Storm Forecast
Valid: Mon 21 Aug 2006 06:00 to Tue 22 Aug 2006 06:00 UTC
Issued: Sun 20 Aug 2006 21:38
Forecaster: TUSCHY

SYNOPSIS

No significant change in upper-level flow pattern... A pool of low geopotential heights is still placed over northern Europe.... One short - wave enters the system from the NW, affecting parts of western Europe, while another disturbance will lift out towards the NE, affecting parts of SE Europe.
While most parts of the western and central Mediterranean will see a relief of the very hot conditions, extreme heat will go on over the eastern Mediterranean.

DISCUSSION

....Bosnia and Herzegovina, Albania and Serbia and Montenegro...

Eastward shifting trough and attendant lift and still steep mid-level lapse rates will cause an environment, favorable for scattered TSTM development .... Up to 1000 J/kg instability , 25-30m/s DLS and locally enhanced LL flow ( also supported by mountainous topography ) will pose a risk of large hail / severe wind gusts and a few tornadoes.
Models like GFS and AFWA MM5 indicate a speed max, crossing the area during the late morning hours and the area has to be monitored for possible MCS development with an attendant severe wind gust risk....a westward expansion of the level-2 could become necessary.

... Bulgaria, parts of Romania, Moldova and Ukraine...

A rapidly eastward moving trough will reach the area during the late afternoon hours and should yield a favorable environment of large-scale ascent ( due to a favorable coupled jet structure and increasing PVA values , spreading eastward).
At lower levels, a weakening and partial very diffuse frontal zone ( still cold front character ) will slowly move towards the east / southeast .... Models indicate, that the front will be more active over the Ukraine, Moldova and Romania, where widespread LL pressure fall should help to shift the front as far east as the Black Sea (mainly during the night hours ).
Further towards the south ( over Bulgaria ), front should also continue its SE-ward drift, but
signals in the moisture convergence charts are more diffuse ( GFS even indicates several signals of convergent zones, crossing the area from NW towards the SE ).
Recapitulating, conditions for widespread lift at upper- / lower - levels will become increasingly favorable during the late afternoon / evening hours .

Optical depth ( dust )analysis and latest SeaWIFS photo indicates a broad belt of Sahara air, covering most parts of central / southern Italy and SE Europe, spreading further towards the NE.... This was also supported by yesterday's 12Z soundings, indicating very steep lapse rates between 800 and 500 hPa.
This airmass will be involved in the circulation pattern of an approaching upper-level trough and hence a continual NE-ward advection over the area of interest will be the result.
No significant boundary layer modification expected and hence instability values of ~ 1KJ / kg look reasonable, although higher moisture content along the coastal areas of the western Black Sea, combined with those steep lapse rates could locally yield even higher instability values.
Thermal axis of warm low-levels ( 20°C @ 850hPa ) should shift eastward during the morning and early afternoon hours, so that stout cap should start a weakening trend....
Initiation should occur first over Bulgaria , developing rapidly northward along the frontal boundary.
DLS of 20-25m/s and very steep lapse rates pose a risk of large to isolated very large hail and severe wind gusts, espcially if a line of storms can evolve.
Although not impressive, boundary layer wind field modification along the front could pose a threat for an isolated tornadic supercell.

Models indicate another convective precipitation maximum under the base of the NE-ward shifting trough.... Shear should be displaced well from the activity and no significant organisation is expected, although TSTMs could pose a marginal hail / isolated severe wind gust threat.

... Belgium, the Netherlands and extreme western part of Germany...

GFS seems to has a good handling with a small disturbance, crossing United Kingdom during the early morning hours, arriving a few hours later in the area of interest.... WV and IR loops ( 20.08. at 20Z ) already show a well developed disturbance, indicating signs of slowly increasing convective activity.... Wind profiler datas of Uist also indicate the approaching disturbance and its well developed circulation pattern.
A pool of cold mid-level air ( ~ -20°C at 500hPa ) will cross the eastern English Channel ( with SST`s of 18-19°C ) with anticipated instability release of ~500 J/kg mainly over Belgium and the Netherlands.
Although DLS will be very weak, a combination of enhanced LL shear / SRH-1km values and a uncapped boundary layer in the level-1 area will pose a risk for isolated tornado / cold-core funnel development.
Models inidcate rapid decrease of instability further inland.... LCL's of ~ 500m and steepening mid-level lapse rates in a nearly uncapped environment under the base of this disturbance should be enough for at least low-end instability release till southern Germany....
Late arrival, weak instability signals and strengthening stable boundary layer preclude a level-1 over parts of S-Germany for the moment.