Storm Forecast
Valid: Tue 21 Jan 2014 06:00 to Wed 22 Jan 2014 06:00 UTC
Issued: Mon 20 Jan 2014 22:36
Forecaster: TUSCHY

A level 2 was issued for parts of SW Italy mainly for excessive rain.

A level 1 was issued for SW Italy and parts of the Tyrrhenian Sea mainly for heavy to excessive rain and waterspouts.

A level 1 was issued for the C-Adriatic Sea and adjacent areas mainly for heavy to excessive rain and waterspouts.

A level 1 was issued for the SE Adriatic Sea and the E Ionian Sea mainly for heavy rain and an isolated tornado risk.

A level 1 was issued for the NE Aegean Sea mainly for large hail, strong to severe wind gusts, heavy rain and an isolated tornado event.

SYNOPSIS

A broad upper trough covers an area from E-France all the way to the Black Sea. Beneath that trough a LL depression has evolved over Italy, which now shifts slowly to the east. This eastbound motion is forced by a stout high pressure area with chilly air to its north (over NE Europe). A pronounced baroclinic zone has evolved between both features and runs from S-Denmark to NE Germany to Belarus, causing havoc due to winter precipitation events like sleet, freezing rain and snow.
A sub-980 hPa depression NW of Ireland approaches this blocking pattern from the west with no serious impact on W-Europe during this forecast period.

DISCUSSION

... Parts of SW Italy ...

A shallow warm-core depression over the Adriatic Sea beneath a weakly sheared base of the large-scale upper trough steers this excessive rainfall event. Given the structure of the depression's core, strong 2 km AGL winds with 20 m/s (local maxima aoa 25m/s) blow plume of 15-20 mm PWs (caused by a long fetch of NW-erly winds atop 16-18 °C SSTs over the E-Tyrrhenian Sea) towards SW Italy. Cross-mountain flow enhances efficiency of rainfall production along uplsope facing mountain crests. Forecast soundings show rapidly diminishing shear aloft with quite favorable conditions for training/organized lines of showers/thunderstorms moving onshore in our level 2 area. Excessive rain aoa 100 mm/24 h is likely and local amounts of 200 mm/24h are possible. A sharp W-E gradient in rainfall amounts is forecast.
Level areas were expanded offshore to reflect conducive setup for waterspouts with strong convergent BL flow, 100-200 J/kg LLCAPE and healthy updrafts (due to 300-800 J/kg SBCAPE). An isolated strong event can't be ruled out.

Ingredients degrade during the night with a decreasing severe risk.

... Adriatic Sea and the E-Ionian Sea (offshore) and an area from S-Croatia to Greece (onshore) ...

This event won't be well focused as belt of 20 m/s winds at 850 hPa slowly shifts south from the S-Adriatic Sea towards W-Greece. Similar shear conditions as the one over SW Italy exist with strongest winds at lowest 2-3 km and weakening winds aloft ... although with reducing trend of LL wind speeds betimes, a quite homogenous vertical wind field (regarding strength) evolves beyond noon. Interaction of a southbound moving front beneath the left exit of an intense mid-layer jet streak (circling the base of the main upper trough) should assist in organized bands of showers and thunderstorms. This event will be progressive with a gradual shift to the south. Hence, a broad swath will see heavy to isolated excessive rainfall amounts with some upslope areas collecting aoa 100 mm/24h. Coastal areas will also see gusty winds as well mixed maritime air advects ashore and an isolated tornado event is possible (overlap of roughly 100 J/kg LLCAPE and enhanced shear along the coasts).

In addition, slow moving/quasi-stationary showers and thunderstorms affect the C-Adriatic Sea and adjacent areas all day long as gradually weakening low/mid-layer vortex spins down with no serious motion to the east. Numerous mesoscale convergence zones, enhanced LLCAPE and up to 500 J/kg SBCAPE also add a more pronounced waterspout risk with an isolated strong event being possible. Not sure how much influence two cold eddies in the central part of the Adriatic Sea may play on CI (5-6 K gradients in SSTs), so added a high probability lightning area for all regions for now. The C-Adriatic Sea could see some minima in lightning activity however. Localized flash flooding is possible along the E-coast of C-Italy and along the coast of the E-Adriatic Sea.

... NE-Aegean Sea, W- and NW-Turkey and Bulgaria, E-Romania and E-Moldova ...

Downstream of the main upper trough an intense mid/upper jet overspreads the area of interest and pushes 0-6 km bulk shear values to 30-40 m/s. A tongue of rich moisture with mixing ratios in the BL approaching 8g/kg advects from the Aegean Sea to the north / northeast, beneath an eastward advecting plume of steep mid-level lapse rates. Hence an extensive area will see substantial MUCAPE build-up from the Aegean Sea to Bulgaria and to the W Black Sea with weakly capped 400-800 J/kg MLCAPE over the E/NE Aegean Sea probably sprawling all the way to the extreme SW Black Sea. Shear and CAPE support organized DMC over the E/NE Aegean Sea with large hail, strong to severe wind gusts, an isolated tornado event and heavy rain. Peak activity will be during the evening hours until midnight, as a wavy front passes by. Alignement of front and shear vectors indicate high potential for rapid clustering of storms with limited time for discrete storm structures. After 00Z the risk for thunderstorms rapidly shifts east over N-Turkey with a decreasing severe risk, as CAPE diminishes and shear becomes a bit more displaced to the south.

We issued a quite large lightning area to the north to account for isolated to scattered elevated thunderstorm activity. 20 m/s effective DLS and modest MUCAPE should support a marginal to isolated large hail risk with strongest storms. Not much modification needed for more surface based storms over NE Bulgaria during the afternoon hours. Strong wind gusts accompany those storms, although low LCLs and augmented SRH-1 could also result in an isolated and short-lived tornado event. This however should occur on an isolated scale and hence the level 1 was not expanded to the north.

The 50% lightning area was split in two pieces with the southern one representing clustering storms from the NE Aegean Sea and the northern one reflecting enhanced elevated activity along the tip of a wavy front and due to some wrap-around moisture in case of a developing depression over the far NW Black Sea.

... Bay of Biscay and W-Ireland ...

Isolated to scattered short-lived and non-severe thunderstorms occur during the overnight hours. Gusty winds and sleet will be the main risk.