Storm Forecast
Valid: Thu 22 Jul 2010 06:00 to Fri 23 Jul 2010 06:00 UTC
Issued: Wed 21 Jul 2010 21:03
Forecaster: GATZEN

A level 2 was issued for northern Sweden and Finland mainly for tornadoes and severe wind gusts.

A level 1 was issued for south-eastern France to south-western Germany mainly for tornadoes and large hail.

A level 1 was issued for southern and eastern Germany mainly for large hail, severe wind gusts and excessive rain.

A level 1 was issued for Eastern Spain mainly for large hail.

SYNOPSIS

Low geopotential expands from the arctic sea to the British Isles. Within the south-westerly flow that stretches from the west Mediterranean to north-eastern Scandinavia, several jet streaks are present. Warm air remains over most of southern and eastern Europe ahead of a cold front that will reach from northern Spain to the central Alps, western Poland and north-western Russia at the end of the period. Except for the regions north of the Black Sea and western Poland, quite rich low-level moisture is forecast. Steep lapse rates are expected from Russia to Poland and southern Germany, over the west Mediterranean region, and over northern Scandinavia. Forcing can be expected especially along the frontal boundary during the period, where QG forcing will be present ahead of the main trough.

DISCUSSION

Sweden to northern Finland

A vort-max actually over the Bay of Biscay will travel north-eastward crossing northern Germany in the morning hours and is expected to affect central and northern Sweden and northern Finland during the day, where the mid-level jet streak will likely exceed 30 m/s. A frontal wave that builds in response to this trough is associated with strong warm air advection into eastern Scandinavia, and low-level moisture is forecast to increase to 11-13 g/kg mixing ratio. This is expected to yield CAPE given rather steep lapse rates especially near the trough centre over the northern portions, while rather warm mid-level air will keep the equilibrium level quite low over the southern portions.

Strong QG forcing is forecast that will be strongest near the warm front over the northern portions until the afternoon hours. The advected air mass will be unstable with CAPE values about 600 J/kg, and thunderstorms are likely. These are forecast to organize rapidly in a strongly sheared environment with about 15 m/s bulk shear and 100 m²/s² storm-relative helicity in the lowest 3 km. Right moving storms that will move eastwards are forecast to produce tornadoes as low-level vertical wind shear is about 15 m/s and adequate low-level buoyancy and favourable hodographs are also in place. Strong tornadoes may be possible due to the strong low-level winds reaching 25 m/s at the 700 hPa level. Large hail, excessive precipitation, and severe wind gusts are also possible.

In the afternoon and evening hours, strong linear forcing is forecast along the approaching cold front. A linear and mostly parallel stratiform MCS is expected that will spread eastward ahead of the trough axis. While the winds will further increase to 35 m/s at the 500 hPa level, bowing segments are expected along this line that will pose a threat of severe wind gusts and tornadoes. Excessive rain will be also possible. The MCS is forecast to produce thunder and severe wind gusts up to the Arctic Sea during the morning hours, while warm mid levels will limit the potential of deep moist convection further south.

South-eastern France to south-western Germany

A mid-level jet streak originating from the east Iberian Peninsula will enter this area in the morning hours. While the low-level cold front will be heading eastward, some QG forcing may lead to weak frontal waves. Along of these waves, the moist low-level air ahead of the front can overlap with the strong vertical wind shear reaching more than 20 m/s in the lowest 3 km. Best lapse rates are centred over southern Germany, where CAPE will reach 1000 J/kg ahead of the front. But some weak instability may be also possible further south over France. Given the convergence along the front, rather moist low levels and low cloud base in combination with 7-10 m/s low-level vertical wind shear and about 200 m²/s² helicity in the lowest 3 km, tornadoes are forecast that may be strong. Main uncertainty is diurnal heating, and an upgrade to level 2 may be warranted in the morning hours. Large hail and severe wind gusts will be most likely over south-western Germany, where the instability is higher. Over south-eastern France, these storms may go on until the afternoon and evening while the severe potential will gradually decrease due to decreasing vertical wind shear and forcing. To northern storms will spread into southern Germany during the day.

Southern and eastern Germany

Initiation is expected in the morning hours over south-western Germany, and due to cold pool formation and the approaching cold front, a linear MCS will likely spread into southern Germany during the afternoon hours. While the MCS will leave the area with strong deep layer vertical wind shear, high instability will be in place over Bavaria due to latest GFS model output (2500 J/kg). A severe linear MCS is forecast to develop that will pose a threat of severe wind gusts in the afternoon and evening hours. Large hail will also be well possible. An upgrade to level 2 seems to be likely as widespread severe wind gusts may likely occur.

Further north, the low-level cold front will cross Germany in the morning hours, leaving westerly surface winds. Ahead of the short-wave trough over south-western Germany, cold air advection will stop during the day and turn into warm air advection. Diurnal heating will likely lead to instability as increasing low-level moisture and the advection of steep lapse rates from southern Germany are expected. In the afternoon hours, some storms may develop over the mountains over eastern Germany given upslope flow and increasing low-level convergence. These storms may be capable of producing large hail. Along the front storms will likely cluster into one or two MCSs spreading north-eastward. Excessive rain will be possible along the north-western flank of the MCS, while severe winds due to the developing rear inflow jet will become more likely along the leading gust front. Large hail is not ruled out in the afternoon hours. Tornadoes are expected to be unlikely initially given rather dry boundary-layer air, but the chance is forecast to increase slightly in the evening hours when strong low-level jets and moist low-levels will overlap over eastern Germany. As for southern Germany, an upgrade may be needed when it turns out that severe weather will be more widespread.

Storms will likely go on during the night with back-building storms as the front will stay over eastern Germany until the morning and a strong southerly low-level jet is forecast ahead of the cold front. Excessive rain may be possible.

Eastern Spain

Ahead of the southern tip of the trough, a strong south-westerly mid-level jet will affect Spain. Strong diurnal heating is expected, resulting in steep lapse rates spreading northward, and a low will likely develop over central Iberia along the cold front entering the northern portions. The associated low-level easterly winds will advect rich low-level moisture from the west Mediterranean into eastern Spain, where instability up to 1500 J/kg 30 hPa MLCAPE is expected. Upslope flow and convergence over the mountains will assist initiation, and storms that develop will have a great potential to become supercells as 25 m/s deep layer vertical wind shear and about 100 m²/s² 0-3km helicity will be present. Main severe threat will be large hail and locally severe wind gusts until the night hours, when storms are forecast to weaken.

Northern Greece and north-western Turkey region to western Romania and eastern Poland/Baltic States

Moderate instability built-up is forecast with MLCAPE locally exceeding 1500 J/kg. With lack of synoptic scale forcing and very weak winds at all levels of troposphere, mostly disorganised, diurnally driven thunderstorms are expected. Local flash flooding, marginally severe hail or wind gusts can not be ruled out from stronger pulse storms or multicell clusters, but coverage should not be high enough to warrant a Level 1.

Central British Isles

Undrneath the through centre, weak tornadoes are not ruled out given weak vertical wind shear and quite good low-level buoyancy and convergence.