Severe snow event in Herault on the 28th of February 2018

Transcription

1 Severe snow event in Herault on the 28th of February 2018 Mediterranean event from Ph. ALBERT METEO FRANCE DIRSE/PREVI (mail:

2 Abstract Background Medium and short range ensemble forecast High resolution forecast Last forecast and Red vigilance Observations and troubles

3 Synoptic context: snow by mediterranean episode A EMMA Satellite Image+ MSLP 28 february Z

4 Ensemble forecast EPS : stable signal far in advance D+5 Q50 Q75 EPS 24/02 00utc for 1/03 06 utc Quantiles of 24h accumulation Rain and snow Q90 QMAX

5 Ensemble forecast PEARP : stable signal in advance Q50 D+3 Q75 PEARP 26/02 06utc Quantiles of 24h accumulation Rain and snow Q90 QMAX

6 Ensemble forecast PEARP : stable signal in advance Q50 D+2 Q75 On 27 th, 34, 30 and 48 vigilance ORANGE PEARP 27/02 06utc Quantiles of 24h accumulation Rain and snow Q90 QMAX

7 Synoptic overview with ARP 27/02 00UTC forecast 36 for 28/02 12UTC 500hPa 950hPa Snow Cumulation 03h Rain Cumulation 03h Morning of 28 th, first weak snowfall forecast over Languedoc

8 Synoptic overview with ARP 27/02 00UTC forecast 42 for 28/2 18Z 500hPa 950hPa Snow Cumulation 03h Rain Cumulation 03h Afternoon of 28 th, milder over Herault/Gard with mostly rain forecast in plains

9 ARP 27/02 00UTC forecast 48 for 01/03 00Z 500hPa 950hPa snow 03h rain 03h Night of 28 th, comeback of snow on eastern Herault, rain intensifying on western Herault

10 ARP 27/02 00UTC forecast 60 for 01/03 12Z 500hPa 950hPa Snow 03h Rain 03h Morning 1 st March, generalization of rain with warm air advection in low levels

11 Vigilance issue : what kind of precipitation over Languedoc? Variability snow / sleet / ice / rain with fields θ w in low levels - level of maximum thermal advection (for example 850hPa) - level of maximum of cold air in very low levels (for example 950hPa) snow ice sleet snow rain Area of transition rain-snow

12 ARPEGE 00 28/02 (animation of 3h accumulation from 28 th 03 utc to 2 d 00 utc) Snow amounts in opacity Rain amounts in transparency Expected scenario over Herault : Snow on coast between 09 and 12utc Rain on coast between 12 and 18utc Snow on coast between 18 et 06utc Rain spread during the day on 01/03

13 ARPEGE 00 28/2 12h accumulation and Day of 28 February on coast 34 (A9 and Montpellier) : 5 à 15 kg/m2 snow Followed by 10 to 20mm of rain between 12 and 18utc Night of 28 to 01 on coast 34 (A9 and Montpellier) East part 5 to 15 kg/m2 of snow then 20 to 30mm of rain between 18 to 06utc West part 30 to 50mm of rain

14 AROME 03 animation of 3h accumulation RR from 28 th à 03 utc to 2 st 00 utc Snow amounts in opacity Rain amounts in transparency Herault AROME simulated rain blocking on coastline by cold air dome Use of field Rrso = snow + graupel

15 Timeline of precipitation according AROME 03 Snow until early afternoon Then, milder: rain with partial melting of snow on ground New important snow event during the following night

16 AROME 06 24h accumulation for 01 st 12UTC RR tt RRso Rain tt Snow Update AROME 06h : axis of rain blocking over the sea on eastern Herault more RRso

17 AROME 12 24h accumulation for 01 st 12 UTC RRtt RRso PLtt NG Update AROME 12: confirmation axis of rain blocking more and more RRso

18 Discrimination of meteors during the event with automatic tool Hydre Blue : snow Green : rain Red : freezing rain Datas from model lead the tool to overestimate rain

19 Vigilance decisions by forecasters At 15h on 28 th : 15 to 20 cm in the northern part of Montpellier. Motorways blocked with a lot of «snow castaways» At 16h : rain and snow mixed on the southern part of Montpellier: it is lated compared to AROME forecast but one still believe melting is possible. At 18h30 change of forecaster: cold air resist. Arome, AromePI, Hydre don t fit reality. 20 h: red vigilance is issued for the next forecast snow falls Before milder temperatures and rain at the end of the night, heavy snow and sleet, with little increase of snow dept (+5 cm in the northern part of Montpellier) but great increase of snow weight (collaspe of roofs). Automatic snow height captors in Beziers and Nimes : no data because of software configuration (0=-50 cm!) No snow Observations! Only social media news!

20 What really happens during the day 28 th of February Precipitations started near 07h30 loc in the area of Montpellier in the form of snow. Snow lasts until the middle of the afternoon. At 13h loc, snow amounts near 10/15cm in the urban area of Montpellier. Rain and snow mixed arriving by west Starting at 15h, rain and snow mixed observed in the area of Montpellier. 13h loc Near 16h, snow accumulation of 10/15 cm in the northern area of Montpellier where snow maintains. With rain, snow compacts in the southern part of the city. 15h

21 What really happens during the night 28 th of February to 1 st of March During the night near Montpellier, snow falls with heavy and sticky snow. On thursady 1 st of March morning, rain gradually takes over in the region of Montpellier. Before rain takes over, amounts of snow reach : - 30/35cm NW of Montpellier - 20 cm in the city of Montpellier - 20/30cm inside Herault (piemont of Cévennes and area Lodève -Bédarieux). 22h

22 Consequences and feedback 2000 cars and trucks blocked on A9, Montpellier Airport closed no bus or tramway, collapses of roofs, fall of trees, power cuts, etc... Bad feedback from those who were criticized by the media Montpellier Mayor, Vinci (motorway) : «snow event was more intense than forecast» Only small snowplows available!

23 Motorway administrator Inconstancy between automatic products and bulletins (message more pessimistic in forecaster bulletins) Forecast not clearly understood during road conferences

24 Conclusion Forecast rather good, not perfect but with gradual improvement No change in behaviour from Public when orange vigilance, only red could be disuasive (?). After 17 years of vigilance, people forget to read bulletins or think only Authorities have to take action Problem of trucks : too many, no respect of driving bans, no snow equipment, etc...