Storm Forecast
Valid: Sun 09 Jun 2013 06:00 to Mon 10 Jun 2013 06:00 UTC
Issued: Sat 08 Jun 2013 21:12
Forecaster: TUSCHY

A level 2 was issued for C Germany and extreme N Czech Republic mainly for excessive rainfall amounts.

A level 1 surrounds the level 2 mainly for heavy to isolated excessive rainfall amounts.

A level 1 was issued for N Italy, Corsica, parts of Austria, Switzerland, S-Germany, parts of the Czech Republic, parts of Poland, Belarus and surrounding areas and the NE Adriatic Sea mainly for large hail, strong to severe wind gusts and heavy rain. An isolated tornado event is possible.

SYNOPSIS

Finally a more dynamic weather pattern takes shape over central Europe as the Iberian cut-off low starts to merge with the amplifying trough over Norway and Sweden....although "merging" might be too aggressive as both features make an approach but remain more or less disconnected through the forecast period. The interaction however results in a very active weather day over parts of Central Europe.
Further east, over E/SE Europe, no significant change in the geometry of the height field is noted. Transient ridging over Italy shifts east and approaches Greece during the overnight hours with stronger ridging extending from Portugal all the way to Scotland.

At the surface, an extensive E-W aligned quasi-stationary frontal zone is analyzed from the Bay of Biscay to France (ill defined) with more healthy looking frontogenesis expected from Germany to the east. This boundary separates high-energy air to its south from drier air to its north. Any oscillation to the north/south of that boundary will be mainly convectively driven.
Numerous transient surface lows are forecast to evolve and ride along that boundary to the east . During the night, departing surface lows should start to pull the western part of the front (France to W-Germany) to the south.

DISCUSSION

... C-Germany ...

A favorable pattern for slow moving thunderstorms with excessive rainfall amounts is about to evolve over the area of interest.

At mid/upper levels, the Scandinavian trough sends a vorticity lobe to the south, while the cut-off low swings in from the SW. Between all those features, a col evolves over C-Germany with healthy upper level mass transport via the entrance region of a 30 m/s 300 hPa jet, which moves towards the Baltic Sea. Also, strong cyclonically curved flow approaches from the SW which should support a prolonged period of divergent flow and forcing.

In the lower tropospheric layer, a W-E aligned frontal boundary sharpens with an extensive surface low pressure channel developing just to its south. Embedded in this channel of low pressure, numerous more or less pronounced vortices are forecast to ride to the east with the final one evolving during the overnight hours, inserting a final push of the boundary to the south.

For initiation purpose, exact placement, strength and BL wind modification becomes hard to pinpoint in various model outputs. Right now, enough signals are present for a cluster of showers and thunderstorms to move into SW Germany during the early morning hours with a gradual shift to the N/NE. Also during the forenoon, CI along the boundary from Rhineland Pfalz to Saxony is likely. Bavaria probably awaits the thunderstorms from the west with no CI ahead expected due to somewhat stronger cap signals, but CI around noon onwards is also likely both due to NE-ward spreading thunderstorms from the SW and also due to CI over the N-Alpine rim. By then, the overall picture of storms becomes very messy. Thunderstorms last well into the night although current thinking is that beyond midnight, the most concentrated shower/thunderstorm activity remains confined to the frontal boundary over C-Germany with much less activity further south.

Regarding ingredients, many parameters point to slow moving/training or back-building thunderstorms with slow storm motion and an high effective rainfall potential. From Bavaria to Saxony, anvil-layer flow remains enhanced with 30-40 kt southwesterlies overspreading the warm sector. In the low-mid troposphere, erratic wind fields are once again forecast along the boundary over C-Germany with reversed shear in the lower troposphere and some veering atop. Further south over Bavaria and Baden-Wuerttemburg, forecast soundings reveal deeply veered profiles until 18z before winds start to back in response to mid-level cooling. This is also reflected in the expected storm motions, which range from 0-10kt along the boundary over C-Germany and increase to 20-35 kt further south. With this wind field distribution in mind, we want to forecast a bimodal storm mode over the area of interest. Storms to the south (e.g. emerging from the Alps to the NE) likely reveal multicellular or even transient supercellular structure and remain discrete for some time before growing upscale into numerous NE-ward moving storm clusters. Large hail and strong to severe wind gusts are possible next to an isolated heavy rainfall risk ... the latter one especially during cell merging.
Thunderstorms to the north, along the boundary, find themselves within effective PW values of 25 - 35 mm, deep moist low/mid-tropospheric profiles, low LCLs, BL mean mixing ratios in excess of 10 g/kg and deep warm cloud depths (regionally exceeding 4 km in depth) for effective collision-coalescence growth. Any slow moving thunderstorm will be able to produce excessive rainfall amounts with more than 60 mm in less than 2 h. Until the afternoon, the highest concern for severe storms will be over Thuringia and Saxony with strongest anvil-layer flow, but thereafter, the risk also rapidly increases further west with strengthening frontogenetic forcing expected. Despite the local scale of those rainfall amounts, we issued a level 2 for the boundary, as regions with repeatedly passing thunderstorms could easily pick up rainfall amounts in excess of 100 mm. Flash flooding is a distinct possibility with that activity.
During the overnight hours, the main focus will be the placement of the final surface low, with ongoing model spread keeping confidence low for now. However, enhanced convergence likely results in excessive rainfall amounts along its northern fringe ... with latest data pointing to C-Germany. Convectively enhanced rainfall amounts once again could reach excessive criteria.

Switzerland is a bit uncertain regarding coverage of storms. Strong mid/upper diffluence, modest CAPE and modest shear should support at least a few severe thunderstorms with large hail and severe wind gusts. Thunderstorms over W/SW Switzerland seem to cluster betimes with an heavy rainfall risk as modest LLJ remains backed for quite some time, before the surface low moves east. This area was added to the level 1.

For all areas within the broad level 1, conditions remain supportive for a few tornado events due to robust LL CAPE support. Various LL vorticity signals can be seen in model fields and this looks reasonable given the presence of various outflow boundaries. which may locally increase LL shear. This time I excluded the term "non-supercell" as I don't want to rule out an isolated supercell tornado event in case storms ride along any boundariy (e.g. synoptic or mesoscale).

... Poland to Belarus ...

The quasi-stationary frontal boundary once again becomes the focus for scattered to widespread initiation. The main change is a gradual increase in deep layer shear just to the north but also along the boundary itself. 6 km bulk shear of 10-20 m/s with 8 km shear locally exceeding 20 m/s (veered profiles) composed with 500-1000 J/kg MLCAPE increase confidence in a few well organized multicell/isolated supercell events with large hail and strong to isolated severe wind gusts during the initiation stage, transforming more into a severe wind gust and heavy rainfall risk after clustering. Both, the upper level forcing and the entrance region of a 300 hPa jet probably support scattered to widespread thunderstorm development, so concerns about discrete and long-lived thunderstorms are low for now. However, with better forcing, a defined boundary and stronger shear in place, we would not be surprised to see a few well organized lines of storms to race east over W/C/EPoland, capable of producing concentrated swaths of strong to severe wind gusts and large hail. Thunderstorms continue well into the night.

For all areas within the broad level 1, conditions remain supportive for a few tornado events due to robust LL CAPE support. Various LL vorticity signals can be seen in model fields and this looks reasonable given the presence of various outflow boundaries. which may locally increase LL shear. This time I excluded the term "non-supercell" as I don't want to rule out an isolated supercell tornado event in case storms ride along any boundariy (e.g. synoptic or mesoscale).

... N-Italy, Austria into the S-Czech Republic and the Balkan States...

The eastward moving cut-off low enters the scene from the west. A 20-25 m/s 500 hPa jet along its eastern fringe spreads east and affects the area of interest during the forecast. Mid-level lapse rates don't look healthy but f.ex. NE Austria might be influenced by a weak Alpine EML plume, which fans out to the NE. Diffluent upper flow regime, approaching forcing and the rough orpography itself should assist in scattered thunderstorm development beyond noon. Forecast soundings reveal deeply veered profiles although the directional shear component weakens a bit during the afternoon with an increase in speed shear expected. Any thunderstorm which evolves from N-Italy into E-Austria will see 15-20 m/s DLS (increasing to 25 m/s over N-Italy), so well organized multicells/isolated supercells are likely. With increasing unidirectional shear expected betimes, splitting cells with large hail and severe wind gusts will be the primary risk.

What could keep the thunderstorm coverage and maybe severity of thunderstorms down a bit is mid-level ridging, which affects E-Austria and the S-Czech Republic until the evening hours, before weakening. Also influx of drier air between 600-700 hPa could limit thunderstorm coverage as entrainment for storms increases. Therefore we kept the probabilities a bit lower from E-Austria to S-Czech Republic.

The risk of a few well organized multicell/supercell events extends all the way down along the E-Adriatic coast, as the trough approaches during the night. 20m/s DLS and 500-900 J/kg MLCAPE support organized multicells with isolated large hail and severe wind gusts. Hence the level 1 was expanded far south and is valid after sunset.

For all areas within the broad level 1, conditions remain supportive for a few tornado events due to robust LL CAPE support. Various LL vorticity signals can be seen in model fields and this looks reasonable given the presence of various outflow boundaries. which may locally increase LL shear. This time I excluded the term "non-supercell" as I don't want to rule out an isolated supercell tornado event in case storms ride along any boundariy (e.g. synoptic or mesoscale).

... SE-Europe ...

Daytime driven thunderstorms are once again forecast with an isolated large hail and severe downburst risk during the mature stage. Strongest storms are forecast from W-Ukraine into W/C Romania, where best forcing and moisture will be present. Isolated thunderstorms continue all night long with a decreasing severe risk betimes.

... S-Sweden ...

A few strong to isolated severe pulsating thunderstorms are forecast with isolated large hail and strong wind gusts. Weak shear and modest CAPE keep probabilities below a level 1. Thunderstorms weaken after sunset.