Storm Forecast

Storm Forecast
Valid: Fri 20 May 2016 06:00 to Sat 21 May 2016 06:00 UTC
Issued: Thu 19 May 2016 19:10
Forecaster: TUSCHY

A level 2 was issued for parts of the Ionian Sea and surrounding areas mainly for excessive rainfall amounts.

A level 1 surrounds the level 2 for a lesser extent of excessive rainfall reports but also for large hail, severe wind gusts and an isolated tornado event.


Gradient maps at 100 hPa feature narrow gradient over the Atlantic and Europe with one trough just west of Europe and another one over E-Europe. The first trough is reflected in tropospheric maps by a broad area with low geopotential heights, covering most of the NE Atlantic. A strong impulse
dives towards the base of that trough and results in further amplification. This results in a transition period to an increasingly more meridional aligned flow regime over SW/W-Europe during the forecast and the following days. However, this more favorable pattern for organized DMC won't have an impact on today's outlook.
Further east, two weakening upper lows affect parts of the CNTRL Mediterranean and E-Europe. Low geopotential heights in between both lows atop modest BL moisture result in an extensive area with thunderstorm probabilities. The main focus for organized DMC remains with the low, which affects the CNTRL Mediterranean area.


... Adriatic and Ionian Sea including surrounding areas ...

Cold-core low drops S/SE from CNTRL Italy towards the Ionian Sea. SSTs increase to 20 C over the Ionian Sea, which explains most robust MLCAPE forecasts especially along the W coast of Greece and Albania (mainly due to enhanced frictional
BL moisture convergence). 800-1000 J/kg MLCAPE is forecast with lower values to the W and N (inlcuding the Tyrrhenian and Adriatic Sea). Two foci exist:

The first one beneath the base of the upper low, where weak DLS and modest MLCAPE exist. Weak background shear and
maximized LL/mid-level lapse rates support numerous strong to temporarily severe pulsating storms with marginal hail and gusty winds. Coastal regions may see an isolated waterspout risk with a few funnel reports over onshore regions. This risk traverses from the Tyrrheanian Sea to the SE. Upstream areas see influx of drier air, but especially the E-CNTRL coast of Italy may see a prolonged period with showers/thunderstorms due to an elongated fetch over the Adriatic Sea (with NE-erly winds).

The second area exists over the N/E-Ionian Sea. Phasing polar/subtropical jets support a prolonged period with 20-25 m/s DLS. Combined with up to 1 kJ/kg MLCAPE, an adequate environment exists for organized/severe DMC. Current idea is that an organized cluster of thunderstorms continues over SE Italy from the night. Persistent 10-15 m/s S-erly LLJ advects a moist/unstable air mass to this MCS, which gradually builds to the SE (towards W-Greece) during the forecast. Forecast soundings indicate a supportive pattern for training storms with deep (0-4 km) southerly flow and modest upper divergence during the forecast. A level 2 was issued to indicate the enhanced probability for very heavy rain in excess of 100 l/qm within a few hours. Tail-end storms and any more discrete cell will pose a large hail/tornado and severe wind gust risk.
SW-Greece offers too may uncertainties regarding timing and coverage of thunderstorms, so no level 2 was yet issued.
However the general setup supports training convection with excessive rain and enhanced flash flooding in case the development of organized DMC materializes.

A sharp level gradient exists over the onshore regions, as strong mid/upper flow advects rain cooled air to the E/NE, which induces rather stable conditions.

...Other lightning areas ...

Weak DLS and roughly 800 J/kg MCLAPE support pulsating strong/severe thunderstorms with isolated large hail and a few strong downburst events. Excessive rain may locally occur due to slow moving thunderstorms, but spotty nature does not justify a level 1 for now. Thunderstorms rapidly decay after sunset.

15% lightning areas over Norway/Sweden, Ukraine and Belarus will see isolated thunderstorms with sub-severe strength. Storms rapidly decay after sunset.

A few elevated thunderstorms may erupt over N-Portugal/NW Spain during the passage of a weak short-wave during the afternoon. This activity moves to the NE (over the Bay of Biscay during the night). Nothing severe is forecast.

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