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Jan 23/24 2016 obs/nowcast - the fight for the North


RUNNAWAYICEBERG

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Solid AM AFD from BOX

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

***MODERATE TO HEAVY SNOW ACROSS CT/RI AND SOUTHEAST MA***

***HEAVIEST SNOW FROM HFD-PVD-PYM TO THE SOUTH COAST***

***STRONG TO DAMAGING WINDS CAPE COD AND THE ISLANDS***

7 AM UPDATE...MADE MINOR UPDATES TO THE POPS...SKY COVER...AND

TEMPERATURE FORECAST TO ACCOUNT FOR LATEST TRENDS. EXTREMELY DRY

AIR AT THE SURFACE /15 TO 20 DEGREE DEWPOINT DEPRESSIONS/ IS

SLOWING THE ONSET OF SNOW ACROSS NORTHERN CONNECTICUT AND RHODE

ISLAND. DESPITE LIGHT RADAR RETURNS OVER THESE AREAS...SNOW HAS

YET TO BE OBSERVED AT THE SURFACE. STRONGER RADAR RETURNS /20 DBZ

OR HIGHER/ ON THE REGIONAL RADAR ARE MORE IN LINE WITH OBSERVED

SNOW AT THE SURFACE. EXPECT SNOW TO MOVE INTO NORTHERN CONNECTICUT

BY 9 AM...AND SOUTHERN RHODE ISLAND AS EARLY AS 730-8 AM.

MODEL TRENDS AND PREFERENCES...

00Z MODEL GUID IN VERY GOOD AGREEMENT AND NOT MUCH CHANGE FROM

PREVIOUS RUN. EVEN THE 00Z NAM WAS WITHIN THE ENVELOPE OF 00Z

SOLUTIONS. THUS FOLLOWED A MODEL BLEND FOR MOST FORECAST PARAMETERS.

IN ADDITION THIS RESULT IN NO CHANGE TO OUR PREVIOUS HEADLINES. JUST

AN FYI...DATA FROM 12 DROPSONDES OFF THE MID ATLC WERE AVAILABLE FOR

THE 00Z GFS AND 5 DROPSONDES FOR THE 00Z NAM.

KEY SYNOPTIC FEATURES AND UNCERTAINTY...

WATER VAPOR SATELLITE IMAGERY AND MESOANALYSIS INDICATES A VERY

IMPRESSIVE MID LEVEL LOW OVER THE INTERIOR MID ATLC AREA WITH A WELL

ESTABLISHED WARM CONVEYOR BELT/FIREHOSE WITH SUBTROPIC ORIGINS

STRETCHING FROM CUBA NORTHWARD UP THE EASTERN SEABOARD. MEANWHILE

COLD CONFLUENT FLOW ALOFT OVER CENTRAL NEW ENGLAND WAS PROVIDING A

COLD AND DRY AIRMASS OVER THE REGION. IN FACT DEW PTS WERE STILL IN

THE SINGLE DIGITS ACROSS MUCH OF INTERIOR CT/RI/MA. INITIALLY THIS

WILL SLOW THE NORTHWARD PROGRESSION OF THE SNOW.

FORECAST HINGES ON HOW FAR NORTH VERTICALLY STACKED/OCCLUDED LOW

WILL TRACK. COMPETING FORCES ARE COLD/DRY CONFLUENT FLOW OVER

CENTRAL NEW ENGLAND THIS MORNING WHICH REMAINS STUBBORN TO RETREAT

NORTHWARD TODAY. IN RESPONSE TO THIS...THE MID LEVEL CIRCULATION

TAKES ON A POSITIVE TILT...BECOMES ELONGATED SW TO NE AND THEN

TRACKS MORE EAST THAN NORTH...REMAINING SOUTH OF NEW ENGLAND. THIS

LIMITS THE NORTHERN PROGRESSION OF THE WARM CONVEYOR BELT

PRECIP...PIVOTING TO ABOUT THE MASS PIKE AREA...THEN COMING TO A

HALT. THUS SNOW TOTALS DIMINISH RAPIDLY AROUND THE MASS PIKE /I-90/

AND POINTS NORTHWARD.

TIMING ...

THE 00Z NAM QPF WAS VERIFYING VERY NICELY AT 06Z AND 09Z WITH

CURRENT OBSERVATIONS...SO FOLLOWED THIS GUIDANCE FOR THE ONSET OF

THE SNOW. AT 09Z SNOW IS ALREADY INTO EASTERN LONG ISLAND. SNOW

SHOULD REACH NORTHWARD TO A HFD-WEST-UUU LINE BETWEEN 6 AM AND 8 AM.

BY MIDDAY THE NORTH EDGE OF THE SNOW SHIELD SHOULD BE AROUND THE

MASS PIKE AND TURNING SOUTHEAST INTO SOUTHEAST MA. BY 1 PM TO 4 PM

SNOW SHOULD REACH THE MA/NH BORDER. SNOW BECOMES STEADIEST AND

HEAVIEST 18Z-00Z. SNOWFALL RATES OF AROUND UP TO AN INCH PER HOUR

WILL BE COMMON DURING THIS TIME ACROSS CT/RI AND SOUTH OF THE MASS

PIKE. POTENTIAL STRONG SNOW BANDS MAY PIVOT NORTHWARD ONSHORE INTO

SOUTHERN RI/SOUTHEAST INCLUDING CAPE COD AND NANTUCKET 18Z-00Z. IN

FACT THERE IS LOW FOR SLANTWISE CONVECTION/THUNDERSNOW OVER THIS

REGION. THIS WOULD YIELD A PERIOD OF SNOWFALL RATES ON THE ORDER OF

1-3" FOR A BRIEF TIME. HOWEVER CONFIDENCE IS LOW ON NAM AND GFS

DIFFER ON LOCATION/MAGNITUDE AND INSTABILITY. IN ADDITION BOTH NAM

AND GFS INDICATE SOME DRY AIR ADVECTING INTO THE SNOW GROWTH REGION

/-10C TO -20C LAYER/. HENCE UNCERTAINTY ON INTENSITY AND

EXACT LOCATION OF SNOW BANDS THIS AFTERNOON.

SNOWFALL...

NOT MUCH CHANGE FROM PREVIOUS FORECAST HOWEVER DID INCORPORATE

SLIGHTLY HIGHER TOTALS INTO SOUTHERN HARTFORD COUNTY AS POTENTIAL

BANDING FROM NYC/SOUTHWEST CT ADVECTS NORTHEAST. ALSO INCREASED

AMOUNTS A BIT ACROSS SOUTHEAST MA NORTHWARD INTO THE SOUTHERN

SUBURBS OF BOSTON /NORFOLK COUNTY/. THIS IS IN RESPONSE TO ALL MODEL

GUIDANCE DEVELOPING SECONDARY MID LEVEL LOW SOUTHEAST OF CAPE COD

WHICH BRIEFLY WRAPS WCB/TROWAL PRECIP ACROSS THE SOUTHERN SUBURBS OF

BOSTON BETWEEN 00Z-06Z. THEREFORE HAVE EXPANDED THE WINTER WEATHER

ADVISORY NORTHWARD INTO NORFOLK COUNTY. AS FOR OUR SNOW PROBABILITY

GRAPHICS...GIVEN THESE MAPS ARE FOR PLANNING PURPOSES AND THE

STORM IS JUST ABOUT UNDERWAY...THESE GRAPHICS WILL BE DISCONTINUED.

HEADLINES ...

AS MENTIONED ABOVE ONLY CHANGE FROM PREVIOUS HEADLINES WAS EXPANDING

THE WINTER WEATHER ADVISORY NORTHWARD INTO NORFOLK COUNTY...SOUTHERN

SUBURBS OF BOSTON. WAS TEMPTED TO EXPAND BLIZZARD WARNINGS INTO

NANTUCKET AND CAPE COD HOWEVER MODEL SOUNDINGS INDICATE BLYR MAY

WARM ENOUGH FOR RAIN TO MIX IN AT TIMES ALONG WITH THIN WARM LAYER

ALOFT YIELDING A POSSIBLE MIX OF SLEET. EITHER OF THESE SCENARIOS

WILL RESULT IN VSBYS TOO HIGH TO JUSTIFY A BLIZZARD WARNING.

NEVERTHELESS DAY SHIFT WILL NEED TO KEEP AN EYE ON THIS.

STRONG TO DAMAGING WINDS...

00Z GUID HAS TRENDED STRONGER WITH LOW LEVEL JET. STRONGEST WINDS

18Z-06Z. 00Z NAM WAS THE STRONGEST MODEL ALTHOUGH REMAINDER OF GUID

NOT TOO FAR BEHIND. NAM SOUNDINGS SUGGEST LOW PROBABILITY OF

HURRICANE FORCE WINDS AT NANTUCKET...SLIGHTLY LOWER RISK ACROSS CAPE

COD. NONETHELESS HIGH WIND WARNING WILL REMAIN IN EFFECT FOR THIS

REGION WITH THE EXPECTATION OF ISOLATED DAMAGING WINDS AND POWER

OUTAGES.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...

TONIGHT...

SNOW BEGINS TO TAPER OFF ACROSS CT AND WEST-CENTRAL MA BETWEEN 00Z-

03Z AND THEN THE REMAINDER OF THE AREA 03Z-06Z EXPECT POSSIBLY

LINGERING UNTIL 12Z CAPE COD AND NANTUCKET.

IN ADDITION...STRONG NE WINDS WILL SHIFT TO THE NORTH AND BEGIN TO

SLOWLY SLACKEN.

&&

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