# Presentation: 679044

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

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

## 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!

## 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.

## 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.

## MCC Study Details

- 10 total MCC cases into composite maps

## Maddox Pre-MCC Initiation Graphics

-                850 hPa                                                  700 hPa

**Notes:**
1.) Characterized by a broad trough over the western states, a ridge over the southeast, and a weak high over the Great Lakes
2.)  Most pronounced feature at 850 hPa is the relatively strong 850 mb Jet (this is a recurrent features of the MCCs’ initation environment and usually provides high moisture inflow to the environment)
3.)  Strong WAA is present at all of the lower levels of the atmosphere.
4.) At 700mb, winds typically veer and weaken with height.
5.) Possibility of a weak shortwave trough moving through the area.

## Pre-MCC Initation Graphics Cont.

-                500 hPa                                              200 hPa

**Notes:**
1.)  500 mb flow characteristically out of the southwest, with an increase in wind gradient from south to north. 
2.)  Normally a downwind trough signal with weak upwind shortwave moving into the initiation area.

3.) 200 mb flow shows a weak jet streak to the north of the genesis region.
4.) Characteristically weak diffluent flow over the genesis region.

## 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

## 

Part 2:  A Localized Study of Nocturnal Convection

- Part 2:  A Localized Study of Nocturnal Convection

## Area of interest

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

## 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.

## Example of lightning plot

## 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.

## 0700 UTC

**0800 UTC**

**0900 UTC**

## 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

## When does convection initiate?

## Where does convection initiate?

## 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

## Pre-Nocturnal Convection Composites

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

## 850 hPa

## 700 hPa

## 500 hPa

## 200 hPa

## Temperature Advection

- 850 hPa                                                 700 hPa

## 700 to 500 hPa Stability

-            Lapse rate                            Differential temperature advection

## 850 to 700 hPa Stability

- Lapse rate			Differential temperature advection

## 850 hPa Moisture transport

## 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.

**Notes:**
1.) large areas of unstable, moist air moving over study area.
2.) By comparing to 700mb theta-es (which is assumed to be a saturated parcel dependant only on temperatures) you can note that warmer, more stable air is progressing eastward.

## 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)

**Notes:**
By looking at this field, you can note basically if you have a large amount of warm, unstable air moving into the area...and you have a large amount of saturated, warm, stable air moving into the mid-levels, taking a difference field will determine where you have the best chances of reaching convective initiation.  

In the above map, there is more warm capping air over the western Dakotas, so that a parcel lifted from 850 mb to 700 mb would have a colder temperature than its environment.
Farther east, an increased amount of LL unstable air at the nose of the LLJ is creating parcel temperatures warmer than that of their 700 capped values (enhancing a broken cap)

## 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

## 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.

## 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?

## 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.