Storm Forecast

Storm Forecast
Valid: Fri 18 May 2012 06:00 to Sat 19 May 2012 06:00 UTC
Issued: Thu 17 May 2012 22:06
Forecaster: TUSCHY

A level 1 was issued for NE-Spain mainly for large hail, strong to severe wind gusts and an isolated heavy rainfall / tornado event.

A level 1 was issued for N-Algeria mainly for large to isolated very large hail and severe downbursts.

A level 1 was issued for parts of the Aegean Sea, NE-Greece, far NW Turkey, parts of Bulgaria and SE Romania mainly for heavy to excessive rainfall amounts.

A level 1 was issued for NW/W-Turkey mainly for isolated large hail, strong wind gusts and heavy rainfall amounts.

SYNOPSIS

A strong cut-off low over SE Europe reveals only marginal eastward net motion, which results in an active day for heavy rainfall over parts of SE-Europe. Another trough over the E-Atlantic approaches Europe during the forecast with strong SW-erlies spreading to the east/northeast. Ridging in-between results in tranquil weather over the C-Mediterranen.

DISCUSSION

... Spain ...

In response to the SW-ward amplifying trough just west of the Bay of Biscay, the mean flow backs from WSW to SW, which results in a noticeable slow-down of the eastward advancing cold front over Spain. This front moves into an area with widespread surface pressure falls, which probably weakens the isallobaric component. Also, strong postfrontal diabatic heating weakens the temperature gradient along the front, which may be compensated however by enhanced convergence due to the interaction between the front itself and inland moving sea breeze fronts. To summarize all that, we see no reason, why modest BL moisture (dewpoints in the mid-tens) should not be available during the afternoon hours in the highlighted area.

E-Spain remains beneath a well mixed layer at mid-levels, so at or above 1 kJ/kg MLCAPE are likely within that area of interest.

A phasing polar/subtropical jet creates moderate to strong speed shear over most of Spain with 6km and 3km shear exceeding 15-20 m/s. Despite a slow weakening trend until sunset, shear remains sufficient for well organized multicells/isolated supercells.

Only weak forcing signals are present until 18Z, with stronger PVA maxima moving in during the night hours. This may assist in isolated to scattered thunderstorms, so likelihood for discrete storms is enhanced. Large hail and strong to isolated severe wind gusts (downburst events) are possible with any storm. There is no hint at an augmented rainfall risk right now, but we won't exclude isolated flash flooding, especially when storms mature/cluster betimes.
As storms move to the E/NE, they encounter an increasingly higher BL moisture content over far E/NE Spain, which lowers LCLs to 1 km or less. Especially any interaction with the inland moving sea breeze front may be the focus for an isolated tornado event. Uncertainty in thunderstorm initiation and ongoing spread in QPF preclude higher probabilities for now.

Thunderstorms gradually decay until midnight with weakly organized clusters of showers/thunderstorms moving offshore and towards SW-France.

... N-Algeria ...

Along and north of the Tell Atlas, BL dewpoints in the range of 15-20 C create a favorable set-up for high MLCAPE release as this area resides beneath a well structured EML plume. Can't argue against 1.5 kJ/kg MLCAPE (higher MUCAPE) forecast of HIRLAM in 20-25 m/s DLS regime as subtropical jet moves through. Forcing will be the missing ingredient, so either diabatic heating or subtle mid/upper waves may spark an isolated thunderstorm over N-Algeria. Any storm with a deep updraft may pose a large to very large hail and severe downburst risk, as supercell structures are well possible. Limited coverage of storms and uncertainty regarding initiation preclude a level 2 for now.

With loss of daytime heating, storms slowly weaken while moving offshore during the night.

... Parts of Bulgaria, SE Romania, NE Greece and NW-Turkey ...

A deep and quasi-stationary cold-core low atop the S-Aegean Sea creates a favorable set up for training thunderstorm activity within the highlighted areas. 850 hPa NE-erly winds with speeds of 15 m/s or more advect a moist and unstable air mass to the area of concern. In the meantime, winds at 4 km AGL and above weaken which lowers the magnitude of the deep layer steering flow, so slow moving and back-building storms are likely...even an eastward building MCS event is possible. Models still diverge in placement of the QPF maxima, with one group placing strongest precipitation offshore over the Aegean Sea and another model group showing a maxima over SE-Bulgaria/NE Greece/ far NW Turkey. GFS also expands the precipitation far to the north, including SE Romania. Hence a broad level area was issued. As virtually all models show 80-100 l/qm/24h maxima within the level 1 area, flash flooding may become a concern (if QPF maximum occurs onshore).

W-Turkey is placed beneath the upper low with cold mid-levels. Partly daytime driven, partly synoptically forced DMC is expected with isolated large hail/strong wind gusts. Heavy rainfall occurs with rapidly clustering thunderstorm activity, which continues well into the night.

With weak background flow and modest LL CAPE, a few waterspout events are possible mainly along the W/SW coast of Turkey and Crete.

... C/E-Ukraine ...

Broad confluent wind field is present over C-Ukraine, which assists in better BL moisture and MLCAPE of 500-1000 J/kg. Shear remains weak (around 10 m/s), so ill defined multicells are forecast. Isolated large hail, strong wind gusts and locally heavy rain will be the main hazard. Despite some weakening during the night, unstable conditions support continued initiation all night long.

... S-C/ France, Benelux and parts of Germany ...

A few strong thunderstorms are forecast to develop during the afternoon/evening hours within the NE-ward expanding warm sector. Marginal CAPE and modest shear may assist in an isolated large hail event, but otherwise marginal hail and strong wind gusts accompany that activity.

... E-Sweden and W-Finland ...

A few non-severe thunderstorms are forecast between noon and sunset as a sharp upper trough lifts to the north. Modest MLCAPE of 300-500 J/kg and weak shear may be compensated by impressive forcing, so even without favorable shear/CAPE overlap, a few better organized multicell storms are possible, which race to the N/NE. Marginal hail and strong wind gusts will be the main hazard. Storms rapidly decay after sunset.

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