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Nov. 22nd/23rd Severe Threat


andyhb

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Following this week's significant event, it appears another potential may exist with a mid/upper level wave crossing parts of the Lower MS Valley/TN Valley Tuesday perhaps into Wednesday, so I thought I'd start this for you guys to perhaps avoid a situation like this past event where the svr wx discussion got absorbed into the general discussion/obs. thread. Dr. Forbes and SPC mentioning this potential.

Forbes:

TUESDAY

The GFS and ECMWF numerical models are still different in eastward speed of the surface low, cold front, and upper trough, so it's hard to be sure of the exact location of the severe threat. Using a compromise of the models, isolated to scattered severe thunderstorms and at least a few tornadoes in east TX, west and north LA, south and east AR, north MS, west and middle TN, west KY, northwest AL.

SPC:

BY DAY-5/22ND-23RD...DIFFERENCES BECOME PRONOUNCED WITH THIS SYSTEM

BETWEEN SPECTRAL/ECMWF...AND AMONGST MREF MEMBERS. CONSIDERABLE

UNCERTAINTY EXISTS ATTM REGARDING BOTH PHASE SPEED AND AMPLITUDE OF

MID-UPPER WAVE...AND RESULTANT POSITIONING/STRENGTH OF FAVORABLE

LOW-LEVEL MASS FIELDS. THEREFORE...ALTHOUGH SOME PORTION OF

MID-SOUTH/DELTA/TN VALLEY REGIONS PROBABLY WILL NEED AT LEAST

15%/SLGT-RISK AREA EVENTUALLY...IT IS TOO SOON TO ASSIGN

UNCONDITIONAL 30% D5 LINE ATTM.

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http://www.talkweath...ek/page__st__25

This certainly has the attention of Fred. He is emphasizing the underestimation of this past Wednesday's event as a possible danger to this one, and with some very impressive winds aloft, what he sees could be a considerable problem.

Things sound like they could start to get interesting... I have to admit I'm not following as closely as I should be for a threat. Are you from the southeast originally or do you just take an interest in severe weather OP? I think it's great you are contributing to out regional thread but if we could get some background on ya that would fill in some blanks regardless of the reason for posting. Thanks in advance.

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Things sound like they could start to get interesting... I have to admit I'm not following as closely as I should be for a threat. Are you from the southeast originally or do you just take an interest in severe weather OP? I think it's great you are contributing to out regional thread but if we could get some background on ya that would fill in some blanks regardless of the reason for posting. Thanks in advance.

Interested in severe weather OP, although I live/am from the Pac NW so that's a little strange :P

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hanukkah eve blowdown is my profile pic, that thing was ridiculous.

but yeah, the hodo from that skew-t that Fred posted there on talkwx is downright mean looking, if this bias towards lower instability/shear that he has been highlighting turns out to be true once again, look out...

Around Double Springs, AL, 18z Tuesday. (Hodos up towards Moulton/Huntsville/S TN are ugly as well):

post-6489-0-42671800-1321771257.png

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Next weekend is also (Friday-Sunday) is also looking good for severe across the southeast.

I'd like to see an more E-W oriented trough with a broader base rather than N-S oriented as the GFS (although it is one mother of a trough) currently shows and while the deep layer wind profiles are extremely strong (80-110 kts at H5, 50+ kts at H85), the moisture return may be a problem due to the relatively short time between systems and the surface low being not far south enough to induce more vigorous moisture advection northwards.

Although the Euro/GGEM look a bit better, with the sfc low farther south and a broader trough base. It looks spectacular on Wundermap's H5 Euro graphic.

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Large Day 3 outlined, with substantial 30% area as well.

post-6489-0-81122600-1321779393.gif

DAY 3 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

0216 AM CST SUN NOV 20 2011

VALID 221200Z - 231200Z

...THERE IS A SLGT RISK OF SVR TSTMS PORTIONS CENTRAL/WRN GULF COAST

TO SRN APPALACHIANS AND OH VALLEY...

...SYNOPSIS...

STG SHORTWAVE TROUGH AND EMBEDDED MID-UPPER LOW -- NOW EVIDENT IN

MOISTURE CHANNEL IMAGERY OFFSHORE CA -- IS FCST TO MOVE EWD ACROSS

GREAT BASIN...SRN ROCKIES AND SRN HIGH PLAINS THROUGH DAY-2. BY

START OF PERIOD...THIS SYSTEM SHOULD ASSUME POSITIVE TILT AT 500

MB...ROUGHLY FROM ERN KS SWWD TO W-CENTRAL TX. ALTHOUGH OPERATIONAL

MODELS AND SREF MEMBERS START TO DIVERGE WITH AMPLITUDE AND SPEED OF

THIS PERTURBATION LATE DAY-2 INTO EARLY DAY-3...DIFFERENCES NOW

APPEAR MESOSCALE IN NATURE THROUGH THIS PERIOD. MID-UPPER TROUGH

SHOULD EXTEND FROM CENTRAL/SRN IL TO CENTRAL/ERN TX BY 23/00Z...AND

FROM OH SWWD OVER AL BY 23/12Z.

AT SFC...FRONTAL-WAVE CYCLONE SHOULD MOVE FROM SERN MO ENEWD ACROSS

LOWER GREAT LAKES/NRN APPALACHIANS REGION THROUGH PERIOD. COLD

FRONT SHOULD EXTEND FROM LOW SWWD ACROSS SERN OK AND CENTRAL/SW TX

BY 22/12Z...THEN FROM SRN INDIANA TO WRN/MID TN AND CENTRAL/SRN LA

BY 23/00Z. FRONT SHOULD REACH CENTRAL/SRN APPALACHIANS AND WRN FL

PANHANDLE BY END OF PERIOD.

...CENTRAL/WRN GULF COAST TO SRN APPALACHIANS AND OH VALLEY...

LATE DAY-2 INTO EARLY DAY-3...EXPECT BAND OF TSTMS TO BUILD SWWD

ALONG SFC COLD FRONT...WITH ATTENDANT RISK FOR OCNL DAMAGING GUSTS

AND ISOLATED HAIL FROM OZARKS TO CENTRAL/ERN TX. THROUGH LATE

AFTERNOON...SVR RISK SHOULD SHIFT EWD...WHILE EXPANDING NWD OVER OH

VALLEY IN STEP WITH EXPANDING WARM-SECTOR REGIME OF AT LEAST MRGL

BUOYANCY. CONVECTION ALSO SHOULD BACKBUILD ALONG/AHEAD OF FRONT

TOWARD COASTAL SE TX AND LA. DAMAGING TSTM GUSTS SHOULD REMAIN

MOST PREVALENT SVR MODE DURING AFTERNOON/EVENING...THOUGH TORNADOES

AND ISOLATED HAIL ALSO ARE POSSIBLE. MAIN UNCERTAINTIES RESULT FROM

DIFFERENCES IN PROGS OF SYSTEM SPEED...AND INVOLVE BOTH TIMING OF

SVR THREAT AREA-WIDE...AND NWD EXTENT OF SVR THREAT INTO OH VALLEY

REGION. SVR THREAT THEREFORE IS MORE CONDITIONAL WITH NWD AND EWD

EXTENT WHERE AVAILABILITY OF SFC-BASED EFFECTIVE LIFTED PARCELS IS

PROGRESSIVELY MORE QUESTIONABLE.

WARM-SECTOR AIR MASS WILL DESTABILIZE DIURNALLY FROM COMBINATION OF

BOUNDARY-LAYER THETAE ADVECTION AND POCKETS OF SFC DIABATIC HEATING.

EXPECT SFC DEW POINTS RANGING FROM LOWER 60S NEAR OH RIVER TO UPPER

60S/LOW 70S OVER COASTAL AREAS OF LA/MS/AL. FCST SOUNDINGS SUGGEST

THIS WILL OFFSET MODEST MIDLEVEL LAPSE RATES ENOUGH FOR MLCAPE

1000-1500 J/KG FROM GULF COAST TO TN VALLEY REGION...DIMINISHING TO

BELOW 500 J/KG OVER OH RIVER. DOMINANT CONVECTIVE MODE SHOULD

REMAIN QUASI-LINEAR...AS WINDS ALOFT PRECEDING POSITIVELY-TILTED

TROUGH CROSS LOW-LEVEL CONVERGENCE ZONE AT SMALL ANGLE.

HOWEVER...WITH LACK OF SIGNIFICANT ELEVATED MIXED-LAYER AIR MASS

AHEAD OF FRONT...MLCINH SHOULD BE RATHER SMALL. THIS MAY PERMIT

DISCRETE OR CLUSTERED DEVELOPMENT AHEAD OF MAIN LINE...WITH ENOUGH

DEEP-LAYER SHEAR TO SUPPORT CONDITIONAL SUPERCELL POTENTIAL AHEAD OF

MAIN BAND...IN ADDITION TO ANY EMBEDDED SUPERCELLS/BOWS/MESOVORTICES

IN QLCS.

CONVECTIVE REGIME WILL SHIFT EWD ACROSS LOWER DELTA...MID-SOUTH...OH

VALLEY AND TN VALLEY REGIONS THROUGH EVENING...IN STEP WITH OR

SLIGHTLY AHEAD OF SFC COLD FRONT. DURING LATE EVENING AND OVERNIGHT

HOURS...FAVORABLE BUOYANCY WILL BE OVERTAKEN FROM N-S AS MAIN TSTM

BAND ENCOUNTERS MORE STABLE BOUNDARY-LAYER TRAJECTORIES EMANATING

FROM RIDGING OVER ATLANTIC COAST REGION. SVR POTENTIAL ACCORDINGLY

SHOULD DIMINISH OVER NRN PORTIONS OF OUTLOOK AREA...AND BECOME MORE

CONDITIONAL/MRGL EWD ACROSS ERN PORTIONS AL/TN INTO WRN GA.

..EDWARDS.. 11/20/2011

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Interested in severe weather OP, although I live/am from the Pac NW so that's a little strange :P

I don't think it's strange :lol:

CAE doesn't seem to be worried....as usual....lol

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...

-- Changed Discussion --MEDIUM RANGE MODELS IN GOOD AGREEMENT MOVING SHORT WAVE AND

ASSOCIATED COLD FRONT THROUGH AREA WEDNESDAY. A GOOD CHANCE OF

SHOWERS EARLY WEDNESDAY...THEN CHANCES DECREASE DURING THE DAY

AS A COLD FRONT MOVES EAST. DECIDED TO NOT MENTION THUNDER

WEDNESDAY AS FRONT MOVES THROUGH EARLY. HIGH PRESSURE BUILDS OVER

THE REGION THANKSGIVING DAY THROUGH SATURDAY AS NEXT SYSTEM

APPEARS TO BE LATER THAN SATURDAY. HIGHS WEDNESDAY IN THE LOWER

70S COOL TO THE LOWER TO MIDDLE 60S THURSDAY THROUGH SATURDAY.

OVERNIGHT LOWS IN THE 40S WEDNESDAY NIGHT DROP TO THE 30S THURSDAY

NIGHT BUT ARE BACK UP AROUND 40 FOR FRIDAY NIGHT.-- End Changed Discussion --

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Tuesday is looking interesting from the Tennessee Valley into the Deep South. Ongoing convection in the Ohio Valley will wreck havoc on the synoptic warm front. I’m a little surprised SPC has not mentioned a secondary Gulf Coast warm front south of the synoptic warm front. Sometimes the Deep South sees the coastal front lift north and it can be a nasty focus for supercells.

For the Tennessee Valley and mid-South I genially echo SPC thinking. Rounds of thunderstorms should bring some hail, locally high winds and maybe an isolated tornado. The synoptic warm sector should make it into middle and eastern Tenn. Western Tenn may rain out. Kentucky is a crap shoot but might be able to spin up one “cold core” type report.

The Deep South has the best chance of a real set-up. Latest WRF hints a decent southern stream jet streak ejecting along the I-20 corridor. Multiple effects could enhance severe potential but a majority are required this time of year. The Deep South jet streak would in theory back surface winds more between I-20 and I-10. If so the secondary coastal warm front would lift north to say the I-20 area. Winds south of that front would be stronger, helping to mix out low clouds. Any stay convection in Tenn would only act to sharpen this coastal front lifting through the Deep South, due to possible cooler outflow from the north side of the front.

We really don’t need this kind of action again in the South. Also the highways mentioned will be filled with holiday travelers. Fortunately I’d say there is only a 1/3 chance of my scenario. Otherwise ongoing slop-vection keeps tornadoes isolated and short-lived in line with a slight risk.

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Looks like CAE is changing thier minds now :lol:

.SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY/...

THE NAM AND GFS KEEP A WARM FRONT AND ASSOCIATED DEEPER MOISTURE

NORTH OF THE FORECAST AREA THROUGH TUESDAY. A COLD FRONT

APPROACHING FROM THE WEST TUESDAY NIGHT SHOULD MOVE THROUGH THE

FORECAST AREA WEDNESDAY. NUMEROUS SHOWERS AND A FEW THUNDERSTORMS

WILL LIKELY OCCUR MAINLY ASSOCIATED WITH THE H85 JET AHEAD OF THE

FRONT. USED THE HIGHER NAM MOS POPS. THE NAM AND GFS INDICATE H85

WINDS OF 35 TO 40 KNOTS. SEVERE THUNDERSTORMS MAY OCCUR BECAUSE OF

THE STRONG SHEAR. THE MODELS SHOW THE JET AND ASSOCIATED MOISTURE

SHIFTING EAST OF THE FORECAST AREA BY WEDNESDAY AFTERNOON.

SPC

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1130 AM CST MON NOV 21 2011

VALID 221200Z - 231200Z

...THERE IS A SLGT RISK OF SVR TSTMS FROM SERN TX...THE LOWER MS

VALLEY...SERN STATES AND SRN PORTIONS OF THE OH VALLEY...

...SYNOPSIS...

UPPER TROUGH NOW OVER THE SWRN STATES WILL MOVE ENEWD...REACHING THE

LOWER/MID MS VALLEY EARLY TOMORROW BEFORE CONTINUING ACROSS MUCH OF

THE REMAINDER OF THE SERN STATES AND OH VALLEY TUESDAY EVENING AND

OVERNIGHT. ATTENDANT SURFACE LOW WILL DEEPEN AND LIFT NEWD THROUGH

THE OH VALLEY...WHILE TRAILING COLD FRONT SWEEPS THROUGH THE SERN

U.S. AND WARM FRONT LIFTS NWD INTO THE OH VALLEY AND MID ATLANTIC.

...SERN TX...LOWER MS VALLEY AND SERN STATES THROUGH THE OH

VALLEY...

THE SERN U.S. PRE-FRONTAL WARM SECTOR WILL CONTINUE TO MOISTEN AND

DEVELOP NWD IN ASSOCIATION WITH ENEWD MIGRATING SSWLY LLJ THAT

SHOULD UNDERGO FURTHER STRENGTHENING AS SURFACE LOW DEEPENS.

DEWPOINTS WILL LIKELY RANGE FROM THE UPPER 60S OVER THE SRN HALF OF

THE SERN STATES TO NEAR 60 OVER THE OH VALLEY. OWING TO EXPECTED

MARGINAL MID-LEVEL LAPSE RATES AND POTENTIAL AREAS OF MULTI-LAYER

CLOUDS...INSTABILITY SHOULD REMAIN MODEST. HOWEVER...POCKETS OF

DIABATIC WARMING WILL LIKELY OCCUR WHICH SHOULD BOOST MLCAPE FROM

1500 J/KG OVER SRN PARTS OF THE GULF COASTAL STATES TO AOB 500 J/KG

OVER THE OH VALLEY. STORMS WILL LIKELY BE ONGOING ALONG AND JUST

AHEAD OF THE FRONT EARLY TUESDAY FROM SERN TX INTO THE LOWER MS

VALLEY WITH QLCS THE DOMINANT MODE AND ISOLATED DAMAGING WIND THE

MAIN THREAT. ACTIVITY WILL REMAIN EMBEDDED WITHIN 45+ KT EFFECTIVE

SHEAR AS IT CONTINUES EWD...AND STORMS MAY UNDERGO SOME

STRENGTHENING AS THE DOWNSTREAM WARM SECTOR DESTABILIZES. POTENTIAL

WILL EXIST FOR EMBEDDED SUPERCELLS AND LEWP/BOW STRUCTURES WITH A

CONTINUING THREAT FOR ISOLATED DAMAGING WIND...BUT TORNADOES WILL

ALSO BE POSSIBLE. THE BEST SHEAR/INSTABILITY PARAMETER SPACE IS

EXPECTED IN THE CORRIDOR FROM THE LOWER MS VALLEY INTO THE TN VALLEY

AREA. ISOLATED SEVERE THREAT MAY SPREAD EWD INTO NRN GA AND SC

OVERNIGHT.

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With each model run it looks less impressive. Need to see what happens with the SPC update and model runs later tonight.

Agreed, I'm not seeing a whole lot of potential for supercells outside of the main line at this point. There's just not enough ascent out in front of the warm sector. There is some guidance that forms a secondary low, but its mainly from outliers. QLCS fest to me. I'd be more worried about the wind rather than the tornadoes in this situation.

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With each model run it looks less impressive. Need to see what happens with the SPC update and model runs later tonight.

Guess you haven't seen this discussion from BMX:

IF THERE WAS A CHANGE IN THE MODEL GUIDANCE TODAY...IT WAS A

FURTHER SLIGHT SLOWING DOWN OF THE ARRIVAL OF THE FRONT ON

TUESDAY. THE LATEST OUTPUT SUGGESTS THAT THE FRONT SHOULD REACH

THE NORTHWESTERN COUNTIES BETWEEN 8 PM AND MIDNIGHT TOMORROW...AND

NOT EXIT THE AREA UNTIL 6 AM OR SO WEDNESDAY. AHEAD OF THE

FRONT CONDITIONS ARE FORECAST TO BECOME MODERATELY UNSTABLE...WITH

SBCAPES PEAKING BETWEEN 500 AND 1000 J/KG. 0-6KM BULK SHEAR AND

0-3KM SRH VALUES APPEAR TO BOTH BE SUFFICIENT TO AT LEAST SUPPORT

ORGANIZED SEVERE THUNDERSTORMS ON THE SYNOPTIC SCALE. ONE THING

THAT IS AT LEAST A LITTLE CONCERNING IS THAT ALL OF THE SHORT

RANGE MODELS AT LEAST HINT AT THE POSSIBLE DEVELOPMENT OF A

SECONDARY SURFACE LOW IN NORTHERN MISSISSIPPI TUESDAY

AFTERNOON/EVENING. IF THIS WERE TO HAPPEN...THIS COULD ENHANCE THE

LOW LEVEL WIND SHEAR JUST A BIT MORE...WHICH COULD LEAD TO BETTER

STORM ORGANIZATION.

AT THIS TIME...THE GENERAL CONSENSUS IS THAT THE PRIMARY MODE OF

SEVERE WEATHER WOULD BE THAT OF A SQUALL LINE IMMEDIATELY AHEAD OF

THE COLD FRONT. HOWEVER...WE`VE GOT TO WATCH FOR THE POTENTIAL OF

CELLULAR DEVELOPMENT AHEAD OF THE MAIN LINE...WHICH WOULD ENHANCE

A TORNADO THREAT. EVEN IF WE ENDED UP WITH A COMPLETELY LINEAR

SITUATION...THERE`S PROBABLY GOING TO BE ENOUGH LOW LEVEL SHEAR

PRESENT TO CAUSE LEWPS...ROTATING COMMA HEADS...OR OTHER

VORTEX-RELATED FEATURES THAT COULD ALSO PRODUCE TORNADOES. AND

FINALLY...WHILE IT APPEARS THAT THE GREATEST SEVERE RISK WOULD BE IN

THE WEST AND NORTHWEST COUNTIES...I WOULDN`T WRITE-OFF THE

EAST/SOUTHEAST ONE BIT.

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Agreed, I'm not seeing a whole lot of potential for supercells outside of the main line at this point. There's just not enough ascent out in front of the warm sector. There is some guidance that forms a secondary low, but its mainly from outliers. QLCS fest to me. I'd be more worried about the wind rather than the tornadoes in this situation.

That's what everyone said before Wednesday's event last week...but look what happened...

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That's what everyone said before Wednesday's event last week...but look what happened...

Actually, it is my view that this is going to be very similar to last Wednesday. And its not what everybody said. There were many, including the local NWS WFOs, that nailed it last week. But people have been overhyping this setup since late last week and if anything, over the last 24 hours, the setup has become less impressive. There have been some, not all by any means, who have made tomorrow's system out to be a major outbreak and that simply will not be the case and it will be mainly a linear system unless something drastic changes.

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Never did say that this was going to be a "major" outbreak, I just think it's a little premature to worry more about wind than tornadoes in this particular case.

This factor is a HUGE if, however.

When all the short range models are showing the possibility (or at least that's what they said in that AFD), it's not such a giant "if", imo. Not sure exactly what models they are talking about, but it's not exactly hogwash to highlight that particular section of the AFD. Keep in mind that this trough was significantly weaker in the runs farther back into last week.

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Never did say that this was going to be a "major" outbreak, I just think it's a little premature to worry more about wind than tornadoes in this particular case.

When all the short range models are showing the possibility (or at least that's what they said in that AFD), it's not such a giant "if", imo. Not sure exactly what models they are talking about, but it's not exactly hogwash to highlight that particular section of the AFD. Keep in mind that this trough was significantly weaker in the runs farther back into last week.

And never once did I single you out as being someone overhyping this into a major outbreak. Actually, I never said anything about you at all. But there have been many people overplaying this event... I'm sorry if this fact offends you in some weird way.

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:huh:

Who?

Actually it hasn't been that bad on this board, but many people on facebook and other boards are freaking people out and it just annoys me. I never singled anyone out on this board. I was just giving some a word of caution that from what I am seeing, the event is trending down, not up.

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Actually it hasn't been that bad on this board, but many people on facebook and other boards are freaking people out and it just annoys me. I never singled anyone out on this board. I was just giving some a word of caution that from what I am seeing, the event is trending down, not up.

Well, that's kind of a given.

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Speaking of models, 00z NAM is out...

And not to stoke the fire or anything, but here's a snippet out of BMX's PIS for tomorrow:

PUBLIC INFORMATION STATEMENT

NATIONAL WEATHER SERVICE BIRMINGHAM AL

750 PM CST MON NOV 21 2011

...DAMAGING WINDS AND TORNADOES ARE POSSIBLE ACROSS CENTRAL ALABAMA

TUESDAY AFTERNOON AND EVENING...

A STRONG STORM SYSTEM WILL BRING THE POSSIBILITY OF SEVERE

THUNDERSTORMS AND TORNADOES TO CENTRAL ALABAMA AS EARLY AS TUESDAY

AFTERNOON. NOW IS THE TIME TO PREPARE FOR THE POSSIBILITY OF SEVERE

WEATHER. REVIEW THE SAFETY RULES AND PUT YOUR PRE-DETERMINED SAFETY

PLAN INTO ACTION! AS THE SYSTEM APPROACHES THE AREA GRAPHICAST ON

OUR WEBPAGE WILL BE UPDATED TO KEEP YOU INFORMED OF THE WEATHER

CHANGES.

Tornado watch issued in Central and Southern AR/Extreme SE OK.

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