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000 
FXUS61 KPHI 110828
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
328 AM EST Tue Dec 11 2018

.SYNOPSIS...
Surface high pressure over northern New England moves offshore 
today. Meanwhile, weak surface low pressure will move through 
the Great Lakes before moving across New England tonight. This 
will drag a cold front through the region late tonight through 
Wednesday morning. High pressure builds northeast of the region 
Wednesday night through Thursday night. Coastal low pressure 
will impact the area this weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Visibilities are lowering somewhat (between 6 and 9 miles) in 
Delmarva and portions of southern New Jersey, where temperatures 
have crashed under clearing skies and slightly higher dew points 
(than farther north). Will be monitoring observations closely the 
rest of tonight and may need to include patchy fog in later updates 
should this become more prevalent.

The synoptic pattern for today features a southern-stream vort max 
moving off the coast while a northern-stream perturbation digs 
southeast from southern Ontario into New York/Pennsylvania by this 
evening. Subtle midlevel ridging/descent will exist in between, 
allowing for another dry and mostly sunny day. As expected, guidance 
was too warm yesterday and has been too warm for the past several 
days. However, the surface pattern will be subtly different, with 
winds primarily light westerly or even southwesterly during the 
afternoon. Combined with considerable sunshine, suspect temperatures 
will be a little warmer today than yesterday, though not by much 
since the low-level advection appears to be mostly neutral. Forecast 
is a blend of bias-corrected statistical guidance with a manual bump 
in temperatures by about 1-2 degrees. Generally, this results in 
temperatures about 1-3 degrees warmer than Monday's highs.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
The northern-stream vort max moves into the northern Mid-
Atlantic and Northeast tonight, and weak ascent will spread into
the region as a result. Clouds should increase overnight, 
though probably not early enough to result in another evening 
crash in temperatures. Nevertheless, lows may occur somewhat 
earlier in the night, especially in the north/west, where 
increasing cloudiness should occur earlier. Operational and 
high-resolution models do not generate much precipitation 
associated with the vort max, keeping light QPF to our 
northwest. However, models generally underplay the ability for 
these systems to produce some (snow) showers, so I did add a 
mention of flurries to the Poconos and far northwest New Jersey 
late tonight. So far, not expecting anything measurable, so kept
PoPs under mentionable values.

Overnight lows will likely be a little warmer than those seen 
early this morning owing to the increased clouds, but given the 
ample opportunity for cooling in the evening, I only kept them a
degree or so warmer than the lows seen this morning for now. 
Confidence is a little lower than average, though, given the 
somewhat uncertain sky forecast.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
On Wednesday, the northern Mid-Atlantic will be well upstream 
of a cold front moving through the area tonight. Transient 
midlevel ridging will ensue, and this should allow for another 
dry but cold day. MOS is slightly warmer for max temps on 
Wednesday (versus today), but given weak cold advection on the 
upstream side of the front, I am somewhat skeptical. I held 
temperatures fairly close to Tuesday's values, somewhat below 
statistical consensus.

By 00z Thursday, a vort max should be working through the Great
Lakes/Ohio Valley, but it will be approaching confluent 
midlevel flow as it continues eastward. The vort max and its 
attendant (weak) surface low will weaken with time, but residual
large- scale ascent via low-level warm advection will generate 
some light precipitation near the surface low Wednesday night. 
By late in the night, much of this precipitation should be 
across western/central Pennsylvania and surrounding locations. 
Though the low and precip will exhibit increased disorganization
with time, operational models are starting to converge on a 
scenario in which some snow showers occur during the day across 
portions of the area (primarily north of the Mason- Dixon Line).
The 00z NAM is much farther north with the vort max but also 
considerably stronger with the downstream ascent, allowing for 
fairly prolonged precipitation in the northern CWA during the 
day. Cannot entirely discount this idea owing to recent runs of 
the CMC/GFS depicting similar solutions (though without much 
lift/precip). The ECMWF, however, has been far more consistent 
the past 24 hours and was used as a starting point for the 
forecast.

With this in mind, retained PoPs in much of the area Thursday 
and increased them to chance PoPs across the northern third or 
so of the CWA while removing them from central/southern Delmarva
given the lack of model support for a farther south solution at
this time. Temperatures will be cold enough for mostly snow in 
the morning but may warm sufficiently in the afternoon for 
either rain or snow in the urban corridor eastward. Amounts 
would be light (generally well under an inch), and there is 
still quite a bit of uncertainty on overall coverage. Fine-
tuning of this "nuisance-looking" event will continue in later 
forecasts given the weak nature of the system and the continued 
model spread regarding the track of the vort max/surface low.

Lowered temperatures a few degrees on Thursday given the 
expected passage of the vort max, as the statistical guidance 
looked much too warm in such a regime. Forecast may still be too
warm, given the latest MET guidance.

Weak low pressure passes off the Mid-Atlantic coast Thursday 
evening. Weak high pressure then builds into the Northeast for 
late Thursday night and Friday, then moves offshore Friday 
night. 

Deep closed upper level low pressure will move through the Gulf
Coast states, and strong surface low pressure develops out 
ahead of it over the Southeast and Mid-Atlantic states. A warm 
front lifts north along the coast and into the region on Friday,
and some light precip will develop with its passage. Initially,
precip may be snow or a rain/snow mix, but precip turns to rain
by Friday afternoon. QPF amounts will be light. 

Deepening low pressure then lifts along the coast and passes 
through the region Friday night and Saturday. Rain may be heavy 
at times, especially from after midnight Friday night through 
Saturday morning. Hydrologic impacts are possible during this 
time. 

There are some quite differences between the ECMWF/GFS/CMC-
GDPS, so for now, will carry highest PoPs Friday night and 
Saturday, and then will carry chance PoPs Saturday night. 

Deep upper trough passes north of the region Saturday night, 
and this will push the surface low out to sea. Meanwhile, the 
closed upper low meanders over the Gulf Coast and Southeast 
states before finally moving offshore on Monday. High pressure 
builds in from the west for the start of the new week.

&&

.AVIATION /09Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas.

Today...VFR with light northerly winds becoming west 5-10 kts 
during the afternoon. Generally high confidence; however, may 
see SCT or even BKN CIGs around 3500-5000 feet develop at 
RDG/ABE during peak heating, but this is far from certain. 

Tonight...Mainly VFR, but CIGs 3500-7000 feet are expected to 
develop after midnight from northwest to southeast. Cannot rule 
out some MVFR CIGs at RDG/ABE, though this is too low of a 
probability for mention in the TAFs at this time. Winds 
generally light and variable, though west winds may stay more 
elevated at KPHL. Low to moderate confidence in CIGs; high 
confidence in winds.

Outlook... 

Wednesday...VFR with northwest winds 7 to 15 kts with a few 
gusts to 20 kts or so possible. High confidence.

Wednesday night...Increasing cloudiness with CIGs approaching 
MVFR thresholds late. Light/variable winds may acquire a more 
easterly component by sunrise. Moderate confidence.

Thursday...Sub-VFR conditions possible with a chance of light 
snow showers, especially in the morning. East winds 5 to 15 kts,
perhaps with a few gusts to 20 kts near the coast. Low 
confidence.

Thursday night...Mainly VFR conditions.

Friday...Deteriorating conditions. Light precip possible, with 
potential for snow initially, then rain by Friday afternoon.

Friday night and Saturday...MVFR/IFR in moderate to heavy rain at 
times. 

&&

.MARINE...
Winds are well below criteria across the waters at this time, 
but seas have been slow to subside at buoy 44009 (now between 6 
and 7 feet). Currently expecting seas to trend downward this 
morning, so the small craft advisory for the southern NJ and DE 
Atlantic coastal waters continues until 10 am.

After a lull in winds/seas this afternoon and this evening, 
northwest winds will increase after midnight as a weak cold 
front moves into/through the area. Another brief period of 
advisory conditions is expected on all waters by this time. Will
likely hoist an advisory later this morning for late Tuesday 
night into Wednesday morning, but will hold off on issuing this 
advisory until the current one is done (and we have one last 
look at newer model guidance).

Outlook... 

Wednesday morning...Residual advisory-level northwest winds 
possible.

Wednesday afternoon through Thursday...Sub-advisory winds/seas 
expected. A chance of rain/snow showers on Thursday.

Thursday night...Sub-SCA winds expected. Seas on the ocean may 
build to around 5 feet late.

Friday...Southeast winds will increase through the day and may 
approach advisory levels late. Increasing chances for rain.

Friday night and Saturday...SCA conditions likely with gales 
possible late Saturday. Mainly southeast winds.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 10 AM EST this morning for 
     ANZ453>455.

&&

$$
Synopsis...MPS
Near Term...CMS

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