Storm Forecast
Valid: Fri 13 Jul 2012 06:00 to Sat 14 Jul 2012 06:00 UTC
Issued: Thu 12 Jul 2012 22:20
Forecaster: TUSCHY
A level 1 was issued for NE-Italy, far N-Adriatic Sea, W-Slovenia and W-Croatia mainly for large hail, severe wind gusts and an isolated tornado event.
A level 1 was issued for E-Slovenia, E-Croatia, N-Serbia, Hungary and W-Romania mainly for large hail and strong to isolated severe wind gusts. An isolated very large hail event is possible mainly over far E-Croatia/SW-Hungary and N-Serbia.
A level 1 was issued for parts of the Ukraine and W-Russia mainly for large hail, strong to severe wind gusts and an isolated tornado event.
SYNOPSIS
A potent cyclonic vortex over NW-Europe steers numerous disturbances to the east. Intense shear and numerous UVV maxima cross most parts of C/N-Europe, but limited BL moisture/marginal CAPE keep the overall severe risk limited.
Rising geopotential heights/warming mid-levels keep any thunderstorm activity suppressed atop the Mediterranean.
DISCUSSION
... NE-Italy, S-Austria, Croatia and Slovenia ...
Window of activity will be confined mainly to 06Z until roughly 12Z. Forecast reasoning mainly controlled by high resolution models as the WRF, which seems to capture the potential scenario pretty well.
At 06Z, a strengthening wave in the mid-levels is already atop the area of interest with a gradual E/NE-ward motion thereafter. Strong subsidence upstream is forecast to spread to the east, which probably suppresses CI after 12Z.
Latest surface analysis places surface dewpoints of mid to upper tens from Lake Garda to the east (20Z). During the night, a diffuse cyclonic depression evolves over NW-Italy, which should assist in enhanced ageostrophic and canalized 10-15 kt NE-erly surface flow over S-Veneto and S-Friuli-Venezia Giulia. This also explains enlarged forecast hodographs from Padova and Udine with forecast SRH-1 at or above 200 m^2/s^2. With mid/upper wind field increasing from west to east during the morning hours, DLS strengthens to 25 m/s and more, so kinematics will be more than adequate for rapid storm organization.
Mangitude of instability will be questionable. NE-erly surface flow during the night may bring surface dewpoints a bit down, which makes surface based initiation questionable. However, up to 1000 J/kg MUCAPE are available, which is adequate for long-lived storms. Any storm, which manages to ingest unmodified moist inflow parcels from the N-Adriatic Sea however will also be able to become surface based.
Current thinking is that a few thunderstorms either continue from the night or evolve early in the morning hours in response to the eastward moving vorticity lobe. WRF has persistent signals of 1 or 2 supercells, which race to the E. Large hail, severe wind gusts and heavy rain will be the main hazard. However, an isolated tornado event can't be ruled out mainly along the coast of the Adriatic Sea and just inland. Storms move rapidly to the east and also affect W-Slovenia and W-Croatia until 12Z. I'm a bit reluctant with weakening those storms as rapidly as models do over Slovenia and Croatia and hence the level 1 was expanded far east to account for probably more elevated activity with large hail and strong wind the main risk. Despite drier air present over S-Austria, I also can't rule out an elevated severe storm to move in from the SW with large hail and strong to severe wind gusts. Hence added also parts of extreme S-Austria for that activity.
Storms rapidly diminish from west to east.
... Hungary, N-Serbia and parts of Romania ...
Overnight's cold front pushed through the area of interest with dewpoints now in the upper singles (20Z), so a lot depends on how strong moisture recovery will be during the day. Latest model output still indicates a strong wave crossing Hungary from SW to the NE. Ahead and during the passage of this wave, WAA and moisture recovery should assist in MLCAPE of 500-800 J/kg with another cold front surge thereafter from NW to SE during the afternoon hours onwards. Heating from C Hungary to the south will be good with thick cloud shield expected to the north (including anvil debris from convection from the west). Expect isolated to scatterd thunderstorms to either move in from the west or evolve over the rough terrain from Croatia and . N-Bosnia and Herzegovina with an eastward motion. DLS of 20-25 m/s and a moderate lapse rate gradient point to a few large hail and strong to severe wind gusts events. An isolated very large hail event is possible over far E-Croatia, SW Hungary and N-Serbia during the discrete stage of any storm. Thunderstorm probabilities diminish from NW to SE as the eastward moving upper wave and southward moving cold front decouple with diminishing prefrontal CAPE.
However during the evening and night hours, a moderate MUCAPE plume and 20 m/s DLS still overlap ahead of the SE-ward moving front, so a few strong to severe storms may also occur as far east as W-Romania. Isolated large hail and strong wind gusts will be the main hazard. This area was also added into a low-end level 1. We expanded the 15-% thunderstorm area far to the E/NE to account for a few elevated storms.
... Far W-Russia and parts of the Ukraine ...
A potent upper trough crosses far W-Russia during the forecast. A tongue of moist and warm air is present ahead of the surface pressure channel but becomes thinner betimes. Also, the mid/upper jet remains bound tightly along the sharp trough axis, which keeps best shear/CAPE fields separated during most of the period. Hence we expect scattered CI early in the day with rapid clustering thereafter. Isolated large hail and strong wind gusts accompany strongest storms, but the heavy rainfall risk will become the dominant one during the afternoon and evening hours, as numerous storm clusters move to the N. Locally enhanced LL shear is present mainly in the capped prefrontal air mass, but an isolated tornadic thunderstorm may occur. Thunderstorm activity decreases from S to N during the night.
... Rest of the lightning areas ...
Weak shear and low to moderate SBCAPE allow scattered to widespread showers and thunderstorms (pulsating) to evolve during the daytime hours. Main hazard will be heavy rain due to slow storm motion. LL moisture values not excessively high, but especially over C-UK, backbuilding activity is possible. Low LCLs and enhanced backing along a strong convergence zone may assist in a few funnels/isolated tornado reports over C-UK (mainly within the 50% lightning area). For now, no level 1 was included.
Next to the aforementioned risk, marginal hail and strong wind gusts accompany strongest storms.
... C-Germany after 03Z ...
For July, an impressive dynamic mid/upper feature crosses Germany from SW to NE. Rapidly increasing PVU values atop the head of the surface wave and a strong UVV maximum indicate rapidly improving environmental conditions for an augmented risk for somewhat deeper convection. GFS also indicates a pronounced dry slot feature, which may assist in potential instability build-up. Hence, shallow convection is possible within a significant LL shear/SRH-1 environment. However, best time frame for a probably increasing tornado risk evolves after 06Z and hence no risk level was issued.
