Scientific Forecaster Discussion
fxus61 kphi 201721
Area forecast discussion
National Weather Service Mount Holly New Jersey
1221 PM EST Wed Nov 20 2019
low pressure will approach the Canadian Maritimes late today,
meanwhile high pressure near the Ohio Valley will build eastward
through tonight. High pressure slides across our area during
Thursday before moving offshore Thursday night, then a cold front
will move through on Friday. Low pressure will move quickly from the
mid-south on Friday night to the Canadian Maritimes by Sunday night,
then high pressure returns to the mid-Atlantic early next week.
Near term /through tonight/...
for the 1230 PM update, the axis of a strong upper-level trough
continues to shift to our east. This feature will continue to
support strengthening low pressure as it tracks toward the
Canadian Maritimes through late today. The cloud cover is
gradually thinning out as drier air advects southeastward and
heating mixes some of the moist layer out. No significant
changes were made with this update, and tweaked the hourly
temperature, dew point and wind grids based on the current
Otherwise, as high pressure builds toward the eastern U.S. Today,
the pressure gradient will increase for some breezy northwest winds.
Decent cold air advection will occur across the area today, but
directions are also favorable for some downsloping, which should
mitigate the cooling effects to some degree. Additionally, the
source region of the incoming surface high is not cold, so
temperatures will not stray too much from seasonal averages today.
Operational/statistical guidance is (are) in reasonable agreement,
so used a blend (including the previous forecast) for highs today.
This should result in temperatures in the 40s for areas north of the
I-295/I-195 corridors and low 50s south of there. However, with
gusts of 20 to 30 mph this afternoon, it will feel a little chilly.
Winds will diminish somewhat tonight as high pressure continues
its approach to the area; however, this will be a gradual
process. Skies will be clearing as large-scale subsidence
approaches the region. The question will be if the winds
diminish enough for more ideal radiational cooling. Current
thinking is that this may indeed be the case for the sheltered
valleys north and west of the urban corridor, but may not for
the urban heat island and points south and east. Latest forecast
incorporates some of the colder guidance northwest of the fall
line but warmer guidance south and east. It is quite possible
the forecast temperatures are not cold enough and warm enough in
these respective regions.
Short term /Thursday through Thursday night/...
a surface ridge will be moving through the region on Thursday
with northwest midlevel flow upstream of the departing large-
scale trough. A surface low will be translating northeastward
from the Central Plains to the Great Lakes and adjacent southern
Ontario during the period, with downstream (and transient)
ridging moving into the mid-Atlantic. This should keep things
generally quiet across the area during the day, with warm
advection increasing after the surface ridge moves through the
area. Cannot rule out some spotty showers in the Poconos as the
surface low races toward the Saint Lawrence valley late in the
night, but think the stronger lift and associated precipitation
will stay mostly north of the region.
Guidance is in reasonable agreement that temperatures will be
near seasonal averages on Thursday, but temperatures will likely
be a little warmer on Thursday night from the previous night as
warm advection and resultant cloud cover increase.
Long term /Friday through Tuesday/...
main concerns in the medium-range period are a weak frontal
passage on Friday and a more potent system affecting the area
A surface low will be racing eastward through southeast Canada
and far northern New England on Friday, with an attendant front
weakening as it impinges on downstream ridging in the western
Atlantic. With the strongest lift well north of the area, think
precipitation coverage will be spotty or scattered with
precipitation amounts light. Warm advection in advance of the
front will keep thermal profiles much too warm for anything
other than liquid precipitation. Best chances for precipitation
are during the afternoon on Friday, but did lower pops
considerably from the previous forecast given the dry-looking
model output in general. Temperatures will be well above
seasonal averages on Friday, but should lower quickly on Friday
night given a transient high moving through the area (and
rapidly clearing skies). Went colder than guidance on Friday
night given concerns for fairly decent radiational cooling.
Attention turns to a southern-stream system ejecting from the
Desert Southwest late this week. As it progresses into the
central/Southern Plains on Friday, a surface low will develop
and lift quickly east-northeast into the mid-south by Friday
night. Models generally indicate some phasing with a digging
northern-stream vort Max in vicinity of the upper Midwest/Great
Lakes on Saturday, which will act to intensify the large-scale
lift and the attendant surface low as it approaches the central
Appalachians Saturday night. Widespread rain should develop in
the warm-advection regime downstream of the low. As phasing
continues on Sunday, there are indications of secondary-low
development in the mid-Atlantic, with fairly decent quantitative precipitation forecast
generated by the strong/intensifying lift via differential
cyclonic vorticity advection and warm advection. Consensus of
model timing appears to be Saturday night, though there remains
some lingering discrepancies among the deterministic model
output. Notably, the speed of the system should prevent quantitative precipitation forecast from
getting too extreme, but widespread 0.5-1.0 inch totals appear
Expecting a large chunk of the precipitation to be liquid, but
there is some concern about the far northern County Warning Area. Models look a
little too warm on the northern fringes of the precipitation
shield (and tend to be in the Poconos anyway). Used temperatures
a little below guidance for Saturday night (closer to the CMC
vs. The gfs), which generates a mix of precipitation
near/north of I-80 given the forecast timing of the low's
passage through the area. Will need to watch this system
closely, as colder-than-projected forecasts would suggest decent
potential for some wintry precipitation on the northern
periphery of the precipitation shield.
Some wraparound precipitation may linger into Sunday, but pops
rapidly decrease during the day as the low lifts well northeast
of the area. However, Sunday may end up warmer than Saturday
given favorable downsloping and improving skies.
High pressure builds into the area early next week, which should
result in mostly dry conditions and warming temperatures.
Aviation /17z Wednesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
This afternoon...VFR with ceilings 6000-15000 feet. Northwest
winds 10-15 knots with gusts 20-25 knots. High confidence.
Tonight...VFR with clouds dissipating. Northwest winds diminishing
to 5-10 knots. High confidence.
Thursday...VFR with northwest winds 5 to 15 kt becoming southwest
late. High confidence.
Thursday night and Friday...restrictions (mainly brief) possible
with a chance of showers. Southwest winds up to 10 kt on
Thursday night increasing to 10 to 15 kt with higher gusts on
Friday. A switch to west or northwest winds is anticipated late
in the day as a cold front moves through. Moderate confidence.
Friday night...mainly VFR with light winds. Moderate confidence.
Saturday...light/variable winds becoming east or southeast late
in the day. Increasing cloudiness, with cigs
lowering/approaching MVFR thresholds late in the day. Low
Saturday night and Sunday...restrictions likely with rain
Saturday night, possibly lingering into Sunday morning.
Improvement is expected by afternoon. Winds backing from east to
north Saturday night (generally light) becoming northwest and
increasing to 10 to 15 kt with higher gusts on Sunday. Moderate
to high confidence on overall evolution; low confidence on
Small Craft Advisory remains in effect for Delaware Bay this
afternoon/this evening and for the Atlantic waters this
afternoon through the overnight hours.
Northwest winds ramping up to around or slightly above advisory
criteria this afternoon as the pressure gradient increases
between low pressure well offshore and high pressure moving into
the eastern U.S. The strongest winds are expected late today
into this evening before diminishing by daybreak Thursday.
Conditions are expected to be sub-advisory on Delaware Bay after
Seas are expected to be around 2 to 4 feet, possibly maxing out
around 5 feet on the open waters well off the coast.
Thursday and Thursday night... sub-advisory conditions
Friday and Friday night...marginal advisory conditions possible
as southwest winds increase in advance of a cold front, becoming
northwest by Friday night. Winds should diminish by Saturday
Saturday...sub-advisory conditions expected.
Saturday night...advisory conditions possible with a good chance
of rain and associated visibility restrictions.
Sunday...advisory conditions possible, with rain generally
Marine...Small Craft Advisory until 7 am EST Thursday for anz450>455.
Small Craft Advisory until midnight EST tonight for anz430-431.