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December 6-7: Rainy Frontal Passage or a Wintry Wave?


moneypitmike

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18z NAM soundings indicate a pretty healthy event for a lot of the region...we'll have to watch this pretty closely over the next 4-5 hours.

 

The sounding even at 03z are really close to parachutes for places like ORH/FIT/BAF...even on the RAP model too.

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How do you read this?  it looks like the temp is just above freezing?  Is 1000mb close to the ground? 

 

There are many who can answer this far better than I can, but I was focusing on the upper air.  You're correct that right near the surface (1000mb), it appears to be slightly above freezing.  I've found model output for surface temperatures to be hit or miss, so I don't sweat a degree or two.

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Maybe far W and NW regions.  Still 49F here, so I expect it to start as plain jane rain.

 

 

I think outside of the high Berkshires and NW CT 1000+ feet, its going to start as rain...the low levels should start cooling pretty quickly once precip sets in as 925mb temps should be around 2C for places like ORH-BDL by then...that should be enough to get it into the mid to upper 30s...then shortly after we should see the CAA plus pretty big omega-induced cooling take over. The low-level CAA is going on the whole time (even if more of an ooze) which helps expedite the process.

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There are many who can answer this far better than I can, but I was focusing on the upper air.  You're correct that right near the surface (1000mb), it appears to be slightly above freezing.  I've found model output for surface temperatures to be hit or miss, so I don't sweat a degree or two.

but even 850mb seems to be above freezing...

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first of the year!!  :snowing:

 

CTZ002-003-MAZ005-011-012-070500-
/O.EXA.KBOX.WW.Y.0014.131206T2300Z-131207T1100Z/
HARTFORD CT-TOLLAND CT-CENTRAL MIDDLESEX MA-EASTERN HAMPDEN MA-
SOUTHERN WORCESTER MA-
INCLUDING THE CITIES OF...HARTFORD...WINDSOR LOCKS...UNION...
VERNON...FRAMINGHAM...LOWELL...SPRINGFIELD...MILFORD...WORCESTER
350 PM EST FRI DEC 6 2013

...WINTER WEATHER ADVISORY IN EFFECT UNTIL 6 AM EST SATURDAY...

THE NATIONAL WEATHER SERVICE IN TAUNTON HAS ISSUED A WINTER
WEATHER ADVISORY FOR SNOW AND SLEET...WHICH IS IN EFFECT UNTIL 6
AM EST SATURDAY.

* LOCATIONS...NORTHWEST CONNECTICUT INTO CENTRAL MASSACHUSETTS.
THIS INCLUDES THE CITIES AND TOWNS OF HARTFORD...WINDSOR
LOCKS...UNION...VERNON...FRAMINGHAM...LOWELL...SPRINGFIELD...
MILFORD AND WORCESTER.

* HAZARD TYPES...SNOW AND SLEET.

* ACCUMULATIONS...SNOW ACCUMULATION OF 1 TO 3 INCHES...ALONG WITH
AROUND A TRACE OF ICE.

* UNCERTAINTY...IF THE CHANGEOVER TO SNOW IS DELAYED BY A FEW
HOURS LESS SNOWFALL WOULD OCCUR. CONVERSELY ONCE THE CHANGEOVER
COMMENCES A BURST OF HEAVIER SNOW IS POSSIBLE AND WOULD RESULT
IN LOCALLY HIGHER AMOUNTS.


* TIMING...RAIN WILL QUICKLY CHANGE TO SLEET THEN SNOW BETWEEN 10
PM AND 1 AM. BRIEF HEAVY SNOW IS POSSIBLE OVERNIGHT.

* IMPACTS...SNOW COVERED ROADS WILL RESULT IN HAZARDOUS DRIVING
CONDITIONS TONIGHT.

* WINDS...NORTH 5 TO 10 MPH.

* VISIBILITIES...ONE QUARTER TO ONE HALF MILE AT TIMES.

* TEMPERATURES...DROPPING INTO THE LOWER 30S.


PRECAUTIONARY/PREPAREDNESS ACTIONS...

A WINTER WEATHER ADVISORY IS ISSUED WHEN SNOW AND/OR ICE IS
FORECAST TO DEVELOP IN THE AFFECTED AREAS...BUT ACCUMULATIONS ARE
EXPECTED TO BE LIGHT. ANY SNOW OR ICE WOULD MAKE DRIVING AND
WALKING DIFFICULT...BUT NOT IMPOSSIBLE...ON UNTREATED ROADWAYS
AND SIDEWALKS. WHEN TEMPERATURES ARE BELOW FREEZING...MOTORISTS
NEED TO BE ESPECIALLY CAREFUL ON BRIDGES AND OVERPASSES WHERE
SLIPPERY SPOTS CAN EASILY DEVELOP.

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Comparing 21z ob at BDL to 21z predicted ob by the 12z NAM using coolwx.com, the 44F is spot on by the model but the dewpoint is already at 0C, but the 12z NAM has it at 2C at 21z. Interesting that the dew is much lower compared to the model giving us the most snow. And to add to this, the 12z NAM has the dew rising from 2C to 3C by 0z at BDL tonight, but since it's already off by 2C, I would bet that will bust too high as well. 

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but even 850mb seems to be above freezing...

 

If my understanding is correct, the temperature isotherms tilt to the right as you go up the chart.   I've reattached the original sounding with the 0 degree isotherm highlighted in blue.  Anything to the left of the 0 degree line is modeled to be below freezing.

 

post-2015-0-11501100-1386364439_thumb.gi

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