Storm Forecast
Valid: Mon 06 Aug 2012 06:00 to Tue 07 Aug 2012 06:00 UTC
Issued: Sun 05 Aug 2012 22:19
Forecaster: TUSCHY
A level 2 was issued for far SE France and NW/N Italy mainly for large to very large hail, severe wind gusts, an isolated tornado event (a strong one possible) and heavy rain.
A level 2 was issued for extreme SE-Germany, N- Austria, the Czech Republic, Poland, Latvia, Lithuania and Estonia mainly for large to very large hail, severe wind gusts, isolated tornadoes and heavy rain.
A level 1 surrounds both level 2 areas for large hail (a sporadic very large hail event possible), strong to severe wind gusts and heavy rain.
A level 1 was issued for parts of Sweden mainly for an isolated large hail event and heavy rain.
A level 1 was issued for S-Finland mainly for an isolated tornado and large hail risk.
SYNOPSIS
Not much change in the synoptic pattern is recognized in model data with deep cold-core vortex over the N-North Sea moving gradually eastwards towards Norway. Complexity starts with at least two major short waves, which lift NE-wards along the downstream side of the main upper trough, dictating today's severe risk. At the same time, wave length of the upper trough shortens and places the C-Mediterranean beneath the anticyclonic side of an intense jet with temporarily increasing geopotential height fields. Ridging and attendant WAA also affect parts of E-Europe. A weak upper trough is placed over Turkey with no real net motion forecast.
At lower levels a cold front runs from the W-Mediterranean all the way to NE Germany. As the amplitude of the upper wave shortens, the cold fronts becomes quasi-stationary over the W-Mediterranean with a faster eastward motion over E-Germany and Poland. Eastward progression of the front along and just north of the Alps is hard to forecast, as complex orography and the evolution of numerous clusters dictate the speed of the baroclinic boundary. An extensive warm sector is present over C-E Poland and extends all the way into Latvia, Lithuania and Estonia. Cold and warm front are bound to a deepening surface depression over Sweden, which moves to the NE. Evolving occlusion is placed over S-C Norway/Sweden with a slow net motion to the north as long as the deepening of the cyclone continues.
DISCUSSION
... NE Spain, Balearic Islands, the Balearic Sea and areas to the N/NE ...
Obviously, dynamics become the main issue with a strong vorticity lobe grazing the area of interest to the north. Hence, timing of best dynamic forcing is confined to the 06-12Z time frame. Model discrepancies remain signficant with high resolution HIRLAM expecting a large overnight cluster over NE Spain, sending numerous outflow boundaries to the SE, which may act as forcing mechanism for isolated convective initiation (CI) over the Balearic Islands. The local MM5 however keeps QPF near zero with isolated CI over NE Spain. Both models however indicate a transient cyclonic wind field atop the Balearic Islands, which keeps surface dewpoints in the mid twenties. Yesterday's sounding of Palma had capped 1 kJ/kg and a similar set-up is forecast today, although better forcing and a 2-4K cool-down at 850 hPa increase probabilities for isolated CI. Cold front itself should not play a role for CI as this feature remains diffuse in most model fields and displaced to the west (E-coast of Spain). Extreme 6 km bulk shear in excess of 40 m/s, 1.5-2 kJ/kg CAPE (EZ, GFS) and a gradually eroding cap indicate a risk of sporadic but well organized DMC. Any updraft, which resists shear/entrainment and becomes longer lived will be a hail producer with very large hail likely next to isolated severe wind gusts. Despite limited time frame and moderate confidence in CI, a low lightning probability level 1 area was issued. After 12Z, subsidence and departing forcing keep CI probabilities very low.
... S-France, NW/N-Italy and far S-Switzerland ...
Gradually eastward moving cold front will induce scattered thunderstorm development mainly until the late afternoon hours (evening hours over N-Italy). However, overnight clusters (e.g. an evolving one over the E-Pyrenees at 21 Z) play a major role in coverage and placement of thunderstorms during the forecast period. Surface cold front at 21Z is already placed roughly along a line Toulouse-Bourges (France) and NE-ward traveling clusters may push this front further to the east compared to what latest model data expect. Nevertheless, a slow-down of this eastward motion is forecast by most models, so at least SE France and for sure NW/N Italy remain in an unstable and strongly sheared prefrontal air mass. 1000 J/kg MLCAPE and 25-35 m/s DLS overlap point to rapid thunderstorm organization with any longer lived storm. WRF evolves somewhat thickening cirrus along the intense jet, but latest IR imagery keeps this cloud shield more to the south and not as thick as forecast, so diabatic heating should occur at least on a temporarily basis.
Regarding the storm mode, uncertainties exist. However, current thinking is that numerous MCSes race to the NE with an active southern part (convective-wise) and a rapidly N/NE-ward expanding stratiform and rain-cooled precipitation shield. Tail-end storms will likely reveal supercell structures with all severe modi possible (large to very large hail, damaging wind gusts, heavy rain and an isolated tornado). If latest LCL forecast (LCL height at or below 1 km) remain on track (reasonable, given regions with dewpoints in the lower twenties), a strong tornado event can't be ruled out. Right now, confidence in those storms is highest from extreme SE France into Piedmont/Lombardy and Trentino-Alto Adice (N-Italy).
Further north over Switzerland, anvil debris and stratiform rain of any ongoing cluster may impact the forecast. As cold front moves in from the west until noon, a small time frame for prefrontal discrete but isolated supercell development is possible from SW into NE Switzerland in case of temporarily insolation. Large to very large hail, severe wind gusts and an isolated tornado can be expected with that activity. However, stalling cold front becomes the focus for repeated CI with a mess of numerous clusters affecting most of Switzerland thereafter. Heavy rain will be a distinct risk.
... Austria and SE Germany ...
Slowly eastward moving cold front again will be the main focus for scattered initiation. Impact of clusters to the south (thick cirrus shield) and transient foehn signals until noon will play a role in how unstable the air mass becomes over SE Germany into N-Austria. If diabatic heating occurs, dewpoints in the mid to upper tens and moderate mid-level lapse rates will be supportive of 500-1000 J/kg MLCAPE build-up within 20-25 m/s DLS. Refering to the paragraph above, storms moving out of Switzerland towards S-Bavaria may be able to intensify into a rapidly forward propagating MCS with potential long-lived supercells along its tail-end. This complex then moves into N-Austria with large to isolated very large hail and severe wind gusts the primary risk. This solution is supported by the minority of model data and a lot depends on diabatic heating, but pattern and slow eastward movement of the cold front until 06z (6th August) keep confidence high enough to go with a small level 2 area (tied to the large level 2).
Next area of concern will be S/SE and E-Austria. Either capped until the late afternoon hours (E-Austria) or awaiting incoming DMC from N-Italy (S-Austria), it may take a while for thunderstorms to initiate. High resolution models like WRF keep CI more isolated, probably as major cluster over the western Alps (ongoing during the afternoon hours onwards) may influence the strength of capping/subsidence. However, EZ and GFS both show significant CAPE in excess of 1.5 kJ/kg with 25-40 m/s DLS, so any storm will be capable of producing large to very large hail, damaging wind gusts, an isolated tornado (e.g. far SE-Austria with enhanced SRH-1) and heavy rain. Limited coverage, strong cap and low QPF signals keep confidence below a level 2 threshold for now, but this will be a high-end level 1, as any strom will be capable of producing even extreme events (e.g. hail).
Further west (western Alps and central Alps), major storm clusters pose an heavy rainfall risk, especially given background of terrible flash flood events in the past. We issued a level 1 for that.
... E-Germany, the Czech Republic, Poland, Latvia, Lithuania and Estonia ...
Ahead of a gradually eastward progressing cold front, an extensive warm sector is in place. Surface dewpoints in the mid to upper tens and moderate mid-level lapse rates assist in widespread 1 kJ/kg MLCAPE and more. 15-20 m/s DLS to the north increases to 25-30 m/s towards the Czech Republic with 20 m/s 3 km shear just along the cold front itself. Two northward lifting short waves will add adequate forcing for CI all day long. Of main concern will be DMC during the daytime hours, as discrete thunderstorms are possible. A mixture of well organized multicells and supercells races to the NE with large to very large hail, severe wind gusts and an isolated tornado risk. The latter risk may be maximized over NE Poland into Latvia/Lithuania, if indeed a weak surface depression evolves along the eastward moving cold front, assisting in backing winds in the lower levels and stronger directional shear. Betimes, storms line-up into well organized NE-ward racing MCS events with severe winds and heavy rain the primary risk. Most models keep QPF signals very isolated over the Czech Republic and this may indeed be the case, when this area resides between two large clusters (as proposed by GFS), but we do not trust model MCSes regarding placement and strength that far out and hence a broad brushed level 2 was issued.
During the night, a large cluster from the Alps may impact the Czech Republic and Poland with heavy rain and strong to severe wind gusts likely.
We added only extreme E-Germany in a level 1, as latest model data indicates the chance that not much time for BL recovery is forecast behind overnight's activity. However, any developing prefrontal storm will be severe before cold front finally moves off to the east.
... Parts of Sweden ...
Especially S/SE Sweden may temporarily be affected by the western fringe of the extensive warm sector with EZ/GFS showing 500-1000 J/kg MLCAPE build-up. Current thinking is that an ongoing cluster of stratiform rain with embedded thunderstorms from the overnight hours continues over S-Sweden with a few severe thunderstorm possible, as diabatic heating occurs. Large hail and strong wind gursts the main hazard next to an isolated tornado risk, given augmented LL shear. Betimes (after 12Z), eastward building occlusion cuts off the inflow of warm and moist air from the S/SE with decreasing thunderstorm and severe proabilities. However, the occlusion slows down with heavy rain becoming an increasing risk fro C-Sweden.
... S-Finland ...
During the late night hours, the northeastward moving depression approaches Finland from the SW with an unstable warm sector spreading to the north/northeast. Augmented LL shear and low LCLs inidcate an enhanced risk of an isolated tornado event during the night, next to an isolated large hail and strong wind gust event with strongest storms. The risk continues beyond 06Z.
... Benelux, far NW-Germany and Denmark ...
Cool mid-levels and a weakening background flow may assist in a somewhat enhanced funnel/short-lived tornado risk mainly along the coast, where LLCAPE is enhanced. However, no level 1 risk will be issued right now, as shear may be too strong for an enhanced non-supercell tornado risk. However, 15 m/s DLS may be adequate for an isolated better organized thunderstorm with marginal hail and strong wind gusts. The activity decays after sunset.
A similar set-up is forecast over E-UK with much weaker DLS, so main thunderstorm activity will be sub-severe with an isolated funnel/short-lived tornado risk.
