1. WEATHER FORECASTING IN MID-LATITUDE REGIONS IN MID-LATITUDE REGIONS 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 group, headed by J-Ch Rivrain and with the support of the scientific expertise provided by J-Ph Lafore, is composed of Mrs Canonici, Mercier, Mithieux and Mr Boissel, Bourrianne, Celhay, Jakob, Hagenmuller, Hameau, Lafore, Lavergne, Lecam, Lequen, Mounayar, Rebillout, Rivrain, Rochon, Robin, Sanson, Santurette, Voisin and many others. Proofreading, references by Jean Paul Billerot.

2. 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

3.  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

4. 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

5. - 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:

6. 00 UTC 12 UTC 06 UTC 18 UTC  ’ w at 850hPa (shaded) 19 Sept 2000 Wind at 500 m

7. 19 Sept 2000 1, 2, 3: Radar images at 12, 15, 18UTC 4: Thunder impacts between 12 and 18 UTC 1 3 2 4

8. ANASYG, 19 Sept 2000 at 12 UTC