Scientific Forecaster Discussion
fxus61 kphi 191137
Area forecast discussion
National Weather Service Mount Holly New Jersey
637 am EST sun Nov 19 2017
low pressure over upstate New York will lift northeast into the
Saint Lawrence valley today. The attendant cold front will move
through the mid Atlantic early this morning. High pressure will
build in from the south and west tonight into Monday before moving
off the southeast coast on Tuesday. A cold front is expected to pass
through the region late Tuesday night or Wednesday morning. High
pressure builds back into the area for Thanksgiving. The high will
eventually weaken and move offshore late in the week as a cold front
approaches from the Midwest. The front will likely move through the
mid Atlantic on Saturday.
Near term /until 6 PM this evening/...
630 am update: front is blasting through the area at this time,
with several reports of 40 to 50 mph wind gusts and downed
trees/power lines coming in near/behind the front. Showers are
rapidly moving along/ahead of it and should be offshore within
an hour or two.
Changes to forecast include tweaks to hourly temperatures and
dew points to match trends/frontal timing and to fine-tune wind
speeds/gusts based on upstream obs. Made considerable edits to
sky cover, as I expect fairly rapid improvement later this
Complex forecast through today as a strong cold front moves through
the area in the next few hours. Strong/deepening surface low in
central New York will progress east-northeast along/just south of the
Saint Lawrence River during the next few hours, with an equatorward-
extending cold front racing eastward through the mid-Atlantic, aided
by strong pressure rises upstream. Showers continue to develop in
the pre-frontal warm sector in the northern mid-Atlantic, with
widespread slab-like lift along the cold front itself contributing
to low-topped convection in central PA. Given the very strong winds
off the surface (850-mb winds 50-75 kts downstream of the front),
these showers will not have a difficult time transporting these
winds to the surface, particularly with the added effects of water
loading and evaporative cooling below the cloud bases. As a result,
pushed up the Wind Advisory for the entire cwa, which started at 1
am. Several gusts of 40 to 50 mph have been reported so far, with
the reports generally clustered in the Lehigh Valley, the Delmarva
Peninsula, and the immediate coast of southern New Jersey so far.
The hrrr is strongly indicative of two waves of stronger wind gusts
through the morning hours. The first is associated with the frontal
band of showers moving through the area between now and 12z (and
perhaps with showers in advance of this band), and the second is
after cold frontal passage, tied strongly to the 2-3 hour duration
of strongest pressure rises immediately upstream of the front. Gusts
of 40 to 50 mph are likely after frontal passage, with scattered
power outages and downed trees expected. The strongest winds will
ease only slowly during the late morning and afternoon hours, as
strong boundary-layer mixing will continue, aided by cold-air
advection and increasing insolation with time.
Meanwhile, the showers should quickly exit stage east early this
morning, with quantitative precipitation forecast generally in the quarter to half inch range for
the overnight and early morning hours today north of the Mason-Dixon
line and perhaps a tenth of an inch or so in far southern portions
of the County Warning Area.
Temperatures have risen slowly in the pre-frontal warm sector,
though there has been sharp cooling in the Rural Valley locations
thanks to higher clouds earlier this past evening. Guidance is
handling this poorly, to say the least, and have had to make several
edits to hourly temperatures already (with more likely to come
before frontal passage). Thereafter, temperatures will likely begin
a slow fall through the day, though increasing insolation and strong
mixing may curtail this to some degree. General forecast is strongly
weighted to hi-res 2-M temperatures, given the unsurprisingly poor
performance of statistical guidance overnight
Short term /6 PM this evening through 6 am Monday/...
main question tonight is the potential for some snow showers in the
Poconos. With a strongly mixed boundary layer and saturation around
900-850 mb, there are indications that snow showers will develop
periodically to the northwest, occasionally progressing into the
southern Poconos. The best time window for this looks to be late
this evening and overnight as the deep-layer flow becomes more
northwesterly (more favorable fetch for lake-effect "streamers").
Additionally, the main vort Max associated with today's system will
be moving through Sunday night, which will supply additional lift
and cold air aloft to aid in the development of a shallow (nearly)
dry-adiabatic boundary layer in the Post-frontal environment. Have
kept and even prolonged slight-chances or chances of snow showers in
this region, with potential for snow squalls given the strong low-
level wind field and favorable low-level thermodynamic profile.
Meanwhile, temperatures will turn colder across the area, though the
bottom will not fall out given the continued elevated winds (though
much lighter than during the day). Current forecast is a mix of
mav/met/ecs MOS, which are in reasonable agreement, and features
temperatures generally a few degrees below seasonal averages.
Long term /Monday through Saturday/...
a rather tranquil and predominately cool weather pattern is in
store for next week.
Monday and Monday night... high pressure builds to our south across
the southeast states on Monday. Westerly winds will still be breezy
(gusts 20-30 mph) owing a tight pressure gradient to the north of
the surface high but not nearly as strong as today. Cold air advection thermal
pattern in the low levels transitions to neutral Monday morning and
eventually even to a warm air advection pattern once the cold pool progress
downstream of the area late afternoon and night. Dry and cool
conditions can be expected with high temps 6-9f below normal and
lows 3-6 below normal.
Tuesday... high pressure moves off the eastern Seaboard. Southerly
return flow around the high will draw milder air into the region.
Highs return to near normal. Winds will once again be breezy,
especially during peak afternoon heating with gusts 20-30 mph.
Tuesday night and Wednesday... a cold front will approach from the
Tuesday evening before moving through the forecast area late Tuesday
night into early Wednesday morning. Models continue to hint at a
band of rain developing over the mid-Atlantic region Tuesday night
in response to frontogenetical lift that occurs in between a
northern stream shortwave trough and a southern stream disturbance
that is tracking off the southeast coast. These systems do not
appear to phase in time to bring more significant rains to our area,
instead provide US with a glancing blow. Pops and quantitative precipitation forecast increase from
northwest to southeast across the forecast area with models in
agreement that the better dynamics pass near the coast. Rain is
forecast to end from west to east during the first half of the
morning as drier air arrives in wake of fropa. Similar to what
happens today, temperatures initially in the 40s and 50s will
struggle to rise much during the day Wednesday as cold air advection offsets
Wednesday night through Friday...quiet weather is in store for
Thanksgiving with high pressure building overhead and even into
black Friday when the high starts to retreat offshore. Below normal
temperatures return with highs in the lower to mid 40s on Thursday
and mid 40s (n/west of the fall line) to lower 50s (s/east of
Friday night and Saturday...a cold front will approach from the
Midwest Friday night and then move through the region sometime on
Saturday. The setup with this system is similar to the predecessor
one Tuesday night/Wednesday morning system where both northern and
southern stream systems fail to phase in time to produce a
significant rainfall event across the forecast area. The airmass
moderates to within a few degrees of climo in pre-frontal warm
sector on Saturday before colder air moves back into the region
later in the weekend.
Aviation /12z Sunday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Today...rapid improvement to VFR with showers quickly exiting
the area early in the morning. Very strong west or west-
northwest winds 20 to 30 kts with gusts to 40 kts or so are
likely through the day, with the strongest winds generally in
the morning. Confidence is well above average.
Tonight...VFR with west winds slowly diminishing to around 10
kts after midnight. Gusts to 25 kts or so possible through the
evening hours. Confidence above average.
Monday through Tuesday...VFR. West winds 10-15 kt with gusts around 25
kt on Monday become SW and weaken to under 10 kt Monday night. SW
winds increase on Tuesday to around 10 kt with gusts to 20 kt during
the afternoon. Forecast confidence: high.
Tuesday night and Wednesday...mainly VFR. However, the chance for
rain increases late Tuesday night into early Wednesday morning,
especially closer to the coast. Cannot rule out MVFR cigs if
steadier rain develops along and especially east of I-95 terminals
between 06-12z Wednesday. Wind shift from SW to northwest will in wake of
cold fropa early Wednesday. Northwest winds gusts to around kt on
Wednesday. Forecast confidence: medium overall; low for MVFR cigs.
Wednesday night and Thursday...VFR. Northwest winds gradually diminishing.
A storm system is currently expected to remain offshore. Forecast
confidence: medium to high.
gale conditions are generally occurring across the waters at this
time, with the strongest winds/gusts expected to occur just after
frontal passage from about 6 am to noon. Winds 40-45 kts are likely
during this time window. Very choppy/erratic seas are likely through
Showers will continue through daybreak, with even a rumble of
thunder possible, but should move out quickly by mid to late
Monday and Monday night...Small Craft Advisory conditions likely with northwest winds
gusting to 25-30 knots around daybreak, gradually decreasing
throughout the day. Seas in the coastal waters will also
subside, starting out at 4-5 ft in the morning, then becoming
2-4 ft by late afternoon.
Monday night...no marine headlines anticipated. Northwest winds drop below
25 kt and seas decrease to 2-3 ft.
Tuesday and Tuesday night...Small Craft Advisory likely beginning late morning and
continuing through the evening. SW winds increase to 15-20 kt with
gusts 25-30 kt gusts. Seas in the ocean waters are forecast to peak
in the 5-6 ft range by early Tuesday evening. Winds and seas
a low water advisory was issued for the Delaware Bay north of a
line from East Point, New Jersey to Slaughter Beach, Delaware (anz430 marine
zone). This was based on the latest total water level forecast
of around -2 ft MLLW near high tide late this afternoon-early
evening. A more westerly wind direction (vs northwesterly) of
the strong winds should limit The Blow Out potential in the
tidal portion of the Delaware River (including philadelphia)
PA...Wind Advisory until 4 PM EST this afternoon for paz054-055-
New Jersey...Wind Advisory until 4 PM EST this afternoon for njz001-007>010-
Delaware...Wind Advisory until 4 PM EST this afternoon for dez001>004.
Maryland...Wind Advisory until 4 PM EST this afternoon for mdz008-012-015-
Marine...Gale Warning until 5 am EST Monday for anz430-431-450>455.
Low water advisory from 2 PM this afternoon to 7 PM EST this
evening for anz430.