Storm Forecast
Valid: Sun 15 Apr 2018 06:00 to Mon 16 Apr 2018 06:00 UTC
Issued: Sat 14 Apr 2018 19:39
Forecaster: TUSCHY
A level 1 was issued for the Tyrrhenian Sea and surroundings mainly for excessive rain, isolated large hail and a low-end tornado risk.
A level 1 was issued for NE-Italy and parts of Slovenia mainly for excessive rain.
A level 1 was issued for SE-Portugal and S-Spain mainly for severe wind gusts and an isolated tornado threat.
SYNOPSIS
Negative tilted trough is analyzed over NW-Europe but deamplifies. Another impulse enters this trough from the W in the shape of an elongated upper trough, which crosses the Iberian Peninsula during the night. An upper trough over the CNTRL-Mediterranean shifts east during the forecast with pronounced WAA downstream, assisting in ridging over the Balkan States. Long-lived blocking high over Scandinavia weakens until 06 UTC.
DISCUSSION
... Tyrrhenian Sea and parts of Italy and N-Tunisia/NE-Algeria ...
NE-ward drifting upper low and a broad LL vortex affect that region during the forecast. Weakly capped 400-800 J/kg MLCAPE exist as cold mid-levels overspread a warm and moist marine air mass. Slow moving storms with heavy rain and an isolated large hail risk are forecast in a weakly sheared environment with modest CAPE. Don't want to exclude a low-end tornado risk over W-CNTRL Italy, where LL flow backs and SRH1/LL shear increase. Some models constantly show a scenario with a blossoming cluster as the LL vortex moves ashore over W-Italy. Temporal training could result in locally excessive rainfall amounts, but kept this event in a level 1 due to missing level 2 QPF support in model data.
...NE-Italy, Slovenia, Croatia and Bosnia and Herzegovina ...
Models show ongoing BL moisture increase N and E of the Dinaric Alps during the day with modest SE-erly near BL flow. Northward fanning EML plume atop that BL with diurnal heating support around 500 J/kg capped MLCAPE with higher peaks over W-Hungary. Cap starts to erode during the night with eastward drift of ridge axis and approaching forcing of the Mediterranean trough. Forecast soundings show weakening and elevated CAPE combined with 15 m/s DLS, so marginal hail and gusty winds will be the main hazard. Elevated storms spread far north or rather evolve along the N-ward fanning MUCAPE plume up to W-Poland. Graupel and gusty winds occur.
An overnight level 1 was added for parts of NE-Italy and Slovenia due to clustering storms with locally heavy rain.
... SE-Portugal and S-Spain ...
High shear/low CAPE setup evolves beyond midnight as progressive and sharpening upper trough plus surface cold front move in from the W. Strong lift and marginal CAPE indicate the chance of a forced line of convection. Degree of DMC activity varies in models with magnitude of prefrontal moisture advection. Forecast soundings show LL inversion at 850 hPa, so given good kinematics (15-20 m/s 0-3 km shear) and some LL T/Td spread, severe wind gust become the main hazard. An isolated tornado event however is possible, as not much BL modification is needed for near surface based activity. Lack of a robust LLJ signal preclude a level 2 for wind right now.
Elsewhere, isolated to scattered non-severe thunderstorms are forecast within the highlighted areas. The main hazards will be gusty winds and graupel/hail.
