Storm Forecast
Valid: Fri 29 Sep 2006 06:00 to Sat 30 Sep 2006 06:00 UTC
Issued: Thu 28 Sep 2006 18:52
Forecaster: TUSCHY

SYNOPSIS

The apparently most prominent synoptic feature is a well developed upper-level trough over the eastern Atlantic, which should slowly move towards the SSE during the next 24 hours.
A weak upper-level trough ( placed over E-Poland, Belarus and the Ukraine ) will continue its eastward shift and its faint southward amplification.
In-between, higher pressure will be present and should cause quiet weather conditions for parts of CNTRL Europe ( beside the mountainous regions of Switzerland and W-Austria, where a few orographic storms will be possible. )

DISCUSSION

....United Kingdom...

++Correction of paragraph 3++

Latest WV imagery already indicates a short-wave trough ( S of Ireland ), marked by a strengthening humidity gradient and this system seems to be on track ( by comparions with GFS 12Z ).... An intense upper-level jet rounded the base of an eastern-Atlantic upper-level trough and should reach the Bay of Biscay during the morning hours.... Strengthening upper divergence and rapidly NNE-ward developing upper-level jet will cause a more organized short wave trough to start a NE-ward motion, crossing United Kingdom (UK) from the SW during the day-time hours.
Handling of this system by the model pool is fine, although there still exists some uncertainties how active this short-wave passage will be ( in regard to the convection ).

At lower-levels, there is no clear signal for a well developed front, but GFS develops a strengthening Theta-E boundary over S/CNTRL UK, moving rapidly towards the east ,but final strength of this boundary will also depend on short-wave strength.

Models try to develop an area of convection over SW- UK ( during the morning hours ), moving rapidly towards the NE ....Right now there appears to be two foci for a few organized storms to form:

The first one will be S/E of Birmingham.... GFS 12Z indicates a few hours of limited diabatic heating and a significant cool-down of the low-levels (700hPa),compared to yesterday....Hence there will be the chance for low-end instability release despite the weak lapse rates.
Latest GFS runs indicated a slight SE-ward shift of the 850 hPa 20m/s speed max and therefore a location in the region of better instability....Wind field should continue to strengthen during the day ... LCLs will stay pretty low and if a southerly or even backing LL wind field could be achieved, a threat for one or two tornadoes would exist.
Placement under the righ exit quadrant and displacement of strongest lift towards the north should limit storm coverage, but expect the threat to be high enough for warranting a level-1.

The other focus can be found over CNTRL UK / S Scotland and the Irish Sea , where kinematic parameters are forecast to be very favorable for organized storms to form..... Main concern will be, if enough instability can be released, but rapidly cooling mid-levels should support at least modest steepened lapse rates and low-end instability release.
DLS up to 25m/s will be present and combined with over 200 m**2/s**2 SRH-1km helicity, an enhanced threat for tornadoes and severe wind gusts will exist.... Isolated large hail will be also possible in stronger cells.

The threat should shift N/NE-ward and will undergo a weakening trend during the late evening / early night hours due to a decline in instability and progressively cooler SSTs (offshore ).


....N-Spain, extreme S-France...

A threat for a few elevated TSTMs exists over N-Spain ( mainly during the afternoon and early evening hours )... A slight cool down at mid-levels and at or above 60% moisture content in lower / mid-levels should be adequate for a few TSTMs to develop... DLS of 20m/s and steepened lapse rates will yield a favorable environment for storms to become severe with an isolated large hail risk.

Further towards the east ( coastal areas of S-France ), weak onshore flow should help to advect slightly higher dewpoints northward and at least low-end SBCAPE values will be possible..... Main risk will be an isolated severe wind gust threat.

...SE Europe...

Weakening upper-level trough should support scattered TSTMs in a weakly shared environment so no storm organisation is expected.