 
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 
high. 


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 
areas. 


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 
mechanism. 


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- 
southwest. 


Outlook... 
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. 


&& 


Marine... 
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. 


Outlook... 
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. 


&& 


Phi watches/warnings/advisories... 
PA...none. 
New Jersey...none. 
Delaware...none. 
Maryland...none. 
Marine...Small Craft Advisory until 6 am EDT Monday for anz450>452. 
Small Craft Advisory until 6 am EDT Monday for anz453>455. 


&& 


$$ 


Synopsis...iovino 
near term...gorse 
short term...gorse 
long term...iovino 
aviation...gorse/iovino 
marine...gorse/iovino