Storm Forecast
Valid: Sat 18 Sep 2010 06:00 to Sun 19 Sep 2010 06:00 UTC
Issued: Sat 18 Sep 2010 05:18
Forecaster: TUSCHY

A level 2 was issued for parts of Slovenia and Croatia mainly for
excessive rainfall.

A level 1 surrounds the level 2 mainly for heavy rainfall. Also, a
few strong to severe wind gust/isolated tornado reports are
expected along the NE-Adriatic coast.

A level 2 was issued for parts of central Italy mainly for
excessive rainfall.

A level 1 was issued for central Italy mainly for large hail,
strong to severe wind gusts, heavy rainfall and tornadoes.

A level 1 was issued from the Balearic Islands eastwards
mainly for large hail, strong to severe wind gusts and
tornadoes.

SYNOPSIS

Large-scale cyclonic vortex remains situated over N-Europe. Numerous showers/thunderstorms evolve beneath this feature.
Strong wind gusts and a few cold-core funnels are possible,
but background shear remains rather strong for an augmented
waterspout risk. Numerous disturbances round the base
of this trough, affecting parts of central Europe. A channel with
somewhat lower geopotential heights runs from France to the
SW and assists in unsettled conditions over SW-Europe. A
weak ridge causes more stable condition over SE-Europe/E-
Mediterranean.

DISCUSSION

... Parts of Slovenia, Croatia ...

Major heavy rainfall event is in full swing. Yesterday's peak
values over W-Slovenia range from 80 to locally up to 300
l/qm/24h, so not much additional rain is needed for more
flooding.

During the daytime hours, the overall risk for excessive rainfall temporarily diminishes, as mid-/upper flow becomes more
zonal/slighly anticyclonic ahead of another, major disturbance,
which consolidates futher to the west (over France). As this
upper trough draws near, gradual intensification of this feature
is forecast, which also assists in a constantly strengthening
surface depression over N-Italy. Again, this vortex seems to
spin up betimes with a gradual deepening circulation and
backing wind field ahead over the N-Adriatic Sea. During the
evening and night hours, the upper trough and attendant
surface vortex move eastwards. In addition, a quasi-stationary
baroclinic zone over the Alps remains in place, which keeps the
very moist air mass along/south of the Alps. Right now, it looks
like the main event sets in after sunset as a weak coupled jet
configuration evolves over the area of interest, next to persistent
influx of a moist Mediterranean air mass from the south and
strong forcing from the west. Models diverge slightly, where to
place highest rainfall amounts with GFS being along the
southern edge of the solutions for the past few runs and it has weaker sums. Nevertheless, all models (mesoscale ones and even global model with coarse resolution) indicate the chance for 80 - 150 l/qm/12h. Despite those values being at the lower-end of our level 2 criterion, we decided to stick with such an high level due to the following reasons:

- not satisfying handling of deep convection, which could increase rainfall amounts significantly on a small scale, easily down to the meso-gamma scale

- history of impressive rainfall amounts, which already fell and reported flooding in this area

Due to the more intense depression and upper trough, the level areas were expanded southwards/eastwards compared to yesterday.

Again, tail-end storms of inland moving thunderstorm clusters pose a risk for a few strong to severe wind gusts and isolated tornadoes.

... Parts of Italy ...

EZ, GFS and WRF all agree in modest to strong SBCAPE/MLCAPE build-up during the day ahead of the strengthening depression to the northwest. Rich LL moisture advects northwards beneath gradually cooling mid-levels, so
roughly 1000 J/kg MLCAPE look reasonable with locally higher values. Wind field at all levels strengthens during the day, as upper wave approaches from the west and the vortex over NW-Italy constantly intensifies. Due to the strong vertical tilt of the surface low and the upper wave, directional shear component also strengthens throughout the day with broad SRH peaks (SRH 1 up to 200 m^2/s^2 and SRH 3 of 200-300 m^2/s^2).
Initiation will start quite early, probably already during the late morning hours with repeatedly development during the day. Activity probably peaks between sunset and midnight, where scattered to widespread thunderstorm activity is forecast. Due to the persistent influx of moisture from the south, prolonged period of thunderstorm activity and mesoscale models indicating 70-100 l/qm/12h along the N-Apennine Mountains, a small level-2 for excessive rainfall was issued. Otherwise, depending on the final strength of the surface vortex, all kind of severe is forecast, including large hail, severe wind gusts and tornadoes. The latter risk could be maximized along the coast, where more discrete storms/tail-end storms of clusters are expected. However, due to the broad nature of the augmented SRH values, tornadic thunderstorms are possible elsewhere in the highlighted area. An high-end level 1 was issued as it remains still unclear, how discrete thunderstorms remain/how fast clustering takes place.

... Balearic Islands ...

Forcing remains weak, but a wavy, quasi-stationary baroclinic zone is present over that area, assisting in isolated to scattered thunderstorm development. This boundary resides somewhere next to/over the Balearic Islands until the early afternoon hours, before accelerating to the southeast. The prefrontal air mass
features high moisture content with up to 1000 J/kg SBCAPE expected. DLS of 20-25 m/s and 0-3 km shear just shy below 20 m/s assist in organized multicells/isolated supercells. In fact, the only inhibiting factor will be weak forcing. Otherwise, conditions seem very supportive for severe weather. Any thunderstorm, which evolves poses a risk of large hail, severe wind gusts and tornadoes. A strong tornado can't be ruled out, given increasing 0-3 km CAPE. Activity moves to the south/southeast during the late afternoon hours. This risk expands well to the east...probably all the way to Sardinia. An high-end level 1 was issued due to the more isolated nature of thunderstorm activity.

As a side note: A mature supercell evolved west of Mallorca (~ 180 km at 1.30 UTC). A strong deviant storm motion, cloud top temperatures at or below -65 °C and an expansive cold-U shaped signature next to the favorable speed/ directional shear of model outputs all hint on a potential supercell. This cell gradually morphed into a line of thunderstorms, which currently affects parts of the Balearic Islands. This increases uncertainties regarding the thunderstorm probabilities for the Balearic Islands until noon/the afternoon hours. Sparse data offshore and synop data from the E-coast all indicate that this supercell rode along/just south of the baroclinic zone, which is still situated to the NW. Mesoscale subsidence with such features and cooling outflow could lower probabilities somewhat...at least temporarily.