Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Area Forecast Discussion (AFD)

Issued by Philadelphia/Mt Holly (PHI)

                            
000
FXUS61 KPHI 180545
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1245 AM EST Fri Jan 18 2019

.SYNOPSIS...
A weak weather system will bring a light snowfall to most of the 
region tonight. A brief window of high pressure and dry weather 
settles in for later Friday and early Saturday. A significant 
storm will impact the region Saturday night and Sunday with a 
mix of snow, ice, and rain followed by a rapid freeze. Bitterly 
cold air is on tap for early next week as that storm pulls away 
and Arctic high pressure moves in. Temperatures will moderate by
the middle of next week with the potential for yet another 
storm at that time.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
The snow has arrived across many areas of the forecast area as
of 1230AM. Vsbys have dropped to less than 2 miles at some
spots. A light coating has accumulated on the grass here at the
WFO in Mount Holly. This light snow will continue through the
overnight with a light accumulation in many areas. Overall,
ongoing fcst on track, so few changes issued for the update. 

Previous discussion...
A weak upper level disturbance and associated surface low will pass 
by the area tonight. This is not a dynamically impressive system and 
it will be a fast mover within the lingering zonal split flow 
regime. However, sufficient lift and decent WAA will allow for some 
light precipitation to impact the region. 850mb temperatures look to 
remain below 0C throughout the event, so despite marginal boundary 
layer temps expect most of this to fall as snow with some mixing 
with rain possible in our far southern zones. QPF is not impressive 
and has trended downwards, between 0.1 and 0.15 inch across the 
region which is expected to translate between and and 2 inches of 
snowfall. Some localized impacts are possible with the Friday AM 
commute, but the bulk of the snow should fall before the peak 
commute hours. This combined with the accumulations being so light 
precludes a WWA issuance, but do be aware some slick spots are 
possible in the morning.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SATURDAY/...
A window of dry weather for late Friday morning into Saturday 
morning. Weak low pressure pulls away Friday morning with a slight 
chance of a lingering rain or snow shower early on in eastern NJ 
before some gradual clearing. Canadian high pressure moves in to the 
north during the day. Initially, somewhat milder air will be in 
place behind the departing low as weak trailing shortwave and 
surface cold front do not clear until around noontime. This will 
mean high temperatures in the upper 30s and low 40s. However, as the 
high to our northwest moves closer later Friday and Friday night, 
gradual CAA will begin to cool temperatures aloft and at the 
surface. Partial clearing and light winds should allow for decent 
radiational cooling Friday night with temps dropping back down 
towards seasonable levels, most places in the 20s.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
Saturday through Sunday night

Split flow will prevail over the area at the start of the long 
term period with an approaching southern stream trough moving NE
into the Ohio Valley while a northern stream trough pivots SE 
through Ontario. These two converging mid-lvl features will set
the stage for a significant winter storm to impact the area 
Saturday PM into Sunday. Although there are still some timing 
discrepancies among guidance (in particular the GFS is 3-6 hours
faster than other solutions), the general consenus is that the 
Sfc.low will track NE from the Ohio Valley on Saturday, passing 
more or less directly over the area by Sunday morning. 
Increasing synoptic ascent ahead of the upper wave will begin to
generate precipitation over western portions of the area by 
Saturday afternoon with precipitation becoming more widespread 
and intense by Saturday evening. Residual cold air will likely 
initiate precipitation as snow in most locations on the NW side 
of I-95, however the transition to rain from south to north 
will begin in earnest as the low-lvl warm front & heaviest 
precipitation begin to push through Saturday night. 

There will almost certainly be challenges with ptype Saturday 
night into early Sunday morning as warm air advects in at low 
to mid lvls. Currently feel the GFS is too quick to erode low-
lvl cold air in northern portions of the CWA, and consequently 
leaned on the NAM thermal profiles for p-type Saturday night. 
Ultimately this results in snow transitioning to primarily 
sleet/freezing rain over northern portions of the CWA namely the
Lehigh valley and portions of NW New Jersey (north of about 
Mercer County) by Sunday morning. Despite the likelihood of some
mixed-precipitation over the aforementioned areas it still 
appears fcst.snow amounts with the initial precipitation push 
( before the warm nose strengthens) are sufficient to justify 
hoisting Winter Storm watches in Eastern PA/NW NJ (additionally 
ice accumulations could also result in winter weather impacts in
the watch area). South of the Fall Line/I-95 corridor the event
should primarily be a rain event, albeit a fairly heavy one for
January standards with model QPF around 1.5-2.5 inches (apart 
from far southern portions of the CWA where fcst amounts are 
closer to an inch). 

The cold front will begin to push through the area from W-E
Sunday morning behind the departing sfc.low, with many locations
in Eastern PA experiencing their daytime highs around dawn.
Strong pressure rises and the resultant strong NW flow with
advect much colder and drier air into the region through the day
on Sunday. Can't rule out some lingering backside precipitation
(which would be primarily snow) but water amounts should be 
meager in comparison to the warm part of the system due to lack 
of moisture. The flash freeze potential is difficult to assess
at this time as that is dependent on the time lag between the
end of precipitation and the arrival of arctic air and this
exact timing is uncertain at this point. The big story Sunday 
night will be very cold wind chills as overnight lows in or near
the single digits combined with brisk NW winds will result in 
sub-zero wind chills areawide (with sub -10 wind chills likely 
in Eastern Pa.

Monday through Thursday 
Very cold and windy conditions continue Monday in the wake of the 
weekend storm that will be over Atlantic Canada by this time. Expect 
NW winds continuing to gust at least 20 to 30 mph through the 
morning into the early afternoon. Most areas will see highs only in 
the single digits and teens but when combined with the winds this 
will result in subzero wind chills. It should be mainly dry though a 
few flurries will continue to be possible across the southern 
Poconos. 

High pressure builds in Monday night leading to diminishing winds 
with mainly clear skies and very cold overnight lows. Lows will be 
mainly single digits to near zero N/W of the I-95 corridor and in 
the low to mid teens farther S/E. 

For Tuesday and beyond, the high moves off the coast through Tuesday 
leading to a moderation in temps though highs will still be below 
average. The next system could impact the region as early as Tuesday 
night into Wednesday as the upper level trough reloads over the 
central CONUS sending another surface low NE toward the area. This 
could take a similar track to this weekend's storm leading to more 
mixed precipitation as temperatures continue to moderate. This 
system could continue to affect the area into next Thursday.

&&

.AVIATION /06Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, 
KMIV, KACY and surrounding areas.

Overnight...Vsby restrictions have arrived at many sites with 
light snow (or a mix of rain and snow southeast of the I-295 
corridor) ongoing across the area. This should continue in most
areas thru 12Z before ending from W to E. IFR VSBYs likely 
during the snow and CIGs dropping to MVFR or worse overnight. 
VSBYs may be somewhat better in general at MIV/ACY, where a 
rain-snow mix is more likely to occur. There could also be some 
patchy fog. Light east winds. Moderate to high confidence.

Friday...IFR conditions will gradually improve to MVFR through
midday. A return to VFR is possible later in the day but
confidence is lower that this will occur by 20Z. Precipitation 
should quickly end by or just after 12z, perhaps a little longer
near the coast. Winds becoming west 5 to 10 kts. Moderate 
confidence.

Friday night thru Saturday morning...VFR expected with increasing 
clouds Saturday. 

OUTLOOK...

Saturday...Conditions deteriorating through the day, with sub- 
VFR conditions likely by late Saturday afternoon/evening at all
sites. Most TAF sites will see prevailing MVFR conditions in 
moderate rain (possibly snow initially) overnight, however 
KABE/KRDG may experienced primarily mixed precipitation. Winds 
generally from the E/SE

Sunday...Conditions improving to VFR by Sunday afternoon, with 
gusty NW winds persisting through the day.

Monday...Mainly VFR. gusty winds

Monday night and Tuesday...mainly VFR.

&&

.MARINE...
Winds and seas will remain below criteria through tonight. 

A rain-snow mix is expected overnight across the waters (mostly rain 
offshore, but more likely a mix to start with on Delaware Bay). 
Visibility restrictions are expected, especially where it is snowing.

Friday...Lingering rain in the morning with some visibility 
restrictions possible, improving by afternoon. Winds becoming west 
or northwest 5 to 15 kts. Seas 3 feet or less.

Friday night into Saturday morning...Sub-SCA. Fair. 

OUTLOOK...

Saturday...E/SE Winds and seas increasing Saturday PM with SCA
conditions likely by the evening.

Sunday...Strong southerly winds becoming NW and increasing to
near or above Gale force by the afternoon, 

Monday...Gale force NW winds likely continuing.

Monday night and Tuesday...Winds and seas diminishing.

&&

.TIDES/COASTAL FLOODING...
Astronomical tides will be high over the weekend due to the full 
moon. It will take a positive tidal departures of +0.7 to +0.9 to 
begin causing minor flooding.

An onshore flow is expected to develop on Saturday, causing tidal 
departures to increase at that time. While the wind is anticipated 
to become southerly on Saturday night, lowering atmospheric pressure 
will help to keep water levels from decreasing quickly. As a result, 
there will likely be some minor coastal flooding with Sunday 
morning's high tide along the oceanfront and along parts of Delaware 
Bay. 

It should be a one tide cycle event. Water levels are expected to 
decrease sufficiently by Sunday evening's high tide to prevent 
another round of coastal flooding.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Storm Watch from Saturday afternoon through Sunday 
     afternoon for PAZ054-055-060>062.
NJ...Winter Storm Watch from Saturday afternoon through Sunday 
     evening for NJZ001-007>010.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...O'Brien
Near Term...Meola/Miketta/PO
Short Term...Miketta
Long Term...Carr/Fitzsimmons
Aviation...Carr/Fitzsimmons/Meola/Miketta/PO
Marine...Carr/Fitzsimmons/Miketta
Tides/Coastal Flooding...