Transcript Case study of a cold air outbreak over the Baltic Sea, 2-3

Case study of a cold air outbreak
over the Baltic Sea, 2-3 November 2006
Galina Bogdanova,
Latvian Environment,
Geology and Meteorology Agency,
Aviation Forecast Division,
Riga, Latvia
[email protected]
Heavy snowfall along the Latvian coast in autumn and winter is very typical during situations with persistent north-westerly flow of a cold air over the
ice-free areas of the Baltic Sea and the Gulf of Riga. These snowfalls cause traffic hazards and communication problems. A proper understanding
and forecasting of these events are considered essential. The event of 2-3 November 2006 was selected.
Synoptic situation
The weather situation is illustrated by NOAA IR image 2006-11-02/20:51 UTC and a set of analysis and prognostic maps. Mesoscale convection bands forming within the cold air mass
behind a cold front over warm sea water of the Baltic Sea and the Gulf of Riga were particularly strong along trajectories ending over the coast areas. The following significant weather
events 2-3 November 2006 were observed in Latvia : snow showers with visibility of 500-1000m and high snowfall accumulations in the central part, gusty winds. Convective
precipitation were confirmed by measurements from the radar at Riga.
SMHI
Sea surface temperature 2006-11-02
2006-11-02/20:01-NOAA IR image
NWP forecast problems
The main convective precipitation band over the Baltic Sea was very well forecasted and
precipitation accumulation near the Latvian west coast were forecasted in agreement with
observations. However, precipitation amount was not so good forecasted for the Gulf of Riga
and the central part of Latvia. Convective activities along the trajectories of cold air over the Gulf
of Riga in comparison to over the Baltic Sea are often forecasted not so well in forecast models
used by forecasters in Latvia.
The orography, sea-ice data and sea-surface temperature are very important initial data for
model systems to be able to forecast quite well mesoscale convective precipitation in these
weather situations. Unfortunately, at present high resolution model outputs are not available for
forecasters in Latvia.
In case of a cold air outbreak over the Baltic Sea for forecasting heavy snowfall the following parameters
shell be considered:
Relative top.+TA
the centre of a thickness trough (H500/1000) over the Baltic Sea and maximum of synoptic scale cold air
advection,
T difference
significant temperature difference between water surface and the air for severe situations not less than 810°C.
Radar and satellite data are essential for short-range forecasting and nowcasting.