Storm Forecast
Valid: Mon 03 Sep 2018 06:00 to Tue 04 Sep 2018 06:00 UTC
Issued: Sun 02 Sep 2018 19:08
Forecaster: TUSCHY

A level 2 was issued across E Spain and parts of the W Mediterranean mainly for very large hail and severe wind gusts.

A level 1 surrounds the level 2 with similar hazards but lower probabilities. In addition, excessive rain can occur.

A level 1 was issued for parts of CNTRL Europe for excessive rain and isolated large hail.

A level 1 was issued across parts of Belarus mainly for excessive rain and isolated large hail.

SYNOPSIS

A blocked pattern continues over Europe. Anomalous positive height anomalies over N Europe face numerous upper cyclonic vortices with various intensities to its south. The most prominent one is still situated just south of the Alps but starts to shift east during the day. While doing this it starts to enter the area of influence of a strengthening upper low, which is placed over the Baltic States and intensifies gradually into the dominant vortex. The net-result will be a broad area of low geopotential heights from the Baltic Sea to the Adriatic Sea with embedded two distinct cyclonic vortices. No reflection in the MSLP field is forecast.

Another upper trough just west of the Iberian Peninsula already impacts Portugal and NW Spain with lower thickness. A flattening ridge downstream holds steady and keeps SE Spain beneath higher geopotential heights. Complex interaction with the terrain probably sparks numerous LL vortices with the strongest one just offshore of and over Portugal and another one probably emerging off the Strait of Gibraltar with an eastbound motion.

As the main pressure activity is in the mid to upper troposphere with weak reflections at the BL, no real synoptic-scale front impacts today's outlook (despite a leisurely west/northwestward moving warm front over CNTRL Europe).

DISCUSSION

... Iberian Peninsula and the far W Mediterranean ...

A combination of a diabatically driven diurnal thermal low and a dynamically driven general drop of the BL pressure ensures a gradually backing of the LL winds over E Spain to S/SE. Long lasting onshore flow and increasing wind speed assist in onshore advection of a very moist marine air mass (e.g. LL mixing ratios of 10-14 g/kg). Combined with a N/NE ward fanning EML plume with steep mid-level lapse rates, model guidance agrees in aggressive MLCAPE build-up over E/NE Spain with values of 1500 - 2500 J/kg MLCAPE and more than 3000 J/kg MUCAPE.
Warm air at 850 hPa assists in strong capping over SE/E-Spain with weakening CIN to the N (e.g. NE Spain). Forecast soundings show thick and fat CAPE profiles with forecast hodographs indicating a DLS increase from 10 m/s over NE Spain to 20 m/s over SE Spain.

With lowering heights during the day and ongoing insolation, CI should occur mainly along the mountainous orography but small-scale short-waves may also temporarily induce a more favorable environmental background for CI along the coastal regions. In general CI becomes more widespread over NE Spain during the evening. Models could struggle in decaying DMC activity over S/E Spain too fast during the evening with expected CAPE magnitude and lowering heights and hence we would not be surprised about scattered DMC activity also further south compared to the latest model thinking.

Initiating storms pose a significant hail threat with very large hail likely in any discrete and longer-lived updraft (main reason for going with a level 2). Also, severe downburst are possible, either due to heavy precipitation loading or due to more inverted-V like profiles towards E/SE Spain. The most likely area for a progressive (eastbound moving) storm cluster is currently forecast to occur over NE Spain and further east, as flattening ridge assist in a deep westerly flow regime over this area. Strengthening 0-3 km flow and still high CAPE values offshore should support a long-lasting eastward racing MCS with severe to damaging wind gusts and isolated large hail during the night.

Constantly weakening CIN from the north could also bring thunderstorms (if they cluster over/east of E-CNTRL Spain) towards the Balearic Islands. Even stronger shear would support a similar risk of a severe MCS with all kind of hazards.

This overall level issuance is more aggressive to what the models propose, as storm dynamics may not be represented well in the current model suite. We did not include the Balearic Islands into a level 2 to at least reflect the more or less absent signals in QPF fields, but a level 1 was added to highlight the potential risk for thunderstorms to move further south and east compared to current model forecast. We were also very aggressive in extending the level 2 into the ridge over SE Spain but any mountain storm could become very severe in the expected setup, although probably more on an isolated scale.

Over Portugal and the rest of Spain, scattered slow moving thunderstorms bring locally excessive rain, next to isolated large hail during their initiation stage.

... CNTRL Europe ...

With a more zonal alignement of the upper low and a broadening of the vortex, an extensive area resides beneath its weakly sheared but moderately to strongly unstable base. Weak CIN, the orography and the general ongoing cyclonic flow support scattered to widespread DMC activity with clustering thunderstorms. Excessive rain becomes a risk especially along and east of the Dinaric Alps but also over CNTRL Italy. Isolated large hail is also a potential hazard. Due to the widespread nature of thunderstorms, the final CAPE magnitude becomes more questionable due to lots of cloudiness and hence we decided to stick with a broad level 1 area.

Another focus for heavy to locally excessive rain exists along the warm front. Its position remains variable in the global models and hence a broad level 1 area was issued from E Austria to the NW. Diffuse synoptic-scale forcing and lots of cloudiness (+ ongoing convection from the previous night) make it impossible to pinpoint any area of maximized severe. In general the rainfall risk will be the main threat (with training convection) but also isolated large hail can't be ruled out with more discrete cells. Beyond sunset the overall risk diminishes with weakening convection.

... Belarus ...

Another level 1 was added with CAPE aoa 1 kJ/kg and somewhat stronger mid-level flow. Organized multicells with large hail and excessive rain area forecast, which grow upscale into slow moving clusters. The activity continues during the night with a gradual shift to the west. An ongoing heavy to excessive rainfall threat is forecast.