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FXUS61 KPHI 051437

Area Forecast Discussion
National Weather Service Mount Holly NJ
937 AM EST Thu Dec 5 2019

High pressure will build into the region through early Friday as a 
weakening area of low pressure retreats northeast into the Canadian 
Maritimes. A strong, but moisture-limited cold front will sweep 
across the region Friday night into Saturday, before high pressure 
builds into the region for the weekend. By Sunday afternoon, a warm 
front will lift northward across the region ahead of a larger storm 
system and attendant cold front that is poised to affect our region 
through the beginning of next week. This cold front is forecast to 
cross the mid-Atlantic Tuesday, with perhaps a secondary cold front 
passing through Wednesday.


The mid-morning update only extended the pops/wx for snow
showers across the Poconos into the morning later. Clouds were
also adjusted somewhat. 

Previous discussion...

A fairly potent shortwave trough is moving through the Northeast 
early this morning. Despite a rather moisture-starved atmosphere, 
sufficient lift has been generated by the vort max to continue to 
produce some scattered/occasional snow showers in the Poconos 
overnight, and these will likely linger through daybreak or so 
before the ascent associated with the trough moves east of the area. 
Meanwhile, cold/dry advection on the upstream side of the trough in 
the low levels is slowly but surely scouring lower clouds in much of 
the rest of the area. This process should continue through the early 
morning, but will quickly be countered by strong mixing as daytime 
heating begins. Should see a midlevel cloud deck develop during the 
day before diminishing late in the day, which will impede insolation 
and warming to some degree.

Regarding aforementioned mixing, expect northwest winds to increase 
today as the pressure gradient generated by the passing shortwave 
trough (via low pressure to the northeast and building high pressure 
to the southwest) increases. Wind speeds of 15 to 25 mph will be 
common, with gusts to 30+ mph or so possible. 

I went slightly lower than consensus guidance for highs today given 
potential for cloud cover to inhibit diabatic heating. This resulted 
in highs in the 30s north of I-80 and in the 40s elsewhere.


Expect the area to be between systems tonight, as the next northwest-
flow shortwave trough moves through the Great Lakes region during 
the period. A surface ridge axis will move through the northern Mid-
Atlantic overnight, which should allow for quickly diminishing winds 
after sunset. This may promote a fairly rapid decrease in 
temperatures during the evening (if the winds decrease quickly 
enough), but this cooling will be stunted overnight by increasing 
mid and high clouds in advance of the Great Lakes system approaching 
the area. As a result, expect temperatures to stall after midnight 
as clouds inhibit radiational cooling.

The temperature forecast is tricky, as readings will be quite 
sensitive to the winds and cloud cover. I was tempted to go somewhat 
above guidance, but I do think there will be a period of fairly 
efficient cooling, so I was not confident to stray too much from 
consensus at this point. I did, however, modify hourly temperatures 
to exhibit a quick drop during the evening and very little change 
after midnight. Forecast lows should be near freezing in the urban 
corridor and at the beaches, and in the 20s elsewhere.

By late in the night, the large-scale lift generated by the 
approaching vort max may reach the Poconos, with generation of 
flurries/snow showers possible by around daybreak. Models appear to 
hold off timing until later Friday morning, but would not be 
surprised to see observations of snow showers by the end of the 


Synoptic Overview...A weak shortwave perturbation will drive a 
clipper-like surface low across the Great Lakes and into the 
mid- Atlantic Friday. With limited moisture, only scattered 
showers and snow showers are anticipated with this system. Zonal
flow will largely dominate the eastern two thirds of the CONUS 
over the weekend as a larger synoptic trough begins to dig into 
the western third of the country. High pressure will begin to 
build across the Northeast Saturday and Saturday night, exiting 
the region Sunday as the aforementioned system draws a warm 
front northward across the mid-Atlantic. A significant warm up 
looks to be in store after the warm front lifts northward, 
though bouts of rain will work against this. The system will 
take its time crossing our region, with the cold front not 
forecast to move offshore until Tuesday evening. High pressure 
looks to build eastward out of the Tennessee Valley Wednesday 
into Thursday as the system pulls off to the north and east. 
There is the possibility a weak secondary cold front may push 
far enough south to bring a few scattered snow showers Wednesday
night into Thursday, though the details still remain somewhat 
uncertain with this. 

Friday...a shortwave trough will slip across the Great Lakes and 
into New England, deepening as it does so by Friday night. A weak 
surface low will slip across the PA/NY border, eventually making its 
way across the northern portions of our CWA Friday night. The main 
area of forcing for ascent still looks to remain north of our CWA, 
however, a few light snow showers with light accumulations will be 
possible across the Poconos and NW NJ. The lack of moisture should 
keep the rest of our area mostly dry through Saturday. Highs should 
range from the mid 30s to near 50 across Delmarva Friday and from 
the upper 20s to low 40s Saturday behind this system. 

A fairly strong surface high will quickly build across the Ohio 
Valley and translate into the Northeast into Sunday, leading to 
warmer weather as a warm front lifts northward across the mid-
Atlantic and return flow ushers in warmer, southwesterly flow. Highs 
look to range from the upper 30s to the lower 50s. 

As the synoptic trough across the Inner-mountain West begins to 
shift eastward, a strong surface low will begin to develop across 
the middle of the country. Several rounds of rain look to move out 
of the Ohio Valley and into the mid-Atlantic as a series of vort 
maxima roll eastward ahead of the evolving system. The first of 
these looks to arrive Sunday night with another Monday afternoon 
into Monday night. There is some disagreement in how quickly the 
front will ultimately move through the region with the GFS being the 
much more progressive of the two, while the EC keeps a more shy 
approach and sluggishly pushes the front through the region, not 
completing fropa until Wednesday afternoon. This could be 
problematic in terms of excessive rainfall and will need to be 
watched over the coming days to see if this trend continues. Highs 
will be quite mild ahead of this system with widespread 50s and 
possibly even 60s into Delmarva Monday and Tuesday. Slightly cooler 
highs Wednesday (depending on the timing of the front, of course) 
with upper 30s and 40s. A similar story Thursday with even cooler 
highs in the upper 20s and 30s.


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

Today...Mainly VFR, though some CIGs around 3500-7000 feet may 
develop/continue during the day. West to northwest winds 10 to 
15 kt with gusts 20-25+ kt. Moderate confidence.

Tonight...VFR with northwest winds diminishing to around or 
below 10 kt by late evening. High confidence.

Friday...generally VFR, though some rain/snow showers will be 
possible mainly near and north of ABE. MVFR CIGS also possible for 
TTN, PNE, RDG, and ABE. Southwest winds around 10 knots, becoming 
west and northwest by late in the afternoon. High confidence in 
winds; moderate confidence in restrictions.

Saturday and Sunday...VFR prevailing. Northwest winds from 5-10 
knots becoming light and variable Saturday night. Southerly winds 
from 5 to 10 knots Sunday. 

Monday...Sub-VFR conditions probable as showers move into the region 
early Monday morning. South-southwest winds from 10-15 knots. 
Moderate confidence.

Tuesday...Sub-VFR conditions, especially in the morning with showers 
ahead of a cold front that will pass through the region in PM. 
Southwesterly winds from 10 to 15 knots with gusts from 20-25 knots 
turning westerly behind the cold front Tuesday night. Low confidence.


A decent pressure gradient has set up across the waters behind the
departing potent midlevel system this morning. The strong northwest
winds will continue on the waters today. Gusts have already reached
advisory criteria on the open waters and on Delaware Bay. Advisory
conditions are expected to continue for Delaware Bay until early
this evening. Here, the advisory expires at 7 pm. On the Atlantic
waters, advisory conditions should persist through late evening,
so extended the advisory through midnight. Winds should diminish
thereafter as high pressure moves in.

Seas of 3 to 5 feet are forecast on the waters today and tonight.

Friday and Saturday...A period of Small Craft Advisory wind gusts is 
possible Friday afternoon and Friday night, then winds will begin to 
diminish into Saturday afternoon, dropping below 25 knots.  

Sunday...Sub-SCA criteria expected with light and variable to 
southwest winds below 10 knots.

Monday...Southwesterly winds increase later in the day, with gusts 
approaching 25 knots. This is less certain however given a warm air 
advection regime. Seas building to 4 to 6 feet.

Tuesday...SCA conditions likely with southwest winds from 15 to 20 
knots and gusts from 25 to 30 knots. Seas will remain elevated 
from 8 to 10 feet.


MARINE...Small Craft Advisory until midnight EST tonight for ANZ450>455.
     Small Craft Advisory until 7 PM EST this evening for ANZ430-


Near Term...CMS/po

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