Storm Forecast
Valid: Fri 21 Jul 2017 06:00 to Sat 22 Jul 2017 06:00 UTC
Issued: Thu 20 Jul 2017 19:04
Forecaster: TUSCHY

A level 2 was issued for Switzerland, S-Germany, parts of Austria and N-Italy mainly for large/isolated very large hail and excessive rain.

A level 2 was issued for SE-Poland and far W-Ukraine mainly for large/isolated very large hail and excessive rain.

A broad level 1 area encompasses all level 2 areas with a similar risk but with less probabilities including isolated severe wind gusts. The level 1 over the W-/N-Ukraine, S-Belarus and CNTRL-Poland also includes an isolated tornado risk.

A level 1 was issued for parts of W-Russia mainly for isolated large hail and severe wind gusts.

A level 1 was issued for N-Algeria mainly for isolated large hail and severe wind gusts.

A level 1 was issued for parts of Ireland mainly for a few funnel/isolated tornado events.

SYNOPSIS

A rather intense and compact upper-level trough for this time of year is analzyed over Ireland, as phase diagrams show a deep cold core vortex during the forecast. This low drifts SE over the Celtic Sea and also affects W-UK during the overnight hours.
Another upper low, but much broader and less defined/intense is placed over Finland with a gradual motion to the SW. In between both vortices, a strengthening ridge builds north over Norway/Sweden towards the Norwegian Sea.

Further south, a subtropical ridge over N-Africa builds NE and affects the CNTRL-Mediterranean. Between the upper-level low over NW-Europe and that subtropial ridge, the mid-level height gradient increases with a quasi-stationary subtropical jet streak evolving over the W-Mediterranean. Further east another upper-level trough over SE-Europe lifts ENE during the forecast with little impact to our current European thunderstorm outlook.

In the low-to mid-troposphere, a wavy and quasi-stationary front runs from CNTRL-Spain to SE-France to S-Germany before morphing into a progressive eastbound moving cold front over E-Germany, the W-Czech Republic and W-Poland. Another quasi-stationary boundary is forecast over NE-Poland to Belarus into far N-Ukraine. Both fronts will play the main role for todays convective forecast. Another wavy frontal boundary over far NW-Russia is the third focus for CI, whereas a strong occlusion/cold front over NW-Europe should stay avoid of any more substantial BL moisture/mid-level lapse rate overlap and hence of a more robust thunderstorm threat.

DISCUSSION

... W-/CNTRL-Mediterranean ...

Forecast soundings show capped loaded gun profiles with a very moist boundary layer (surface dewpoints in the lower to mid twenties and mean LL mixing ratios of 10-15 g/kg) beneath a NE-ward streaming EML plume (providing steep mid-level lapse rates). Despite most models remaining avoid of any QPF signals, the concern exists mainly over the NW-Mediterranean to see a few intense and elevated updrafts. A quite healthy looking short-wave and a tad cooler mid-levels may squeeze out isolated thunderstorms mainly between the Balearic Islands and the Ligurian Sea. 20 m/s DLS and strong CAPE build-up support organized updrafts with a large hail threat. This activity may also spread onshore over N-Italy and start to root into a unstable BL with a large hail/severe wind gust and excessive rainfall threat. This activity merges with more storms over N-Italy, described later in this outlook.

... E-/NE-Spain to S-Germany ...

A belt of strong to intense thunderstorms is forecast with excessive rain and large hail the dominant threat. The main uncertainty will be how strong the atmosphere will be worked over by the activity from the previous day. Residual anvil debris and regionally more stable outflow air including outflow boundaries will have a substantial impact on today's thunderstorm activity and distribution. Due to all those uncertainties, numerical guidance remains very divergent with cluster development and motion, reducing confidence in the region with the highest severe threat somewhat. Numerical models however show improving conditions for diabatic heating during the day and hence confidence in adequate CAPE increases.

For S-/SE-France ... enhanced BL moisture due to persistent confluent flow/moisture pooling is still present in the vicinity of the quasi-stationary front, already described in the SYNOPSIS part. Mid-level lapse rates remain meager, so MLCAPE will be on the moderate side with 400-800 J/kg, locally up to 1kJ/kg. 15-20 m/s DLS due to a brisk SW-erly flow regime along the boundary add enough shear for organized convection. A NE-ward moving short-wave (well visible in WV imagery) will probably be the focus for most widespread CI. Multicells are forecast from the Pyrenees to SE-France with large hail/isolated severe wind gusts and excessive rain. Some models try to develop a large cluster over SE-France with a motion to the NE, which seems plausbile given the short-wave's interaction with an unstable air mass over the W-Alps. This activity diminishes during the night.

Over E-/CNTRL-Spain ... a similar setup evolves, as onshore flow of moist marine air from the W-Mediterranean advects beneath steep mid-level lapse rates, created over the mountainous region of E-/NE Spain. This combination may push CAPE to locally more than 1.5 kJ/kg, which, coupled with 20 m/s DLS, provides favorable conditions for severe thunderstorms. A few well organized multicells or isolated supercells are forecast with large/isolated very large hail and severe wind gusts. With not much steering flow expected, local flash flood producing rain may become another hazard. This activity diminishes during the night. Despite the risk of isolated extreme severe (e.g. very large hail) no level 2 was issued due to the isolated nature of thunderstorms.

For the Alps and surrounding areas .... Moderate BL moisture is present along and ahead of this boundary with some enhancement during the day (evapotranspiration and convergence along the front). With SW-erly mid-level cross Alpine flow (although not strong), a diabatically and dynamically driven lowering of surface pressure over SE-Germany and the S-Czech Republic is forecast with enhanced confluence/increasing BL moisture north of the Alps. With somewhat steeper mid-level lapse rates aloft, widespread 800 - 1500 J/kg MLCAPE should evolve with expected diabatic heating. Potential hindering points were already discussed in the first paragraph and could locally influence the final CAPE build-up.
Both, surpassing of the convective temperature and a passing short wave from the SW spark numerous thunderstorms over Switzerland and W-Austria around noon with an increase in coverage thereafter. 15 m/s DLS support organized convection with large/isolated very large hail for maturing storms. In addition, flash flood producing rainfall amounts with slow moving/clustering storms pose an additional risk. Severe wind gusts may stay confined to local intense downdraft/downburst events.
Either a developing thunderstorm cluster over Switzerland or Alpine convection move towards S-Germany, where interaction with a very unstable air mass is anticipated. In addition isolated thunderstorms may also evolve ahead of any approaching cluster over the mountainous region (e.g. Black Forest). Intense updrafts pose a similar risk to the Swiss and W-Austrian activity. Betimes, strong cold pools may evolve and push an MCS north with an enhanced strong to severe wind gust risk. As this activity moves N, it outruns the CAPE plume and decouples from strognest forcing. The air mass, where not worked over, remains unstable however for a continued risk of nocturnal thunderstorms over CNTRL-/SE Germany into the Czech Republic ... probably in form of a large cluster. The main hazard shifts back to excessive rain and isolated large hail.

Despite a rather weak shear setup, a broad level 2 was issued to reflect the enhanced excessive rainfall and large hail risk with any developing storm in a moderate to high CAPE environment.

... N-Italy to S-/E-Austria ...

The approaching short-wave from the W and a favorable CAPE/shear space (800-1500 J/kg and 15 m/s DLS) result in well organized multicells/isolated supercells, which evolve over N-/NE-Italy and far S-Austria. These storms pose a large/isolated very large hail, strong to isolated severe wind gust and excessive rainfall risk. In addition, isolated thunderstorms from the Ligurian Sea may spread onshore and to the NE. The strength of the short-wave indicates a risk of upscale growth into numerous clusters, which enter S-Austria during the afternoon hours. A continued risk of large hail, excessive rain and strong to isolated severe wind gusts exists. During the evening and night, this activity outruns strongest forcing and is forecast to weaken betimes as CAPE diminishes.

... E-Czech Republic, Slovakia and Poland to S-Belarus and W-Ukraine ...

Numerous short-waves cross a broad warm sector over most of CNTRL/-/SE-Poland and interact with the W-E aligned frontal boundary over NE-Poland, S-Belarus all the way to the NW-Ukraine. In addition, a long-lived nocturnal cluster may enter SW-Poland in a weakening stage (daytime driven) before noon before re-strengthening later-on over SE-Poland/W-Ukraine.
500-1000 J/kg MLCAPE and 15-20 m/s DLS indicate a risk of multicells and isolated supercells with large hail, excessive rain and strong to severe wind gusts. As the upper streamline pattern remains divergent, a long-lived cluster is expected to cross the level 2 from SW to NE/E. Training convection is possible in an environment with very high effective PWATs and weak steering flow, increasing the flash flood risk. In addition, an isolated tornado risk exists over E/SE Poland, far W-Ukraine and S-Belarus in proximity to a weak depression and the synoptic boundary, as LL shear increases while LCL heights remain low.

... Far NW-Russia ...

Scattered to widespread thunderstorm activity with 400-800 J/kg MLCAPE and very weak DLS result in pulsating and rapidly clustering convection with isolated large hail and locally excessive rain. Despite a local severe risk, no broad level 1 area will be issued for now. The activity weakens during the night but with an ongoing activity as the air mass remains unstable and weakly capped.

A small level 1 however will be needed where 20 m/s DLS and up to 800 J/kg MLCAPE overlap. Isolated severe including severe wind gusts and large hail is possible.

... Ireland and far W-UK ...

An isolated funnel/short-lived tornado risk exists beneath the cold-core low with augmented LL CAPE. Forecast soundings look promising for a few funnel/isolated tornado events and therefore a rather broad level 1 was added for Ireland.