Severe Weather Outbreak

MODERATE THREAT FOR SEVERE WEATHER TODAY

Destructive thunderstorms will develop and move through the Mid-Atlantic and Northeast to start the weekend off on a turbulent note. While widespread damaging wind gusts will be the main hazard, tornadoes cannot be ruled out.

This deadly Low Pressure system is responsible for four (4) deaths and wind damage reports yesterday from Oklahoma to Ohio and western New York.



This Low will strengthen today in southeastern Canada, it will send a cold front diving into the heavily populated Mid-Atlantic and Northeast Interstate 95 communities. Strong southerly winds ahead of this front are ushering in warm and tropical air. At the same time, westerly winds behind the front are driving a fall-like air mass toward the region.

The clash of these air masses will lead to violent upward motion in the atmosphere, and the result will be Severe thunderstorms capable of damaging wind gusts, hail larger than golf balls is expected in many areas. Where these air masses collide, a furious twisting or rotation motion will occur in the atmosphere caused by the sudden changes in wind direction near and just ahead of the front. This will lead to a real threat of tornadoes, mostly from New York City to Allentown northward to Albany areas. This is a potential violent set-up for the Northeast.



Areas surrounding the elevated severe weather danger zone won't escape lightly either. Residents from central and southern Maine to the Carolina Piedmont and northeastern Georgia will also be in harm's way this afternoon and evening.

The SPC Convective outlook



DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1227 AM CDT SAT SEP 08 2012

VALID 081200Z - 091200Z

...THERE IS A MDT RISK OF SVR TSTMS PORTIONS CENTRAL/ERN
NY...CENTRAL/SRN VT...SWRN NH...CENTRAL/WRN MA...MUCH OF
CT...NJ...NRN DE...NERN MD...AND ERN PA...

...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING MDT RISK
AREA...FROM NRN NEW ENGLAND AND NY SWWD TO SC AND PORTIONS ERN GA...

...SYNOPSIS...
UPPER-AIR PATTERN OVER CONUS WILL FEATURE WRN MEAN RIDGING AND ERN
TROUGHING. LATTER WILL BE REINFORCED BY STG SHORTWAVE NOW ADVANCING
SEWD ACROSS LS...IA...AND CENTRAL PLAINS. PRIMARY 500-MB VORTICITY
CONCENTRATION ASSOCIATED WITH THIS PERTURBATION IS OVER WI ATTM.
ENHANCED VORTICITY FIELD IS FCST TO PIVOT CYCLONICALLY ACROSS LOWER
MI...NRN INDIANA AND OH THROUGH 08/18Z...THEN WEAKEN SOMEWHAT AND
EJECT NEWD TO NRN QUE BY 9/12Z. MEANWHILE...TRAILING/NARROW RIBBON
OF MAXIMIZED MID-UPPER LEVEL VORTICITY WILL MOVE GENERALLY SEWD OVER
SRN PLAINS...TN VALLEY/MID-SOUTH REGION...AND CENTRAL/SRN
APPALACHIANS...ALSO GRADUALLY WEAKENING. THESE PROCESSES WILL OCCUR
AS UPSTREAM TROUGH -- NOW EVIDENT IN MOISTURE CHANNEL IMAGERY OVER
NRN MB -- DIGS SEWD AND AMPLIFIES...REACHING LOWER GREAT LAKES AND
OH VALLEY BY END OF PERIOD.

SEASONALLY STG SFC COLD FRONT IS EVIDENT ATTM FROM LE SWWD ACROSS
OH...NEARLY COLLOCATED WITH SQUALL LINE FROM OH TO ARKLATEX
REGION...AND OTHERWISE SURGING SWD ACROSS NERN/W-CENTRAL TX AND
TRANS-PECOS REGION. VIGOROUS SFC CYCLOGENESIS SHOULD OCCUR ALONG
FRONT BY EARLY IN PERIOD OVER SERN ONT. RESULTING LOW THEN SHOULD
DEEPEN AND EJECT NEWD OVER NRN QUE...SLIGHTLY AHEAD OF LEADING
500-MB VORTICITY LOBE. BY 9/00Z...TRAILING COLD FRONT SHOULD REACH
ERN NY...DE RIVER VALLEY...WRN NC...NRN GA...AND SERN LA. FRONT
SHOULD MOVE OUT OF ERN MAINE AROUND END OF PERIOD...WHILE SRN
PORTION MOVES SEWD ACROSS ERN CAROLINAS AND SRN GA.

...NY/NEW ENGLAND SWWD TO SRN PIEDMONT REGION...
SCATTERED-NUMEROUS TSTMS ARE EXPECTED TO SWEEP EWD ACROSS THIS
REGION ALONG/AHEAD OF SFC FRONT. MAIN BAND OF SVR CONVECTION WILL
OFFER DENSE CONCENTRATION OF SVR/DAMAGING GUSTS...SOME OF WHICH MAY
REACH 65 KT...AS WELL AS A FEW TORNADOES...IN AND NEAR MDT RISK
AREA. TORNADOES ALSO ARE POSSIBLE WITH ANY OF SEVERAL STORM-SCALE
CIRCULATION REGIMES. HAIL RISK APPEARS TERTIARY IN IMPORTANCE.
COMBINATION OF INFLUENCES FROM AFOREMENTIONED MID-UPPER SHORTWAVES
WILL RESULT IN HEIGHT FALLS...TIGHTENING HEIGHT GRADIENTS
ALOFT...AND STRENGTHENING WIND FIELDS ALOFT OVER WARM SECTOR DURING
PERIOD OF DIURNAL HEATING. MEANWHILE...EXPECT LOW-LEVEL MOISTURE
TRANSPORT AND WAA THROUGHOUT PREFRONTAL MID-ATLC...WHERE MAIN BELT
OF DEEP ASCENT WILL ENCOUNTER RESERVOIR OF MID-60S TO LOW-70S F SFC
DEW POINTS. STG SFC DIABATIC HEATING IS EXPECTED FROM SERN CONUS
NEWD AT LEAST INTO DELMARVA/TIDEWATER REGION. CLOUD COVER MAY LIMIT
INSOLATION FARTHER N...THOUGH SUFFICIENT HEATING SHOULD OCCUR TO
WEAKEN CAP FAVORABLY OVER NYC AREA...LOWER HUDSON VALLEY AND SRN NEW
ENGLAND.

WIND-DAMAGE AND SPORADIC TORNADO THREAT WILL EXIST WITH PRIMARY
SQUALL LINE OVER MID-ATLC REGION...ERN NY AND WRN NEW ENGLAND.
TORNADO POTENTIAL SHOULD DIMINISH SWD INTO CAROLINAS...WITH DAMAGING
GUSTS STILL PRIMARY THREAT. SVR SHOULD BE MOST CONCENTRATED AROUND
BOW/LEWP FEATURES...EMBEDDED SUPERCELLS...AND RELATED
MESOCIRCULATIONS...GIVEN STRENGTH OF LOW-LEVEL SHEAR INTO WHICH THIS
CONVECTION WILL BE MOVING OVER MID-ATLC REGION. FCST SOUNDINGS SHOW
AREA OF 200-350 J/KG 0-1 KM SRH PRIOR TO FROPA...FROM NERN PA/NJ NWD
UP HUDSON VALLEY AND INTO WRN NEW ENGLAND. THAT AREA WILL LIE ON
NRN FRINGE OF STRONGEST BUOYANCY...WITH ENOUGH HEATING AND RICH
LOW-LEVEL THETAE TO YIELD 1000-2000 J/KG MLCAPE OVER WRN NY/WRN NEW
ENGLAND. BUOYANCY WILL INCREASE SSWWD TOWARD CHESAPEAKE BAY AND
BEYOND WHERE LOW-LEVEL SHEAR WILL BE WEAKER. OVERALL SVR THREAT
WILL BECOME LESS ORGANIZED AND SHORTER IN DURATION FROM GA SWWD TO
CENTRAL GULF COAST REGION.

IN MID-ATLC REGION...CONSIDERABLE UNCERTAINTY REMAINS REGARDING
NUMBER AND LONGEVITY OF RELATIVELY DISCRETE TSTMS AHEAD OF MAIN
LINE...WHICH WOULD POSE MORE ORGANIZED SUPERCELLULAR TORNADO RISK.
FCST BREADTH OF WARM SECTOR...LIKELIHOOD OF RELATIVELY WEAK MLCINH
BY EARLY AFTN...AND ENVIRONMENT CHARACTERIZED BY LARGE
HODOGRAPHS...SUGGEST SOME TSTMS MAY HAVE TIME TO MATURE INTO
DISTINCT SUPERCELLS. THEREFORE...RELATIVE MAX IN TORNADO
PROBABILITIES IS DRAWN FROM ERN NY/WRN NEW ENGLAND INTO SRN NEW
ENGLAND...ERN PA AND NJ.

OVERALL SVR POTENTIAL SHOULD DIMINISH AFTER DARK...THOUGH STG
DEEP-LAYER WINDS AND AT LEAST MRGL BUOYANCY WILL SHIFT EWD OVER MOST
OF NEW ENGLAND DURING LATE EVENING/OVERNIGHT HOURS. AS SUCH...SLGT
RISK PROBABILITIES ARE CARRIED EWD TO COAST...EXCEPT FOR MRGL RISK
OVER ERN MAINE LATE IN PERIOD.

..EDWARDS/LEITMAN.. 09/08/2012

CLICK TO GET WUUS01 PTSDY1 PRODUCT

NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1300Z

The line on the 12z NAM more or less "explodes" right over the area and stays strong as it pushes through New Jersey and eastward through Long Island




Now to get a little more technical. This a very impressive synoptic setup with lots of large-scale ascent from a potent trough swinging to the east, creating height falls and very fast mid and upper level winds with strong 250mb divergence. Most of the northeast will also be in the right entrance region of the jet, also promoting large-scale ascent.The large trough and storm system will also help to promote strong southerly flow at the surface, helping to advect rich low-level moisture, with dewpoints in the low 70s. Some areas will see a SE flow, but considering how warm the ocean is in the early fall, the ocean provides a good moisture source without really providing too much in the way of stabilization away from the immediate coast.

However, one thing to note is that at 500mb, the shortwave energy and area of vorticity appears to escape to the northeast a bit quicker than previous runs. Instead of the strongest area of vorticity being in Pennsylvania at 00z, Sunday, it is now located in NW Upstate NY. This will result in some of the best shear escaping to the northeast a bit faster, and not extending as far to the south into New Jersey.



With the vorticity escaping to the northeast a bit faster, this also means that the best low-level helicity will also be confined to areas north of the immediate Metro region, but still not too far away. The values of 200 meters squared per second squared are confined to the southern Hudson Valley and adjacent areas, as opposed to extending southward into New Jersey. That being said, there is still enough low-level helicity for isolated spin-ups in any part of the Metro region, but the best threat for that appears to be to the north of New York City.



A sounding that really stood out was the 21z sounding at KSWF, which is Stewart International Airport, located in the southern Hudson Valley, near Newburgh, NY.



As one heads to the valleys, south and southeast winds are often “trapped”, leading to increased low-level helicity and thus a higher tornado threat. The winds increase from 15 knots out of the SE at the surface to around 45 knots out of the SW at 850mb, as a strong, closed 850mb low in SE Canada really helps to get the low-level jet cranking. Additionally, the CAPE is 2085J/KG. It would honestly surprise me if there were no tornado reports in the Hudson Valley tomorrow based on these soundings. There is also very good 0-6km shear, though the directional shear is a bit lackluster once you go above 850mb. The extremely fast low-level jet and very fast mid and upper level winds will help provide strong winds in the mixed layer that can easily be mixed down with any form of convection, and a fast-moving squall line which will be able to stay organized with the sufficient shear and instability. The forward speed of the squall-line will only add to the very strong winds that are already possible. Additionally, as I mentioned in yesterday morning’s discussion, the dry punch of air at the mid-levels helps to support strong downdrafts and downbursts because of the evaporational cooling that would take place when it rains. This leads to the surrounding environment becoming colder than the temperature of an air parcel, leading it to sink rapidly.

There is lots of large-scale ascent from the powerful trough, meaning that an area of ascent can race well out ahead of the squall line, when instability should still be strong as well as helicity and shear. When you combine that with a strong 250mb divergent flow, it is certainly possible for discrete cells to form to the left of the low-level jet, much more-so than what was seen on 7/26. This is another reason why everyone in the Metro area needs to be wary of tornadoes, though as previously mentioned, the strongest risk for tornadoes should be in the southern Hudson Valley. I’d also put NE PA in a relatively strong risk area for tornadoes as well, and this zone should extend to the north and east until about eastern upstate NY near the Massachusetts border.

Even if the discrete convection does not work out, there will still be enough shear and helicity in association with the squall line that there may be several tornadoes embedded. The difference between a 5% tornado zone and a 10% tornado zone essentially comes from the discrete supercells that could form before the squall line hits. So this could be a two-part deal, which is why I strongly urge everyone remain wary of the situation.

If you have friends or family that lives in the Northeast, today might be a good day to call or email them to keep a watchful eye out to the skies today.

good read.....am on my toes here today as the front approaches

Cool shot of Isaac.

Thanks E. Looks like there's gonna be some KA-POW going on today. Hopefully nothing too serious when it's all said and done.

Very thorough blog, this morning!! Thanks e...... Based on this setup, it could be a very long weekend for some folks, while we, here in the SE states, will see, what appears to be, "just precip"........

Thanks E. Great job!

BULLETIN - EAS ACTIVATION REQUESTED
TORNADO WARNING
NATIONAL WEATHER SERVICE NEW YORK NY
1154 AM EDT SAT SEP 8 2012

THE NATIONAL WEATHER SERVICE IN UPTON NY HAS ISSUED A

* TORNADO WARNING FOR...
FAIRFIELD COUNTY IN SOUTHERN CONNECTICUT...
EASTERN WESTCHESTER COUNTY IN SOUTHEAST NEW YORK...

TORNADO WARNING FOR...
WESTCHESTER COUNTY IN SOUTHEAST NEW YORK...
ROCKLAND COUNTY IN SOUTHEAST NEW YORK...

TORNADO WARNING FOR...
QUEENS COUNTY IN SOUTHEAST NEW YORK...
KINGS (BROOKLYN) COUNTY IN SOUTHEAST NEW YORK.

Re-Issue
* TORNADO WARNING FOR...
QUEENS COUNTY IN SOUTHEAST NEW YORK...
KINGS (BROOKLYN) COUNTY IN SOUTHEAST NEW YORK...

According to The National Weather Service, the tornado has touched down on the Flatbush Avenue area of the Belt Parkway (and is moving eastward). Queens and Brooklyn have been put on tornado alert until 11:30 a.m. They recommend that residents immediately go indoors and/or to the lowest floor of your building for shelter, and to stay away from windows.

According to reports, there is already "a lot of damage in the area," including cars ontop of other cars.