Severe storms: 20th of May and over Serbia, Hungary and Romania
Authors:
- Mária Putsay
- Kornél Kolláth
- Jochen Kerkman
- Jarno Schipper
Fortunately well forecasted (highest alarm for Hungary on MeteoAlarm) on 20th May 2008, rapid development of severe convective storms were observed over Hungary. On the ground these storms where associated with torrential rainfall, flash floods, hail and even tornados. The enhanced Meteosat 9 images show very the devlopment of nice ring storms! The explosive growth and the severe updrafts are also nicely reflected in the T-Reff plots and also plumes were visible in the HRV images! This material and also the rapid scan of Meteosat 8 is presented in this case study.
Meteosat 9 IR10.8 with weather overlays- 20 May 2008 1200UTC
Meteosat 9 Dust RGB with Showalter Index- 20 May 2008 1200UTC
Above two images of the situation on the 20th May at 1200 UTC. To the left we see the IR10.8 image with the weather at that moment. Some quite great contrasts in temperatures are found over central Europe. Only 9°C in Vienna, but 29°C in Sofia. In between the convection over Hungary has started. The image on the right is the so-called Dust RGB. This RGB is not only usefull to monitor Dust, but also to monitor convection as it provides a nice high contrast. The cells itself appear as red indicating the thick iceclouds, the anvils, or cirrus shields, appear as black due to the IR12.0; IR10.8 brightness temperature difference. The Showlaterindex is overlaid on this latter image. Negative values are an indication for thunderstorms. In this case the SI is up to -3!
In this case the relevant satellite imagery from the SEVIRI instrument and Radar will be presented and described. The case was associated to reports of thunderstorms, hail and tornados!
This footage of a tornado was taken near the city of Gátér in the southeastern part of Hungary (click here to start the movie)
KNMI Surface Chart - 20 May 2008 1200UTC
Meteosat 9 Airmass RGB - 20 May 2008 1200UTC
From ESTOFEX the following forecast for May 20 was issued for Central Europe:
SYNOPSIS
Three low MSL-pressure areas are present on the map today, the main one centered over Italy, a weak one over Scandinavia, and an Atlantic low far away from the continent. A weak ridge stretches from Iceland via the UK towards Portugal.
Most interesting is the Mediterranean low with its extension over the Balkan filled with an unstably stratified airmass with predicted MLCAPE values in the 500-1500 J/kg range. Southeast of an elongated stationary occlusion over Russia/southeast Belarus towards Slovakia and Slovenia thunderstorms will concentrate near zones of enhanced low level convergence. Over this zone, a southwesterly mid level flow creates locally moderate deep layer shear, especially over the Balkan where warm air advection creates veering winds with height favorable for supercell and long-lived multicell clusters.
West of the surface warm front, over eastern Austria and southern Czech Rep. elevated CAPE is present and some thundery convection may be embedded in stratiform rain.
DISCUSSION
…central and northwestern Balkan…
GFS theta-e fields at 700 hPa shows an advance of lower values northward over the Adriatic Sea into western Hungary. At lower levels this is less pronounced, but it seems this feature is a weak cold front. A weak jetstream follows the same curved path. However, this means the area of instability resides mostly at the right exit side of the jet and may experience some subsidence at higher altitudes.
GFS has consistently produced moderate MLCAPE and moderate deep layer shear (about 14-18 m/s 0-6 km) coming together in the region around Hungary. The surface front is sharply defined, where LCL heights are low. 0-1 km shear seems to be predicted to be favorable to tornadogenesis (>10 m/s) - however - probably on the cold side of the front with no instability. SREH-3 km is enhanced through an area more overlapping with instability, according to the model in the eastern half of Hungary - NW Romania corner. This depends mostly on 10m winds from northerly or easterly directions. Storms developing in this area have increased probability of becoming supercells with large hail, gusts and flash floods. But also persistent multicell clusters, possible in a larger region, bear a good chance of large hail. Would not be surprised to see an isolated report of hail beyond 4 cm.
Meteosat 9
Meteosat 9 - Enhanced IR10.8: time sequence This first loop shows the convective development using the IR10.8 channel. The images are artificially enhanced to ensure a better discrimination of the several convective stages.
Meteosat 8 - Enhanced IR10.8: Rapid Scan
Again a loop with in which the convective development using the IR10.8 channel is shown. The advantage however is that these images were taken with the Rapid Scan of Meteosat 8, so with 5 minute time interval!
Meteosat 8 - HRVIS: Rapid Scan
The rapid scan of HRVIS gives another view on the onset of the convection over Serbia and Hungary. Overshooting tops and the development of anvils as the cells reach their mature stage can well be monitored.
Meteosat 8 - Severe Convection RGB: Rapid Scan
The convective development over Central Europe is here pictured using the so-called “severe convection RGB”. On red the brightness temperature difference (BTD) of the two water vapour channels 6.2 and 7.3. On green the BTD of the infrared channels 3.9 and 10.8 and on blue the BTD of the two visible channels 1.6 and 0.6, respectively, are pictured. The yellowish colors here correspond to the areas of higher 3.9 micron reflectivity.
The images consist of four parts; upper left IR10.8; upper right WV6.2-IR10.8; bottom left IR10.8 +radar Zmax > 25dBz; bottem right IR10.8 + 10 minute lightning
With the T-reff plot (Available operational in the MSG RBG program of Rosenfeld and Lensky) Jochen Kerkmann made a plot over the convective region in Hungary and Serbia. The resulting scatterplot shows that the region may be characterised by very severe updrafts that help us to understand the exceptional weather events. You can read more on T-reff “here”.
Especially with the smaller scale cells the high resolution channel onboard Meteosat 9 allows a very good monitoring of the convective development. For this case the HRV (High Resolution Visible) of 1150UTC of shown which shows two large cells over Serbia and Hungary.
The K-index is a widespreaded method amongst meteorologists to make a stability analysis of the atmosphere. If you click the image the K-index as computed by the GII algorithm is presented in 1 hourly interval.
GII Precipitable Water Contents
One final product to be presented is the precipitable water. For a Meteorologist this product can be of extreme value when doing a nowcast. It represents the total atmospheric water vapor contained in a vertical column of unit cross-sectional area extending between any two specified levels, commonly expressed in terms of the height to which that water substance would stand if completely condensed and collected in a vessel of the same unit cross section.