Scientific Forecaster Discussion
fxus61 kphi 160647
Area forecast discussion
National Weather Service Mount Holly New Jersey
247 am EDT Mon Sep 16 2019
a cold front will stall south of our area through today while
high pressure weakens to our west. A weak area of low pressure
slides near our region on Monday. High pressure builds across
the northeast Tuesday into Wednesday, then shifts southward
across the mid-Atlantic region Thursday. The high is forecast to
shift to our south Friday, then offshore next weekend.
Near term /until 6 PM this evening/...
245 am update: dense fog has spread into Talbot and Caroline
counties in Maryland and Kent County in Delaware. Have expanded
the advisory to include these areas. Fog has become patchier in
New Jersey than observed earlier, possibly owing to a weak
shortwave moving through the area at this time. Still seeing
local observations down to a quarter mile, so will keep advisory
going here for now. However, may be able to cancel before
sunrise for the New Jersey portion.
Dense fog has set in along the coast. Webcams and surface
observations are deteriorating, with coverage high enough at
this point to warrant a dense fog advisory. Expecting the fog to
continue (though with variations in density and coverage)
through the overnight but to dissipate quickly after sunrise.
The fog appears to stretch from Ocean County southward to Sussex
County in Delaware, but may spread further inland in Delmarva
(at least, according to hi-res model simulations). Notably, the
models are doing a rather poor job with the timing and evolution
of the low clouds and fog, so not overly confident on spatial
extent and overall duration.
The other concern today is a low-amplitude northwest-flow
perturbation moving through the northeast. Latest model
simulations depict strong enough lift to generate some showers
in the northern cwa, though likely quite light and fairly
scattered. Nevertheless, confidence is increasing with
sufficient coverage to increase pops a little bit (mainly north
of Interstate 78) today. Farther to the south, will likely
observe a sufficient period of partial sunshine to see
appreciable diabatic warming before increased/lower cloud cover
sets in. This makes the temperature forecast somewhat tricky,
with MOS showing a fair amount of spread across the region.
Incorporated a fair amount of hi-res guidance into the highs for
today, as these seem to have reasonable agreement on evolution
of clouds/precipitation today. This resulted in a forecast quite
close to continuity, perhaps a degree or two colder overall.
Other concern today is the sea breeze and the timing of the cold
front associated with the perturbation. Temperatures along the
coast will likely be much cooler than areas farther inland,
owing to the sea breeze's push onshore this afternoon.
Additionally, the cold front will bring fairly Stout cold
advection into the area immediately upstream, so should see a
fairly sharp gradient in temperatures (including highs) in the
northern County Warning Area.
Short term /6 PM this evening through 6 am Tuesday/...
cold front will continue south tonight, with large-scale ascent
possibly remaining sufficient for some showers in the southern
half of the County Warning Area. There are subtle indications of increasing
vorticity in the northwest flow overnight, which may explain why
shower coverage appears to increase south of the front after
dark. Not overly sold on this potential, but did increase pops
in Delmarva and far southern New Jersey to at least mentionable
during the evening/overnight period.
Immediately downstream of the front, may see a resurgence in low
clouds and fog. However, this potential remains rather unclear
owing to uncertainty with the frontal position and shower
coverage. As such, have held off mentioning this in the
forecast. Meanwhile, for areas north of the front, temperatures
should be colder, especially as skies clear. However, there may
be enough of a breeze to keep temperatures from going to values
suggested by the colder guidance, so kept the forecast close to
a statistical blend.
Long term /Tuesday through Sunday/...
An extended period of quiet weather looks likely over our area
for the long term. At the mid-lvls a fairly amplified ridge will
reside upstream of the area for essentially the entire long-
term period (although at times oscillating in orientation and
mean-position). At the surface the upstream ridge positioning
will promote anticyclogenesis with the initial surface high
centered north of the area. By the end of the workweek the high
will re-Orient itself south of the area shifting the low-lvl
flow from onshore to offshore with a corresponding increase in
The boundary will be largely south of the area by Tuesday
afternoon with highs generally in the 70s areawide.
Wednesday/thursday: should be dry and mild with onshore flow
prevailing. Highs will generally be near climo (low to mid 70s)
with lows in the low to mid 50s. Mostly sunny outside of some
occasional stratus (most prevalent near the coast).
Friday/saturday: the high will relocate south of the area
towards the late week-early weekend with the flow shifting more
westerly as a result. This will usher in a warming trend into
the weekend with maxes increasing into the upper 70s/lower 80s
on Friday, and then increasing further into the mid 80s on
Saturday. Precipitation chances remain essentially nil. A
mostly quiet long term period is expected, with only a small
chance of precipitation at the beginning of the period..
Aviation /07z Monday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Rest of tonight...expect periods of IFR/LIFR at Acy and miv as
low clouds and fog have moved inland much more readily than
forecast. VFR conditions should occur elsewhere. Light/variable
winds. High confidence.
Monday...mainly VFR, with cloud cover increasing/lowering from
north to south through the day. Cannot rule out a sprinkle or
shower, especially at ridge/Abe/ttn, but negligible impacts are
expected at the terminals. Light/variable winds becoming
northeast up to 10 kt as a cold front moves through the area
late in the day. Moderate confidence.
Monday night...mainly VFR, but cannot rule out some sub-VFR
conditions in advance of the southward-moving cold front at
Acy/miv during the evening. Winds generally northeast up to 10
kt. Low confidence.
Tuesday-Wednesday night...mostly VFR during the day, with a
chance of MVFR conditions overnight in stratus. East to
northeast winds 5-10 knots
Thursday-Thursday night...generally VFR conditions expected with
east-southeast winds 5-10 kts.
areas of dense fog are occurring near the coast on Delaware Bay
and the Atlantic waters. Coverage appears sufficient to warrant
a dense fog advisory through 8 am; however, conditions may be
quite variable and may improve away from the coast. The low
clouds and fog may linger through the morning, so will be
monitoring surface observations closely to determine if the
advisory needs to be extended.
Regarding winds/seas, sub-advisory conditions are expected
through tonight. However, a cold front will be moving through
during the late afternoon and evening hours. Winds will switch
to northeast, becoming 10 to 20 kt overnight. Though conditions
are expected to remain below advisory thresholds, cannot rule
out an isolated occurrence of 25 kt wind gusts around daybreak.
Tuesday-Thursday...Small Craft Advisory conditions potentially developing as
early as Tuesday but likely by Wednesday. Seas will increase to
5-7 ft by late Wednesday with northeasterly wind gusts likely
increasing to 25 kts or so as well.S
There is a low risk for the development of dangerous rip
currents at New Jersey and Delaware beaches on Monday due to
light southerly flow and waves of 2-3 feet in the surf zone.
New Jersey...dense fog advisory until 8 am EDT this morning for njz020>027.
Delaware...dense fog advisory until 8 am EDT this morning for dez002>004.
Maryland...dense fog advisory until 8 am EDT this morning for mdz019-020.
Marine...dense fog advisory until 8 am EDT this morning for anz430-431-