Storm Forecast
Valid: Wed 13 Jan 2010 06:00 to Thu 14 Jan 2010 06:00 UTC
Issued: Tue 12 Jan 2010 22:25
Forecaster: PUCIK

A level 1 was issued for Western and Southern Iberia mainly for severe wind gusts and tornadoes.

A level 1 was issued for Northern Morocco for excessive precipitation.

SYNOPSIS

A split-flow pattern has established over Europe with the main blocking feature being a high pressure system that stalls over Scandinavia and Western Russia. The main branch of the flow is diverted to Southeast with strong Northwesterly jet-stream observed over Southwestern Europe both at mid and upper levels. This jet will dig further Southeastwards during the forecast period. Two important disturbances are forecast - the first one will form in the exit region of the progressing jet over the Balearic Sea. The second one will form over the Atlantic Ocean in the region of subtropical frontal zone and is expected to affect Iberian penninsula by Wednesday evening. Most of Europe is under the influence of cold, modified continental polar airmass while the main frontal zone will lie over the Mediterranean.

DISCUSSION

... Western and Southern Iberia, Northern Morocco ...

The aforementioned disturbance poses serious uncertainities - not regarding its track but its depth - while GFS and WRF-NMM are quite agressive with surface pressures below 990 hPa, ECMWF predicts surface pressures of only around 1000 hPa. Both GFS and WRF models are also pretty optimistic regarding the instability factor with at least low-end CAPE values also inland, especially over NW and SW parts of Iberia, in contrast to ECMWF, producing very spotty instability. Dewpoints in subtropical airmass might climb over 16°C over southern parts of the region ahead of the approaching cold front. Quite high Delta Theta-E values are simulated - over 16 K in the vicinity of the cold front.

Very strong flow at all tropospheric levels is predicted with over 30 m/s at 850 and 700 hPa level and over 40 m/s at 500 hPa level. With ageostrophic component of the flow in the warm sector of the low, enhanced SREH values are predicted and they might reach over 300 J/kg in 0-3 km layer. Low level shear will reach 15 m/s, locally even higher values up to 20 m/s are possible. This scenario is very favorable for well organised, rapidly progressing storms, probably in the form of a squall line. Bowing segments might develop, with enhanced threat for damaging wind gusts or tornadoes. Due to the rapid storm motion, over 25 m/s and downard transport of momentum, severe wind gusts might be expected at a number of locations. Detrimental factor for tornadoes might be almost unidirectional wind shear profile and a questionable boundary layer rooted instability.

Another question mark regarding this day is the forcing - while northern part of region should be under a strong DCVA, southern part will not experience strong synoptic scale forcing and convergence fields do not suggest widespread initiation along the boundary. Nevertheless, widespread precipitation is simulated by all NWP outputs, so storms should develop.

Strong coastal convergence and upflow of unstable airmass over Atlas mountains should result in high amounts of precipitation over Northern Morocco, close to the Strait of Gibraltar.