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FXUS61 KPHI 200907

Area Forecast Discussion
National Weather Service Mount Holly NJ
407 AM EST Wed Nov 20 2019

High pressure will migrate slowly through the eastern U.S. through 
Thursday before shifting offshore Thursday night. A cold front will 
move through the area on Friday. A surface low will move quickly 
from the Mid-South on Friday night to the Canadian Maritimes by 
Sunday night. High pressure will return to the Mid-Atlantic early 
next week.


A large-scale trough is moving through the Northeast at this
time, with embedded vorticity maxima moving eastward through and
east of the Mid-Atlantic. A predecessor vort max was migrating
well offshore, with substantial lift aiding in the development
of a strong surface low well east of the Virginia coast. A
kicker vort max was digging into the southern Mid-Atlantic,
which will force the leading perturbation northeastward, taking
the ocean storm with it. The core of the large-scale trough will
move through the northern Mid-Atlantic late tonight through
around midday, with lift/moisture sufficient for a midlevel
cloud deck through much of this period.

As high pressure builds into the eastern U.S. today, the
pressure gradient will increase for some breezy northwest winds.
Decent cold 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.


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.


Main concerns in the medium-range period are a weak frontal
passage on Friday and a more potent system affecting the area
this weekend.

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 QPF 
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 QPF 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 CWA. 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.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas. 

Rest of tonight...VFR with CIGs lowering to around 5000 feet or
so by daybreak. Light northwest winds. High confidence.

Wednesday...VFR with CIGs around 5000 feet for much of the day.
Northwest winds increasing to 10 to 15 kt with higher gusts.
High confidence.

Wednesday night...VFR with clearing skies. Northwest winds
diminishing to around 5 to 10 kt. 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


No changes to the small craft advisory in effect for Delaware
Bay this afternoon/this evening and for the Atlantic waters this
afternoon through the overnight hours.

Light northwest winds early this morning will ramp up to around
or slightly above advisory criteria by 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 midnight.

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
moving out.


MARINE...Small Craft Advisory from 11 AM this morning to 7 AM EST 
     Thursday for ANZ450>455.
     Small Craft Advisory from 11 AM this morning to midnight EST 
     tonight for ANZ430-431.


Near Term...CMS

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