Weather Photos blog

A very dangerous setup is setting up over southern Alpine region during the next 48hrs. An impressive very large deep trough is placed over western half of Europe and strong SW-erly zonal flow is established over southern Europe and Mediterranean sea. Numerous disturbances/frontal systems are continuously traveling towards northern Mediterranean.

One of the surface depressions crossed southern Alpine region and Slovenia yesterday during the day and previous night which resulted in large precipitation amount. Whole western Slovenia received from 50-120mm of total rainfall accumulation in 36hrs period (22.12. 18z - 24.12. 6z), while some location along the western side of Dynaric mountain range received 150-200mm in the same period. The rainfall amounts are definatelly an impressive value but additional concern is the strong WAA which melted all snow accumulated during the previous weekend. Temperatures at 850mb level have risen for more than 20°C which is a strong warming for such a short period of 72hrs.

Main rivers over west-central Slovenia have risen rapidly and local flooding already occured today. Here are today’s max reached water flow and max reached heights for some rivers:

• Soca river at Solkan station: 1900 m^3/s, 1039 cm
• Soca river at Log Cezsoski station: 237 m^3/s, 283 cm
• Vipava river at Dolenje-Ajdovscina station: 199 m^3/s, 349 cm
• Sora river at Suha station: 320 m^3/s, 360 cm
• Selska Sora river in Zelezniki station: 70.6m ^3/s, 235 cm
• Sava river at Sentjakob station: 992 m^3/s, 752 cm
• Sava river at Medno station: 958 m^3/s, 419 cm
• Reka river at Cerkvenikov mlin station, Buje: 157 m^3/s, 445 cm
• Reka river at Skocjan II station: /, 307 cm
• Idrijca river at Podroteja station: 142 m^3/s, 329 cm
• Ljubljanica river at Moste station: 220 m^3/s, 242 cm
• Gradascica river at Dvor station: 32.6 m^3/s, 237 cm

Rainfalls have mainly stopped during the day today when the first depression shifted NE-wards. Another strong surface depression was blasting across southern Iberian Peninsula earlier today and was rapidly approaching Northern Mediterranean. Its expected to cross our regions this night. Again, several tens of milimeters are expected, mainly over western Slovenia with peaks along the mountain range (NW Slovenia-Posocje and SW Slovenia-Ucka and Sneznik area). There, 50-100mm are indeed possible. But this system will be less intense than the previous one.

500mb heights:

6hrs accumulated rainfalls Wed/Thu night:

Strong jet streak within the main zonal SW-erly flow will provide quite an impressive wind shear (50-70m/s of DLS) and SR helicity (300-600m^2/s^2). Due to strong LL jet (called Sirocco wind), very high LL shear will be present as well.

H7 winds…

DLS/LLS Thu 00z…

SREH3 Thu 03z…

According to these impressive values of shear nad helicity, additional help to persisting upslope flow may allow precipitation to be more intense. Thats why expected rainfalls are often higher than those models predict (ignoring the fact that models are more global with poor resolution). More to the south, over Adriatic sea and coastal areas some weak instability could release and a couple of storms might gain organization with such strong shear. A tornado over water cannot be excluded. But overall, the main threat will remain excessive rainfall.

Slovenian National Meteorological Service EARS issued yellow and orange warnings for SW Slovenia, mainly for excessive rainfalls and strong winds.

METEOALARM DAY 1

METEOALARM DAY 1 - NW SLO

METEOALARM DAY 1 - SW SLO

This new surface depression overnight will additionally rise the rivers, but the more interesting and serious situation comes later during the afternoon of Thursday and especially overnight to Friday. Then, a new and stronger surface depression arrives into Northern Mediterranean and high amount of precipitation are expected.

500mb heights:

As can be seen on today’s model maps, the Northern Mediterranean with North Italy and Slovenia will be placed right under the left-jet exit region which will provide widespread ascent beneath the upper-level divergence. Intense rainfalls will become very likely!

300hPa winds + divergence - left exit region over northern Mediterranena sea:

Here are some wind maps across different levels where strong persisting LL jet will provide strong upslope flow and intense orographic rainfalls along the Dynaric mountain range as well as along the western side of Apennini mountain range…

H7 winds…

850mb winds…

925mb winds…

10m winds…

When mid-level cooling and dry intrusion enters the area towards the morning hours of Friday, steepening lapse rates will provide marginal instability and convective precipitation, including some storms with intense rainfalls will be possible. Strong DLS, LLS and SREH should allow some organized convection, but mainly it will provide strong forcing for orographic precipitation.

CAPE and Lifted Index:

DLS/LLS Fri 03z…

SREH3 Fri 03z…

High amount of water troughout the troposphere just confirms that rainfalls will be strong and large accumulations are very likely. See the attached rainfall accumulations from different models which highly agree with the main precipitation max over NW Slovenia-Posocje area.

Effective precipitable water…

6hrs accumulated rainfalls Thu evening as of ALADIN model today…

ECMWF/NMM 12h accumulated precipitation from Thu 12z until Fri 00z…

ECMWF/NMM 12h accumulated precipitation from Fri 00z until Fri 12z…

GFS total rainfall accumulation during the next 72 hrs confirms the main precipitation max along the western side of Dynaric mountain range…

BOLAM model 12hrs accumulated rainfall show above 150mm in NW Slovenia…

What is another great concern with this strong depression is even more intense WAA which will rise temperatures for additional few degrees and snow height should be several hundred metres higher than today. It was between 1600 and 2000m. Its quite possible, that overnight to Friday, snow height will be well above 2000m which will result in melting snow up to these altitudes. As from the latest data, quite a lot of snow cover is at those altitudes and will bring additional water amount into the rivers.

H85 temperatures Fri 00z…

For this reason of high probability for intense and excessive rainfalls, EARS has issued red warning for SW Slovenia, mentioning additional more than 100mm during the 24hrs period. From our experiences, this number could easily be doubled locally especially where convective rainfalls will eastablish and enhance precipitation. More details about the warnings for tomorrow afternoon and overnight to Friday can be seen below:

METEOALARM DAY 2

METEOALARM DAY 2 - NW SLO

METEOALARM DAY 2 - SW SLO

To show that this upcoming event is very serious is the fact, that EARS has published a public statement on their website as well. Unfortunatelly the statement is only in English, but it mainly covers all what I discussed above through the model maps and predictions.

22. december 2009

As for the evolution of events and flooding, it can be said that the first depression will bring marginal changes in river heights/flow, while the second stronger depression could become a dangerous flooding event with return period of 50 or more years. All river mentioned above will likely have extremely high flow and major flooding is expected. Again, it has to be mention that both high amounts of precipitation and melting snow up to 2000m ASL will be the main reason for major flooding. Of course also taking in account the fact, that two periods of excessive rainfalls has already gone through and ground has a hard time holding such water amounts.

An update might come tomorrow afternoon/evening when the main depression starts to blasts Slovenia, but I am not completely sure wherever I will be outside. Definatelly its a day of monitoring and observing this historic event.

Stay tuned for future updates!