Storm Forecast
Valid: Fri 27 Jun 2008 06:00 to Sat 28 Jun 2008 06:00 UTC
Issued: Thu 26 Jun 2008 22:31
Forecaster: TUSCHY

SYNOPSIS

A belt of strong westerlies sprawls over most parts of the northern / central forecast area. High pressure and hot / stable conditions prevail over most parts of the Mediterranean. At the surface, a cold front runs from the Alpine region to Belarus and separates a cooler and more stable airmass to its north and west from a very warm and humid airmass to its south and east. This boundary will be the main focus for enhanced thunderstorm chances.

DISCUSSION

.... Croatia, Bosnia and Herzegovina, parts of Serbia, S - Hungary , eastern Slovenia and western Romania ....

There will be a risk for significant hail, strong to severe downbursts and torrential rainfall with each thunderstorm in the level - 2 area.

Despite minor interceptions, the airmass over the 22- 26 °C warm central Mediterranean modified during the past few days and withal a missing true tropcial source region we think that at least parts of the central Mediterranean experience an airmass which is comperable to a true tropical airmass. Dewpoints mainly along the coastal areas of Italy and also Corsica and Sardinia have dewpoints in the low to mid twenties and even mixing further inland only reduces dewpoints into the upper tens / lower twenties. This is also valid for the area of interest.

The attention then turns to the mid - levels, as yesterday's soundings had very steep mid - level lapse rates as a result of a northward advected and slowly modified EML and there is no reason why those lapse rates should weaken during the forecast period. This combination of a nice EML atop a very moist BL results in a concentrated area of high to extreme instability values as MLCAPE values will reach 1500 - 2500 J/kg and SBCAPE values top out at or above 3000 J/kg. GFS 12Z run increased instability again with ICAPE values in excess of 5000 kJ/m^2 and MLCAPE values well above 2000 J/kg. Those values are subject to local moisture pools / areas with strong mixing so strong fluctuations of the magnitude of the instability release on a small scale are likely. However, strong diabatic heating will help to remove the LL cap and initiation will be underway during the midday and afternoon hours dependant on the topography. Another focus for initiation will be a southward sagging convergence zone / weak cold front which will cross the area from the north during the evening and night hours. Finally a weak short wave trough at upper - levels opens up while crossing the area from the west during the forecast period, resulting in weak lift.

So in summary we think that daytime driven thunderstorms will develop over a broad area as initiation will be dependant on the local topography but thunderstorms will start to organize more and more along the convergence zone. Shear is not that strong with 6km bulk shear values between 10 and 15m/s so multicell storms / isolated supercells will be the primary storm mode. Well mixed subcloud layer, LCLs well above 2km, high instability values and locally strong veering with height all favor a significant hail risk especially when storms stay more discrete. Another concomitant phenomenon with such parameters are strong to severe downdraft wind gusts. Betimes and especially when the convergence zone overtakes storms, clustering will take place. Very rich BL moisture and abundant moisture inflow into the thunderstorms will help to raise the threat for torrential rain and attendant flash flooding. Despite slowly weakening instability values after sunset, conditions remain favorable for large hail and strong wind gusts.

... Areas surrounding the level - 2 area and parts of central / northern Italy ...

Atmospheric conditions remain the same despite somewhat lower instability values. The issued level - 1 for Romania is a high - end one as conditions for severe hailstorms look favorable with at or above 1000 J/kg MLCAPE and enhanced storm relative helicity values in the lowest 3km. The main reason for not upgrading is the BL moisture, which slowly gets mixed out but dewpoints at or above 15 °C are still present. If latest model data confirm the enhanced shear and scattered nature of the thunderstorms than we have to consider an upgrade. Large hail and strong to isolated severe wind gusts will be also the main threat over N - Italy.

... Western Ukraine, Moldova, parts of Belarus, Slovakia and SE / E Poland...

Instability release is weaker than further towards the south, but stronger DLS should compensate that at least partially. An eastward moving cold front will cross the area during the afternoon and night hours and scattered thunderstorms will evolve mainly after 15Z as cold front reaches the far northward stretching instability axis. A few large hail / strong wind gust reports are possible and as LL shear increases somewhat during the night hours, an isolated tornado can also develop. There is an area over SW Ukraine and Moldova, where GFS has intense veering and stronger LL shear. If thunderstorms evolve in this environment and if GFS is indeed on the right track ,an update for this area may be needed. Thunderstorms along the east / northeastward moving cold front will go on during the night hours although the severe weather risk should diminish.

... Central Norway and Sweden ...

The atmosphere is nearly uncapped and a well mixed BL and an overall cold atmosphere result in scattered thunderstorms with marginal hail and an isolated and short lived tornado / funnel report. This activity is diurnal driven and will weaken considerably after sunset.

... N - Ireland / S - Scotland and N - UK ...

A tricky forecast for those regions. An upper - level disturbance shifts eastwards, approaching Ireland during the midday hours from the west. As this feature crosses Ireland and Scotland, it undergoes rapid weakening. The main inhibiting parameter will be the instability as LL and mid - levels stay quite warm. EL temperatue analysis also has a rapid warming as the feature crosses the area. For now we will stick with a general thunderstorm area as the occurrence of deep convection does not look likely. The shear would be more than adequate for severe wind gusts and tornadoes but weak LL lapse rates and aforementioned missing buoyancy are reason enough to stick with a general thunderstorm area.


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