Storm Forecast
Valid: Sun 02 Sep 2018 06:00 to Mon 03 Sep 2018 06:00 UTC
Issued: Sat 01 Sep 2018 09:42
Forecaster: TUSCHY

A level 2 was issued across E Croatia to the SE Czech Republic mainly for excessive rain, large hail, severe wind gusts and an isolated tornado threat.

A level 1 surrounds that level 2 with similar hazards but lower probabilities.


SYNOPSIS

An eastward ploughing sharp upper trough over Iceland, accompanied by an autumn-like intense mid/upper jet, causes rising geopotential heights over N Europe (e.g. the North Sea, Sweden and Norway) per downstream development. This finally chokes an upper low off of the westerlies, which meanders over S-CNTRL Europe (e.g. N Italy). This feature is still tilted with height and surrounded by numerous weak mid-level short waves, which circle that vortex. A stronger wave passes trough the southern base of that cut-off and approaches the Adriatic Sea during the night from the west. This causes a coupled jet with rather impressive upper divergence along its eastern fringe (the N Balkan States to E Austria).
An upper trough just west of Portugal gradually draws near from the west and encroaches on the Iberian Peninsula during the forecast.
Along the tip of a subtropical ridge a strong subtropical jet impacts far SE Europe (e.g. Turkey). High thickness and synoptic-scale subsidence suppress DMC activity.

Regarding synoptic-scale fronts, the only feature of interest is a warm front along the eastern fringe of the Italian vortex, which pushes to the N/NW during the forecast (towards the Czech-Republic and E-Austria). Trajectories in that warm sector show air mass origin from the S-CNTRL Mediterranean with warm/moist air spreading N/NW.


DISCUSSION

...E Croatia, parts of Serbia, W Hungary, Slovakia, far E Austria to far SW Poland and far E Czech-Republic...

Leisurely NW-ward moving warm front and immediate postfrontal warm sector features LL mixing ratios in excess of 10 g/kg beneath meager mid-level lapse rates of up to 7 K/km. Temporal diabatic heating ensures MLCAPE values of 800-1200 J/kg with peak values probably over W-Slovakia, W Hungary and E-Croatia. In fact, cloudiness is an uncertainty with this event, as ongoing convection from the previous night and thick clouds in the warm conveyor belt are handled differently in parts of the model data. Hence the confidence in CAPE magnitude decreases along the W fringe of the CAPE tongue (e.g. E Austria to E Slovenia).

Shear at all levels is adequate for organized convection with 20 m/s DLS and enhanced LL shear (including curved hodographs especially in the lowest 3 km AGL). These values are maximized along the warm front.

CI is possible all day long and supported either by the orography, the warm front itself, outflow boundaries from the previous night and regions with better diabatic heating.

Despite the ongoing uncertainties where the highest severe risk evolves (given ongoing convection from the previous night) the general environment favors severe thunderstorms, including a few supercells. Large hail and severe wind gusts are possible during the initiation stage and in thunderstorms, which stay more discrete for a longer time period. In addition, strong directional shear along the warm front and favorable 0-3 km SRH next to abundant DLS and low LCLs indicate the chance for tornado development especially with discrete thunderstorms or with those, which cross vorticity-rich mesoscale (outflow) boundaries.
In addition, augmented LL flow with up to 30 kn at 850 hPa and moisture rich inflow indicate a chance for training convection along the warm front, where excessive rainfall amounts and flash flooding is possible. This risk extends from E Croatia to E Austria/W Slovakia into the Czech Republic. This risk diminishes only gradually during the night as strong CAPE-rich inflow persist with little E-W motion of the synoptic scale front.

Despite the uncertainties, we decided to upgrade parts of the broad level 1 to a level 2, where model consistency and confidence in organized severe and excessive rain is the highest.

... Italy to the N/CNTRL Adriatic Sea ...

Cool mid-levels atop a seasonable moist BL assist in widespread SBCAPE of 500-900 J/kg and even higher values over the Adriatic Sea. Shear next to the vortex' center is weak, so the main threat will be locally excessive rain with slow moving and clustering thunderstorms. During the night the threat of a few waterspouts increases over the N Adriatic Sea as the lower part of the synoptic-scale vortex gradually shifts offshore next to excessive rain with thunderstorm clusters. A broad level 1 covers that threat.

S Italy was added to the level 1 but here mainly for an isolated large hail and severe wind gust threat as DLS ramps up to 20 m/s. Despite the overall straight hodograph structure, some enlargement of the curvature is noted along the coasts, where an isolated tornado event can't be excluded.

... Portugal and Spain ...

Mixed signals are present over the Iberian Peninsula during the forecast with respect to CI. Upper ridge weakens during the forecast as the trough approaches from the west, but it remains questionable if the forcing of the trough already impacts the area of interst until 06 UTC. Most of the area resides beneath the ridge and hence only a few orographic events are forecast (also due to strong capping). CAPE increases towards the coasts, but due to only a low-end chance for an isolated large hail event no upgrade was performed in this forecast.
Also another cluster between NE Spain and the Balearic Islands may move off the coast and impact mainly offshore areas while paralleling the thickness gradient (triggered by the cut-off over CNTRL Europe). Gusty winds and isolated hail is possible next to heavy rain.

... Other lightning areas ...

Neither ingredients nor coverage support further level areas. Thunderstorms are accompanied by graupel and gusty winds with heavy rain on the local scale.