Storm Forecast
Valid: Sat 29 Oct 2011 06:00 to Sun 30 Oct 2011 06:00 UTC
Issued: Sat 29 Oct 2011 01:53
Forecaster: VAN DER VELDE
A level 1 was issued for eastern Spain mainly for excessive convective rainfall.
A level 1 was issued for Tunisia mainly for excessive convective rainfall and large hail.
SYNOPSIS
Low pressure over the southern part of the western Mediterranean Sea steers increasingly unstable air toward Balearic islands and the east coast of Spain, while pressure rises and the low shifts to Tunisia.
DISCUSSION
...Balearic Sea and east coast of Spain...
Models agree about the tendency of higher CAPE advecting to the west. GFS tends to be rather conservative, with less than half that observed in WRF and HiRLAM models. In 12Z HiRLAM, CAPE appears inflated as result of rapid BL mixing ratio rises near the Balearic islands, totally absent in GFS. There is also a difference in flow strength, GFS being weak, with little advection into land, while HiRLAM creates a stronger flow and orographic moisture.lifting. In any case, a persistent convergence of storms from the northeast can lead to local excessive rain sums between Valencia and Benidorm with flash floods. In the morning arrival from weakening storms/MCS from the southern Mediterranean likely causes a first peak in the level 1 area. Around 09Z the low-level vertical wind shear and storm-relative helicity are supportive of rotating updrafts, with some chance of a large hail or tornado event, but deep layer shear is quite weak.
Along the entire eastern Spain coast waterspouts may be observed, particularly if the weak flow scenario of GFS becomes reality, and a convergence line just offshore.
...Tunisia and northeastern Algeria...
Large storms will likely persist for many hours along the coast and over land in the high CAPE area (1500 J/kg) and large quasi-geostrophic forcing, with a chance of local flash floods. Moisture inflow from the east-southeast also lifts over higher terrain, increasing flash flood chances. Farther to the south in Tunisia (off the map), where a jet overlays the low level flow, deep layer shear >20 m/s and SREH around 250 mē/sē offer good potential for supercellular storms with large hail as primary threat.
