Storm Forecast
Valid: Fri 12 Aug 2011 06:00 to Sat 13 Aug 2011 06:00 UTC
Issued: Fri 12 Aug 2011 02:35
Forecaster: TUSCHY
A level 1 was issued for Portugal and Spain mainly for large hail and severe downdrafts. A few significant hail events are possible.
SYNOPSIS
Split flow regime continues over Europe. Numerous upper troughs cause unsettled conditions with scattered showers and thunderstorms. Belt of better winds remains to the west of Europe, so most of those storms remain weakly organized. Winds may eventually pick up over UK/Ireland and Scotland during the night, as strengthening depression moves in from the west.
DISCUSSION
...Portugal and Spain ...
A weakening upper low is embedded in ongoing split-flow regime. Overall tendency for this feature is to merge with a modest deep layer jet to its north, which causes this low to move more to the east, while opening up.
Strongest height gradients / shear remains confined to the southern fringe of this feature, covering all of far S-Spain and adjacent areas. However, with a track forecast over far S-Spain, strong WAA advects a dry and hot air mass northwards, probably capping most of the area with strongest shear over far S-Spain (DLS up to 20 m/s). Hence, initiation is confined to areas further north, including all of central and parts of northern Spain, where scattered initiation is expected...probably more isolated further south.
Advection of a well mixed and dry African air mass to the forecast area results in steep mid-level lapse rates. BL moisture is not impressive at all with surface dewpoints in lower to mid tens, but that's adequate for 500 - 1000 J/kg MLCAPE. Where pockets of better moisture exists (e.g. along mesoscale convergence zones), those values may be exceeded. As upper trough moves in, wind field veers with increasing SRH forecast. Therefore, multicells are well possible with isolated supercells not ruled out, given 10 to 15 m/s DLS ( with highest supercells risk probably in the low thunderstorm probability area over southern Spain, where DLS increases to 15-20 m/s). Large to very large hail is forecast, next to severe downbursts, given well mixed and deep subcloud layer with augmented DCAPE. For now we think a broad level 1 may cover that risk despite the fact that a few significant hail events are well possible. However uncertainties in mesoscale evolution of surface flow pattern (e.g. placement of convergence zones) preclude a level 2 for now. Storms keep going all night long while spreading to the northeast.
... N-Italy ...
Combination of weak lee cyclogenesis over far NW Italy and onshore/upslope flow along the S-Alps results in a belt of augmented CAPE within the highlighted area. No confined speed max crosses that area during the period, so 10-15 m/s DLS may be the best to get. A few stronger storms / multicells / are possible with isolated large hail. No level was issued for that risk. Storms diminish after sunset.
... Parts of Germany into Poland and the Czech Republic ...
Flat upper trough moves in from the NW during the forecast and continues to move off the east. Mid-levels cool down somewhat which keeps mid-level lapse rates steep enough for some CAPE. Also, BL air mass will be moist, so roughly 500 J/kg SBCAPE are a reasonable forecast. Shear will remain weak so the main risk will be marginal hail and strong wind gust mainly around peaktime heating. Thunderstorms gradually decay after sunset.
...NE Poland, Lithuania, Latvia, Estonia and parts of Belarus ...
Modest SW flow covers those regions with influx of some modest BL moisture from the south. 500-800 J/kg MLCAPE and 10 m/s DLS allow a few stronger storms to form with marginal hail and strong wind gusts. Won't rule out an isolated large hail event due to modest CAPE values. Thunderstorms decrease after sunset.
