Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
933 PM EDT sun Oct 23 2016
a cold front will slide through our region early Monday morning.
High pressure centered over central Canada will build to our north
during mid week. A warm front, followed quickly by another cold
frontal passage is expected through our region late Thursday or
early Friday. High pressure follows to our south across the
Carolinas next Saturday with another cold frontal passage likely
Saturday night or Sunday.
Near term /until 6 am Monday morning/...
A potent mid-level short wave will continue to dive southeastward
from the eastern Great Lakes overnight. Much of the forcing along
the path of the shortwave appears to be tied to warm air
advection on the nose of a strong 850 mb jet. This will tighten
the thermal gradient and where this sets up will be the placement
of a ribbon of frontogenetic forcing. Our northern zones look to
be on the southern end of the precipitation axis, however are
expected to see at least some showers as this system zips east-
southeast. As a result, we continued the idea of quickly
increasing pops after midnight but also tightened the gradient
southward. Looks like much of the measurable quantitative precipitation forecast is north of I-80
northward. Farther southward, just some increase in cloud cover is
anticipated overnight. Raised pops with the 9:30 PM update taking
into account the radar trends along with the latest rap and hrrr
model runs which inched the shower activity slightly further
As a low-level jet arrives overnight along with accompanying warm
air advection, temperatures are not expected to drop off as much and
may even rise a bit toward morning. Lows were raised a couple of
degrees in spots with the last update to take this into account.
Short term /6 am Monday morning through 6 PM Monday/...
a quick moving short wave and weak surface low will exit to our
northeast early in the morning. This feature will pull a cold front
offshore to start the day and will also amplify the trough aloft
across the region. As a result, any showers up north end very early.
As cold air advection increases during the day combined with
cyclonic flow and a tightening pressure gradient, there will be a
gusty wind once again. It looks like gusts should peak at around 30
mph especially from late morning on.
The increasing cold air advection combined with just enough moisture
should result in some stratocumulus lingering or developing. This
could be fairly extensive across the northern areas especially as
another strong short wave is forecast to arrive during the
afternoon. We therefore kept more clouds across the northern areas
through the day. Since the cold air advection is forecast to be more
notable in the afternoon, many areas should have temperatures get
into the 60s before the cooling starts. We used mostly a MOS blend
for the high temperatures.
Long term /Monday night through Sunday/...
500 mb: a cold trough in the northeast USA will weaken to the
Maritimes late Wednesday as a short wave moves through the Great
Lakes- Ohio Valley region. That short wave crosses the mid Atlantic
coast Friday with a broader trough to move east across our area
Temperatures: October through the 22nd has averaged around 4 degrees
above normal. The chill during midweek will make a significant Dent
in that above normal departure but still leave the month as a
whole, averaging around 2 degrees above normal. Calendar day
averages Tuesday should be around 3 to 4 degrees below normal, 5
and 10 degrees below normal Wednesday and Thursday, thereafter
within 3 degrees of normal Friday through Sunday.
Forecast basis: unless otherwise noted, a 50 50 blend of the 12z/23
GFS/NAM MOS guidance Monday night-Tuesday night, thereafter the
12z/23 GFS mexmos Wednesday and then the 15z/23 wpc guidance
Wednesday night -Sunday, at times blended with the 12z/23 GFS mexmos
Monday night...clear to partly cloudy except there could be a sprinkle
or flurry in the Poconos toward dawn Tuesday. Seasonable. A gusty
northwest wind to 20 mph high terrain and I-95 region to the coasts.
Confidence: above average except below average any pcpn.
Tuesday...scattered sprinkles/flurries possible during the morning,
mainly Poconos, otherwise...considerable diurnal driven cloud
cover, breezy and cool. Northwest wind gusts to 25 mph or 30 mph.
Confidence: above average except below average any pcpn.
Tuesday night...freeze and or frost is an above average
likelihood for E PA and New Jersey Countryside (probably excluding
Philadelphia center city) and adjacent sections of NE Maryland and north Delaware
as northwest winds diminish. -4c at 850. Confidence: above
average. The 12z/23 NAM boundary layer winds tend to show a little
bit of stirring (transfer) most of the night east of I-95. That
may indicate a cool bias in the NAM temps near the shore. On the
other hand, sref modeled precipitable water which is near .3" inches early
Tuesday morning, lowers to less than .25" from Acy- phl northward,
indicating The Columns cold and dryness. This very low pwat tends
to be a prerequisite for below normal night time cold. Continues
to be highlighted in the severe weather potential statement. Hourly temps sampling <36 for areas
of frost and <33 for widespread frost. Since there may be a slight
stirring of the wind, the frost might be spottier than anticipated
but the freezing temp fcst looks pretty solid along and north of
Wednesday...quite cool with highs 5 to 10 degrees below normal
despite partly to mostly sunny skies. Northwest to north winds.
Confidence: above average.
Wednesday night...there will be another risk for freeze and or frost
on Wednesday night. However, unlike Tuesday night, increasing
high clouds after midnight may somewhat limit radiational
cooling. If the clouds come in early enough, this could inhibit
frost development. For now, continue a mention of patchy frost for
areas with a forecast min of 35 degrees or less, since temps will
plummet during the evening,despite bands of thin cirrus. Thicker
cirrus should not arrive until after 2am when the damage may
already be done. Confidence: average
Thursday and Thursday night...low pressure and its waa pattern
will run into marginally cold airmass over NE PA and NW New Jersey where
a a little snow, sleet or freezing rain is possible during the
morning, conditional on spotty precipitation arriving prior to 11
am. Will add the potential for a few slippery spots in the severe weather potential statement
that area. Used the partial thickness tool for ptype, manually adding
snow. There is quite a bit of model discrepancy regarding whether
the column will still be cold enough for snow to reach the ground
but it is possible. No accums expected, other than a trace of sleet
and/or snow in the hills and mountains.
Small chance of an isolated thunderstorm during the evening on the
Delmarva but not mentioned in the grids.
Otherwise, Max temps may not occur until sometime late in the day
or evening. Am suspicious that the fcst Max temps issued at 330 PM
along and north of I-78 are 3f too warm.
Please see wpc for qpf amounts (around half an inch give or take a
quarter inch). I count this as somewhat beneficial rain for our
Confidence: average except below average on any mixed ptype vcnty
Friday...partly sunny and seasonable. Northwest wind gusts 15 to
20 mph. Confidence: average.
Saturday and Sunday...fair and seasonable. A cold frontal passage
is probable but with model timing differences. Confidence: below
average, especially Sunday.
Aviation /02z Monday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
Tonight...VFR with an increase in clouds, especially north of kphl.
Lower VFR ceilings should be confined from about kabe to kttn on
northward. The main shower activity after 04z should stay north of
kabe and kttn. West to southwest winds diminishing to 10 knots or
less, although some increase may occur toward daybreak Monday with a
wind shift to the northwest.
Monday...VFR ceilings around at or above 5000 feet. Northwesterly
winds 10-15 knots with gusts up to 25 knots.
Monday night through Wednesday night...mostly VFR conditions expected.
Gusty northwest winds to 25 kt expected Tuesday afternoon.
Confidence: above average.
Thursday...VFR ceilings Thursday should lower to MVFR conds in periods
of rain (with possible localized ifr) Thursday night. East wind becoming
southeast. Confidence: above average.
Friday...VFR. Northwest wind gust to 15 or 20 kt. Confidence: average.
Intense low pressure will continue to move well north of New England
through tonight, while an upper-level disturbance arrives from the
northwest toward morning. This will allow the winds to decrease
through this evening from south to north, however a low-level jet is
forecast to arrive toward morning before shifting offshore during
Monday morning. The strongest low-level flow is forecast to occur
during warm air advection, therefore it is questionable how much
mixing can occur down to the surface. Cold air advection then
increases during Monday especially in the afternoon. The combination
of both of these is expected to contribute to advisory level gusts
through Monday. Eventhough we have a lull here in the late evening
with the winds, they are expected to increase again toward
sunrise. Will maintain Small Craft Advisory headline through the overnight.
Since the stronger winds aloft are forecast to move through during
a less favorable mixing signature, gusts were kept below gale
force. While there should be a lull for awhile this evening,
especially on Delaware Bay, it should be short enough and
therefore a Small Craft Advisory is in effect for all zones
Monday night and Tuesday...Small Craft Advisory conditions are expected to continue
intermittently as northwest winds occasionally gust above 25 kt
(though seas may drop off below 5 ft. Small Craft Advisory headline as of the 330
PM forecast was extended through Monday evening with future
shifts to rereview for extensions. Confidence: above average.
Wednesday and Thursday...no marine headlines anticipated. Northwest
to north winds gusting to 15 kt Wednesday becoming east to southeast
Thursday. Confidence: average.
Thursday night...southeast to south winds could increase above Small Craft Advisory
criteria on the Atlantic coastal waters late. Confidence: average
Friday...no marine headlines anticipated at this time. Northwest
wind gusts to 15 or 20 kt. Confidence: average
our forecast indicates October should average at least 2 degrees
above normal for phl and Abe. Some of the recent forecasts have
been colder and that may mean phl and Abe slip to around 20th
warmest October on record, which is only noteworthy in that the
excessive monthly warmth of July, August, September eased in
October. Will reevaluate tomorrow and Tuesday.
Marine...Small Craft Advisory until midnight EDT Monday night for