Storm Forecast
Valid: Mon 20 Sep 2010 06:00 to Tue 21 Sep 2010 06:00 UTC
Issued: Sun 19 Sep 2010 22:59
Forecaster: DAHL

A level 1 was issued across E Spain and the SW Mediterranean Sea mainly for severe wind gusts and large hail.

SYNOPSIS

A broad upper westerly flow is present over Europe with two separate branches, which each exhibit largely independant wave disturbances. The northern branch covers northern and north-central Europe, curving into N Russia, while the southern branch extends across the Mediterranean regions. A northern-stream trough will cross the North Sea on Monday, supporting SFC cyclogenesis, but otherwise the large-scale SFC pressure field will remain rather quiescent.

DISCUSSION

... SW Mediterranean ... E Spain ...

The few available west-Mediterranean soundings suggest that the LL moisture is quite shallow, resulting in very weak or non-existent MLCAPEs. However, it may be speculated that locally the moisture is deeper than in the soundings and that appreciable CAPEs could be present.

DCVA-related adscent should overspread Iberia and the western Mediterranean during the day, which on the one hand will support convective initiation, but on the other hand will bring about extensive cloudiness and possibly some rain. Overall it thus seems that instability should remain on the weak side but that deep convection will occur.

Deep-layer shear over eastern Spain and the western Mediterranean will likely be in the 20 to 25 m/s range, which is more than adequate for well-organized multicells and supercells. It seems that the main threat with this activity will be severe wind gusts and large hail, as well as local flooding. The coverage of the severe events is uncertain given the limited instability and the possibility of the storms to be elevated and imbedded in stratiform rain. Currently, a LVL1 threat seems to be sufficient, but an upgrade to LVL2 may be required, should the stratiform cloudiness weaken and instability become larger than currently anticipated.

... North Sea and the coasts of NW Germany and Denmark ...

The trough crossing the North Sea on Monday does not seem to be associated with a region of low-level baroclinity at its downstream side. Latest satellite imagery indicates a split-fron structure and it is speculated that the SFC cold front loses its identity as the system progresses eastward. GFS simulates all the instability in the thermal-trough region. Given a lack of low-level baroclinity, it thus seems unlikely that a line of strongly-forced convection will result despite the strong DCVA.

Deep shear is simulated to be weak, but strong low-level buoyancy and 10 m/s LLS suggest that a few short-lived funnels/tornadoes could occur, especially along the N-German and Danish North-Sea coasts. The overall severe threat seems to be loo low for a LVL1, though.

... W Black Sea ...

It seems that a few storms may develop over the W Blask Sea in a favorably-sheared environment late in the period. However, the forecast precip signals are quite weak, so the confidence in widespread convective development is rather low and a LVL1 does not seem to be warranted. Still, an isolated large-hail/severe-wind event or two could occur.