shelf, rainbow, 49th thunderstorm day

One week later, on Sunday September 26th, the situation again became interesting. With a strong southerly flow, moist warm air was rapidly advected northward over eastern Europe. Even without notable sunshine the temperature easily reached 24 C in the early afternoon.
A coldfront was approaching from the west-southwest, as can be seen on this weathermap:


Surface analyses for 26 september 2010, 00 UTC.













And yes, in the late afternoon this storm came in from the south:





















When the rain and gusts were over, an extremely bright rainbow (category 5) was visible in easterly direction. For me it was the brightest rainbow in years.
These are two images of the rainbow (no photoshop here):
































Till so far, NO thunder was observed with this "storm". However, when it moved further east, it activated enormously and I observed thunder & lightning at the back of the storm, so I could write the 49th thunderstorm day of 2010 in my observation-book.
A nice squall line with embedded discete cells at the front and trailing stratiform precipitation, developed over the eastern Carpathian mountains in the course of the evening:

last thunderstorm of the year ?

Sunday evening, September 19th, a coldfront came in from the west-northwest. On the front edge of the precipitation area thunderstorms developed. For the time of year they were rather active thunderstorms. Lightning activity was around 15-20 flashes per minute at it's peak. Also there were a few exceptionally bright CG's (cloud to ground lightning) nearby. Even with a setting of f/10 on my photo-camera they caused a completely white-out of the image.....
Because of the heavy rain and because the thunderstorms were a bit unexpected, I couldn't make much images.


















Autumn is a short season in Transylvania, so it is likely that this was the last thunderstorm of the year.

high shear, very low cape

In the early evening of Friday, august 27th 2010, nice altocumulus lenticularis forms were observed:


Here, these clouds are often indicative that a coldfront is approaching.










On Saturday there were minor chances for thunderstorms. But if they developed, they could be supercells, i.e. storms with a rotating updraft.
Chances were low, because the dry air in the lowest layers and high temperatures aloft, created a thermodynamic profile with almost no CAPE (a measure of instability). This can be seen in the prog-sounding for my location. The vertical windshear is strong with a windspeed 58 kt at 500 hPa.




















This situation was also well forecast by ESTOFEX:

...Romania, Ukraine...
Large wind shear and curved hodographs are favorable for supercells with large hail, but it is unlikely that with such low instability as indicated (<200 J/kg) something can develop.


Nonetheless, late in the afternoon I observed an interesting cloud-structure:



Same photograph as above. An inflow-band (orange arrow) is feeding this "wannabe storm" from the north. As often the low-level wind was modulated by the river Mures valley and not from the northwest as indicated by the gfs-model. In light blue the cloud movement at the lower mid-levels. In dark blue the cloud movement at "anvil" level.


It was a light shower by then and I felt a few raindrops falling out of the cloudband. Thunder was not observed.


The shower struggled and struggled, but due to the low instability it didn't succeed in developing into a storm. Finally it collapsed due to the strong windshear and entrainment.







On radar I saw that about the same time, behind the eastern Carpathian Mountains near Bacau, one storm succeeded in becoming a strong cell. Perhaps this one caused severe weather over there?