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

2. 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!

3. 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.

4. 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.

5. MCC Study Details 10 total MCC cases into composite maps

6. Maddox Pre-MCC Initiation Graphics 850 hPa 700 hPa

7. Pre-MCC Initation Graphics Cont. 500 hPa 200 hPa

8. 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

9. Part 2: A Localized Study of Nocturnal Convection

10. Area of interest • Convection must develop between 94o - 101o W and 42o and 46oN.

11. Using lightning data • Obtained lightning data January 1998 – September 2003 • Divided study area into 0.25o x 0.25ogrid • 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.

12. Example of lightning plot

13. 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.

14. 0700 UTC 0800 UTC 0900 UTC

15. 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

16. When does convection initiate?

17. Where does convection initiate?

18. 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

19. Pre-Nocturnal Convection Composites • Focusing on the big three aspects of convection: • Lift • Moisture • Overall Stability

20. 850 hPa

21. 700 hPa

22. 500 hPa

23. 200 hPa

24. Temperature Advection 850 hPa700 hPa

25. 700 to 500 hPa Stability Lapse rate Differential temperature advection

26. 850 to 700 hPa Stability Lapse rate Differential temperature advection

27. 850 hPa Moisture transport

28. 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.

29. 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)

30. 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

31. 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.

32. 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?

33. 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.