Storm Forecast
Valid: Sun 07 Apr 2019 06:00 to Mon 08 Apr 2019 06:00 UTC
Issued: Sat 06 Apr 2019 12:07
Forecaster: TUSCHY

A level 1 was issued across parts of W-Greece for excessive rainfall.


SYNOPSIS

A blocking 2-3 sigma anticyclone remains anchored over Iceland with an ongoing disturbed streamline pattern over Europe.
Numerous smaller scale waves merge into a broad trough over NE Europe whereas another upper trough impacts a region from France to the CNTRL Mediterranean. Overall negative geopotential height anomalies over parts of W/SW and S Europe result in a broad area with thunderstorm probabilities.

Most synoptic-scale fronts over Europe won't play a substantial role in today's outlook as augmented vertical temperature gradients will be the dominant mechanism. The one exception will be an eastbound moving occlusion/cold front over the Ionian Sea, which serves as focus for enhanced and potential severe thunderstorms in combination with a forming LL vortex.

DISCUSSION

... France to the CNTRL Mediterranean ...

Already described setup with neutral to slightly steeper mid-level lapse rates with cold mid-levels atop a seasonable moist BL creates a broad area with SBCAPE in the 400-800 J/kg range. There is not much shear to work with but the amount of weakly capped CAPE and lowered WBZ level probably cause numerous graupel/hail reports with a few approaching our severe criterion (e.g. SW France, far NE Spain and N-CNTRL/NW Italy). Missing foci on the mesoscale for more concentrated severe precluded an upgrade for now. Gusty winds and heavy rain accompanies this activity, which may temporarily grow upscale into numerous disorganized clusters. A rapid weakening trend is forecast beyond sunset.

Corse and Sardegna probably see scattered diurnal driven quasi-stationary thunderstorms between 10-18Z with locally excessive rain/large amounts of small hail. The main limiting factors for an upgrade were the marginal LL moisture content with LL mixing ratios barely exceeding 6 g/kg and the coldest mid-level air displaced to the west.

A few funnel/short-lived tornado reports are possible with robust LL CAPE (locally approaching 200 J/kg in the lowest 2 km).

... Ionian Sea and W-Greece ...

Between 00Z and 06Z (continuing during the following day) a favorable setup for excessive rain and clustering thunderstorms evolves. The approaching negative tilted upper trough and attendant diffluent/increasingly divergent upper flow pattern overspreads a moist and unstable air mass (300-700 J/kg MLCAPE). Scattered to widespread thunderstorms initiate and grow upscale into slow moving clusters. No distinct LL speed max is forecast which lowers the training risk. Timing remains a bit uncertain and level 1 rainfall amounts may not yet be achieved until 06Z in case of a delay in CI by a couple of hours (e.g. ongoing model discrepancies in how fast the LL vortex intensifies S of S-Italy). Nevertheless augmented confidence in an intensifying heavy rainfall event with at least a few excessive rainfall reports caused a level 1 upgrade fow now.