Storm Forecast

Storm Forecast
Valid: Fri 23 Jun 2017 06:00 to Sat 24 Jun 2017 06:00 UTC
Issued: Thu 22 Jun 2017 19:40
Forecaster: TUSCHY

A level 2 was issued for parts of Hungary, Slovakia, SE-Poland, W-/CNTRL-Ukraine, W-Romania/Moldova and N-Serbia mainly for damaging wind gusts, large/very large hail and excessive rain. A few tornadoes are possible and a strong event can't be excluded.

A level 1 was issued for E-CNTRL France, S-Germany Switzerland, N-Italy, Slovenia and Austria mainly for large/isolated very large hail, excessive rain and severe wind gusts.

A level 1 was issued for the Pyrenees and NE-Spain mainly for large/isolated very large hail and damaging wind gusts.


SYNOPSIS

A zonal flow regime dominates the forecast. One large vortex south of Iceland intensifies and moves slowly E/SE while another vortex over NW Russia departs to the north. In between both vortices, flat ridging covers most of Scandinavia although it flattens out during the forecast while progressing east.

A potent short-wave continues to pass along the base of the aforementioned ridge to the SE. It is forecast to exit NE-Germany during the start of the forecast and enters far W-Ukraine during the evening hours. This rather stable wave features subtle intensity variations although a general weakening trend during the overnight hours over SE Europe is anticipated by the majority of the models.

Meanwhile a strong ridge extends from Morocco to the W-Mediterranean with hot and dry air affecting the Iberian Peninsula. Subtle mid-level waves round that ridge during the forecast.

During the night, another mid-level wave enters NW-Europe and some phasing is shown in models with a pool of rather cold mid-levels over the far E-Atlantic. The result is a developing upper trough over the Bay of Biscay. Model solutions diverge with this trough and uncertainties remain rather high.

The surface pressure field yields a complex configuration over E-CNTRL Europe (Poland to Belarus/Ukraine). Overall, a coupled upper jet configuration results in a broad area of low pressure between Poland and Belarus. Initially east/southeastbound moving short-wave seems to be accompanied by a somewhat better defined surface low, which however weakens betimes along with the short-wave and departs more to the NE (e.g. it merges with the broad area of low surface pressure). Latest GFS run re-strengthens that depression over the Ukraine during the night, but we think this is mainly a convective-feedback problem with QPF maximum attached to it. Despite those discrepancies, the result will be an highly meridional aligned and progressive surface trough which extends from far W-Russia to Bulgaria.
Elsewhere, no major / distinct surface features will play a role in our convective forecast as a nearly vertically stacked low south of Iceland remains to the west of our forecast area.

At 12 UTC, a warm front is expected to extend from W-Belarus to the CNTRL Ukraine with a cold front racing SE over Poland to Romania and becoming more wavy/quasi-stationary along the Alps and further west. More synoptic fronts enter the forecast area from the W (affecting NW Europe during the night) but they won't play a big role in today's convective outlook due to a limited overlap of ingredients.

DISCUSSION

... Poland, the E-Czech Republic, Slovakia, Hungary, Romania, Ukraine and Moldova ...

Ingredients for an outbreak of organized/severe thunderstorms merge as the short-wave interacts with a plume of very unstable air. Especially for Poland and the Czech Republic the outcome of the convection of the previous night from Germany casts some uncertainties about the final degree of today's severe risk but in general, ingredients support organized DMC activity.

The low-tropospheric moisture mixes out a bit along its northern part (e.g. Poland to Ukraine), but still offers values of 9-11 g/kg ML mixing ratios. With gradually weakening mid-level lapse rates, MLCAPE should be in the 400 - 800 J/kg range, with regionally higher peaks (around 1 kJ/kg). As BL moisture content and steeper mid-level lapse rates increase further south, MLCAPE of 1-2 kJ/kg is expected from Slovakia, Hungary to W-Romania and N-Serbia.

As the short-wave passes by, 500 hPa winds increase to 30 m/s and hence 0-6 km bulk-shear strengthens to 20-25 m/s. Low-tropospheric winds feature values of 15 m/s or more over a broad area, so LL shear is also encouraging for organized convection. The final geometry of the LL depression/trough dictate how much the SRH will be enhanced. Forecast hodographs mostly show rather straight signatures but stronger looping at low/mid-levels especially in proximity to the warm front and to a variable degree in the warm sector.

Regarding CI, DMC activity most likely is ongoing from the previous night and an extensive thunderstorm cluster (or numerous smaller ones) is/are forecast to affect Poland and the Czech Republic at 06 UTC and potentially as far east as Hungary and W-Ukraine. This scenario raises the question about how much diabatic heating materializes and how much the air mass ahead of the cold front will be already worked over by preceeding thunderstorms. This is especially the case for Poland. However, CAPE/shear space already supports severe thunderstorms with all hazards including excessive rain, large hail, severe wind gusts and an isolated tornado. Overall the severe risk increases during the day and further to the E/SE, as air mass destabilization continues. Regarding the storm mode two foci exist:

a) Thunderstorm clusters from the previous night move E/SE and re-strengthen dring the day as they encounter the very unstable air mass E of the Alps. A long-lived MCS with severe/damaging wind gusts is possible, affecting a broad area from Slovakia to the CNTRL Ukraine. A derecho event is possible with widespread damaging wind gusts given strong forcing and the attendant wind profile! Howevere there is also the chance of any MCS, which loses connection to the synoptic forcing to start to build more into the very unstable air mass over E-Hungary and W-Romania, which would favor another swath of severe/damaging wind gusts more in a SE-erly direction (towards Romania). Striking is the enhanced 1-3 km flow, which could indeed result in a forward propagating MCS event and level 3 conditions could occur over W-Romania. However too many uncertainties remain for upgrading such a confined region for now.
Beside the severe wind gust threat, large hail and excessive rain will be another hazard.

b) Ahead of any cluster/MCS, more discrete CI is also possible in the warm sector from Hungary to W-Romania and W-/CNTRL-Ukraine. Conditions will be prime for long-lived supercells with all hazards, including very large hail (hail diameter in excess of 5 cm), damaging winds and a strong tornado or two.

A decoupling of the more ESE-bound moving short-wave and high CAPE environment further to the W result in a gradual decrease of organized DMC during the night over SE-Europe towards Serbia and Bulgaria. However an organized cluster of storms spreads east towards the Black Sea during the night. The severe risk diminishes on a slow rate and hence the level 1/2 were expanded far east. All hazards remain possible.

... Alpine region to the Massif Central, N-Italy to the Balkan States ...

Mainly the eastern parts of the broad area (e.g. Austria, Slovenia and Croatia) will be grazed by the passing short-wave. Further west, not much synoptic-scale forcing can be expected during the forecast. However, a PV streamer accompanies the wave and slows down over the Alps. In addition, the cold front continues to move south and interacts with the Alps. Models tend to indicate a quasi-stationary front somewhere over/south of the Alps, but expected thunderstorm activity could blur the frontal's structure during the forecast. With all that in mind, enough support exists for numerous severe thunderstorms in a very unstable and weakly/moderately sheared environment.

0-6 km shear increases from 10 m/s over E-France and N-Italy to 15 m/s over E-Austria and Croatia. The wind field at mid-/low-levels yields rather low values. With LL moisture of more than 10 g/kg mixing ratios beneath moderately steepened mid-level lapse rates, widespread 1-2 kJ/kg MLCAPE are forecast with regional spots of more than 2.5 kJ/kg. In fact, some local models indicate peaks of more than 3 kJ/kg over N-Italy with very fat mid-level CAPE profiles. Forecast soundings also show LCL heights of 1.5-2.5 km and dry/deep subcloud layers with more than 1000 J/kg DCAPE. However rising heights from SW and subsidence keep activity in check. In addition, a stout cap south of the Alps lowers confidence in CI. Nevertheless, any storm in the level 1 area will pose a distinct large to isolated very large hail threat with mature multicells. There will be also a risk of severe/damaging downburst events and of excessive rain.
Thunderstorms continue during the night with a diminishing severe risk.

... Pyrenees...

Diurnally driven thermal low over Spain/Portugal advects rich marine air from the W-Mediterranean towards NE-Spain and the Pyrenees. This moisture sneaks beneath steep mid-level lapse rates and result in 1-2 kJ/kg MLCAPE. 850 hPa temperatures in excess of 20 C place a stout cap over the area, but subtle short-waves, rounding the mid-level ridge, lowering thickness and CI along the orography support the idea of scattered thunderstorms during the afternoon/evening. 15 m/s DLS is more than adequate for well organized multicells/isolated supercells with large/very large hail, damaging downbursts and excessive rain (although the rainfall risk may be tempered somewhat by the rather dry 1-3 km subcloud layer).

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