Scientific Forecaster Discussion
fxus61 kphi 210205 cca
Area forecast discussion...corrected
National Weather Service Mount Holly New Jersey
905 PM EST Wed Nov 20 2019
high pressure will continue building to the south of the forecast
area through Thursday. Low pressure will pass from the Great Lakes
toward the Canadian Maritimes Thursday through late Friday pushing a
cold front across our region on Friday. Low pressure will develop
across the southern Appalachians and pass just to the south of the
forecast area and offshore late Saturday into early Sunday. High
pressure then returns through early next week.
Near term /until 6 am Thursday morning/...
a strong upper-level trough now to our east will lift across the
Canadian Maritimes tonight. This will take surface low pressure with
it. Meanwhile, a ridge to our west will approach our region toward
morning which will allow for surface high pressure to also start
building into the mid-Atlantic late tonight.
A tightened pressure gradient will remain in place through tonight
given the surface high not arriving over our area until during the
day Thursday. Despite this, the gusty northwesterly winds are
expected to diminish this evening with the gusts ending as the depth
of the mixing layer shrinks. In addition, the cold air advection
will end and become weak warm air advection overnight with the
pronounced warm air advection up the Ohio Valley. Now that our
region is on the western side of the upper-level trough, increasing
subsidence and drier air will allow for the clouds to dissipate
through the overnight.
This trend continues this evening with many areas of New Jersey/Delaware
mostly clear already with clouds lingering across Berks County
and the souther Poconos. Low temperatures were not adjusted
much, but the hourly temps were changed in areas where the
clouds are lingering the most.
Short term /6 am Thursday morning through Thursday night/...
as an upper-level ridge slides across the east during the day
Thursday, surface high pressure moves across our region before
shifting offshore late in the day and at night. This will allow an
initial northwest flow to become west and southwest, however given a
more relaxed pressure gradient for much of the day the winds are
expected to be on the lighter side.
As the flow backs to more southwesterly, the strong warm air
advection up the Ohio Valley will spread eastward and arrive across
our area especially later in the day and at night. The leading edge
of this stronger warm air advection will result in a warm front
arriving Thursday night. The strongest push in tandem with some
short wave energy may result in a few showers clipping the Pocono
region into far northwestern New Jersey, with perhaps a few
sprinkles a little far southward. However, the stronger lift and
associated precipitation should stay north of our region. The warm
air advection will result in increasing and gradually lowering
clouds especially Thursday night. Temperatures will climb into the
lower 50s for many areas Thursday afternoon, then not as cold at
night due to the warm air advection and more cloud cover.
Long term /Friday through Wednesday/...
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). The most likely
outcome of precipitation types at this point will be rain
changing to a light wintry mix of snow/sleet/freezing rain
across the highest elevations of the southern Poconos and northwest New Jersey
on Saturday night which would change over to all snow or a
rain/snow mix toward daybreak as the system departs. 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
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 is forecast to be slightly 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 through
Aviation /02z Thursday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Tonight...VFR with clouds dissipating. Northwest winds diminishing
to 10 knots or less, becoming locally light and variable late. High
Thursday and Thursday night...VFR with ceilings developing mainly
late in the day and at night, lowering to about 5000-10000 feet at
night. Northwest winds 5-10 knots becoming southwest in the
afternoon, then becoming locally light and variable at night.
Friday...restrictions (mainly brief) possible. Isolated rain
showers overspreading the region throughout the day ahead of a
cold front. Southerly winds becoming west late around 10 kts
with gusts up to 20 kts. Moderate confidence.
Friday night...VFR expected with northwest winds around 5 kts.
Saturday and Saturday night...VFR initially with MVFR
developing late in the day through the overnight. Periods of IFR
possible. Rain overspreading the region late in the day and
through the overnight hours. Winds 5 kts or less initially west
becoming variable. Low confidence.
Sunday...lingering sub-VFR restrictions likely in the morning.
Clearing skies throughout the day. Winds becoming northwest
around 10 kts with higher gusts. Low confidence on timing.
we will drop the Small Craft Advisory for the Delaware Bay at
930 PM. The winds continue to diminish and gusts have dropped
below 25 kts attm. Fair weather expected overnight.
The Small Craft Advisory remains in effect through the overnight
hours for the Atlantic coastal waters. Cold air advection and
deeper mixing is resulting in wind gusts in the 25-30 knot range
across the ocean waters. This will continue through tonight,
however the cold air advection wanes overnight therefore the
mixing is expected to weaken with time.
For Thursday, surface high pressure slides across our area before
shifting offshore late in the day and at night. This will result in
less wind for a time but also a wind shift from northwest to
southwest. The flow then increases during Thursday night, however
this is within a warm air advection regime. The southwesterly winds
though may gust to advisory level especially toward Friday morning.
Given the ongoing advisory and also lower confidence with the amount
of mixing within the warm air advection regime, held off on a new
one at this time.
Friday and Friday night...winds and seas will possibly reach
Small Craft Advisory criteria with winds gusting 20-25 kts and seas possibly
building to near 5 feet throughout the day. Winds and seas will
diminish slowly toward daybreak Saturday.
Saturday...sub-sca conditions expected.
Saturday night...marginal Small Craft Advisory conditions developing with rain.
Sunday...marginal Small Craft Advisory conditions diminishing toward the end of
the day. Rain ending early.
Marine...Small Craft Advisory until 7 am EST Thursday for anz450>455.