Storm Forecast
Valid: Fri 04 Mar 2016 06:00 to Sat 05 Mar 2016 06:00 UTC
Issued: Thu 03 Mar 2016 22:21
Forecaster: TUSCHY

A level 1 was issued for the NE Ligurian Sea mainly for excessive rain but also for an isolated tornado and large hail risk.

A level 1 was issued for the E-Ionian Sea and the E-Aegean Sea mainly for isolated excessive rain and a few waterspout events.

SYNOPSIS

Stratospheric maps indicate a pronounced branch of the polar vortex hovering over Europe. Complex interaction of numerous vorticity maxima within the area of influence of that trough result in unsettled conditions for vast areas.
Two vortices will play a role in today's outlook. A stout smaller scale trough over the Adriatic Sea rotates east towards Turkey while weakening. Another trough over the W-English Channel continues its journey towards the south/southeast. Ongoing amplification results in an extensive upper-level trough configuration, which affects areas from NW Europe all the way to the Iberian Peninsula during the night. This trough will be the dominant feature on European weather maps during the overnight hours. Geopotential heights increase downstream and cause transient ridging from Italy to Belarus.

Both mid-level vortices will be accompanied by low-tropospheric low pressure areas, which continue to weaken during the forecast. During the night, a third LL vortex evolves over far S-France and moves east towards NW Italy until 06 UTC. Numerous frontal intrusions in the past caused meager BL mixing rations over the Mediterranean and surrounding areas. Any peaks in BL moisture will evolve along frontal boundaries. Over land, BL moisture will be too sparse for DMC, excluding France.

DISCUSSION

... CNTRL/E- Mediterranean ...

Eastbound moving trough with 500 hPa temperatures down to -30 °C cause scattered DMC activity. Strong forcing affects the Ionian Sea during the start and the Aegean Sea during the afternoon/evening hours. Magnitude of the vertical temperature gradient and modest BL moisture support 800 J/kg SBCAPE over offshore areas. Despite weak DLS, isolated large hail and gusty winds can be expected with strongest updrafts. In addition, coastal regions may see an augmented waterspout risk with weak low-tropospheric background flow beneath the center of the vortex. Therefore broad level 1 areas were issued. Due to the progressive nature of the low/trough, no serious rainfall risk is forecast. However, coastal areas could see heavy rainfall due to repeatedly onshore moving showers/thunderstorms. The activity ends from W to E during the forecast.

... Bay of Biscay, France and the Ligurian Sea ...

Another progressive vort max will cause isolated to scattered thunderstorm activity over a broad area. Most of this activity remains sub-severe due to meager CAPE and displaced shear. Repeatedly onshore moving showers/thunderstorms could bring heavay rain to the Cordillera Cantabrica, but this will not justify a level 1.

A cold front crosses W/SW France until the early afternoon. A temporarily better organized band of enhanced convection could evolve along the cold front, but neither shear nor modest forcing should cause severe wind gusts with that activity. Postfrontal activity may produce a few stronger cells during the afternoon with marginal hail and gusty winds. Rapid weakening is anticipated during the evening hours.

The main focus for organized DMC arises over the NE Ligurian Sea during the night. In response to a developing LL depression over S-France, backed low tropospheric flow assists in N/NE-ward advection of BL moisture towards NW Italy. Focus for CI will be the leisurely eastbound moving depression and a wavy front, which bisects the Ligurian Sea from SW to NE. Between 00-06 UTC forecast soundings indicate a good chance for temporarily training coastal convection and an MCS may evolve. Excessive rain will be the main hazard. Model guidance keeps QPF amounts below a level 2 mainly due to aforementioned frontal intrusions and meager BL mixing rations. As BL flow responds to the strengthening depression, backed low-tropospheric flow (especially along the front) may support an isolated tornado event. Any discrete storm could also produce large hail due to 25 m/s DLS. A level 1 was issued and was expanded well inland to cover the rainfall risk.