Precursors to the Initiation of Nocturnal Convection in the Eastern Plains

1
Precursors to the Initiation of Nocturnal
Convection in the Eastern Plains

• Matthew Dux
• March 1, 2006
• WFO Pleasant Hill, MO

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Objectives

• Understand the impacts of forecasting nocturnal
  convection
  
• Review synoptic trends prior to nocturnal MCC
  initiation as based on previous studies
  
• Review pre-initiation synoptic conditions as
  apparent in localized studies
  
• Put it all together in the end!

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Nocturnal Convection Impacts/Concerns

• Usually a smaller staff base during the late
  evening and overnight hours
  
• Calling people in to work?
• Will you need to coordinate with your awareness
  network?
  
• Spotter network shrinks once the overnight hours
  are reached
  
• Reduced awareness of meteorological impacts of
  weather
  
• Heavily rely on law enforcement, media, and
  emergency managers
  
• Normally an advanced forecast can be issued
• Uncertainty in strength and type of convection
  leads to large portions of an area
  
• Television, radio, and severe weather sirens are
  usually less effective past 2200 LT.
  

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Precursors to MCC Development

• One of the most significant nocturnal events
• Primary threat of intense rainfall and flash
  flooding
  
• Can produce hail, wind, and even tornadoes.
• Looking at synoptic and mesoscale models of MCC
  development (Maddox 1980) forecasters are guided
  to look at
  
• Large low-level moisture content
• Weak low-level warm-air advection
• High equivalent potential temperature advection
• These ingredients are commonly noted as
  intensifiers for nocturnal convection.
  

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MCC Study Details
10 total MCC cases into composite maps
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Maddox Pre-MCC Initiation Graphics
850 hPa
700 hPa
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Pre-MCC Initation Graphics Cont.
500 hPa
200 hPa
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Summary of Signals

• Ongoing convection lies ahead of a weak mid-level
  shortwave trough
  
• Long wave ridge pattern dominates flow aloft
• However, strong low-level WAA is the predominate
  source for sustaining and organizing convection
  
• Abundant moisture usually pooled into the area by
  a strong low-level jet at 850 hPa
  
• Jet continues to veer and strengthen as night
  progresses sustaining convection
  
• 10 g/kg average common prior to development

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• Part 2 A Localized Study of Nocturnal
  Convection
  

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Area of interest

• Convection must develop between 94o - 101o W and
  42o and 46oN.

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Using lightning data

• Obtained lightning data January 1998 September
  2003
  
• Divided study area into 0.25o x 0.25o grid
• Every hour counted the number of lightning
  strikes in each bin
  
• Data written to GEMPAK files
• If less than 10 strikes across the area, hourly
  data was not saved.

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Example of lightning plot
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Criteria for convective initiation

• Must initiate between 0200 UTC and 1400 UTC.
• 10 lightning strikes in 0.75o x 0.75o box.
• Majority of must be within the area of interest.
• No lightning strikes in adjacent boxes during the
  current hour and the previous hour that can be
  traced from convection that developed outside the
  box or prior to 01Z.
• There can be no lightning strikes in the same box
  the previous two hours.
  
• Continuity
• Must produce at 10 strikes in 0.75o x 0.75o for 2
  consecutive hours.
  
• The 0.75o x 0.75o box must be adjacent for
  consecutive hours.
  

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0700 UTC
0800 UTC
0900 UTC
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Climatology Results

• 262 days identified
• 43 days per year
• For each storm, following were tracked
• Time of initiation
• Location of initiation to nearest 1 degree
• Time of dissipation
• Number of severe weather reports
• Number of flash flood reports

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When does convection initiate?
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Where does convection initiate?
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Creating composite maps

• Observed soundings collected for each case.
• Soundings objectively analyzed for each case
• Used a Barnes analysis to a 1 x 1 degree lat-lon
  grid.
  
• Data quality controlled by examining grids.
• If bad data found, it was removed and data
  reanalyzed.
  
• Pre-initiation data averaged to create a
  composite grid
  
• 0200-0700 UTC period uses pre-event 1200 UTC
  sounding
  
• 0800-1400 UTC period uses pre-event 0000 UTC
  sounding
  

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Pre-Nocturnal Convection Composites

• Focusing on the big three aspects of convection
• Lift
• Moisture
• Overall Stability

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850 hPa
21
700 hPa
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500 hPa
23
200 hPa
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Temperature Advection
850 hPa
700 hPa
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700 to 500 hPa Stability
Lapse rate
Differential temperature advection
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850 to 700 hPa Stability
Lapse rate Differential temperature advection
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850 hPa Moisture transport
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Equivalent potential temperature advection

• By comparing 850 hPa theta-e to 700 hPa theta-es
  one can estimate cap strength.
  
• 850 hPa theta-e dependent on temperature and
  moisture.
  
• 700 hPa theta-es dependent on temperature only.

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Low-level stability

• Difference in 850 hPa theta-e advection and 700
  hPa theta-es
  
• advection shows best area to overcome capping
  inversion
  
• Positive values indicate an area where lower
  levels of the atmosphere can overcome capping
  inversion at 700 hPa (more unstable)
  
• Negative values show the opposite (more stable
  air dominates)

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Conclusions

• Initiation of nocturnal convection is common in
  the eastern plains.
  
• 262 nocturnal events in a 6 year period
• 43 days per year of new convection
• Two peaks in convective initiation
• Late evening and late night.
• Convective initiation favored near the Missouri
  River near in south central South Dakota
  
• No apparent change in location by time.
• Many of the same signals of MCC enhancement are
  initiators to new nocturnal convection
  

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Synoptic scale conditions prior to new initiation

• Long wave ridge moving into Great Lakes
  downstream of study area
  
• 850 hPa trough developing in lee of Rockies,
  ridge over the SE
  
• Low-level southerly flow increasing (nocturnal
  jet)
  
• Convection commonly initiates at the nose of the
  LLJ
  
• Low level flow increasing moisture into the
  eastern Plains
  
• Convection initiates on the northern edge of the
  moisture surge
  
• Increasing mid-level instability
• Thermal advection acting to increase mid-level
  lapse rate (7H-5H)
  
• Weak mid-level shortwave progressing through the
  area
  
• Strong low-level thermal advection
• Synoptic scale forcing for lift seen prior to
  convective development
  
• 850 hPa theta-E advection strong enough to
  overcome strengthening cap at 700 hPa.
  

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Things to Think About

• As time progresses, how does nocturnal convection
  alter the environment?
  
• How will the future evolve?
• What makes daytime convection different than that
  of nocturnal convection?
  

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References

• Maddox, R. A., 1980 Mesoscale convective
  complexes. Bull. Amer. Meteor. Soc., 61,
  1374-1387.
  
• Maddox, R. A., 1983 Large-scale meteorological
  conditions associated with midlatitude, mesoscale
  convective complexes. Mon. Wea. Rev., 111,
  1475-1493.