CONVECTI ON

WEATHER FORECASTINGWEATHER FORECASTING IN MID-LATITUDE REGIONS IN MID-LATITUDE REGIONS

Prepared in close collaboration with the “Working Group on Convection” in the Prepared in close collaboration with the “Working Group on Convection” in the frame of the Plan de Formation des Prévisionnistes program of Météo-France. This frame of the Plan de Formation des Prévisionnistes program of Météo-France. This group, headed by J-Ch Rivrain group, headed by J-Ch Rivrain and with the support of the scientific expertise and with the support of the scientific expertise provided by J-Ph Laforeprovided by J-Ph Lafore, is composed of Mrs Canonici, Mercier, Mithieux and Mr , is composed of Mrs Canonici, Mercier, Mithieux and Mr Boissel, Bourrianne, Celhay, Jakob, Hagenmuller, Hameau, Lafore, Lavergne, Boissel, Bourrianne, Celhay, Jakob, Hagenmuller, Hameau, Lafore, Lavergne, Lecam, Lequen, Mounayar, Rebillout, Rivrain, Rochon, Robin, Sanson, Santurette, Lecam, Lequen, Mounayar, Rebillout, Rivrain, Rochon, Robin, Sanson, Santurette, Voisin and many others. Proofreading, references by Jean Paul Billerot. Voisin and many others. Proofreading, references by Jean Paul Billerot.

• At mid latitudes, over France, the development of severe convective systems is most of the time associated with a synoptic forcing and with convective potential instability.

• Intense convective storms occur if the synoptic situation allows for convective ascents developing over the whole depth of the troposphere, up to the tropopause.

Convection and Synoptic Situation

Upper troposphere

Anomaly (dynamical) at the tropopause

Jet Streak “right-hand” entry “left-hand” exit

Dry air advection low ’w ~ 500hPa

Low troposphere

All conditions favorable for convective development:

warm advection

area of maximum of warm air

= occurrence of a pseudo-front

moist advection

proximity of a front

zone of convergence at low levels

Synoptic Characteristics Favorable to

Storm Development

METEOSAT Water vapor

image,

19 Sept 2000 at 12 UTC

Superimposed:

- Geopotential at 1.5 PVU

- Jet on the 1.5 PVU surface

Upper level circulation

favorable to ascending motions

- Right entry of the Jet Streak flowing north toward the British Islands

- Left exit over the Mediterranean

- Dynamical anomaly of the tropopause

- to a maximum of warm air at low levels: ’w = 18°C at 850 hPa

- with a clearly defined line organization in the convergence zone between southerly and westerly flows

- to a cold air advection at mid levels: ’w = 14° at 500hPa

Radar image, 19 Sept. 2000 at 15 UTC

Superimposed:

’w at 850 hPa (red) and at 500 hPa (blue)

- wind at 500 m height

The main convective activity

corresponds:

00 UTC

12 UTC

06 UTC

18 UTC

’w at 850hPa (shaded)

19 Sept 2000

Wind at 500 m

19 Sept 2000

1, 2, 3: Radar images

at 12, 15, 18UTC

4: Thunder impacts between

12 and 18

UTC

1

3

2

4

ANASYG, 19 Sept 2000 at 12 UTC