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fxus61 kokx 191542 
afdokx 


Area forecast discussion 
National Weather Service Upton New York 
1042 am EST Wednesday Nov 19 2008 


Synopsis... 
a series of disturbances in the northwest flow aloft will maintain 
an unseasonably cold airmass through this weekend. These cold air 
surges will be preceded by a couple of weak areas of low pressure... 
also known as Alberta clippers. The first of which moves through 
early Thursday...and then another on Friday. A strong polar high 
over the Mississippi Valley then builds eastward through the weekend. 


&& 


Near term /until 6 PM this evening/... 
forecast on track this morning...with just a few minor adjustments 
to reflect latest observations around the region. 
Otherwise...negative height anomalies across eastern North America 
will maintain an unseasonably cold airmass for several days to 
come. 


A vigorous upper trough departing the East Coast today will 
briefly be replaced by short wave ridging and a weakening surface high 
building eastward from the Ohio Valley. In addition... a blocking 
high over the central Atlantic...which extends northward into 
Greenland...will steer a deepening oceanic low well south and east 
of Li up into the Canadian Maritimes the next 24h. On the backside 
of the system...a deep-layered northerly flow will sustain the cold 
airmass in place with daytime highs again 10 to 15 degrees below 
normal. Highs will range from the middle 30s north and west of 
NYC...to around 40 along the coast. 


Gusty north winds will gradually diminish over the course of the 
day...but the combination of the cold airmass and wind will 
produce wind chill values in the teens this morning...even with 
sunny skies. 


&& 


Short term /6 PM this evening through Saturday/... 
a weakening shortwave/clipper system approaches region tonight 
from the Great Lakes and crosses on Thursday. Some good 
radiational cooling conditions to start the night...allowing temperatures 
to fall quickly in the evening into the lower 20s/teens in the 
outlying areas. 


Then increasing cloud cover with scattered snow showers developing 
across far western sections of the area with weak warm advection 
late tonight. Northwest flow should confine the snow showers to the higher 
elevations north and west of NYC. Some of this precipitation may spill over 
as flurries along the coast by morning. 


Scattered snow showers will linger on Thursday over the hills north and west 
with flurries at the coast. No accumulate is expected at this time. 
Instability strato-cumulus will likely hold on for much of the day 
Thursday...particularly western half of region. Temperatures again running 8 
to 12 degrees below seasonable. 


Another shortwave/clipper system digs through the Great Lakes to 
the East Coast Thursday night into Friday. This system shows no real 
defined surface low...however...deep-layered cyclonic flow and the 
upper trough moving overhead will pose another chance of scattered 
snow showers across the higher elevations north and west. From 
experience...it is not often that these snow showers make it down 
to the coast...but perhaps some flurries again. 


Vertical thermal profile through the period will support snow. 


&& 


Long term /Saturday night through Tuesday/... 
upper trough axis swings across the region overnight Friday into 
early Saturday...with best chance of flurries over eastern areas. 
Temperatures will remain below normal through Sunday with northwest winds 
and cold air advection in control. Surface high pressure shifts 
off the coast on Monday...bringing return flow and moderating 
temperatures. 


A wave of low pressure is forecast to develop across the central U.S. 
And move toward the region late Monday and Tuesday. There is 
considerable model disagreement about the timing and intensity of 
this low. European model (ecmwf) is warmer and wetter...with the heaviest rain 
occurring Tuesday afternoon. GFS is quicker with precipitation starting 
overnight Monday into early Tuesday...possibly as light snow or mix 
over interior areas. Have leaned toward drier GFS solution though 
confidence is low at this time. 


&& 


Aviation /16z Wednesday through Sunday/... 
VFR through 12z Thursday. 


Brisk northerly winds will gradually subside today as strong low 
pressure offshore lift north towards the Canadian Maritimes. 
North of 10 to 20 knots...with gusts between 20 and 25 knots can be 
expected across NYC metropolitan terminals and coastal terminals until 
late afternoon. The winds will also back to the northwest...then 
west/northwest by late in the day. 


Just a few cumulus around during the daylight hours as upper trough 
swings east. 


High clouds approach this evening ahead of next clipper and 
gradually lower and thicken to low end VFR overnight. Winds will 
lighten. 


Outlook for 12z Thursday through Sunday... 
generally VFR. There is a low chance of snow showers Thursday 
morning...with local MVFR conditions. VFR returns Thursday night 
through sun. Another clipper late Friday/early Saturday will need 
to be watched for possible sub VFR conditions briefly. 


&& 


Marine... 
winds have fallen below gale force this morning. Therefore...gales 
have been dropped and an Small Craft Advisory is now up for all area waters until 
23z today. Winds continue to be from the north/northwest around 25 
knots with gusts up to 30 knots. Seas remain around 6-8 feet on the 
ocean. Winds will gradually subside through the afternoon falling 
below Small Craft Advisory conditions overnight. We will re-evaluate the need to 
extend any Small Craft Advisory beyond 23z with the afternoon package. 


Otherwise...marginal Small Craft Advisory gusts possible on the ocean waters Thursday 
night and Friday from cold air advection...but a prolonged period is more likely 
(and may include the sound...Harbor and bays at times) from Friday 
night through Sun night as the pressure gradient tightens between 
a strengthening offshore low pressure system and strong Canadian 
high pressure building in from the west. 


&& 


Hydrology... 
no significant precipitation (0.5 inches or more in a 12 hour 
period) is expected over the next 7 days. 


&& 


Okx watches/warnings/advisories... 
CT...none. 
New York...none. 
New Jersey...none. 
Marine...Small Craft Advisory until 6 PM EST this afternoon for anz330- 
335-338-340-345-350-353-355. 


&& 


$$ 

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