Storm Forecast
Valid: Fri 17 Oct 2008 06:00 to Sat 18 Oct 2008 06:00 UTC
Issued: Thu 16 Oct 2008 15:57
Forecaster: TUSCHY
SYNOPSIS
Main feature of interest remains a sharp upper trough, centered over Poland during the start of the forecast, while shifting gradually eastwards during the following 24 hours. Strongest mid-level cold pool is still present along the base of this trough, so intense geopotential height gradients prevail. Otherwise, ridging persist with tranquil conditions over most parts of Europe although there will be a few spots, where initiation should occur and which were highlighted on the map. Either shear or thermodynamics are too weak for anything severe.
DISCUSSION
... Parts of central Ukraine and SE Belarus ...
As aformentioned upper trough earns a more negative tilt, the geopotential height gradients become even stronger along the SE quadrant, resulting in a strengthening upper streak over SE Poland and Belarus with readings of 45m/s at 500hPa. At the surface, a cold front shifts to the east under the strong jet-winds, so environment along the cold front is strongly sheared with DLS values of well above 30m/s, but instability release is scarce. However, there exists a chance for isolated initiation over the highlighted area, as diurnal heating and a weakening tongue of better prefrontal BL moisture assist in the release of some low-end instability. Directional shear is most pronounced just along the front and in the warm sector (enhanced by a surface trough, which is slightly displaced into the warm sector), and decreases rapidly in the postfrontal sector , so as surface front lags behind the surface trough, conditions seem not favorable for rotating storms. Another point is that the tilt of the upper trough does not allow mid-levels to cool down significantly, so deep convection is not likely. Showers and thunderstorms, which evolve along the front should stay shallow. 15-20m/s 0-3km shear is still enough to worry about a few strong to severe wind gusts, but environment seems to be too hostile to go with a 1 right now. Vertical CAPE distribution has a slight maximum in the lower levels, but expected magnitude of CAPE and attendant weak updraft strength should keep the risk for funnels/short-lived tornadoes very low. Instability diminishes rapidly during the evening hours and so does the risk for thunderstorms.
... Poland ...
A well mixed and quite humid post frontal airmass overspreads Poland from the NW during the day and as diabating heating starts, scattered showers/thunderstorms will evolve. In fact, timing is perfect as strong mid-level cold pool still affects Poland during the midday / afternoon hours with a fetch of better moister streaming in from NE Germany. Shear is slowly decreasing but still strong enough to mention strong to severe thunderstorms. Of enhanced concern will be a belt of stronger directional and speed shear over W/SW Poland. Global models like GFS displace best shear/instability fields, but confidence is low that a global model can indeed resolve such small details so there will be a chance for rotating storms and an isolated tornado over the level area. This area was expanded well to the north and east (e.g. central and SE Poland ) to include the isolated strong to severe wind gust risk.
... NE Adriatic Sea ...
During the early morning hours of Friday, there will be a very short time-frame for thunderstorm development well inland ( Croatia and SW Slovenia). DLS is strong enough for storm organisation and deep convection is possible. The main risk will be a strong wind gust threat. The reason for not issuing a 1 were the overall bad thermodynamics but also the rapid decay of this instability tongue.
