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January 17-18th Cape Scrape and Inverted Trough


USCAPEWEATHERAF

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Here is your 3pm update posted at 5:23PM from BOX: 

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
300 PM UPDATE...

* ACCUMULATING SNOW TONIGHT INTO MON AM COMMUTE
* WINTER WEATHER ADVISORIES FOR EASTERN HALF OF SNE

LIGHT SNOW WAS WORKING ITS WAY UP COAST IN ADVANCE OF MID LEVEL
SHORT WAVE. MOST OF LEADING EDGE IS ALOFT BUT SNOW IS REACHING
GROUND ACROSS NYC METRO AREA. HIGH-RES MODELS HAVE GOOD HANDLE ON
THIS AREA OF SNOW...BRINGING IT TO S COAST AROUND 5 OR 6 PM. IT
SHOULD START AS LIGHT RAIN ON NANTUCKET AND PARTS OF CAPE COD DUE
TO MARGINAL SURFACE TEMPERATURES...BUT EVEN THERE WILL CHANGE TO
SNOW EARLY TONIGHT AS LOWER LEVELS COOL.

SNOW WILL THEN SPREAD INTO REST OF SOUTHERN NEW ENGLAND BEFORE
MIDNIGHT...BUT STEADIER SNOW WILL BE FOCUSED FROM CENTRAL MA/NE CT
INTO EASTERN MA WHERE MODELS SHOW BEST FRONTOGENESIS...LOW LEVEL
CONVERGENCE AND STEEPER LAPSE RATES IN ASSOCIATION WITH NORLUN
TROUGH. WHERE EXACTLY IN THIS REGION HEAVIER SNOW BANDS SET UP IS
STILL UNKNOWN...BUT CONSENSUS OF HIGH-RES MODELS AND 12Z
GFS/ECMWF/RGEM SUGGEST BANDING GETTING NEAR I-495 FOR A TIME
BEFORE SHIFTING TOWARD COASTLINE OVERNIGHT. CROSS SECTIONS SHOW
MODEST LIFT IN SNOW GROWTH ZONE...AGAIN ESPECIALLY IN E MA.

AT SAME TIME WE ALSO HAVE POTENTIAL FOR SNOW SQUALLS NEAR
BERKSHIRES DUE TO ARCTIC FRONT. THIS COULD BRING QUICK COATING TO
AN INCH TO MUCH OF WESTERN NEW ENGLAND.

WE STILL THINK NE MA IS FAVORED FOR HIGHER TOTALS /2-4"/ BUT
ONSHORE FLOW COULD END UP GIVING COASTAL PLYMOUTH COUNTY AND UPPER
CAPE SOME ADDED ENHANCEMENT. SHOULD ALSO SEE SOME ACCUMULATION ON
ISLANDS AND OUTER CAPE BUT MILDER SURFACE TEMPS WILL PREVENT MORE
THAN AN INCH OR SO.

WE SHOULD NOTE SYSTEM IS OVER PERFORMING TO SOME DEGREE ACROSS MID
ATLANTIC WHERE WE HAVE SEEN REPORTS OF 2" ACROSS DE/NJ. IF WE WERE
TO TALK ABOUT WORST CASE SCENARIO...IT WOULD BE FOR AS MUCH AS
5-7" WHICH IS REFLECTED IN OUR PROBABILISTIC SNOW FORECASTS ON
WINTER WEATHER WEB PAGE.

WE ARE ISSUING WINTER WEATHER ADVISORIES FOR EASTERN MA INCLUDING
CAPE COD BUT WILL ALSO EXTEND BACK INTO CENTRAL MA...RI AND NE CT
DUE TO SOME UNCERTAINTY AS TO FAVORED BANDING AREAS...AND FACT
THIS WILL HAVE SOME IMPACT ON MON MORNING COMMUTE.

KEEP IN MIND MODELS CAN STRUGGLE WITH DETAILS WITH NORLUN TROUGHS
AND LOCATIONS OF QPF AXIS CAN BE OFF. IT IS USUALLY NOT UNTIL WE
SEE BANDS FORM ON RADAR THAT WE KNOW EXACTLY WHICH AREAS THEY WILL
AFFECT. HOWEVER INGREDIENTS ARE THERE FOR MODERATE /2-4"/
ACCUMULATIONS TONIGHT.

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Radar looks pretty good. Couple of inches looks good even without Norlun

 

Somewhere near or just NW of the canal may try for advisory without the inverted trough...but that extra boost from IVT will definitely be needed most places for advisory amounts. A lot of disagreement on short term guidance on what happens overnight...latest RPM shows the IVT snows pretty weak and transient, so not much, but RAP is hammering E MA pretty hard....HRRR is decent too, though maybe not quite as bullish as RAP.

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