Severe Weather Outbreak of 252008

1
Severe Weather Outbreak of 2/5/2008

• Clark Evans
• Current Weather Discussion
• 6 February 2008

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Outline

• Synoptic-Scale Features
• Pros and Cons
• Daytime Evolution
• Convective Initiation
• Long-Track Tornado Event
• Summary

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Synoptic-Scale Features

• Early Morning, 5 February 2008
• Sharp Mid-Upper Level Trough ejecting out along
  the front range of the Rockies
• Significant low-level jet and moisture transport
  from the western Gulf of Mexico over a prolonged
  period of time
• Elevated mixed layer and warming aloft
  contributing to a cap around 700 hPa

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Initial SPC Outlook

• 0600 UTC Moderate Risk across lower Mississippi
  River valley from near Paducah, KY to the I-20
  corridor in LA
• 15 tornado threat with enhanced probabilities
  (moderate risk)
• 45 wind threat (moderate risk)
• 30 hail threat (slight risk)
• Did not go with a high risk due to a few
  uncertainties in the data and guidance

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0600 UTC Uncertainties

• Storm development locations
• Would storms remain confined to cold front or
  would they also develop ahead of it in the warm
  sector?
• Convective mode
• Would storms remain more discrete or would they
  tend to evolve more linearly?
• Warm sector, discrete development would favor an
  increased tornadic threat and warrant a possible
  upgrade to the threat probabilities

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0600 UTC SPC Day 1 Outlook
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1300 UTC SPC Outlook

• Upgrade to HIGH risk as aforementioned 1200 UTC
  soundings came in
• Most of AR and just south of Memphis in MS
• Expanded the MDT risk into western KY
• Reasoning mixed-layer convective inhibition
  relatively weak in warm sector forcing and
  heating should be enough to allow for discrete
  cell development

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1300 UTC SPC Day 1 Outlook
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Daytime Evolution

• Upper trough and surface low were slow in
  ejecting out of the southern Plains
• Daytime heating and ample surface moisture
  allowed for weak showers to develop across
  AR/LA/MS/AL/TN but could not break the cap
• 1800 UTC soundings suggested cap was lifting but
  still present in warm sector

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Afternoon Uncertainties

• 1630 UTC SPC Outlook HIGH risk maintained,
  expanded NE into TN
• 1716 UTC SPC Mesoscale Discussion 157
• THERE STILL REMAINS CONSIDERABLE UNCERTAINTY ON
  HOW THE UPCOMING CONVECTIVE SCENARIO WILL PLAY
  OUT THIS AFTN OWING TO CONFLICTING MODEL
  GUIDANCE.
• Lead shortwave trough was entering from Dallas
  area impact on thermal fields uncertain until
  1800 UTC soundings available
• Inhibition expected to weaken through time and
  allow storms to root in boundary layer, likely
  able to become tornadic from there.

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SPC Mesoscale Discussion 157
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Initiation

• Around 1800 UTC, activity began to fire along the
  cold front near Dallas in response to the
  upper-level forcing
• Around 2000-2100 UTC, activity began to fire well
  ahead of the cold front in LA/AR as rising
  motions became sufficient to break the cap

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Mid-Afternoon Activity

• 2100 UTC Jackson, MS sounding cap essentially
  gone
• Activity was able to root in warm sector, away
  from strongest forcing (cold front upper
  trough), in an environment of significant
  low-level shear
• Storms quickly became tornadic, particularly
  along the Mississippi River

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Level of Free Convection

• Analysis here at 2100 UTC shows the big
  breakthrough
• Earlier times, LCLs were similar to previous
  image
• However, LFCs were much higher
• This suggests buoyant motions were not able to be
  strong enough to break the cap before this time

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Radar Sweeps

• First set initial activity in S. Arkansas
• Second set discrete activity near Memphis, TN
• Third set discrete activity continuing into
  Kentucky and near Nashville, TN
• Fourth set transition to linear convective mode
  near front along Mississippi River

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Winding Down at Night

• Tornadic threat continued for several hours after
  sunset
• However, as upper trough neared the region,
  activity further to the west gradually grew into
  a squall line
• Wind damage became the more significant threat as
  mid-level dry air associated with the trough
  entered the picture

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SPC Mesoscale Discussion 169

• WHILE LOW-LEVEL FLOW FIELDS MAY TEND TO WEAKEN
  SOME...LOW-LEVEL SHEAR WILL REMAIN MORE THAN
  SUFFICIENT TO MAINTAIN A RISK FOR TORNADOES WITH
  CELLS WITHIN...AND JUST AHEAD OF /BEFORE MERGING
  INTO/...LINE.
• HOWEVER...WITH A CONTINUED STRENGTHENING OF
  SOUTHWESTERLY MID-LEVEL FLOW...WITH 500 MB SPEEDS
  IN EXCESS OF 100 KT...INTO THE REAR OF
  LINE...DAMAGING WIND POTENTIAL MAY BECOME THE
  PRIMARY THREAT...AS EVAPORATIVE COOLING IN DRY
  MID TROPOSPHERIC LEVELS AIDS DOWNWARD MOMENTUM
  TRANSPORT TO THE SURFACE.

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Winding Down at Night

• Further east, with the loss of daytime heating,
  surface-based instability gradually weakened.
• As this occurred, the discrete storms began to
  weaken and become non-tornadic
• The southernmost cell of this activity held out
  the longest, passing through Nashville, TN and
  into S. Kentucky as it cycled

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Winding Down at Night

• Further south, a quasi-linear squall line
  traversed across LA, MS, and AL, resulting in
  occasional tornado warnings through the evening
  hours.
• Even despite being night, areas just ahead of the
  squall line/cold front were able to maintain
  500-100 J/kg of surface-based CAPE.

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The Totals

• All figures as of 12p ET 2/6/08
• 67 tornado reports
• 189 wind reports
• 109 hail reports
• 25 mesoscale discussions, 9 watch boxes before
  midnight
• Numerous reports of injuries, particularly SE of
  Memphis and in Jackson, TN, and over 50 deaths
• Union University in Jackson female dorms
  completely destroyed, male dorms damaged with
  students trapped

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Summary and Conclusions

• An unseasonably moist and unstable air mass was
  present for a couple of days prior to the event
  across the lower Mississippi River valley
• Warming aloft associated with the subtropical
  ridge off of Florida and an elevated mixed layer
  advected in from the highlands of Mexico kept
  activity subdued all morning and into the early
  afternoon
• Daytime heating and large-scale synoptic forcing
  for rising motion overswept the region
  mid-afternoon, allowing for widespread convective
  development

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Summary and Conclusions

• Significant low-level shear, particularly speed
  shear but also directional shear, allowed for the
  development of several deadly tornadic supercells
  in AR, TN, MS, and KY.
• This discrete activity continued through the
  early evening hours, impacting the Memphis and
  Nashville metropolitan areas with confirmed
  tornado touchdowns
• As the upper-level forcing intensified and
  mid-level dry air entered the region, activity to
  the west nearer the cold front began to become
  linear in nature

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Summary and Conclusions

• This linear activity would contribute to a number
  of wind reports along the Mississippi River as
  the night progressed
• Further east, discrete activity began to wane
  through the nighttime hours as it moved into a
  region of weaker instability in N. Tennessee and
  S. Kentucky
• The southern extent of the linear activity
  occasionally produced tornado warnings and
  reports, particularly north of I-20 in MS and LA

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Summary and Conclusions

• In the end, three notable tornadic supercells
  developed in a fashion similar to Plains states
  outbreaks and tracked for hundreds of miles,
  resulting in numerous fatalities, injuries, and
  significant damage.
• The current fatality count of 50 is the highest
  from a tornado outbreak since May 31, 1985.