Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 kphi 180738 
afdphi

Area forecast discussion
National Weather Service Mount Holly New Jersey
338 am EDT Tue Jun 18 2019

Synopsis...
a nearly stalled frontal boundary will continue to provide episodes
of unsettled weather through Wednesday. Low pressure will likely
track near or north of the region late Thursday, with a cold front
pushing offshore Thursday night. High pressure should gradually
build in for Friday and may last through most of the weekend. By
late in the weekend or early next week, another frontal system will
approach, with a warm frontal passage expected towards Sunday night
and Monday.

&&

Near term /until 6 PM this evening/...
we'll continue the Flash Flood Watch and even extend the area
northward to cover the rest of NE PA and nrn New Jersey today. The very
humid airmass remain in place and the front will again trigger more
showers and tstms across the region today. Models, especially the hi-
res, are favoring the nrn areas this morning and then back to the southeast
PA/srn New Jersey and Delmarva areas late and into the evening. The 00z GFS
shows this trend also, but the NAM continues to show mostly a nrn
event today. Confid in placement of the greatest quantitative precipitation forecast is not very
great, but feel confid that excessive quantitative precipitation forecast will occur across parts of
the County Warning Area today.

Storm Prediction Center has placed much of the area in a marginal risk for severe
weather today. Model caps values are low/moderate on the GFS and
moderate/high on the NAM. Decent convective available potential energy will probably be realized if
breaks develop in the clouds today. This will be not-common with the
relative humidity values in the atmosphere rather high today. Shear values
will not be too favorable early, but improve later today,
especially across Delmarva.

High temperatures will range from the mid/upper 80s across Delmarva
and srn New Jersey where there will be more breaks in the clouds. Across the
north, readings will top out in the mid/upper 70s.

&&

Short term /6 PM this evening through 6 am Wednesday/...
the disturbance passing thru today will move offshore tonight. A
decrease in tstm/shower activity is expected by late evening and
activity will be at a lull overnight. More fog will develop,
especially in areas that get decent rainfall today. Lows will remain
mild with mostly upper 60s/low 70s. Across the north readings in the
low to mid 60s are expected.

&&

Long term /Wednesday through Monday/...
overview...

Active weather will continue to dominate at least early in the long
term period, with some signs of changes going towards next week as
the Continental U.S. Pattern undergoes a shuffling. Initially, for the middle
part of the week, we are looking at more of the same. The stalled
front will continue to waver nearby on Wednesday, although by then
it will really be starting to lose definition. The weaker forcing
and indications for a good deal of cloud cover to limit instability
will probably keep convective activity on Wednesday rather tame. A
more intriguing setup presents itself by Thursday, and that is where
the most time was concentrated for this portion of the forecast.
This is another in a series of setups we have seen the past couple
weeks that are in many ways more becoming of the cold season than
the warm season. Synoptically, a shortwave trough is likely to
amplify as it moves across the plains and into the Ohio Valley, with
surface cyclogenesis occurring as it does so, supported by low
positioning within the right entrance region of a jet streak moving
over the Great Lakes. This low then looks to track east-northeast
with the mid-level flow towards the Great Lakes, and may eventually
phase with another trough over southern Canada as it begins to move
away from our area. It is what happens before this that could be of
interest to US. Some guidance, particularly the 18.0z NAM, indicates
an unusually intense low, at least given the season and its
location, developing by Thursday. The warm sector environment of
such a cyclone would have considerable potential for severe weather,
but confidence on outcomes regarding the low track is not especially
high. Regardless of its evolution, this system should pull away on
Friday but lingering showers and gusty northwest winds are possible
for that day as the low departs and the trailing upper trough swings
through.

By Friday night and Saturday, weak ridging looks to build in over
the Great Lakes, placing our region downstream of this weak ridge
and upstream of the departing trough. This should yield drier
weather for the weekend, though we will still have to watch an
active convective regime to our west. What is most evident as early
as the end of this week and continuing into next week is a major
pattern change over the west, where the persistent ridging over the
West Coast of late will be replaced by troughing. This should in
turn favor more ridging downstream which would increase our chances
for sustained hotter weather. These changes may be a bit slow to
manifest in our region in part due to the continued tendency for
blocking over Greenland, which has favored persistent troughing over
southern Canada. This in turn has played a role in keeping our
pattern unsettled. If this blocking pattern starts to break, I
expect we will see more in the way of deep Summer heat next week.
That would likely fall mostly outside of this forecast period,
though temperatures may warm towards the mid and upper 80s by early
next week as a warm front moves through in association with the next
frontal system to impact US.

Dailies...

Wednesday-Wednesday night... the long-lingering front will be mostly
washed out by Wednesday, though what's left of it continues to
meander nearby to US. Mid and high level cloud cover will probably
be fairly extensive on Wednesday, and mid-level buoyancy will be
weak. Thus, given weak forcing and lower instability, not expecting
much if any in the way of severe potential Wednesday, and the Storm Prediction Center
has maintained just general thunder probabilities. Pwats continue to
hover just below 2" so any showers or storms that develop will still
have localized heavy rain potential especially since storm motions
will likely be on the slow side. However, in general Wednesday
should not feature as much risk potential as some other days we have
faced recently. Highs near to just below seasonal norms due to the
cloudiness, with another warm overnight as dew points don't budge
out of the upper 60s.

Thursday-Thursday night...the synoptic idea for Thursday was
described above, as developing low pressure moves out of the Ohio
Valley and towards the southern Great Lakes. The track and structure
of this low will dictate our weather for Thursday afternoon and
evening. The two extremes out of the 18.0z guidance are the NAM and
the GFS. The NAM shows a very intense (for June standards) low
pressure developing and tracking roughly over lakes Erie and
Ontario. The GFS, at the opposite end, shows a much more strung out
and more southerly tracking low. The ec and Gem are compromise
solutions though generally look and feel a little closer to the NAM.
The NAM scenario would present the possibility of a rather high end
severe weather event Thursday owing to an unusual combination of
very strong dynamic forcing intersecting a moist and highly unstable
warm sector air mass. The GFS solution would still present some
severe risk, but it would be more limited and confined further south
in the region. Per collaboration with spc, a slight risk has been
introduced for a portion of the region on day 3. At this stage, it
should be cautioned against putting too many eggs into any one
basket on the outcome. What can be taken out of the NAM solution is
that there is a higher than normal ceiling potential for severe
weather on Thursday. However, it is far from certain the ingredients
will come together to achieve this, and with it still being a couple
days away the best course of action for now remains to monitor
trends over the next day or so as more of the hi-res guidance comes
into range. Heavy rain potential certainly exists again on Thursday
with any storms, though we would more likely be looking at faster
moving cells moving more perpendicular to the approaching cold
front, which would help temper that threat.

Friday... Friday looks to be a transition day as the low pressure
system pulls away. It will likely take all day Friday for the upper
trough associated with the low to swing through, helping keep
temperatures on the cool side of normal. Exactly how slow this
evolution is will determine how unsettled Friday turns out to be. If
the Thursday system does end up more strung out like the GFS shows,
we would see more wrap-around showers on Friday. A more compact and
faster moving low would likely yield faster drying and keep the
shower risk Friday lower. This would also yield gustier northwest
winds on Friday. We'll have to keep an eye on the gusty wind
potential for Friday as that gets closer. Split the difference for
the time being with a breezy and cooler than average day with a
shower risk decreasing into the afternoon.

Saturday-Sunday... as mentioned above, we should see some ridging
build in to our west for the weekend, yielding northwesterly or
westerly flow. There have been some previous model solutions sending
one or more mcss our way embedded within the northwest flow over the
weekend. However, the trend on the 18.0z guidance is drier, and this
seems to make more sense for this pattern as we should see high
pressure build in downstream of the ridge over southern Canada.
Expecting this high to be enough to send any ridge-riding
disturbances to our south over the weekend. Thus, expecting mainly
dry weather to prevail for the weekend. Continued northwest flow
with a rather chilly Canadian air mass to our north will keep
saturday's highs near to below normal, with some gradual warm air advection
beginning Sunday as flow turns more westerly, which will make Sunday
the warmer of the two weekend days.

Monday-Tuesday... by early next week, there is good agreement that
another frontal system will begin to affect the region, though it is
certainly too early to speculate on any details of that one. It is
behind this system, towards the middle and latter part of next week,
that long range ensemble guidance is in relatively good agreement on
longwave ridging building in over the central and eastern US and
potentially ushering in hotter weather. For now, we will just have
to watch and wait to see whether that signal proves correct, though
as discussed above the evolution across the Pacific and the western
US should start to favor such an outcome.

&&

Aviation /08z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.

Today...expect plenty of low clouds and fog early this morning at
most of the terminals with IFR/LIFR possible. Improvement will
arrive after sunrise when showers arrive from northwest and push southeast by late
morning. Conditions will probably improve to MVFR in most areas.
Tstms are expected this afternoon with the Delaware valley and southeast
sites expected to get most of the activity. Terminals have thunderstorms in the vicinity in
them for now with confid in many details of the tafs quite low attm.
Light winds this morning then S or SW expected by afternoon.

Tonight...still plenty of uncertainty in the tafs with
showers/tstms across Delmarva and srn New Jersey decreasing overnight. There
will likely be more fog overnight, but the degree and areas extent
still uncertain.

Outlook...

Wednesday-Wednesday night... prevailing conditions should be VFR but
patchy lower ceilings possible with scattered showers and
thunderstorms also possible.

Thursday-Thursday night... showers and thunderstorms likely in the
afternoon and evening. Southerly wind around 10 kt becoming westerly
overnight.

Friday-Friday night... MVFR possible early but improvement to VFR
likely. Breezy, with northwest winds gusting up to 25 kt.

Saturday-Sunday... VFR. Winds mostly out of the west or northwest
with gusts generally 15 kt or less.

&&

Marine...
no big changes on the waters with mostly south winds expected today
and tonight with speeds mostly 5 to 10 knots. Wind directions could
trend a little more southeast this morning at times. Seas will be
around 3 ft on the ocean and 1 to 2 ft across Delaware Bay. Sct
tstms with higher winds and seas will be the main weather hazard
today and tonight.

Outlook...

Wednesday-Thursday... sub-Small Craft Advisory conditions expected. Scattered tstms
with locally higher winds and waves possible both days.

Thursday night-Friday... a period of Small Craft Advisory conditions is likely as
northwest winds gust up to 30 kt by Friday and seas build 4 to 6 ft.
Tstms possible Thursday night.

Friday night-Sunday... lingering Small Craft Advisory conditions possible early
Friday night but the trend will be towards sub-sca. High confidence
in sub-Small Craft Advisory conditions for the weekend.

Rip currents...

A low risk for the development of dangerous rip currents is
expected to continue through today with waves in the surf zone
3 feet or less.

&&

Phi watches/warnings/advisories...
PA...Flash Flood Watch through late tonight for paz054-055-060>062-
070-071-101>106.
New Jersey...Flash Flood Watch through late tonight for njz001-007>010-
012>027.
Delaware...Flash Flood Watch through late tonight for dez001>004.
Maryland...Flash Flood Watch through late tonight for mdz008-012-015-019-
020.
Marine...none.

&&

$$
Synopsis...O'Brien

National Weather Service Glossary of Abbreviations