(from "Nowcasting Mesoscale Phenomena" by John McGinley;
Mesoscale Meteorology and Forecasting; ed. Peter S. Ray; 1986)

1. Accurately analyze morning soundings.

It should be of no surprise that the early morning soundings will indicate the presence of convectively unstable air, the strength of the capping inversion, and the surface heating needed to generate convection. Do other mechanisms exist that will help penetrate the cap?

2. Determine the time of first convection.

The first hint of a mesoscale destabilizing mechanism is clumped cumulus. If lift exists, we might expect stratified (layered) clouds, so the presence of cumulus clouds indicates the effectiveness of surface heating. If the presence of clouds does not occur until late in the day, this may indicate too strong of a cap or dry air in the boundary layer.

3. Watch for the evolution of sea breezes or urban heating which may locally enhance or suppress the cap.

These mesoscale events are good indicators of instability in the environment. Therefore, urban heating may produce thermals which may generate convection.

4. Look for evidence of moist layer increase.

Indications from PIREPS (pilot reports) or satellite imagery may indicate the increase of low level moisture content. Remember, to get storms, we need instability and moist air.

5. Be alert to the presence of fronts or jets.

These are good signs that there will be dynamic lifting. Since jets indicate wind shear and wind shear is vital to the production of severe weather, their presence is crucial. Also, keep in mind that exit and entrance regions can produce divergence and help generate lift.

6. Look for old outflow boundaries.

On a local scale, the outflow air from thunderstorms may act as a trigger for new convection. These may help destabilize the cap by helping with more lifting.

7. Look for existing thunderstorms to provide new outflow.

Again, new convection may be triggered by the outflow of a thunderstorm. Also, the presence of the first thunderstorm indicates that the cap is able to be broken. If this occurs in the late afternoon, additional surface heating will enhance destabilization.

8. Watch for indications of the establishment of a low-level nocturnal jet.

After the presence of thunderstorm development, a low-level jet can act to maintain multicellular activity and the possibility of the development of an MCC. This nocturnal jet reaches its maximum and most westerly flow around midnight.

To determine severity of thunderstorms:

1. Radar display information.

A) Reflectivity over 45 dBZ at mid-levels.
B) At least a 6 km overhang over the low-level reflectivity gradient.
C) The presence of a WER or BWER with the storm top over the strong low-level reflectivity gradient.
D) The formation of the hook echo at low levels.
E) The TVS appearing as an anomaly in a Doppler display.

2. Penetration of the storm top significantly higher than the calculated EL.

The higher the storm, the stronger the updraft, the stronger the storm.

3. Clusters of cells that merge into one.

4. Cells that travel slower or to the right of the mean winds as indicated by their previous movement.

5. Cells that split and intensify.

These intensifications generally occur to the right.

6. Watch the nature of the gust front.

The intersection of two gust fronts may give rise to new explosive convection. Also, a swift moving gust front indicates a strong down draft and hence, gusty surface winds.

7. Configurations of storm systems.

A) Bow echoes often indicate mesoscale organization which may lead to downbursts in the reflectivity gradient ahead of the line.
B) Right flank storms in a squall have the greatest potential for severity due to the unblocked inflow.
C) An isolated storm ahead of the squall may lead to supercell development.

To determine probability of tornadoes:

1. Tornadic storms are often isolated.

Because of the nature of severe storms which tend to move slower than the mean winds, an isolated storm, often means a supercell. These isolated storms are able to maintain their growth and strength because they are not affected by the outflow environment of other storms.

2. Look for 90° veering in the winds within the lowest 4 km.

This is often the case when studying dry line tornadoes in the panhandle region of Texas and Oklahoma and the western Great Plains. This should be coupled with strong moist low-level inflow. This must occur in a region where surface temperatures maintain sufficient warmth to sustain tornado circulation to the ground.

3. Look for rotation in the mesocyclone.

Storm rotation is a must for tornado development. Often times beneath the rain-free base a cloud appendage lowers slightly. This is called the wall cloud. The wall cloud mark the entrance of strongest winds entering the updraft. The tornado will usually form as rotation in the wall cloud is noted. The tornado does form in the wall cloud region, but there are times when the wall cloud is nearly imperceptible. As a general rule, a persistent, rotating wall cloud usually precedes the tornado.

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