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FXUS61 KPHI 100206

Area Forecast Discussion
National Weather Service Mount Holly NJ
906 PM EST Mon Dec 9 2019

Low pressure passes north of the area tonight into Tuesday, and a
cold front will then pass through the region Tuesday evening.  
Strong high pressure builds in from the west late Wednesday into 
Thursday and moves offshore on Friday. Low pressure affects the 
Northeast Friday night into Saturday, followed by high pressure 
for the start of the following week.


900 PM Update...No major changes to the forecast. Rain 
continues at this hour over much of the area but it will taper 
off west to east overnight as a warm front draped across the 
region lifts north. Minor changes were made to T,Td and POP 
grids based on latest obs and trends. 

Previous Discussion...An upper level jet positioned to our west
and south will continue to provide forcing for persistent 
stratiform rainfall through at least the evening hours. This is 
the period forecast to bring the highest rainfall totals through
the area, with amounts of 0.50" to 1.00+" along and north/west 
of I-95 and less than 0.50" south/east of I-95. A warm front is 
lifting north through the region this afternoon and warm air 
advection will continue to increase as will southerly winds 
through the overnight period. As a result, temperatures will 
rise into the mid to upper 50s outside of the higher terrain by 
daybreak Tuesday. As the jet pivots toward our north through 
early Tuesday, we will lose the best forcing for widespread 
precipitation overnight through Tuesday morning, so a lull in 
the widespread rainfall is expected. However, a few showers are 
still possible during this period.

With continued warm air advection, temperatures will continue to 
rise through around noon Tuesday until the cold front arrives, 
peaking in the low 60s along and south/east of I-95. The front will 
cross the forecast area from northwest to southeast through the 
afternoon hours with temperatures falling and winds shifting 
northwest behind it. An enhancement in precipitation is forecast as 
the front pushes through, mainly during the afternoon. Rainfall 
amounts with this round of rainfall should be generally less than 
0.25". Most of the rain will be moved out of the area by Tuesday 
evening with chances for a few lingering showers into the


Tuesday night the surface cold front will be well off the coast
with not much in the way of coverage initially expected. Late 
Tuesday night into very early Wednesday the 700/600 MB thermal 
gradient will begin to tighten as the shortwave approaches. 
Latest GFS and NAM FGEN fields are fairly impressive with lift
increasing starting over Maryland and spreading northeast. The 
NAM and the other high res models are depicting this as
precipitation spreading over the area rather rapidly Wednesday 
morning. The NAM is slightly further south with the strongest 
forcing for ascent compared to the GFS and ECMWF, but overall 
agreement is not to bad. The main concern will be precipitation 
type issues. 

Over the southern Poconos thermal profiles support all snow for
the event, but the strongest FGEN signal remains just southeast
of this area. The best forcing for ascent is usually towards the
warm side of the FGEN band, which would favor limiting snowfall
totals here. Towards the I-95 corridor the best chance of
precipitation exists, but thermal profiles initially support
rain. Both the NAM and GFS show initially rain changing over to
a rain/snow mix and then finally all snow. NAM forecast 
soundings indicate slightly stronger 700/800 mb winds and 
implied WAA. A brief transition to sleet is indicated via the 
NAM, but the most likely scenario appears to be rain/snow to 
snow as max temperatures aloft on the NAM seem to warm compared 
to the GFS and ECMWF. The most likely snowfall totals across 
the area are about 1 to 2" with slightly higher totals over the
northwestern zones. The change over to snow and subsequent 
totals will be highly sensitive to the thermal profile and QPF 
footprint though. 

Wednesday afternoon the FGEN band will head offshore with the
dry conveyor belt heading east. This will bring a rapid end to
the precipitation from the west. Expect highs Wednesday in the
upper 30s for most of the area (colder over the southern Poconos
and warmer along the coast).


Summary: Cold and dry Thursday, warming up on Friday with rain 
likely Friday night into Saturday. Slightly cooler with some 
lingering light rain chances on Sunday, then likely dry and
seasonable on Monday.


Thursday... Strong (1040 mb) High Pressure builds in from the
west on Thursday with cold and dry conditions resulting. 
Although it will be mostly sunny highs will only reach the 20s 
in the Poconos and generally low to mid 30s elsewhere. Overnight
mins will generally be in the teens to low-mid 20s, although 
these could be slightly warmer (particularly in the south) if 
low-lvl cloud cover begins to move in (as suggested by the 

Friday... The high will move into the Gulf of Maine and towards
the Canadian Maritimes on Friday, with east to southeast flow 
developing behind the it. Temperatures warm back up to near and 
slightly above normal levels on (e.g. from the upper 30s in the
Poconos to lower 50s in Delmarva).

Meanwhile, low pressure will develop over the southeast U.S. 
and begin to lift north. A warm front will develop out ahead of
that low and will lift towards the Mid-Atlantic and Northeast 
during the day Friday. Although Friday starts out dry, some 
light overunning precip may develop late in the day. It should 
be warm enough for mostly rain, but can't rule out some frozen 
precipitation Friday evening in the far north given that initial
low-lvl wet-bulb temps will be fairly low. There are still some
timing discrepancies among the global models with the CMC being
very fast, the GFS intermediate, and the UKMET/EC slower. 
Consequently retained the thinking of the previous forecast with
respect to PoPs leaving mostly slight-chance/Chc PoPs during 
the day on Friday, as suspect that most of the initial moisture 
surge will manifest itself as virga rather than measurable 
precip (particularly up north).

Friday night into Saturday... Guidance has come into better
agreement that precipitation will be likely in the Friday night
into Saturday morning timeframe as the surface low and warm (or
potentially occluded front) will pass through/near our area 
(there are still some slight discrepancies in the low's 
positioning). Guidance depicts us drying fairly rapidly behind 
the cold front on Saturday so cut PoPs a bit after 18Z but left 
low-end Chc PoPs in case timing changes. We will start Saturday
in the warm sector and even behind the cold front the cold 
advection isn't particularly extreme so most locations should 
top out in the 50s.

Sunday/Monday... A secondary cold front associated with the
northern branch low moving over the Great Lakes will cross the
area on Sunday, with maybe some light precipitation accompanying
it (but suspect most of the area remains dry). Temps will be a 
bit cooler than Saturday but still generally mild (upper 40s to 
lower 50s).

Monday currently looks to be a dry day as high pressure builds 
near the area, while the (likely) next storm system to impact 
our area begins to take shape over the Southern US. Generally
expect highs near climo.


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

Tonight...There will continue to be some fluctuations between
IFR and MVFR this evening before most sites should go to
predominate MVFR overnight with even VFR possible at times. Winds
generally S/SW 10-15 with gusts of 20-25 knots...especially 
overnight. Moderate Confidence in forecast evolution.

Tuesday...MVFR with periods of IFR ceilings possible after 16Z.
Rain showers developing after 16Z. This next round of rain
should move out late day between 21 and 0z. Southwesterly winds
around 10 kts turning westerly after 18Z. Moderate confidence.


Tuesday night...MVFR/IFR in rain changing to snow overnight.
Winds W/NW 5-10 knots. Moderate confidence.

Wednesday...Widespread MVFR/IFR in rain/snow in the morning. 
Conditions should gradually improve to VFR in the afternoon. 
West winds near 10 kt.

Thursday... VFR. Light northwest winds becoming northerly on 
Thursday. High Confidence.

Thursday night-Friday... Generally VFR early, but the chances
for sub-VFR conditions will increase on Friday.. Light 
northeasterly winds Thursday night veering more easterly (around
5kts) on Friday. Moderate confidence

Friday night-Saturday... Sub VFR conditions likely Friday
night/Saturday AM in rain, maybe some improvement Saturday
afternoon behind a cold front.. Winds veering more southerly 
Saturday AM and then more westerly on Saturday afternoon. 
Moderate confidence


Tonight...A small craft advisory continues for the Atlantic 
coastal waters tonight as winds will increase to 20-25 knots, 
with gusts of 30 knots. Strong winds just above the surface do 
exist, 40 to 50 kts around 1 kft, but low level warm air 
advection will likely limit mixing and keep winds in SCA 


Tuesday and Tuesday night...The small craft advisory will continue 
through Tuesday for the Atlantic coastal waters with a diminish in 
winds forecast Tuesday night. Wave heights will then fall below SCA 
criteria Tuesday night. Current small craft advisory for the coastal 
waters might have to be extended for a few of hours to account for 
the slower timing in wave height reduction. 

Wednesday...Conditions should stay below SCA criteria, though some 
westerly gusts to 25 kt are possible.

Wednesday night into Thursday morning... A brief period of SCA wind 
gusts will be possible late Wednesday night into Thursday morning as 
the low level jet strengthens. For now the episode looks rather 

Thursday... Sub SCA conditions expected with northerly-northeasterly 
winds gusting 15-20kts. 

Friday...Seas potentially increasing above SCA criteria Friday PM 
but uncertainty is high. Winds becoming easterly and gusting 15-20 
kts by Friday afternoon.

Friday night into Saturday... Easterly winds will continue to 
increase Friday night with SCA conditions possible starting early 
Saturday morning. Elevated seas will then persist through Saturday 
afternoon as winds turn from the west.


MARINE...Small Craft Advisory until 6 PM EST Tuesday for ANZ450>455.
     Small Craft Advisory until 6 AM EST Tuesday for ANZ430-431.


Near Term...Fitzsimmons/Staarmann

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