[dsmythy]

I choose the word 'depression' out of confusion as to what they are supposed to be called.

I'm talking about the storms that form in the Western Med as regular depressions before sometimes attempting to form a warm core and ditch it's associated fronts. The sea temperature required for them seems to be lower than a tropical storm and they seem to occur January-August, often forming off the coast of Libya before heading east.

What exactly are they? Simply powerful depressions? Subtropical storms? "Tropical" storms? "Polar" lows? Some form of hybrid?


January 1982




January 1995




October 1996

[snowstreams]

They sure do look like hurricanes alright. They must be something related to polar lows. Especially the ones that occur during January etc.
There can be some air masses with very cold upper temperatures that come down over the eastern med. They must feed the tropical characteristics of the lows.

[dsmythy]

Just for the fun of it I'm going to put this low pressure area, currently forecast to come north off Libya, up for small potential of forming something interesting eventually to the SE of Italy for the 9th/10th. Cue bog standard low pressure

[Deep Easterly]

Nice analysis of a similar depression in the Med on the 26th September 2006:

http://www.mmm.ucar.edu/people/rotun...es/weather.pdf

As Snowstreams says, they are basically a Mediterranean version of a Polar Low .

This satellite image was taken earlier that morning before the cyclonic feature developed fully off the SE coast of Italy:



Fax analysis for 0000 UTC - 27th Sept 2006:


shows that the mesoscale depression formed in a cold sector, similar to Polar Low features in the North Atlantic*

EDIT: *Actually, that is not strictly true in this case. A further look at the DWD analysis charts shows that this mesoscale feature developed more or less along the actual surface cold front rather than behind it which would make its formation less Polar Low like, although the feature did seem to separate itself from the front and into the cooler sector as it matured later in the day. (open to contradiction)

2 metre temp analysis for 6z:

shows a tongue of very warm air being drawn up from N Africa ahead of the front which no doubt helped to feed energy into the developing low. The small scale low developed more or less along the 15c isotherm along the south coast (Italy) which is roughly where the cold front lay around that time.

[Su Campu]

Here's an ASCAT windmap for the area from 04:36 UTC that morning, several hours before the peak, showing some 50 knot vectors over the sea.

[Deep Easterly]

Here's a nice on the spot daily run down on the 'TMS' (Tropical-like Mediterranean Storm) that developed in the western Med between the 5th-7th this month, with a detailed discussion on both the sub & broader-scale synoptic processes involved in its formation:

http://translate.google.com/translat...embre-2011.htm

European satellite animation for period 4th to 8th November. One big factor in triggering off this particular storm is the deeply cool and very unstable mP air mass that spilled into the western Med on the 4th & 5th:


Image: AeMet.is



Some further reading on these unusual storms:

http://www.eumetsat.int/Home/Main/Pu...05_luque_v.pdf

ABSTRACT:
"Tropical-like storms in the Mediterranean Sea with a clear eye surrounded by an axisymmetric cloud structure are quite unusual. Almost one case per year on average is identified in satellite images in all the Mediterranean basin. These storms, once generated over the sea, can affect islands and continental coastal lands. Although, documented tropical-like cyclones have not usually achieved hurricane intensity, their potential for damage is high due to the densely populated Mediterranean coastal regions."



ftp://texmex.mit.edu/pub/emanuel/PAP..._etal_2007.pdf

ABSTRACT:
"Tropical-like Mediterranean storms grow and evolve from a combination of deep convection and typical mid-latitude baroclinic processes. The axisymmetric cloud-resolving model assumes a homogeneous atmosphere. This assumed background atmosphere is also temporally invariant during the period of simulation. Spatial homogeneous and temporal invariant assumptions are inappropriate in the Mediterranean basin"


Basically, while structurally similar to Tropical Stroms, due to the conditions that allow TMS's to form. they can be classed no more that 'Sub-Tropical Storms', which is fair enough I suppose, since the Mediterranean is not in the tropics..


An excellent site listing numerous case studies of TMS's with satellite and synoptic charts and animations as well! (more excellent links listed in both of the above papers)

http://www.uib.es/depart/dfs/meteoro...GIA/MEDICANES/

[dsmythy]

Some decent looking convection building south of the Balearics at this time. Any chance of development? I don't recall any plunge of colder air down to the area though in the last few days that might have otherwise fueled this low to something stronger?

http://www.sat24.com/sp

[Su Campu]

That activity is the usual type of convection that occurs when an upper low is near or over Iberia. There is actually warm air advection of the Med at the moment, so a different setup altogether. The thunderstorms are being generated by upper features.

[Su Campu]

In fact, ESTOFEX explain quite nicely what's going on there today. A good chance of waterspouts over much of the Med.

Storm Forecast
Valid: Mon 21 Nov 2011 06:00 to Tue 22 Nov 2011 06:00 UTC
Issued: Mon 21 Nov 2011 05:29
Forecaster: VAN DER VELDE
A level 1 was issued for eastern Spain and Balearic islands mainly for excessive convective precipitation.
A level 1 was issued for Sardinia and Sicily mainly for severe convective wind gusts, large hail and possibly a tornado.

SYNOPSIS
A low pressure area has now installed itself in the western Mediterranean with increased amounts of CAPE. The mid level potential vorticity maximum which entered Africa will start digging into the southern Mediterranean during the afternoon and develop a cold/dry front subsequently moving over Sardinia and Sicily. The Balearic islands will be near the almost stationary occlusion with where models trigger quite a lot of precipitation.

DISCUSSION

...eastern Spain...
The region south of Valencia will see most orographic moisture lifting as the low level wind direction turns from easterly to more northerly directions. As has already been the case during the night, persistent storms should continue there for some more hours and cause locally excessive rain sums. Storm motion should remain very weak.

...Sardinia and Sicily...

GFS and HiRLAM disagree on the areal distribution of vertical wind shear and helicity, but are quite in agreement on the amount of CAPE (800-1500 J/kg). While shear is around 15 m/s over a deep layer, SREH can reach 300 m2/s2, most sure over Sicily. Both models agree on a band of >22 m/s mean winds between 1-3 km AGL over Sardinia. A linear convective system with severe wind gusts is a likely scenario. Embedded supercells could also create large hail and perhaps a tornado
Waterspouts are likely to be observed here and there over a large section of the western Mediterranean.

HiRLAM Precipitation

[Su Campu]

Widespread thunderstorms, some with hail, are occuring in Sardinia and Tunisia this evening. A ship is reporting 38 knot winds and squall just off the north coast of Tunisia.



Latest radar from Sardinia.

[dsmythy]

http://www.estofex.org/cgi-bin/polyg...rmforecast.xml

Italy, Corsica, Sardinia potentially under threat from strong winds and extreme rainfall. Be interesting to see if a 'medicane' occurs with this system.

[Su Campu]

This is from Lightning Wizard's (Estofex) Facebook page, when they were getting excited about this system in Saturday's Hirlam run.

It seems to be a bit of a downgrade on latest runs but nevertheless a lot of rain for Sardinia, Corsica and much of central and northern Italy.

[Deep Easterly]

Some scattered but intense looking storms over parts of Italy right now.

http://www.sat24.com/en/it

(click on 'Lightning' option at bottom right hand of satellite image for sferic data)

[Su Campu]

The 12Z 0.05° Hirlam shows a distinct warm-core system forming in the Tyrhennian Sea tomorrow and affecting much of the coastal areas of central Italy, eastern Corsica and northeastern Sardinia. Instantaneous contraction map shows fluid trapping and banding around an eye-like centre, generating humungous rain rates.






Theta-e charts also show it as a warm-core system, so it would tick the boxes of a sub-tropical storm-like system, with the wind field a little displaced from the centre.



Upslope rainfall enhancement on the high terrain of Corsica and central Italy is going to cause some real problems of flash-flooding.

[tylercollins]