Sept. 30th - Oct. 2nd Severe Weather Thread

[andyhb]

Figured I'd start a thread given that there hasn't been much discussion on this. The 09z SREF is popping a 50 STI for Monday (also at 18z and 00z).

Day 1 Outlook:

...LOWER MS VALLEY/CNTRL GULF COAST...

WIDESPREAD CLOUDINESS WILL LIMIT HEATING/DESTABILIZATION ACROSS MUCH

OF THE REGION TODAY BUT SOME BREAKS IN CLOUDS IN SRN LA AND POSSIBLY

INTO FAR SRN MS WILL ALLOW FOR GREATER DESTABILIZATION FURTHER S.

RESULTING POOR LAPSE RATES WILL LIMIT MUCAPE VALUES TO AROUND

750-1500 J/KG DESPITE VERY MOIST AIRMASS. WHILE MARGINAL

THERMODYNAMIC ENVIRONMENT IS EXPECTED...DEEP LAYER SHEAR WILL

INCREASE THROUGH THE DAY AND INTO THE OVERNIGHT HOURS AS UPPER

TROUGH SHIFTS EWD AND AMPLIFIES. AT THE SAME TIME...SFC LOW WILL

STRENGTHEN WITH A RESPONDING INCREASE IN LOW LEVEL WINDS...WITH A

SLY LOW LEVEL JET AROUND 30-35 KT FORECAST TO DEVELOP ACROSS FAR SE

LA INTO SRN/CNTRL MS/AL TONIGHT. FURTHERMORE...VEERING VERTICAL WIND

PROFILES WITH INCREASING SPEED SHEAR WILL LEAD TO A THREAT FOR A FEW

TORNADOES AND STRONG/DAMAGING WINDS AS LINE SEGMENTS DEVELOP AHEAD

OF EWD ADVANCING COLD FRONT FROM SRN LA/MS THIS AFTERNOON INTO SRN

AL/FAR WRN FL PANHANDLE TONIGHT. FURTHER NORTH...ADDITIONAL

THUNDERSTORM DEVELOPMENT IS EXPECTED IN WAA REGIME ALONG AND NORTH

OF A W-E ORIENTED WARM FRONT FROM CNTRL/NRN MS/AL. IT SHOULD BE

NOTED THAT STRONGER FORCING FOR ASCENT WILL BE DISPLACED SLIGHTLY TO

THE NORTH...TN/NRN MS/AL...OF BEST SEVERE POTENTIAL...PERHAPS

LIMITING COVERAGE OF STRONGER/SEVERE STORMS GIVEN MARGINAL

THERMODYNAMIC ENVIRONMENT.

Day 2:

...GULF COAST STATES/SE TN...

AN UPPER-LEVEL LOW CENTERED OVER ARKANSAS WILL OPEN AND MOVE SLOWLY

EWD ON MONDAY. A 40 TO 50 KT MID-LEVEL JET SOUTH OF THE LOW WILL

EJECT NNEWD ACROSS THE WARM SECTOR MONDAY AS A SFC LOW AND COLD

FRONT ADVANCE EWD INTO THE CNTRL GULF COAST STATES. MODELS SHOW

MODERATE INSTABILITY IN PLACE TO THE SOUTHEAST OF THE DEEPENING SFC

LOW WITH SFC DEWPOINTS IN THE UPPER 60S AND LOWER 70S F ACROSS ERN

MS...AL AND GA. THUNDERSTORMS APPEAR LIKELY TO BE ONGOING NORTH OF

THE LOW CENTER ALONG THE AXIS OF THE LOW-LEVEL JET SUNDAY MORNING.

FROM THE LOW SEWD...MODEL FORECASTS SHOW A LINE OF THUNDERSTORMS

WITH THIS LINE MOVING GRADUALLY EWD ACROSS THE WARM SECTOR DURING

THE AFTERNOON.

FORECAST SOUNDINGS ACROSS THE WARM SECTOR IN ERN AL AND WRN GA

MONDAY AFTERNOON SHOW MLCAPE VALUES OF 1000 TO 1500 J/KG WITH 0-6 KM

SHEAR AROUND 45 KT. IN ADDITION...LCL HEIGHTS ARE FORECAST TO BE

NEAR 500 METERS WITH 0-1 KM SHEAR OF 25 TO 30 KT. THIS ENVIRONMENT

SHOULD SUPPORT ROTATING CELLS ELEMENTS EMBEDDED IN THE LINE OR

DISCRETE SUPERCELLS AHEAD OF THE LINE ACROSS THE WARM SECTOR. AS THE

ENVIRONMENT BECOMES MORE FAVORABLE DURING THE DAY...AN ISOLATED

TORNADO THREAT IS EXPECTED TO DEVELOP MONDAY AFTERNOON. WIND DAMAGE

COULD ALSO OCCUR WITH THE MORE INTENSE SEGMENTS OF THE LINE. THE

GREATEST TORNADO POTENTIAL MAY OCCUR SUNDAY AFTERNOON ACROSS NE AL

AND NW GA INTO SE TN WHERE LOW-LEVEL WINDS ARE FORECAST TO BE BACKED

TO THE SOUTHEAST WHICH WILL ENHANCE LOW-LEVEL SHEAR PROFILES.

[andyhb]

Tornado watch, Southern MS/Southeastern LA:

URGENT - IMMEDIATE BROADCAST REQUESTED

TORNADO WATCH NUMBER 656

NWS STORM PREDICTION CENTER NORMAN OK

135 PM CST SUN SEP 30 2012

THE NWS STORM PREDICTION CENTER HAS ISSUED A

TORNADO WATCH FOR PORTIONS OF

SOUTHEAST LOUISIANA

SOUTHERN MISSISSIPPI

COASTAL WATERS

EFFECTIVE THIS SUNDAY AFTERNOON AND EVENING FROM 135 PM UNTIL 700

PM CST.

TORNADOES...HAIL TO 1 INCH IN DIAMETER...THUNDERSTORM WIND GUSTS

TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.

THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 80 STATUTE

MILES EAST AND WEST OF A LINE FROM 50 MILES NORTH NORTHWEST OF

PINE BELT MISSISSIPPI TO 30 MILES WEST SOUTHWEST OF SLIDELL

LOUISIANA. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE

ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU6).

REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR

TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH

AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR

THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS

AND POSSIBLE WARNINGS.

DISCUSSION...STORMS DEVELOPING AHEAD OF COLD FRONT AND EAST OF NEWD

ADVANCING MESOLOW CIRCULATION MAY BECOME CAPABLE OF PRODUCING

ISOLATED TORNADOES AND DAMAGING WIND GUSTS. MLCAPE FROM 1000-1500

J/KG AND 0-1 KM STORM RELATIVE HELICITY FROM 150-200 M2/S2 WILL

SUPPORT THE POTENTIAL FOR ORGANIZED STORMS INCLUDING LOW LEVEL

MESOCYCLONES AND BOWING SEGMENTS AS ACTIVITY CONTINUES THROUGH THE

WARM SECTOR.

AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL

SURFACE AND ALOFT TO 1 INCH. EXTREME TURBULENCE AND SURFACE WIND

GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 400.

MEAN STORM MOTION VECTOR 22030.

...DIAL

[SmokeEater]

Nice couplet in the S MS TW'd cell.

[andyhb]

TOG, southern MS:

AT 330 PM CDT...THE PUBLIC REPORTED A TORNADO ON THE GROUND NEAR

CEMETERY ROAD HEADING NORTH TOWARDS HIGHWAY 53. THIS STORM WAS

LOCATED 12 MILES SOUTHWEST OF SAUCIER...OR 11 MILES NORTHEAST OF

DIAMONDHEAD...MOVING NORTHEAST AT 15 MPH.

[andyhb]

15z SREF now has four straight frames with a 50 STI area in AL/GA.

[SmokeEater]

Very strong couplet now in Stone County, MS.

[andyhb]

Strong couplet in Southern MS:

[SmokeEater]

Tornado may have just struck McHenry, MS, damage reports starting to come in, dispatch notifying hospitals for ambulances.

[andyhb]

0425 PM TORNADO MCHENRY 30.71N 89.14W

09/30/2012 STONE MS EMERGENCY MNGR

DAMAGE REPORTED ON PINE AVE IN MCHENRY. DETAILS UNKNOWN

AT THIS TIME.

[tornadotony]

15z SREF now has four straight frames with a 50 STI area in AL/GA.

Be very careful using that product right now. DVD (from NWS TAE) pointed out to me earlier (I didn't even realize it) that SPC lowered the CAPE threshold from 1000 J/kg to 500 J/kg and raised the MLLCL threshold from 1000 m to 1500 m. What does this mean? Since this is a probability-based product, it's going to be easier to get higher values since the probabilities are based off the thresholds.

[andyhb]

Be very careful using that product right now. DVD (from NWS TAE) pointed out to me earlier (I didn't even realize it) that SPC lowered the CAPE threshold from 1000 J/kg to 500 J/kg and raised the MLLCL threshold from 1000 m to 1500 m. What does this mean? Since this is a probability-based product, it's going to be easier to get higher values since the probabilities are based off the thresholds.

The LCLs won't be a problem, although I'm wondering why they decided to lower the threshold for CAPE.

[SmokeEater]

Strong couplet once again SE of Wiggins, MS.

[SmokeEater]

Structural damage in McHenry, MS.

[WilkesboroDude]

[tornadotony]

The LCLs won't be a problem, although I'm wondering why they decided to lower the threshold for CAPE.

Mathematically, the LCL adjustment is a problem, because it will also skew the numbers higher for low-LCL cold season events, like today and tomorrow.

[andyhb]

Mathematically, the LCL adjustment is a problem, because it will also skew the numbers higher for low-LCL cold season events, like today and tomorrow.

Good point. Do you have any theory on why they decided to do this?

Also, MCD:

MESOSCALE DISCUSSION 2010

NWS STORM PREDICTION CENTER NORMAN OK

0509 PM CDT SUN SEP 30 2012

AREAS AFFECTED...PARTS OF SE LA...CNTRL/SRN MS AND ADJACENT W

CNTRL/SW ALABAMA

CONCERNING...TORNADO WATCH 656...

VALID 302209Z - 302345Z

THE SEVERE WEATHER THREAT FOR TORNADO WATCH 656 CONTINUES.

SUMMARY...SEVERE WEATHER POTENTIAL...INCLUDING THE RISK FOR

TORNADOES...MAY INCREASE FURTHER INTO THE EVENING HOURS...INCLUDING

PORTIONS OF SOUTHWEST...PERHAPS WEST CENTRAL...ALABAMA. WITH A

WATCH LIKELY TO BE NEEDED ACROSS AREAS EAST OF WW 656...AND SEVERE

POTENTIAL LIKELY TO CONTINUE ACROSS WW 656 BEYOND 01Z...WW 656 MAY

BE REPLACED EARLY WITH ONE NEW WATCH WITHIN THE NEXT HOUR OR SO.

DISCUSSION...WIND FIELDS...PARTICULARLY AT MID-LEVELS...ARE STILL

SOMEWHAT WEAK WITH REGARD TO SEVERE WEATHER POTENTIAL...BUT

STRENGTHENING OF SOUTHERLY 850 MB WIND FIELDS WITHIN THE WARM SECTOR

IS EXPECTED THROUGH THE 02-04Z TIME FRAME. AS THE SUB-1000 MB

SURFACE LOW CENTER SLOWLY MIGRATES NORTHEASTWARD INTO/THROUGH

SOUTHWESTERN MISSISSIPPI DURING THE NEXT FEW HOURS...MODELS SUGGEST

THAT THE AXIS OF STRONGEST WINDS /UP TO 40+ KT/ WILL ALIGN NEAR THE

MISSISSIPPI/ALABAMA BORDER AREA. GIVEN HIGH BOUNDARY LAYER MOISTURE

CONTENT /70F SURFACE DEW POINTS/...ENLARGING CLOCKWISE CURVED

LOW-LEVEL HODOGRAPHS MAY SUPPORT FURTHER INCREASE IN POTENTIAL FOR

TORNADOES...PARTICULARLY AS STORMS REMAIN DISCRETE IN NATURE.

HIGHEST POTENTIAL MAY LINGER WHERE LOW-LEVEL CONVERGENCE IS ENHANCED

INLAND OF SOUTHEAST MISSISSIPPI AND SOUTHWEST ALABAMA COASTAL AREAS.

..KERR.. 09/30/2012

[SmokeEater]

Strong couplet making a beeline for McLain, MS.

[andyhb]

D1, 5% tor for most of AL, the western half of GA, FL Panhandle and SE TN.

DAY 1 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1243 AM CDT MON OCT 01 2012

VALID 011200Z - 021200Z

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS MUCH OF AL...FL

PANHANDLE...WRN/CENTRAL GA AND SERN TN...

...SYNOPSIS...

A DEEP CLOSED LOW WILL TRACK THROUGH THE NERN STATES TO THE CANADIAN

MARITIMES THIS FORECAST PERIOD. AS THIS SYSTEM MOVES NEWD AND THE

SRN EXTENT OF A NRN STREAM TROUGH ADVANCES EWD THROUGH THE DAKOTAS/

MN...THE RESULTANT WEAK HEIGHT FALLS EXPECTED OVER THE MIDWEST/MID

MS VALLEY WILL ALLOW THE LOWER MS VALLEY TROUGH TO PHASE SOMEWHAT

WITH THE NRN STREAM TROUGH. LITTLE LONGITUDINAL MOVEMENT TO THE E IS

EXPECTED WITH THIS LATTER DEEP TROUGH...BUT MODELS TEND TO AGREE THE

SYSTEM/S CENTER WILL MOVE SLOWLY NNEWD REACHING THE MID SOUTH BY 12Z

TUESDAY.

AT 12Z TODAY...AN AREA OF LOW PRESSURE SHOULD BE CENTERED OVER EAST

CENTRAL MS BETWEEN MEI/CBM...WITH THIS LOW MOVING NNEWD INTO MIDDLE

TN BY LATE TONIGHT/EARLY TUESDAY MORNING. MUCH OF THE

CONVECTION/TSTMS THIS FORECAST PERIOD ARE EXPECTED TO LOCATED ALONG

AND E OF A PRE-FRONTAL TROUGH/CONFLUENCE ZONE. THIS GENERALLY N-S

ORIENTED BOUNDARY WILL MOVE EWD ACROSS MUCH OF AL AND THE WRN FL

PANHANDLE THROUGH 02/00Z...AND REACH WRN GA THIS EVENING.

...FL PANHANDLE/AL/WRN GA/SERN TN...

SSWLY MIDLEVEL WINDS LOCATED IN THE ERN PERIPHERY OF THE MS VALLEY

LONGWAVE TROUGH ARE EXPECTED TO STRENGTHEN THROUGH THE MORNING/

AFTERNOON TO 50-60 KT FROM THE CENTRAL GULF COAST /WRN FL PANHANDLE

AND SRN AL/ TO THE SRN/CENTRAL APPALACHIANS. THESE WINDS COMBINED

WITH SLY LOW LEVEL WINDS UP TO 40 KT ACROSS AL/WRN GA TO ERN/SERN

TN...THROUGH MAINLY THE FIRST HALF OF THE PERIOD...WILL RESULT IN

STRONG EFFECTIVE BULK SHEAR OF 40-55 KT. DESPITE WEAK DEEP LAYER

LAPSE RATES...A MOISTURE-LADEN AIR MASS SPREADING FARTHER INLAND

COMBINED WITH SOME DIABATIC HEATING WILL SUPPORT A WEAK-MODERATELY

UNSTABLE WARM SECTOR/SLIGHT RISK AREA.

CONVECTION WITH EMBEDDED TSTMS SHOULD BE ONGOING AT 12Z TODAY INVOF

THE SURFACE LOW AND EXTENDING SWD ALONG THE PRE-FRONTAL CONFLUENCE

ZONE NEAR THE MS/AL BORDER. AFOREMENTIONED THERMODYNAMICS AND

FAVORABLE DEEP LAYER AND LOW LEVEL SHEAR SUGGEST ORGANIZED STORMS

WILL BE POSSIBLE TODAY INTO THE EVENING ALONG THE LEAD CONFLUENCE

ZONE AND WITHIN THE WARM SECTOR. THE PRIMARY SEVERE WEATHER THREATS

WILL BE STRONG WIND GUSTS AND AN ISOLATED TORNADO POTENTIAL.

...SC...

A WARM FRONT IS EXPECTED TO MOVE NWD ACROSS MUCH OF SC TODAY...WITH

A MOISTENING BOUNDARY LAYER COMBINED WITH DIABATIC HEATING IN THE

WARM SECTOR RESULTING IN MODERATE INSTABILITY. STRONGER SSWLY FLOW

ALOFT /40 KT AT 500 MB/ WILL SUPPORT SUFFICIENT BULK SHEAR FOR STORM

ORGANIZATION. HOWEVER...THE LACK OF GREATER FORCING ALOFT FOR STORM

DEVELOPMENT AND MODEL UNCERTAINTY IN OVERALL STORM COVERAGE PRECLUDE

INTRODUCING A CATEGORICAL SLIGHT RISK AT THIS TIME.

[CandymanColumbusGA]

There's already been two tornado warnings in Alabama, one in Coosa County (north of Montgomery) and the other in Coffee County (south of Troy). Haven't seen or heard of damage or confirmations yet. With the jet expected to get stronger this morning/afternoon, I expect we'll see more warnings.

[andyhb]

Lots of breaks in the clouds over AL, SBCAPE is already up to 1000-1500 J/kg across much of the slight risk area in AL/GA.

[toxictwister00]

Tornado Watch #658 in effect for AL/GA/TN hasn't been graphically updated on SPC yet.

http://www.spc.noaa....tch/ww0658.html

EDIT: On a side note, it looks like some sun is trying to come out here after that good downpour of heavy rain we had.

[Thrasher Fan]

With the warm front lifting north of ATL at the moment things to the west and south should begin to fire up.

[Thrasher Fan]

First TOR warned storm of the afternoon and it is no where near the Watch area.

Tornado Warning

SEVERE WEATHER STATEMENT

NATIONAL WEATHER SERVICE WILMINGTON NC

305 PM EDT MON OCT 1 2012

SCC051-011930-

/O.CON.KILM.TO.W.0006.000000T0000Z-121001T1930Z/

HORRY SC-

305 PM EDT MON OCT 1 2012

...A TORNADO WARNING REMAINS IN EFFECT UNTIL 330 PM EDT FOR CENTRAL

HORRY COUNTY...

AT 304 PM EDT...LOCAL LAW ENFORCEMENT REPORTED A TORNADO. THIS

TORNADO WAS LOCATED NEAR ADRIAN...OR 7 MILES NORTH OF CONWAY...MOVING

NORTHEAST AT 5 MPH.

LOCATIONS IN THE WARNING INCLUDE BUT ARE NOT LIMITED TO BAYBORO AND

ALLSBROOK.

[jrips27]

We have had some on and off sunshine over the past hour. Getting another shower now.

[Disc]

Not bad.

[a5ehren]

Doing an amateur reading of the mesoanalysis products, right now it looks like the potential isn't too bad because the areas of high shear and CAPE aren't in the same areas at the moment.

Are we expecting the high CAPE values over North AL to spread over North GA as the afternoon moves on?

[Thrasher Fan]

Severe weather is turning into a bit of a dud. Parts of the watch have already started to be canceled across middle Alabama.

On the bright side abundant rain has fallen around the ATL metro today. Much needed for sure.

[SouthernNJ]

ATL TDWR recently picked up a couple of low-level circulations NE of

Logansville and WSW of Oxford. Neither were warned for or long-lasting,

but the one close to Oxford looked threatening at 447 PM.

[Witness Protection Program]

Welp, got a tor warn east of Nashville (just east of Watertown, TN.)

And there was much excitement.

(BTW, my hair does not look like that.)

[Jon]

SE NC