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Potential Major Storm 9/18


NEG NAO

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everything looks on track to me

are you not concerned about the lack of discrete convection and the low SB CAPE? Though perhaps the CAPE can still increase and I think the localized low-level convergence increase won't happen until around 20 or 21z...maybe that can help with our lift ahead of the line.

But I still would have really liked more clearing and SB CAPE values of ~1000 J/KG right now. The 0z NAM last night had SB CAPE values above 1000 J/KG for us at this time.

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Its the convergence issue more than CAPE. There are not even very low clouds yet. Obviously we are not tapping anything yet. The CAPE thats there is enough if we increase the convergence which will happen later this afternoon.

are you not concerned about the lack of discrete convection and the low SB CAPE? Though perhaps the CAPE can still increase and I think the localized low-level convergence increase won't happen until around 20 or 21z...maybe that can help with our lift ahead of the line.

But I still would have really liked more clearing and SB CAPE values of ~1000 J/KG right now. The 0z NAM last night had SB CAPE values above 1000 J/KG for us at this time.

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Its the convergence issue more than CAPE. There are not even very low clouds yet. Obviously we are not tapping anything yet. The CAPE thats there is enough if we increase the convergence which will happen later this afternoon.

Right. The 00z NAM forecast last night had localized increases in low-level vorticity at around 21z in E NJ/NYC/western LI. That might help. I'm definitely not giving up yet, but the NAM last night did show clearing and higher SB CAPE...it would be nice to not have to rely completely on the convergence and to have gotten help from the CAPE/instability like what the NAM was showing last night.

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BULLETIN - IMMEDIATE BROADCAST REQUESTED

TORNADO WATCH OUTLINE UPDATE FOR WT 646

NWS STORM PREDICTION CENTER NORMAN OK

340 PM EDT TUE SEP 18 2012

TORNADO WATCH 646 IS IN EFFECT UNTIL 1100 PM EDT FOR THE

FOLLOWING LOCATIONS

CTC001-005-009-190300-

/O.NEW.KWNS.TO.A.0646.120918T1940Z-120919T0300Z/

CT

. CONNECTICUT COUNTIES INCLUDED ARE

FAIRFIELD LITCHFIELD NEW HAVEN

MAC003-190300-

/O.NEW.KWNS.TO.A.0646.120918T1940Z-120919T0300Z/

MA

. MASSACHUSETTS COUNTIES INCLUDED ARE

BERKSHIRE

NYC001-021-027-035-039-041-043-057-059-079-083-091-093-095-103-

111-113-115-119-190300-

/O.NEW.KWNS.TO.A.0646.120918T1940Z-120919T0300Z/

NY

. NEW YORK COUNTIES INCLUDED ARE

ALBANY COLUMBIA DUTCHESS

FULTON GREENE HAMILTON

HERKIMER MONTGOMERY NASSAU

PUTNAM RENSSELAER SARATOGA

SCHENECTADY SCHOHARIE SUFFOLK

ULSTER WARREN WASHINGTON

WESTCHESTER

ANZ330-335-340-345-350-353-355-190300-

/O.NEW.KWNS.TO.A.0646.120918T1940Z-120919T0300Z/

CW

. ADJACENT COASTAL WATERS INCLUDED ARE

LONG ISLAND SOUND EAST OF NEW HAVEN CT/PORT JEFFERSON NY

LONG ISLAND SOUND WEST OF NEW HAVEN CT/PORT JEFFERSON NY

PECONIC AND GARDINERS BAYS

SOUTH SHORE BAYS FROM JONES INLET THROUGH SHINNECOCK BAY

MORICHES INLET NY TO MONTAUK POINT NY OUT 20 NM

FIRE ISLAND INLET NY TO MORICHES INLET NY OUT 20 NM

SANDY HOOK NJ TO FIRE ISLAND INLET NY OUT 20 NM

ATTN...WFO...OKX...ALY...

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I like the appearance of the line of storms over Eastern PA this past half hour. It will be interesting to see if they push east into West/Central New Jersey.

Entering the environment of strong wind shear, will be interesting to see if the line begins to push out and bow. Although lowering the likelihood of tornados, it would increase the chance of more widespread severe and damaging straight line winds.

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I think the line further south from MD into northern VA is the one we need to keep an eye on.

I think that might be too far south. The area of convection from N MD into Southeast PA has taken on an east-northeast trajectory. Now there's even some discrete stuff ahead of it moving north.

It will be interesting to see if the strengthening low level jet triggers more precipitation around the line and forces it to become englufed.

*Edited for north/south typo.

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BULLETIN - EAS ACTIVATION REQUESTED

SEVERE THUNDERSTORM WARNING

NATIONAL WEATHER SERVICE MOUNT HOLLY NJ

423 PM EDT TUE SEP 18 2012

THE NATIONAL WEATHER SERVICE IN MOUNT HOLLY NJ HAS ISSUED A

* SEVERE THUNDERSTORM WARNING FOR...

EAST CENTRAL BERKS COUNTY IN EASTERN PENNSYLVANIA...

EASTERN LEHIGH COUNTY IN EASTERN PENNSYLVANIA...

THIS INCLUDES THE CITY OF ALLENTOWN...

NORTHAMPTON COUNTY IN EASTERN PENNSYLVANIA...

THIS INCLUDES THE CITY OF EASTON...

SOUTHERN MONROE COUNTY IN NORTHEAST PENNSYLVANIA...

WARREN COUNTY IN NORTHWEST NEW JERSEY...

* UNTIL 515 PM EDT

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I think that might be too far north. The area of convection from N MD into Southeast PA has taken on an east-northeast trajectory. Now there's even some discrete stuff ahead of it moving north.

It will be interesting to see if the strengthening low level jet triggers more precipitation around the line and forces it to become englufed.

The line in southeast Pa. / northern Md. might hold on in it's intensity. Anything that has developed east ahead of the squall line thru central NY and central/eastern Pa has not intensifed into descrete cells as of yet. The winds have picked up here in the Hudson valley to being sustained in the mid 20's and the gusts are more frequent into the mid 30's here. We'll see :popcorn:

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mcd1970.gif

MESOSCALE DISCUSSION 1970

NWS STORM PREDICTION CENTER NORMAN OK

0321 PM CDT TUE SEP 18 2012

AREAS AFFECTED...ERN PA...DE...NJ...S-CNTRL/ERN NY...WRN CT...WRN MA

CONCERNING...TORNADO WATCH 643...645...646...

VALID 182021Z - 182215Z

THE SEVERE WEATHER THREAT FOR TORNADO WATCH

643...645...646...CONTINUES.

SUMMARY...THE THREAT FOR SVR WINDS AND TORNADOES CONTINUES ACROSS

TORNADO WATCHES 643...645...AND 646.

DISCUSSION...THE SVR THREAT ACROSS WW/S 643/645/646 IS EVOLVING IN

TWO REGIMES:

/1/ ONE REGIME IS ALONG A SHALLOW SQUALL LINE AXIS EXTENDING FROM

THE ADIRONDACKS SWD INTO S-CNTRL NY AND ADJACENT PARTS OF FAR NRN

PA. AN INTENSE LOW-LEVEL JET STREAM ASSOCIATED WITH 50-55 KT OF

0-1-KM BULK SHEAR AND ELONGATED/LOOPING LOW-LEVEL HODOGRAPHS PER

AREA VWP DATA WILL SUPPORT A THREAT FOR SVR WINDS AND TORNADOES WITH

THE EWD-ADVANCING LINE.

/2/ THE OTHER REGIME IS WITHIN THE BROAD ZONE OF WARM ADVECTION

LEADING THE SQUALL LINE FROM THE MID ATLANTIC TO THE HUDSON VALLEY

AND SWRN NEW ENGLAND. THE ISALLOBARIC RESPONSE TO SFC PRESSURE FALLS

OF 3-4 MB PER 2 HRS OVER NERN NY/NRN VT WILL CONTINUE TO DRIVE

MODEST BOUNDARY-LAYER DESTABILIZATION NWD -- MLCAPE OF 250 TO 500

J/KG -- IN ADVANCE OF THE SQUALL LINE OWING TO THE INFLUX OF

RICHER/MARINE-LAYER MOISTURE. SIMILAR LOW-LEVEL SHEAR AS MENTIONED

ABOVE SHOULD SUPPORT ROTATING STORMS EVOLVING IN BOTH LINEAR

SEGMENTS AND PERHAPS SEMI-DISCRETE SUPERCELL MODES.

..COHEN.. 09/18/2012

ATTN...WFO...BOX...BTV...OKX...ALY...PHI...BGM...BUF...CTP...

LWX...

LAT...LON 38677574 39617634 40167641 41287606 42807563 43417531

43957504 43997443 43687368 43367344 42867328 42487318

41907296 41437246 41257217 41047204 40867244 40557320

40157338 39737365 39077401 38587474 38677574

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The LLJ increasing in strength is making the boundary layer more moist, helping to increase our SB CAPE without sunshine. It might not be enough, but it's an improvement...1000 + J/KG in lots of S NJ and heading for Long Island. Maybe now is our window for discrete cells? They would probably start to form in S NJ.

sbcp_sf.gif?1348000292667

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It almost feels like the few hours as Irene was passing to our West here in SE Nassau (Wantagh). SSE winds gusting into the 40's to near 50 right on the water. Granted the winds during Irene at the point were slightly stronger it has that same general feel. Irene also did a good jo tree trimming any trees with a weak South facing side so we are seeing less branches down then normal for wind of this strength and direction during full leaf season.

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