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Area forecast discussion 
National Weather Service Taunton Massachusetts 
500 PM EST Thursday Dec 4 2008 


Synopsis... 
a cold front will cross the region this evening. Dry colder than normal 
weather prevails Friday into Saturday as high pressure arrives. Great 
Lakes low pressure late Saturday will combine with developing low pressure 
southeast of Cape Cod to become a large gale in the Canadian Maritimes late Sunday. 
This should bring a period of snow showers to our area Saturday night 
or Sunday. Cold dry high pressure follows into the area Monday. Thereafter... 
low pressure in the lower Mississippi Valley Tuesday will be heading into 
the northeast USA Wednesday...possibly with heavy weather. 


&& 


Near term /until 6 am Friday morning/... 
appears that the cold front is just starting to enter New England 
out of New York state at 20z. Steady light rain has moved into the 
Berkshires with a few showers from near kore to kfit. 


As precipitation moves into the region...best upper level support runs out 
ahead of it so expect continued trend of dissipating showers to 
continue. Already noting this trend on NE radar mosaic. Have a brief 
likely period of showers over SW New Hampshire/northwest Massachusetts early this evening... 
otherwise will see scattered showers push across the region tonight. 
Will be on light side with best shot of measurable precipitation over 
central and western areas. Expect less than 0.1" with this feature. 
Also expect precipitation to push off the Cape Cod coast by around midnight. 


Still noting wind gusts to 25 knots or so over Cape Cod and the 
islands...otherwise appears that the core of the low level jet has 
pushed east. Expect winds to continue to diminish as they shift to 
west tonight. 


Models indicating good shot of cold air working into the region. 850 mb 
temperatures drop -6c to -11c by 12z Friday. Used blend of mavmos guidance 
and metnam for overnight lows...which will mainly be in the 20s. 


&& 


Short term /6 am Friday morning through Friday night/... 
Friday... 
cold air advection continues through most of the day...though 
appears that the core remains over northern New England. 850 mb temperatures 
will still be on order of -10c to -12c. So...will see some diurnal 
clouds developing. Otherwise...expect a dry day with temperatures running 
at least 5 degrees below normal. High pressure ridge builds out of the Ohio 
Valley. 


Friday night... 
high pressure remains in control. Models start to show development of 
low pressure off the southeast U.S. May see some precipitation develop well offshore 
late at night. Otherwise...expect another dry and chilly night. 
Overnight lows mainly in the teens...except 20s along the immediate 
coast. 


&& 


Long term /Saturday through Thursday/... 
***uncertainty continues for Sat night - sun evening snow risk*** 


Updated from 1pm...with little change in thinking. 


Hazards... 
1/ "possible" wxa for snow southeast coastal mass and S coastal Rhode Island Sat night 
or sun. 


2/ "possible" Wind Advisory for Cape Cod and ack Sun night 


3/ possible southerly flow high wind/Wind Advisory/flooding situation 
for a portion of southern New England next Wednesday. 


4/ possible minor southerly flow coastal flooding buzz and narr Bay next 
Wednesday high tide. 


Details follow through 12z op runs as noted. 


Snow situation: models...both successive cycle same...and from within 
the international suite...have been variable on sharpness of digging 
short wave that eventually GOES negative tilt off the New England coast. 
06z/12z and now the 18z NAM dried out after 18z and 00z NAM cycles from 
yesterday and the updated 15z sref ensembles which offer 12 hour probs for 
1 inch of snow through 06z/Monday have only a 2 to 13 percent probability of 1 inch 
of snow in Rhode Island and parts of eastern Massachusetts Sunday...mainly before 
18z. 


12z op GFS targets southeast mass and Rhode Island to cirrocumulus for 1-4 inches of snow but we 
have boundary layer temperature problems that are far from being resolved. 
The increasing likelihood is that there will be some melting Saturday night 
or Sunday...especially immediate coast and it may be warm enough for rain 
showers. Also...low level inflow is weak and limited in duration. The 12z 
UKMET and ggem were dry. 


12z European model (ecmwf) continues wet... but I have my doubts about the European model (ecmwf)...have 
seen it good and bad. And for those who are thinking the GFS will 
drift to an European model (ecmwf) solution...I have not experienced confidence in 
this trend lately...seeing the GFS be more solid as an overall 
performer. 


So...if the 00z/5 UKMET gets wetter for southern New England...then that will offer more 
confidence in measurable precipitation. The UK is a decent model...and 
in my opinion recently more predictively stable and accurate when compared 
to the European model (ecmwf) through 72 hours. 


Note that through 12z..the GFS ens mean quantitative precipitation forecast has backed to its original 
Tuesday Dec 2nd forecast of .2 for Cape Cod and the islands and less than 
that elsewhere. I do know for several cycles Wednesday the 3rd that 
it spread .2 back to the Berkshires and 0.4 to the mass coast. 


However...I think this mean quantitative precipitation forecast can be biased high if just a couple 
of members have large amounts...with most of the members light. That 
can happen if one or 2 members track much closer to the coast or are 
stronger with resultant heavier quantitative precipitation forecast. 


At this time snow growth looks good for several hours midday Sunday and thats 
when I think we could get an inch or 2 of MDT to heavy snow showers. 


I did not code in heavy snow showers at this time...reserving that for later 
forecasts...if confidence increases. I still have my doubts about 
anything more than .05 most 0f southern New England except southeast mass and coastal Rhode Island. 


Quantitative precipitation forecast...did not use the heavier HPC quantitative precipitation forecast. I could not find any support 
in 12z GFS ens or sref. Instead I used the 12z GFS quantitative precipitation forecast. 


This converts to one half inch to 1 inch CT River Valley northeastward 
with our bullseye of about 1.5 to 2 inches from fmh to tan and ijd. 
Confidence on these amounts is below average...thinking that while it may 
snow briefly heavy...over an inch will be tough with surface temperatures as 
forecast as well as little inflow. 


12z GFS fgen while deep and organized...is weak...there is some instability... 
as seen in 850-500 6.5c/km lapse rates and divergence of q increase which 
pass through southern New England 18z Sunday! 


What I think will happen...instability flurries and snow showers 
interior and much of the coast Sat night or Sunday. We will look for 
a possible 12 hour inverted trough focusing ocean effect somewhere 
in Rhode Island/southeast mass late Sat or early Sunday but there is bl shear and 
inflow is weak. This may turn out to be more of a windex situation. 


Confidence as of this writing is average to above average that there will not be 
any snowfall amount exceeding 2 inches in southern New England except "possible" 4 inch 
related ocean effect somewhere south of a pvc pym pvd line. Have 
been wrong before. We will look at this constantly the next several 
days. I just dont think there is enough digging soon enough...no 
confluence of importance for our area to our north and limited 
moisture inflow. Climatology for most Boston snows is around 1 inch. 
In my opinion...this system just does not fit a pattern for productive 
greater than 2 inch snow generation. 


Sun night...Wind Advisory mainly pvc and ack. Low probability at this time. 


Next Wednesday...noticed GFS for 3 cycles predicting near 100kt 850 mb 
winds somewhere in southern New England or along the southern New England coastal waters. European model (ecmwf) has 
strong southerly flow of nearly 100 knots as well. 7 days out and this 
could easily shear and break down so anything said here is of limited 
value. Tides are coming up...astronomically and surface pressure is modeled 
low with poor surge forecasts for Fox Point in southerly flow situations. For 
Plum Island on NE mass coast. Cant envision much of an erosion 
problem on strong southerly flow. 


The grids have lots of interior freezing rain in them. This will 
eventually be weeded out as model guidance probably warms 




Remainder of the forecast where no comment is a 50 50 blend of 12z 
mavmos guidance with the 4am kbox forecast. 


Sustained winds Sat and sun were a 50 50 blend of NAM and GFS. 


Cloud cover forecast..especially Saturday is low confidence. 


&& 


Aviation /22z Thursday through Tuesday/... 
18z tafs maintain VFR through period. 


Radar continued to indicate light rain showers with no visibility 
restriction moving into western New England at 1730z. These were 
moving NE and will slowly edge their way across region this 
afternoon and tonight as cold front moves through. Ceilings mostly 
4-5kft at worst and only prime area for MVFR ceilings appears to be 
far northwest Massachusetts and SW New Hampshire per 09z sref data. There is also a low 
probability of MVFR ceilings kfmh/khya/kack 06z-09z but confidence 
was not high enough to mention in tafs. 


Wind gusts will remain a concern through 20z across southeast Massachusetts with SW 
30kt gusts likely...though as thicker cloud cover arrives winds will 
diminish rapidly. Expect wind shift to west around 00z all terminals 
before veering to northwest 05z-08z as cold front moves through. 


Front will be offshore by 12z Friday with northwest winds at or below 10kt...backing 
to northwest Friday afternoon. 


Kbos terminal...widely scattered showers 22z-06z but no visibility 
restriction. SW winds diminish to 12g20kt by 20z and become west at or below 
10kt by 00z Friday. 


Friday and Friday night...VFR. 


Outlook...Saturday through Tuesday... 


Saturday...VFR. Low probability of MVFR conditions in scattered late afternoon 
snow or rain showers along the southeast coast. 


Saturday night into Sunday...periods of MVFR or IFR developing in 
snow/rain showers mainly Rhode Island and southeast Massachusetts airports. 


Sunday night into Monday...any lower ceilings/visibilities improve to VFR by 
morning. Wind gusts up to 38 knots possible late Sunday into Sunday 
night across Cape Cod and the islands. 


Tuesday...VFR day degrade MVFR or IFR at night in warm air advection rain...possible 
freezing rain Connecticut River valley. Precipitation type confidence low. 


&& 


Marine... 
tonight...winds will continue to diminish. Will see gusts to 25 knots 
this evening mainly on the eastern waters as winds shift to west. 
Seas will remain high...upwards to 10 feet on the southern outer 
waters early...but will slowly subside. Continued Small Craft 
Advisory through tonight for all waters except Boston Harbor and 
Narragansett Bay. May be able to discontinue for the south sounds 
and bays earlier depending upon how quickly seas subside. 


Friday...may need to continue a small craft for seas on the outer 
waters for leftover 5-6 feet seas. Otherwise...expect west-northwest winds at 15 
knots or less. 


Friday night...as high pressure center builds west of the waters...west winds 
will be at 15 knots or less. Seas will be 3 feet or less. Depending upon 
timing of high...winds may start to shift to SW after midnight west 
of Marthas Vineyard. 


Outlook...Saturday through Tuesday... 


Winds and seas below small craft criteria Saturday as a high pressure 
crosses the waters. A coastal low pressure is expected to pass 
southeast of the waters Sunday. 


Sun...winds and seas will increase...with a Small Craft Advisory 
needed by Sunday night...with probably a gale for gusts. Too far in 
advance to Post a gale watch. 


Winds and seas gradually diminish Monday as a high pressure builds 
across the waters. 


Tuesday...quiet day and then increasing wind and seas at night. 


&& 


Box watches/warnings/advisories... 
CT...none. 
Massachusetts...none. 
New Hampshire...none. 
Rhode Island...none. 
Marine...Small Craft Advisory until 5 am EST Friday for anz231>235-237- 
250-254-255. 


&& 


$$ 
Synopsis...drag 
near term...evt 
short term...evt 
long term...drag 
aviation...drag/jwd 
marine...drag/evt 

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