Storm Forecast
Valid: Tue 17 Feb 2015 06:00 to Wed 18 Feb 2015 06:00 UTC
Issued: Tue 17 Feb 2015 00:17
Forecaster: TUSCHY

A level 2 was issued for Sicily and far SW Calabria mainly for excessive rain.

A level 1 surrounds the level 2 and includes N-Tunisia/NE-Algeria mainly for excessive rain and an isolated tornado threat.

A level 2 was issued for N-Morocco mainly for excessive rain and an isolated tornado event.

A level 1 surrounds the level 2 mainly for excessive rain and an isolated tornado event.

SYNOPSIS

Blocking pattern continues for most parts of Europe as geopotential heights once again increase over NW Europe (Ireland, UK and Scotland), while substantially lower values remain in place over the W/CNTRL Mediterranean. Numerous vortices are embedded in this area of lower heights. One is about to exit S-Spain during the start of the forecast with a rapid motion to the SW. Another one remains more or less in place over Tunisia and reveals a broad cyclonic circulation all the way down to the surface. A third vortex and by far the most intense one approaches Turkey fom the north. Frigid air mass spreads south and creates a zone of enhanced baroclinity over S-Turkey (between the warm Mediterranean and the cold air from the north). A broad LL-vortex evolves along that zone but that's to the east of our forecast area.

A more progressive pattern continues over far N-Europe, where a sharp trough with very cold mid-level air races to the east. It will cross most of Scandinavia during the forecast period from west to east with windy and wet/snowy conditions also spreading east.


DISCUSSION

... Malta, Sicily and adjacent regions ...

Despite benign nature of the big 'syoptic' picture, a high-end threat exists for flash flood producing convection. Quasi-stationary low over N-Tunisia weakens during the forecast and so does its circulation. However, with frigid air mass from the Balkan States spilling south into the Ionian Sea, baroclinity increases in our area of interest. A pronounced NW-SE aligned convergence zone strengthens and remains in place during most of the forecast. With LL cold air outrunning the coldest mid-level air over Turkey and the Aegean Sea, especially the lowest 2-3 km AGL will see ongoing and increasing frontogenesis with the strongest wind field. As a result, this convergence zone will see persistent 15-20 m/s SE-erly winds all day long beneath 700 hPa.

Overall, moisture content of the BL air mass remains sparse, but increases a bit in depth along that convergence zone (moisture pooling). This assists in a confined band of 200-600 J/kg MLCAPE. Not much for deep updrafts, but forecast soundings show transient chances fro updraft depths up to 8-9 km AGL.

Inverted shear profiles are forecast with winds decreasing above 700 hPa. This points to a favorable setup for training/back-building convection. Given very slow motion / only marginal E/W-oscillation of that convergence zone, the risk for excessive rain exists all day long for Sicily and adjacent regions. No surprise that model guidance agrees well in rainfall amounts of 100-200 l/qm/24h especially over E-Sicily and far SW Calabria. Would not be surprised to hear from substantially higher rainfall amounts in local training MCS events with final amounts approaching or exceeding 400 l/qm/24h. However, those extreme rainfall amounts occur in confined regions only.

This will be a long-lasting and life-threatening excessive rainfall and flash flood event. A broad level 2 was issued.

Tail-end charlies may produce an isolated tornado event...especially along the convergence zone itself and along the coasts.

Malta is more uncertain as far as timing goes...when will the convergence zone affect that island and for how long. Kept this area in an high-end level 1 to show enhanced uncertainties. Also, strongest LL winds will remain north, so degree of thunderstorm organization remains less certain.

... NE Algeria and N-Tunisia...

The level 1 was expanded far west to cover the coastal areas of N-Tunisia and NE-Algeria. Persistent upslope flow of less organized convection also brings heavy/excessive rain to this area. Expected amounts of 50-100 l/qm in 24 h also indicate a big chance for flash flooding!

... N-Morocco ...

The same is true for parts of N-Morocco, where scattered showers/thunderstorms bring heavy/excessive rain. Missing foci like a confined LLJ lowered confidence of a broad level 2, but anticipated rainfall amounts probably result in numerous flash flood events. This risk is enhanced by persistent upslope flow along the rough orography and that's the area where a level 2 was placed (QPFs of 100-200 l/qm/24h). The overall threat diminishes during the night. An isolated spout-type tornado event is possible along the coasts of N-Morocco.

... S-Black Sea/NW Turkey ...

A great setup for significant sea effect snow evolves for NW-Turkey. Still elevated SSTs (5-10 °C with highest values to the south) ensure warm/humid sea/air interface. Atop, frigid air mass spreads south with impressive readings (850 hPa temperatures at or below -15 °C and 500 hPa temperatures approaching -40 °C). Long fetch and rather unidirectional shear in the low/mid troposphere assist in organized bands of heavy snow showers. Forecast soundings also show very steep lapse rates and CAPE profiles up to 600 hPa, which involves the graupel zone. Therefore a few lightning strikes may occur in those bands. Very heavy snowfall is expected with confined regions seeing amounts of 100 cm or more, but this won't be covered by our risk scheme. Hence only a general lightning area was issued to cover the lightning activity.

... Scandinavia ...

Numerous general lightning areas were issued mainly due to the enhanced sea/low-tropospheric air mass temperature difference and therefore low-end SBCAPE build-up. Graupel and gusty winds accompany this convection.