Monday, May 09, 2011

Forecasting the 4/27/11 Tornado Outbreak in Alabama

This is a look back at excerpts from some forecast discussions in the days leading up to the historic outbreak of tornadoes in Alabama on April 27, 2011. Forecast models indicated the possibility of severe weather over a week in advance. Local media and National Weather Service offices in Huntsville and Birmingham did an outstanding job alerting us to the potential severity of the event. Following are some samples of their work...

Tuesday 4/19 AM Weather Xtreme Video, James Spann:
"Wednesday the 27th, that could be a pretty active period, maybe, for strong or severe storms. We'll be watching developments toward the middle of next week. Hey, this is April in Alabama, when things like that can happen and often will."

Tuesday 4/19 PM Weather Xtreme Video, James Spann: Toward the middle of next week we could have some issues with severe weather."

Wednesday 4/20 AM Weather Xtreme Video, James Spann: "Very significant severe weather outbreak moves in here...Clearly late Tuesday and Wednesday next week severe weather is on the board here.

Wednesday 4/20 PM Weather Xtreme Video, James Spann: "Clearly a chance of severe weather Tuesday night or Wednesday of next week."

Thursday 4/21/11 NWS Birmingham AFD: THE SECOND SURFACE LOW WILL DEVELOP FARTHER SOUTH AND BRING INCREASING CHANCES OF SEVERE STORMS TO ALABAMA ON WEDNESDAY.

Friday 4/22 PM Weather Xtreme Video, James Spann: "Look at Wednesday (the 27th)! That's a pretty good looking severe weather setup for April in Alabama. Digging trough to the west, difluent flow aloft, surface low near Memphis - 996 mb. In fact it doesn't get more classic than that. That would suggest a chance of supercell storms Wednesday afternoon with the possibility of tornadoes, and a squall line Wednesday night.

Friday 4/22/11 NWS Birmingham AFD: THE BIG EVENT IS STILL SHAPING UP FOR TUESDAY AND WEDNESDAY. THERE IS FAIRLY GOOD MODEL CONSENSUS WITH REGARD TO TIMING AND POSITION OF SURFACE FEATURES. THERE STILL APPEARS TO BE TWO WAVE OF STORMS THAT WILL AFFECT THE AREA. THE FIRST WAVE MAY BE MORE OF A HEAVY RAIN EVENT...AS FIRST LINE OF STORMS SLOWS WEAKENS AND BECOMES STATIONARY OVER NW ALABAMA ON TUESDAY. THERE WILL BE ENOUGH INSTABILITY AND SHEAR FOR THE POSSIBILITY OF SEVERE STORMS...BUT BETTER FORCING AND SHEAR WILL HAPPEN ON WEDNESDAY AS UPPER LOW AND SURFACE LOW DEEPENS UPSTREAM OF ALABAMA.

Friday 4/22/11 NWS Birmingham HWO: CONFIDENCE HAS INCREASED FOR WEDNESDAY...WHEN THERE MAY BE A THREAT OF TORNADOES. SPOTTER INFORMATION STATEMENT...ACTIVATION OF STORM SPOTTERS AND EMERGENCY MANAGEMENT MAY BE NEEDED NEXT TUESDAY AND WEDNESDAY DUE TO THE POSSIBILITY OF SEVERE STORMS.

Friday 4/22/11 NWS Huntsville AFD: BUT THE MAIN STORY WILL STILL BE WITH THE MAIN TROF AXIS AS IT LEADS TO RAPID LEE CYCLOGENESIS OVER THE ARK-OK-TX REGION ON WEDNESDAY. THIS WILL BE THE STRONGER OF THE TWO SYSTEMS THIS COMING WEEK AND LIKELY DEVELOP STRONG TO SEVERE CONVECTION ACROSS MS/TN/AL LATE WED MORNING INTO THE EVENING HOURS. THE GFS IS MUCH MORE BULLISH IN THE DEEPENING OF THIS SECOND LOW AND INDICATES A VERY STRONG LL JET DEVELOPING AHEAD OF THE SYSTEM. THE ECMWF STILL LOOKS FAVORABLE FOR STRONG TO SEVERE DEVELOPMENT ON WEDNESDAY BUT PAINTS A SLIGHTLY WEAKER LL JET /50KTS AS OPPOSED TO 75+KTS JUST TO OUR N WED AFTERNOON/. SVR WX PARAMETERS SPIKE ARND 18Z WED WITH CAPE VALUES IN EXCESS OF 1000 J/KG AND A NICELY CURVED HODOGRAPH...BUT A SIGNIFICANT CAP IS IN PLACE WED MORNING AT ARND
750-800MB.

Saturday 4/23/11 NWS Birmingham AFD: AS THE DEEP LAYER JET FIELDS ROUND THE BASE OF THE UPPER TROUGH AND ENTER THE WARM SECTOR...IT IS LOOKING BETTER FOR SUPERCELLS FOLLOWED BY A DECENT BROKEN SQUALL LINE. I TEND TO FOCUS ON SEVERAL ITEMS THAT COULD BE CONSIDERED SYNOPTIC TO GET SOME IDEA OF STORM MODE. FIRST IS THE 0 TO 6 KM BULK SHEAR...MAGNITUDE AND ORIENTATION. THE VECTOR ORIENTATION IS AT AN ALMOST PERFECT 45 DEGREE ANGLE WITH THE SURFACE FRONT AND INCREASING TO 55 TO 65 KTS INDICATING SUPERCELL DEVELOPMENT BETWEEN 18 AND 21Z WEDNESDAY...BUT IT DOES BACK MORE SOUTHEAST AS IT GETS CLOSER TO SUNSET...SUGGESTING THAT A SQUALL
LINE IS MORE LIKELY WITH TIME OR ALONG THE FRONT. NEXT WOULD BE THE 0 TO 8 OR 10 KM BULK SHEAR...WHICH IS INDICATIVE OF LONGER TRACKED SUPERCELLS.AND IT IS AN IMPRESSIVE 75 KTS FROM 18 TO 00Z. SO THE SHEAR DOESN`T SEEM TO BE MUCH OF AN ISSUE AT THIS POINT.

Sunday 4/24/11 PM Weather Xtreme Video, Brian Peters: "The trough really digs in...as it comes across Arkansas...then it moves to the vicinity of Memphis midday Wednesday...that's going to be our big day...probably tornado watches on Wednesday in the afternoon and evening hours."

Sunday 4/24/11 NWS Huntsville AFD: WE MAY NEED TO RAISE THE POTENTIAL FOR +RA GIVEN AMPLE MOISTURE INFLUX ASSOC WITH THE LLJ. SUPERCELLS MAY BE PSBL AS WELL...SPCLY W OF I-65 ALONG WARM FRONT GIVEN A GOOD COMBO OF ML-CAPE/6KM BULK SHEAR. 0-1KM SRH VALUES INCREASE ABOVE 300 M2/S2 SUGGESTIVE OF A TORNADO THREAT AS WELL LATE TUE NIGHT. WED LOOKS TO BE A HIGH IMPACT WX DAY...PERHAPS CONTAINING AN ONGOING MORNING EVENT...FOLLOWED BY A WDSPRD SQLN WED AFTN/EVNG. WINDY CONDITIONS ARE LIKELY WED GIVEN A VERY TIGHT PRESSURE GRADIENT...THE NAM SUGGESTS THE WARM FRONT AND OVERNIGHT CONVECTION SHOULD LIFT N OF THE AREA BY 12Z. THIS IS STILL A QUESTION MARK THOUGH. THE GFS PROGS A LARGE MCS MOVG INTO MIDDLE TN AND PSBLY CLIPPING NRN AL AS WELL. SOMETHING TO WATCH CLOSELY IN FUTURE SHIFTS AS IT COULD CERTAINLY INCREASE OUR +RA/FLOOD THREAT. IN ANY EVENT...MODEL SOUNDINGS INDICATE A LOW CLOUD LAYER BENEATH A STRONG CAPPING INVERSION THRU THE MORNING HRS. THIS SHOULD PROHIBIT PRE-FRONTAL CONVECTIVE REDVLPMT AND ALLOW FOR DECENT HEATING DURG THE AFTN. A 5H JET ACCELERATES TO 90-100KT BY MIDDAY INTO THE AFTN...CREATING VERY STRONG DEEP LAYER SHEAR. WITH A STRONG SHORTWAVE/-DIV Q PROVIDING AMPLY FORCING ALONG THE COLD FRONT...CONDITIONS APPEAR FAVORABLE FOR A LONG-LIVED SQLN TO DVLP WED AFTN/EVENING. DAMAGING WINDS AND MESO-VORTICES / QLCS TORNADOES WL BE CONSIDERABLE THREATS.

Sunday 4/24/11 NWS Birmingham AFD: WEDNESDAY...WE WILL RAPIDLY CHANGE AS ANOTHER DECENT COLD CORE SPOKE DIGS WAY SOUTH AND QUICKLY NEGATIVE TILTS TO OUR NORTHWEST. AS IT DOES SO...THE HEIGHT FALLS QUICKLY DEEPEN A SURFACE LOW FROM LITTLE ROCK TO DETROIT. HERE IN LIES THE FIRST SIGNIFICANT DISAGREEMENT INTO EXACTLY HOW LOW WE WILL GO. I`VE SEEN EVERYTHING FROM 996 TO 985...AND ALL NUMBERS IN BETWEEN. THIS WILL MAKE AN OBVIOUS SIGNIFICANT DIFFERENCE IN THE WIND FIELDS...IN PARTICULAR...THE LOWEST 1 KM HODOGRAPH TURN. DON`T WANT TO GET INTO THAT MUCH MESOSCALE THIS EARLY...BUT IT IS A LITTLE TROUBLING TO HAVE THAT MUCH VARIATION AS WE ARE GETTING WITHIN THE 3 DAY WINDOW. REGARDLESS...I AM STILL IMPRESSED WITH THE 0 TO 6 KM AND 0 TO 8 KM BULK SHEAR SPEED AND VECTOR ORIENTATION. IT STILL SHOWS ISOLATE LONG-LIVED SUPERCELL FAVORABLY INTO WEDNESDAY AFTERNOON...ESPECIALLY WEST OF I65...BUT DOES HINT AT QUICKLY TRANSITIONING AND BECOMING COLD POOL DOMINATED AS IT ACCELERATES ACROSS THE STATE...IN THE MEAN TIME...THE CAPE PROFILES CONTINUE TO IMPRESS...

Sunday Afternoon 4/24/11 NWS Birmingham AFD: THINGS WILL REALLY GET TO ROCKIN` AND ROLLIN` ON WEDNESDAY, AS A STRONG LOW PRESSURE SYTEM DEVELOPS OVER ARKANSAS AND LIFTS INTO NORTHERN KENTUCKY BY 00Z/THU. STRONG DYNAMICS ASSOCIATED WITH THIS SYSTEM WILL CREATE THE POTENTIAL FOR A SIGNFICANT SEVERE WEATHER EVENT FOR THE MID STATE, INCLUDING THE POSSIBILITY FOR ISOLATED TORNADOES. A STRONG FORECAST 850MB JET, OF 60 TO 65 KTS, COUPLED WITH 0-3KM STM RELATIVE HELICITIES NEAR 500 M2/S2 COULD SET THE STAGE FOR THE BEST POSSIBILITY FOR ISOLATED TORNADOES THAT WE'VE SEEN SO FAR THIS SEASON.

Monday Afternoon 4/25/11 NWS Birmingham AFD: LOOKING AHEAD TO WEDNESDAY...SOME OF THE SYNOPTIC PARAMETERS ARE LOOKING DOWNRIGHT DANGEROUS. IF THINGS GO ACCORDING TO THE NUMERICAL OUTPUT...THE CAP SHOULD CONTINUE TO GROW AND DEVELOP DURING THE EARLY MORNING HOURS...WHILE ENERGY BEGINS TO BUILD AS BREAKS OF SUN HEAT THE SURFACE THROUGH MUCH OF THE MORNING AND EARLY AFTERNOON HOURS. CAPE VALUES ARE SIGNIFICANT AS THEY LOOK TO RANGE ANYWHERE FROM 1000 TO 3000 J/KG. THE CAP IS RATHER WEAK AND CONTINUES TO TREND WEAKER WITH TIME SUGGESTING THAT IT WILL EASILY BE BROKEN BETWEEN 1 AND 3 PM. THAT IS ABOUT THE SAME TIME THAT THE JET MAX BEGINS TO NUDGE IN FROM THE WEST...OR AS THE 0 TO 6 KM AND THE 0 TO 8 KM BULK SHEAR VECTORS INCREASE TO GREATER THAN 60 KTS...AND MORE IMPORTANTLY THEIR ANGLE OF ATTACK SUGGESTS SUPERCELLS AND LONG-LIVED ONES AT THAT. MY FINAL QUESTIONS SURROUND THIS LOWEST SHEAR LAYER FROM 0 TO 1 KM. SYNOPTICALLY...WOULD LIKE TO SEE MORE CONSISTENCY BETWEEN MODELS AND INDIVIDUAL RUNS ON EXACTLY HOW MUCH VEERING WITH HEIGHT WILL EXIST. THIS IS DIRECTLY RELATED TO THE MESSY SURFACE PRESSURE FROM THE DOUBLE HELIX LOW AND FROM VARYING DEGREES OF THE STRENGTH OF THE LOW. ADD TO THAT...EVEN MORE UNCERTAINTY OF WHETHER THIS FRONT BEGINS TO GO LINEAR. OVER THE
PAST SEVERAL FORECASTS...WAS THINKING THAT WE START SUPERCELLS THROUGH MUCH OF THE AFTERNOON AND EARLY EVENING ONLY TO TRANSITION LINEAR AFTER SUNSET...BUT A RE-EXAMINATION OF THE VECTOR ANGLES SHOWS THAT IT MAY NOT HAPPEN UNTIL FURTHER EAST INTO GA AND NORTH FL. SO...WHILE IT MAY BE A LINE...IT MAY ACTUALLY BE A BROKEN OR MORE CELLULAR LINE AS THE EVENT TRANSLATES EAST OF I65. REGARDLESS...MY BEST FORECAST IS THAT THE GREATEST OPPORTUNITY FOR SUPERCELLS AND TORNADOES WILL LIKELY BE ALONG AND WEST OF I65 FROM 1 UNTIL 6 PM WEDNESDAY. PLEASE STAY TUNED TO THE WEATHER EVENTS THROUGH WEDNESDAY EVENING. THISIS A DANGEROUS SITUATION THAT WARRANTS ATTENTION. DO WHATEVER IS NECESSARY TO PROTECT LIFE AND PROPERTY...NOW IS THE TIME TO REVIEW SAFETY RULES AND PLANS AS IT MAY SAVE YOUR LIFE!

Monday 4/25/11 ABC 33/40 Forecast Discussion, James Spann: "SPC, in their day three convective outlook, already has much of North Alabama under a moderate risk. Sure looks like all of the ingredients will be there; a surface low will form north of Memphis, which will give very favorable low level shear, and it looks like the capping inversion should break by mid-afternoon with scattered supercells forming over North and Central Alabama, which will have potential for large hail, damaging wind, and a few tornadoes. Based on the forecast parameters, a few strong, long track tornadoes will be possible Wednesday afternoon and Wednesday evening, so it will be yet another day on which you will need to be close to a good source of severe weather information. If you read this blog, we don’t worry about you, but be sure and pass the word along."

Tuesday 4/26/11 AM Analysis, Dr. Tim Coleman: "…MAJOR TORNADO OUTBREAK LIKELY IN SOUTHEAST OVER THE NEXT 48 HOURS…The morning model runs are in now, and it still looks like a significant severe weather outbreak will occur over the SE US today and tomorrow. I will primarily talk about Alabama here, but this is a large-scale event. The models disagree on the timing of this low, one moving it through Memphis early morning then into KY and OH, the other not until afternoon. That will be one key factor tomorrow, because the highest storm-relative helicity values will be closer to the surface low and low-level jet, making storm rotation more vigorous. Taking the average of the two, the largest storm-relative helicity, a very high 400 or more m2/s2, will be centered from northern MS into NW Alabama and middle TN by noon tomorrow. The thing that makes this event more worrisome is that we’re pretty deep into Spring now, with strong sunshine and a warmer Gulf. Dewpoints tomorrow will rise to near 70 degrees, and assuming we get sunshine, temperatures will rise into the 80s. With cold upper-level temperatures, this will cause very unstable air. CAPE will range from 2,000 to 4,000 J/kg over most of Alabama by early afternoon. With that combination of wind shear and instability, severe storms will develop, and in the areas of strongest shear, the storms will rotate vigorously and produce tornadoes. Pinpointing the exact areas that will be the most vulnerable is still tough at this time, since it will depend some on where clouds linger longer in the morning, where today’s storms leave boundaries, etc. But, the stage will be set most favorably for tornadoes over the northwest half of Alabama this time (it was the south half on April 15). We are talking about areas northwest of a line from Demopolis to Clanton to Anniston to Gadsden."

Tuesday 4/26/11 ABC 33/40 Forecast Discussion, James Spann: "All of the synoptic elements for a major outbreak are in place. A deep (sub-1000 mb) low west of Memphis, steep lapse rates, strong veering of the wind with altitude in respect to projected storm motion, strong wind fields at the surface and aloft, dry air in the mid levels, and a very deep, long wave upper trough that is somewhat negatively tilted enhancing diffluence aloft over Alabama. Projected soundings show the classic “loaded gun” look, meaning that a cap should keep storms at bay through the morning hours, but when that cap breaks early in the afternoon, storms will quickly become severe with all modes of severe weather possible. This means potential for large hail, damaging winds, and a few violent, long track tornadoes. This is dangerous weather setup. Of course, long time readers know that mesoscale features play a huge roll in the ultimate severity of a severe weather day, and we really won’t know about those until early tomorrow. TIMING: This is an event where it doesn’t make much sense to ask when storms will arrive in your county or home town; nobody knows since the initial storms will be cellular in nature. Just understand a severe storm is basically possible anytime from now through 3:00 a.m. Thursday. The storms tonight will mostly pose a threat of high winds and large hail, although an isolated tornado cannot be ruled out.Tomorrow, the main risk comes from about 12:00 noon to 12:00 midnight. Again, we can’t rule out morning storms, but the most intense and dangerous thunderstorms will come during the afternoon and nighttime hours. The severe storms should merge into a long squall line late tomorrow night, with the main threat becoming damaging straight line winds after 10:00 p.m. or so."

Tuesday 4/26/11 SPC Day Two Outlook: HIGH RESOLUTION MODEL DATA IS STRONGLY SUGGESTIVE THAT A SIGNIFICANT CLUSTER OF STORMS COULD BE IN PROGRESS WEDNESDAY MORNING EAST OF THE MIDDLE AND LOWER MISSISSIPPI VALLEY. IF THIS VERIFIES...IT IS UNCERTAIN WHAT INFLUENCE IT WILL HAVE ON SUBSEQUENT BOUNDARY LAYER DESTABILIZATION ACROSS THE OHIO/TENNESSEE VALLEY REGION. THIS UNCERTAINTY PRECLUDES AN OUTLOOK UPGRADE TO HIGH RISK PROBABILITIES AT THE PRESENT TIME. HOWEVER...MOST OTHER INDICATIONS ARE STRONGLY SUGGESTIVE THAT A MAJOR OUTBREAK OF SEVERE THUNDERSTORMS COMMENCING TONIGHT ACROSS PARTS OF THE OZARK PLATEAU/LOWER MISSISSIPPI VALLEY WILL CONTINUE AND EXPAND NORTHWARD AND EASTWARD TOWARD THE APPALACHIANS DURING THE DAY WEDNESDAY. THIS WILL INCLUDE THE POTENTIAL FOR LONG-LIVED SUPERCELLS CAPABLE OF STRONG TORNADOES...AND ORGANIZED STORM CLUSTERS CAPABLE OF STRONG DAMAGING WIND GUSTS.

Tuesday Afternoon 4/26/11 NWS Birmingham AFD: AND NOW FOR THE MAIN EVENT. A SIMILAR SETUP WILL EXIST FROM WHAT WE SAW BACK ON APRIL 15TH...AS SEVERAL WAVES OF ENERGY WILL BE MOVING INTO OUR AREA FROM THE ARKLAMISS WITH A 65KT 850MB JET DEVELOPING. WE WILL HAVE TO CLOSELY WATCH THE DEVELOPMENT OF THE LINE OF THUNDERSTORMS...AS THIS WILL HAVE A LARGE IMPACT ON THE DEVELOPMENT OF SEVERE THUNDERSTORMS LATE IN THE DAY. IF THE LINE HOLDS TOGETHER...SEVERE PARAMETERS WILL BE GREATLY REDUCED ONCE THE LINE PASSES. OUR CONFIDENCE IN THAT SOLUTION IS VERY LOW HOWEVER...AND WE ARE EXPECTING THE LINE TO BE GREATLY WEAKENED BY THE TIME IT REACHES OUR AREA...AND WILL ALLOW FOR A RAPID DESTABILIZATION OF THE ATMOSPHERE AFTER MIDDAY TOMORROW. AFTER TALKING WITH SPC...THE
UNCERTAINTY IN THE STRENGTH AND LOCATION OF ONGOING CONVECTION TOMORROW MORNING IS THE ONLY FACTOR THAT IS PREVENTING THE ISSUANCE
OF A HIGH RISK OF SEVERE STORMS AT THIS TIME. LOOKING AT THE LATEST FORECAST SOUNDINGS...NOT MUCH HAS CHANGED IN RELATION TO PARAMETERS THAT WE HAVE BEEN ADVERTISING OVER THE PAST FEW DAYS. ONE WORD COMES TO MIND AFTER LOOKING AT SEVERAL SOUNDINGS. DANGEROUS. A WEAK CAP SHOULD BE IN PLACE THROUGH THE EARLY MORNING HOURS...THEN RAPIDLY ERODE DURING THE LATE MORNING AND EARLYAFTERNOON. AMPLE TURNING OF THE WINDS WITH HEIGHT...DRY AIR ALOFT...AND STEEP LAPSE RATES WILL BE PREVALENT ACROSS ALL OF CENTRAL ALABAMA TOMORROW AFTERNOON. THE MOST UNSTABLE AREAS WILL BE GENERALLY ALONG AND NORTH OF INTERSTATE 20...BUT I MUST STRESS THAT ALL OF CENTRAL ALABAMA WILL BE UNDER THE GUN. MOST SEVERE WEATHER PARAMETERS ARE OFF THE CHARTS TOMORROW AFTERNOON. THE CRAVEN/BROOKS SIGNIFICANT SEVERE PARAMETER...WHICH INCORPORATES 100MB MLCAPE AS WELL AS THE 0-6KM SHEAR...IS BETWEEN 60000 TO 80000. USUALLY YOU WILL SEE SIGNIFICANT SEVERE EVENTS OCCUR WHEN THE VALUE IS ABOVE 20000. THAT ON TOP OF ALMOST 3000J/KG OF SBCAPE IS A VERY DANGEROUS MIXTURE. 0-6KM SHEAR OF 60-80KTS...WITH VECTORS SUGGESTING SUPERCELL DEVELOPMENT REMAINS IN PLACE AHEAD OF THE FRONT. ALONG THE FRONT...SHEAR VECTORS CONTINUE TO INDICATE THE DEVELOPMENT OF SUPERCELLS UNTIL LATE IN THE EVENING AS THE FRONT APPROACHES EASTERN ALABAMA. THE SHEAR VECTORS THEN BECOME SLIGHTLY MORE PARALLEL TO THE FLOW...INDICATING A MORE LINEAR DEVELOPMENT...BUT TORNADOES ALONG THE LINE WILL CONTINUE TO BE POSSIBLE. ONCE AGAIN...THE DEVELOPMENT OF STORMS TONIGHT WILL HAVE A MODEST IMPACT ON THE MESOSCALE MORROW...AND HOW THE EVENT PLAYS ITSELF OUT. I MUST ALSO STRESS THAT NOW IS THE TIME TO REVIEW YOUR SEVERE WEATHER PLAN TO PROTECT YOUR LIFE AND PROPERTY...TOMORROW WILL BE TOO LATE!

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