Weather

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Area forecast discussion 
National Weather Service Taunton Massachusetts 
1025 PM EST Monday Jan 5 2009 


Synopsis... 
high pressure will build into the region tonight and Tuesday. A 
significant winter storm will impact interior southern New England 
late Tuesday night through Wednesday night with snow and ice. Mainly 
dry and colder weather returns for the end of this week. Another 
round of winter weather is possible Saturday as a clipper moves into 
New England. Much colder air should follow for Sunday and Monday. 


&& 


Near term /until 5 am Tuesday morning/... 
no major changes to ongoing forecast. Have tweaked temperature and 
sky cover trends based on latest observations. 


Quieter weather tonight as a high pressure across PA moves farther 
east. Clearing skies will result in good radiational cooling. 
Previous forecast seems to have this handled well. 


&& 


Short term /5 am Tuesday morning through Tuesday night/... 
high pressure overhead Tuesday with light winds...but overrunning high 
clouds will be moving in from south to north during the day so 
increasing clouds expected. We scaled back probability of precipitation and kept it dry 
through Tuesday as cross sections Show Low level dry layer which will 
need to be overcome. Maximum temperatures ranging from near 30 northwest to upper 30s 
coastal plain. 


Overrunning moisture begins to spread NE across southern New England 00-06z...likely 
reaching all but SW New Hampshire and NE Massachusetts by 06z. The column is cold enough 
for snow to start...then warming in the 850-700 mb layer will likely 
introduce more sleet near the South Coast 03-06z. 


&& 


Long term /Wednesday through Monday/... 
*** a significant winter storm remains in the forecast for 06z 
Wednesday through 06z Thursday across interior southern New 
England...with a combination of snow and ice *** 


Hazards: marginal Winter Storm Warning criteria...enhanced by public 
impact has prompted the issuance of a Winter Storm Watch for much of 
the region along or northwest of the Rhode Island east mass Interstate 95 corridor. 


Confidence for any specific element criteria being equaled or 
exceeded is below average. Precipitation phase guides the amounts. 
Just too much model uncertainty to accurately define the amounts of 
snow... amount of sleet...amount of freezing rain. The ballpark 
figures are attached to the west-southwest and posted to web. These total snow 
and sleet amounts may be an inch or 2 on the high side. 


18z GFS concurs with the watch and is starting to drift toward a NAM 
solution of later. 


Want to emphasize for Road crews... this is a long duration of several 
different precipitation types and will require multiple Road treatments 
of snow to ice and back to snow. My confidence on this aspect of the 
forecast is above average and I see difficult travel redeveloping 
Wednesday night from Boston Gloucester to Norwood west to the Berks 
and northwest to snh where precipitation has let up and the redevelops but 
its colder! 


This forecast was constructed using the 12z GFS MOS and partial 
thicknesses. We did blend temperatures at times with the 4am kbox forecast 
to keep the trends stable. 


In addition to the 12z NCEP operational models we did review the 09z 
sref probs for 12 hour quantitative precipitation forecast...12 hour snowfall...gale and Small Craft Advisory thresholds. 
The 09z sref probs for .01 /an excellent predictor again for last 
nights freezing rain/ and in previous light precipitation cases...were relied 
upon to adjust any probability of precipitation...especially for Wednesday evening. The 12z 
GFS ens probability of precipitation for .05 were also used for pop adjustments. 


Finally...the ggem- seen as too warm aloft too fast. The UK is warm 
as well. The European model (ecmwf) was reviewed as well and those temperatures were blended 
into the forecast process. 


Wind Advisory hazard potential Wednesday afternoon Cape Cod and 
Nantucket where we may see gusts 43 knots. 


Tuesday night into Wednesday night... 
essentially warm air advection at middle levels will battle with strong 12z-18z Wednesday 
fgen lift to determine how much snow before the inevitable warming 
between 850 and 700mb changes snow to sleet and then freezing rain 
interior and rain coast. We think enough instability and fgen to 
allow for a 1 to 3 prd of 1 inch/hour snowfall rates Massachusetts Pike northward Wednesday 
morning. 


Extended the watch into Wednesday evening...not so much to indicate heavy 
precipitation Wednesday evening but to indicate that this is a long duration event 
and that even if there is melting on pavement Wednesday afternoon...there should 
be a transition back to an accumulation of snow Wednesday night as the trough 
aloft approaches with its divergence of q...large middle level lapse rates and 
therefore moist unstable sounding. 


The expectation is for a messy Wednesday morning with on-going freezing 
precipitation. Wednesday evening commute might be pretty decent but northeast Massachusetts 
needs to be alert for a bit of sneaky trapped shallow cold air in 
southern New Hampshire replacing middle to upper 30s middle afternoon temperatures in 
Essex County as well as Middlesex and Suffolk counties of Massachusetts 
Wednesday evening. 


The Thursday morning commute may have delays due to on going or just 
finished accumulative snow showers... especially hfd to orh to lwm 
nwwd. 


Caution: since the trough is so far west on Wednesday...models might be a 
little too deep too fast with too much warming and therefore snow 
lasting longer than now forecast. There was only 1 GFS member similar 
to the 12z NAM but it is noted that the primary trough aloft does not 
really bite into the NE USA till late Wednesday night and now the 18z GFS is 
slowing down with significant Wednesday evening precipitation. 


Therefore the watch and lack of enthusiasm for locking into specific 
details. 


Can there be a half inch of glaze and power outages? Emphatically yes. 
Most likely region...thinking high terrain of northwest hfd County to 
possibly north Tolland County and Northwest Hills of Rhode Island into Middlesex...southern 
and central orh County to the high terrain of Hampden County. Still 
not confident on this being a proper placement. 


S New Hampshire and far northern region of Massachusetts may see an inch of 
sleet cutting back freezing rain there as there should be at least a 
2500 foot below freezing layer under the above zero layer found 
between 700mb and 800mb...which allows refreezing of rain as sleet. 


Showalter index suggest thunder S coast midday Wednesday but not in grids and the 
strong deep fgen with -epv layered suggests isolated thunder might 
occur in the s+ region forenoon Wednesday Massachusetts Pike northward. 


Point and click has the attempt at details. The storm total graphic 
has posted. 


The watch for the nbt to bos owd area is a fringe watch and not much 
higher than 40 percent chance of verifying but we thought it best to 
include should colder solutions emerge in the future. Appears we will 
need a Winter Weather Advisory for much if i95 to the cape Wednesday morning. 


Across the coastal plain...melting snow and the potential for 1 
inch of rain Wednesday afternoon may result in some typical minor street and 
poor drainage flooding. However...no major problems are expected. 


As the previous shifts mentioned...a period of sleet and freezing 
rain may result in down tree limbs and power outages in parts of the 
watch area of Massachusetts northwest Rhode Island and north CT with one quarter to one 
half inch of icing on top of the snow/sleet. However...this event 
will feature a period of snow followed by sleet before the freezing 
rain. This means that the amount of freezing rain we receive should 
be less and not as destructive/widespread as what we saw a month ago. 
Nonetheless...this still has the potential to cause significant 
problems over a portion of the region that was hard hit last month. 




Thursday and Thursday night...cold trough aloft may generate bands of 
moderate to heavy snow showers...especially northwest Massachusetts and SW New Hampshire and also 
S coast. We saw the 12z European model (ecmwf) and saw an operational run of the GFS 
say the same thing ydy for the cape and islands. Probability of precipitation modified higher 
than guidance to account for this drier but significant short term 
snowfall. 


Friday...a return to cold dry weather with below normal temperatures in 
a blustery northwest flow. 


Saturday through Monday...12z models are in decent agreement during 
this time period. Low pressure moves eastward across the Great Lakes 
Friday night into Saturday. The result will be increasing warm air 
advection throughout the column across southern New England. This may 
result in a light accumulating snow event for much of southern New 
England starting late Friday night into a good portion of Saturday. 
Colder and drier air arrives in the wake of the low Saturday night 
into Sunday and lasts through Monday as high pressure moves in. 


&& 


Aviation /03z Tuesday through Saturday/... 
overnight...VFR with middle/high clouds departing most of southern New 
England to the east by 06z. 


Tuesday...VFR...increasing high clouds. 


Outlook...Tuesday night through Saturday... 


Tuesday night through Wednesday night... 
MVFR to IFR conditions in snow changing to rain on the coast and ice 
across the interior. Brief wind gusts between 25 and 35 knots on the 
coast...mainly Wednesday afternoon and Wednesday night and primarily kack 
kfmh and khya. Low level wind shear problem possible midday Wednesday kack where also gust 
43 knots possible. 


Thursday... VFR ceilings occasionally MVFR conds in bands of snow showers. Few west 
wind gusts 25 to 35 kts. 


Friday...VFR. Northwest wind gusts 25 to 30 kts daylight diminishing at 
evening. 


Late Friday night into Saturday...MVFR/IFR conds possible in snow. 


&& 


Marine... 
dropped Small Craft Advisory for Cape Cod Bay and Nantucket Sound this 
evening based on latest observations. Small Craft Advisory continues 
for Boston Harbor and coastal waters east of Massachusetts. 


Tuesday...diminishing winds but lingering Small Craft Advisory seas eastern outer waters 
through middle morning. 


Outlook...Tuesday night through Saturday... 


Tuesday night and Wednesday... 
northeast winds increase to Small Craft Advisory very late Tuesday night and continue 
into Wednesday with secondary coastal low pressure system taking 
over. We are likely to see a period of gales for a few hours on 
Wednesday...especially S coast. GFS gust guidance gave 52 kts S of Rhode Island midday 
Wednesday. We capped at 45 kts but issued gale watch. 


Wednesday night and Thursday... 
moderate to high confidence that we will see west to southwest gale 
force wind gusts behind the departing storm system. 


Friday...W-NW gales are possible-very low probability at this time-behind a 
cold front. 


Saturday...low pressure approaches from the west and crosses New 
England late in the day or at night. This will likely result in southwesterly 
Small Craft Advisory winds/seas...turning west/northwest as the low passes. 


&& 


Box watches/warnings/advisories... 
CT...Winter Storm Watch from Tuesday evening through Wednesday 
evening for ctz002>004. 
Massachusetts...Winter Storm Watch from late Tuesday night through late 
Wednesday night for maz002>006-026. 
Winter Storm Watch from late Tuesday night through Wednesday 
afternoon for maz007-013>015. 
Winter Storm Watch from Tuesday evening through Wednesday 
evening for maz008>012. 
New Hampshire...Winter Storm Watch from late Tuesday night through late 
Wednesday night for nhz011-012-015. 
Rhode Island...Winter Storm Watch from Tuesday evening through Wednesday 
evening for riz001. 
Marine...gale watch from Wednesday morning through late Wednesday night 
for anz231>234-250-254-255. 
Small Craft Advisory until 3 am EST Tuesday for anz230. 
Small Craft Advisory until 9 am EST Tuesday for anz250-254. 


&& 


$$ 
Synopsis...kjc/drag 
near term...kjc/drag/Belk 
short term...kjc 
long term...drag/ekster 
aviation...kjc/drag 
marine...kjc/drag 

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