Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
705 PM EDT Tuesday Sep 23 2014
high pressure system over our forecast area will strengthen as it
moves northeast...reaching the Canadian Maritimes on Wednesday. A
low pressure system will develop off the Carolina coast and lift
northeast into Delaware-Maryland-Virginia on Thursday before moving offshore on
Friday. Another high pressure from the Great Lakes will build into
the region for the weekend. A cold front is expected to move
through the area Monday night followed by a high pressure system
building into northern New England on Tuesday.
Near term /until 6 am Wednesday morning/...
630 PM estf: have lowered temperatures a bit over the Delaware-Maryland-Virginia as the
first batch of thick cirrus peels away to the east leaving a slot
of mostly clear sky...except the southeast corner of Delaware where thick
high cloud cover should hold firm. Winds were decreased slightly
early this evening permitting ideal evening radiational cooling
over most of our forecast area. The dewpoints were raised slightly
per reality against model output statistics. Ran the ups crossover
fog tool and only found some isolated spotty fog potential in
Sussex County of far northwest New Jersey early Wednesday morning.
So from the 330 PM discussion...
fair weather is expected tonight as high pressure will
be across the region. Skies will be mostly clear across the
north...and partly cloudy across the south where some high cirrus/cs
clouds will be observed. A slow increase in high clouds will be
seen overnight. Low temperatures will again be cool with decent
radiational cooling expected. I went below mav MOS in most areas.
Lows will range from the low/middle 40s north...to the low/middle 50s
over the Delaware-Maryland-Virginia and metropolitan Philadelphia. Winds will be light and
variable...then become east/NE at 5 to 10 miles per hour by dawn.
Short term /6 am Wednesday morning through 6 PM Wednesday/...
the 630 PM estf raised probability of precipitation in southern Talbot and Caroline
counties as well as southern Sussex County Delaware where it should be
raining at a pretty heavy clip very late in the day Wednesday.
That within the 850 easterly flow of 35-40 knots.
The surface high will be east of the area Wednesday morning. Low pressure
will be deepening off the Carolina coastal areas and beginning to
track north. Clouds will increase across the region during the
day...but the only chance for rain will only be across southern Delaware
and the southern coastal areas of New Jersey. These rain chances would be
during the afternoon only. Winds will increase from the east at 10 to
20 miles per hour during the day with gusts around 25 miles per hour...possibly 30 along
the coast by days end. High temperatures will mostly be in the low
70s region- wide.
Long term /Wednesday night through Tuesday/...
the long term has become much more complicated with a greater
impact over our County Warning Area than was the case a couple of days ago. All of
the models are transitioning to what the European model (ecmwf) and can ggem
originally had. The problem remains in the details as Gulf Stream
convection with this developing system has resulted in very
little run to run continuity as to the locations of heavy rain.
That being said, the deep/dt on the system has been toward a faster
closing off of the low (which has already occurred) and thus a
more westward tug by it on the surface low. The split off the 12z
run is whether or not the low aloft will close again over PA.
This makes ending of precipitation chances different. The can ggem quantitative precipitation forecast
verified the best through 12z this morning, while the GFS looked off
to the best start of the 12z models in the southeast.
Since the op NAM looked fast given the low has closed farther west
than earlier projections, we went closer to the GFS timing. The
UKMET remains the most benign model with this event, but given its
by itself now was given little weight.
Wednesday night, rain, heavy at times will be overspreading our County Warning Area.
Probability of precipitation were increased to Cat east where all the models have decent
measurable rain. Heavy rain mention was kept east based on modeling
consensus and location of their 250mb divergence aloft and better
middle level fgen features. While some models are showing a secondary
precipitation maximum in PA, the predicted precipitable waters are about half of locales
closer to the coast. Surface based instability is not there, but some
instability is being predicted aloft. For now the instability is
trailing the Omega vs being coincidental in our cwa, so no mention
of thunder. That being said the predicted convection to our south
is not doing well for model quantitative precipitation forecast verification. There has not been
much run to run continuity with regards to placement of heavy
rain. This model run suite did not seem to suffer from what looked
like feedback pblms that occurred with some of the 00z and 06z
runs. We are entering this event extremely dry, even for September
and would Need More than the 1 to 3" predicted for this event for
streams and rivers to start having problems. Some headwater
guidance is near 6 inches/12 hours. Biggest flooding issues may
be along the immediate coast if heavy rain coincides with the high
tide cycles. There should also be a rain impact on the morning
commute Thursday, especially in the northern two thirds of our
County Warning Area.
On Thursday, the synoptic scale push of heavier rain is expected to
move through our County Warning Area by noon. Secondary precipitation maxes are now either
offshore or non generated. Now that the models have keyed in for
two successive sounding runs on the closed low in the se, there
has been a slightly faster evolution of this event going forward
and even vs the night run. Differences arise as to how quickly
will the trough or closed low exit the region. Given the drier and
faster trend on the latest WRF and European model (ecmwf) (although the latter is
still among the slowest), we decreased probability of precipitation faster on Thursday
afternoon and night and have Friday dry.
Another well timed weekend as all of the models have US dry and
unseasonably warm in the wake of the departing low. The next
backdoor cold front should come through later Monday or Tuesday.
Probability of precipitation were kept in for Monday night and added for Tuesday.
Aviation /00z Wednesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Tonight...VFR mostly clear north and VFR scattered-broken cirrus south at or above 20000
feet. Light north-NE wind.
Wednesday...VFR broken-overcast cirrus at or above 20000 feet to start with a scattered-broken
deck near 3000 feet developing northward late morning and afternoon.
East-NE wind gust 20 knots interior and near 25 knots coast...especially
Wednesday night and Thursday...MVFR to IFR ceilings are likely with
MVFR visibilities over much of the region as a low passes just east of the
region. Some gusty east winds are also possible for coastal
airports. The terminals expected to have the longest and most
significant impacts are kmiv and kacy with airports in New Jersey and Delaware
overall most affected.
Thursday night and Friday...becoming VFR. Greatest impact on
Thursday night and early Friday should be fog.
Friday night through Sunday. Primarily VFR conditions expected. Some
early morning fog possible at more rural airports and terminals.
no headline tonight. Last good night to be meteorologically safe
on the Atlantic waters...possibly through Saturday.
Wednesday...Small Craft Advisory wind gusts to 25 knots spread northward during the
morning with 30 knots gusts by late afternoon. Seas build to 5 feet
nnj waters during the afternoon and to near 8 or 9 feet late day
southern New Jersey and Delaware Atlantic waters. Light rain will arrive over
the southern waters during Wednesday afternoon...then spread north/west during
Wednesday night and Thursday...a gale watch was issued for most
of our coastal waters as there should be a period when the
gradient tightens to the point that gales are possible. Lowest
confidence in upper Delaware Bay and our northern waters where a
Small Craft Advisory remains in effect through Wednesday night.
Slow improvement with the winds on Thursday, decreasing south
first. Delaware Bay most likely to go below Small Craft Advisory conditions on
Thursday night through Friday night...winds on the ocean should
be below Small Craft Advisory criteria by Friday morning with Delaware Bay
remaining below Small Craft Advisory criteria. Seas may be another problem as they
are predicted to be slow to subside. Small Craft Advisory conditions for seas are
possible through Friday night.
Saturday and Sunday...sub Small Craft Advisory conditions on
Delaware Bay. Seas should subside below Small Craft Advisory
criteria on the ocean on Saturday and remain below for the rest of
a 6 to 18 hour period of intermittent gale force east to northeast
wind gusts (35kt) is expected late Wednesday into Thursday and will
probably cause minor coastal flooding during one or two of the 3
high tide cycles beginning Wednesday evening. The tidal inundation
flood risk will linger through the Thursday night high tide cycle.
Normal astronomical predictions without any meteorological influence
do not indicate any unusually high tides.
Tides versus predicted late early this Tuesday evening are currently
running near or slightly below the forecast values.
However...positive surge departures are expected to grow rapidly
Wednesday evening and sustain that 1 to 2 foot Wednesday evening
high tide positive departure through the Thursday night high tide
If the surge is 1.8 to 2 feet and coincides with the high tide...
minor flooding would occur.
There is a small chance that the surge at the time of Thursday
morning-midday high tide will be 2.3 feet in which case moderate
coastal flooding would occur.
Timing is everything in this situation.
This is no slam dunk for a moderate event ... for minor the
model guidance tends to be resistant to forecasting exceeding
minor threshold. GFS/NAM continue conservative on their 18z runs
and the only the Stevens Institute ssws checks in with minor at
Lewes and barely so!
No coastal Flood Watch since the moderate coastal flood threat
appears to be marginal lower end at worst, and its possibility as
of this writing appears to be less than 25 percent along any
stretch of the coast.
If model guidance and forecaster confidence for moderate coastal flooding
increases in future forecasts...a coastal Flood Watch will be issued.
In the meantime...minor coastal flooding appears to be a good bet
for the Thursday morning high tide cycle, if not already for the
Delaware Atlantic coast Wednesday evening.
Depending on the position of the surface low... the coastal flood
threat may linger Thursday night or even Friday morning as the
strong onshore wind field initially moves north toward Long Island
Thursday and then slides southeastward with the eastward turning surface low
Since the wind event has yet to begin and we're not seeing the
reality of easterly flow on the coastal tide gages and even minor
flooding is not yet a guarantee ... are not yet posting an
advisory and this was checked against the 18z models.
One additional note...some beach erosion is likely and waves of 10
to 12 feet may add to splashover onto the typically vulnerable
coastal beach properties Wednesday night and Thursday.
Reviewing: the primary guidance for this analysis discussion was
the 18z GFS estofs and etss which shows a 1-2 foot surge. The NAM
via dbofs is less threatening...with only around a half foot
surge! The Stevens Institute ssws was checked and looks reasonable
and predicts a 6.5 foot tide for Lewes Delaware Wednesday evening
at 936 PM.
we will issue a high risk for the formation of dangerous rip
currents in our 8 PM forecast for surf zone forecast for Wednesday.
A moderate or high risk for the formation of the dangerous rip
currents will linger Thursday.
The enhanced risk for rip currents might persist into Friday even
as winds diminish and seas slowly subside.
Marine...gale watch from Wednesday evening through late Wednesday night
Small Craft Advisory from 6 am to 8 PM EDT Wednesday for
Small Craft Advisory from 6 am Wednesday to 6 am EDT Thursday
near term...drag/O'Hara 705