Storm Forecast
Valid: Mon 09 Jul 2012 06:00 to Tue 10 Jul 2012 06:00 UTC
Issued: Sun 08 Jul 2012 22:11
Forecaster: TUSCHY
A level 2 was issued for Belarus and areas SW of Moscow mainly for large to very large hail, excessive rainfall amounts and strong to severe wind gusts.
A level 2 was issued for the area SE of the White Sea mainly for tornadoes, severe wind gusts and large hail.
A level 1 was issued for W-Ukraine mainly for excessive rainfall amounts and isolated large hail/strong wind gusts.
A level 1 was issued for parts of Finland and the Republic of Karelia mainly for heavy rainfall amounts and an isolated tornado event.
A level 1 was issued for parts of Norway and Sweden mainly for heavy rainfall and an isolated large hail/tornado event.
A level 1 was issued for parts of Austria, NW Italy, S-Switzerland and far SE France mainly for heavy rainfall amounts, isolated large hail and strong wind gusts.
A level 1 was issued for parts of NE Spain and the Pyrenees mainly for large hail and severe wind gusts.
SYNOPSIS
Active SW-erly flow regime continues over Europe. A temporal weakening of the geopotential height field gradient and a SE-ward moving surface cold front allow for some relaxation of the overall severe risk for most of Europe as best shear/CAPE fields become more separated.
DISCUSSION
... Finland and Republic of Karelia ...
Complex synoptic pattern present with convergent flow atop and a SW/NE elongated surface depression moving gradually to the ENE. Influx of warm/moist air from the SE continues until noon, which supports a well structured deformation zone/the development of a concentrated area with heavy rainfall over parts of C-Finland. This deformation zone gradually weakens, but still assists in heavy rainfall within the level 1 area (also in the non-thunder level 1 area). Daytime driven MLCAPE build-up over far east C-Finland and the Republic of Karelia is expected (up to 800J/kg), which in combination with 20 m/s DLS and 15 m/s LL shear favors a few strong multicells. Large hail, strong wind gusts and an isolated tornado event will be possible with the highest risk over the Republic of Karelia. During the afternoon hours, both the heavy rainfall risk and the severe risk decrease from west to east.
A substantial tornado risk may evolve SE of the White Sea during the afternoon hours. LL shear in excess if 15 m/s, strong SRH-1 and 25 m/s DLS will be a good set-up for a few tornadic supercells with large hail and severe wind gusts. A level 2 was issued for that activity, which gradually diminishes during the evening hours.
... Slovakia, W-Ukraine and Belarus ...
A cold front continues its E/SE-ward motion and affects the area of interest during the forecast period. At the same time, forecast trajectories show anticyclonic parcels entering the area from the east with an increase of the convergence along the synoptic cold front during the day. The air mass ahead of the cold front is characterized by rich and deep moisture, so despite only moderate lapse rates atop, expect another day with widespread 1000-2000 J/kg MLCAPE. 10-15 m/s DLS over most parts hints at a few well organized multicells with large hail and strong wind gusts (e.g. Slovakia, parts of Romania and NE Bulgaria). However, DLS increases to 20 m/s over Belarus with also strengthening anvil layer shear, so supercells with very large hail, severe wind gusts and torrential rainfall amounts are well likely in the level 2 area.
Given amount of LL convergence and strong diffluence aloft, a large cluster of storms is expected over Belarus during the night hours. This cluster tends to build to the E/NE, but we expect a gradual backbuilding into the thetae-axis over W-Ukraine during the night (and hence SE-erly motion of the cluster). Given the slow motion of the synoptic front, slow thunderstorm motions, the tendency for back-building and the exceptionally rich moisture content in the lower troposphere all raise the concern of excessive rainfall amounts also over parts of the NW-Ukraine.
For now we issued the level 2 area mainly for the supercell risk during the day and the risk for excessive rainfall amounts over S/SE Belarus and the border to the Ukraine during the night, but we may expand it a bit more to the south in further updates, if confidence in a major overnight rainfall event over W-Ukraine increases during the day.
... Parts of Norway and Sweden ...
A quasi-stationary W-E aligned synoptic boundary and a surface depression moving along that boundary to the E/NE will be the reason for slow moving and heavy rainfall producing thunderstorms during the forecast. Any shear will be weak with MLCAPE around 500 hPa, so pulsating multicell-type thunderstorms are expected. The main hazard will be heavy rainfall as storms reveal very low storm motions and back-building tendencies. Also, backed low-level flow along the synoptic front locally increases LL shear, so combined with low LCLs, an isolated tornado event can't be discounted. The thunderstorm risk diminishes after sunset, but locally heavy rain (ongoing isentropic lift atop the boundary) keeps going well into the night hours.
... Alpine region ...
SE-ward pushing cold front still lingers along the Alps and keeps BL moisture high over N-Italy, Slovenia and most parts of Austria. During the daytime hours, pretty undisturbed mid-level W/SW-erly flow regime dominates with weak divergent streamline pattern forecast. Hence, probably another round of daytime driven thunderstorms is forecast over the Alpine crest with slow net motion to the east. Mid-level lapse rates are weaker compared to yesterday and so will be the DLS. Hence the main risk will be heavy rainfall next to isolated large hail with a few better organized mutlicells. We expanded the level 1 far to the west with isolated CI over parts of Switzerland and far SE France. 20 m/s DLS and moderate CAPE allow for isolated large hail.
During the evening hours onwards, a short wave approaches Austria from the west and probably increases the overall thunderstorm activity from west to east. GFS/WRF/EZ all indicate an overnight cluster to form out of this activity with an eastward motion. Heavy rain, strong wind gusts and isolated large hail will be the main hazard.
... Pyrenees and NE Spain ...
As mid-layer height fields tighten during the day, shear at mid and upper levels increases (e.g. 500 hPa up to 25 m/s). Strong diabatic heating takes place during the day, which assists in enhanced onshore flow with richt BL moisture advecting to the NW. Hence, regions like Catalonia and Aragon (Spain) will see capped 1-1.5 kJ/kg MLCAPE although cap weakens from west to east.
Forcing will be tricky with only subtle waves detected in the SW-erly flow regime. However a somewhat better pronounced PV streamer crosses the area during the afternoon hours, which may assist in isolated thunderstorm activity over the Pyrenees and the rough topography of NE Spain. This activity may move into the lower terrain and into the aforementioned favorable environment for well organized thunderstorms. Large to very large hail and severe wind gusts will be the main hazard. Kept a 15% lightning area for now to reflect uncertainties regarding initiation.
... Rest of the lightning areas ...
Numerous regions in Europe were added in the lightning areas as either shear or CAPE will be too marginal for severe. A definite large hail risk may arise with any thunderstorm over N-Algeria (surface dewpoints in the mid twenties, very steep lapse rates atop and 25 m/s DLS). However, a stout cap is present and no lifting mechanism is forecast, so we kept this area in a low probability thunderstorm area for now. Isolated DMC over the mountains may erupt, but past events showed limited life time with ongoing entrainment and movement into strongly capped air mass.
S-Denmark and far N-Germany experience strong gradient flow with an eastward moving LL depression. Forecast soundings indicate that not much BL modification is needed for somewhat deeper updrafts within a strongly sheared environment. Confidence in DMC is too low for any level areas. However we do not want to exclude an isolated funnel report with shallow convection.
