Storm Forecast
Valid: Tue 27 May 2008 06:00 to Wed 28 May 2008 06:00 UTC
Issued: Mon 26 May 2008 23:45
Forecaster: SCHLENCZEK

SYNOPSIS

On the east side of a large-scale upper trough over the Bay of Biscay, a shortwave trough will move northward into central France. Ahead of the shortwave, a 35 to 45 m/s jet streak at 300hPa points at central France. Another jet streak that belongs to this upper trough stretches from southern Ireland via Morocco towards northeastern Algeria with a speed max of 55 m/s at 300hPa. On the warm side of the jet streak, a plume of hot and moist air is advected northward and insolation as well as increasing BL moisture will create a significant amount of instability during the day. 12 UTC sounding of Munich on 26th May shows a well-defined EML and huge CAPE after reaching the convective temperature but the moist layer is very shallow within the warm sector airmass over Germany.

Between a stout ridge over the eastern-central Mediterranean and an upper low over southern Finland, a jet streak with 50 m/s at 300hPa, stretching from the southern Baltic Sea via Belarus and the western Ukraine towards northern Turkey, will translate eastward during the period. Scattered - probably non-severe - cold air convection with gusty winds and / or small hail is expected over southern Finland, Estonia, Latvia, northern Belarus and western Russia. Stable conditions will prevail over large portions of the central and eastern Mediterranean.

DISCUSSION

...Central France, Belgium, the Netherlands, southern UK, western Germany...

Around 06 UTC, GFS hints on a new MCS that may form over southern France in the morning hours while the old system over Belgium, the Netherlands and southern UK will likely decay. Surface dewpoints in the upper tens lead to moderate instability with about 1 kJ/kg MLCAPE over central France in the afternoon. A surface low ahead of the shortwave trough over southern France will induce a very strong wind field at lower levels on its eastern flank that should support convective cells in the whole level-two area. The limiting factor will be the stratiform precipitation and mid-level cloudiness that hinder insolation over parts of central France. GFS suggests that at least a small area over eastern-central France will have low cloudiness and more than 1 kJ/kg SBCAPE may form. Given 30 m/s deep layer shear and more than 400 J/kg SRH3, isolated storms will likely be supercellular and should be capable of producing large hail and isolated strong tornadoes as LLS exceeds 20 m/s in some places and strong SRH1 is present. If the convection develops more elevated, tornadoes should be rather unlikely but widespread severe gusts may occur as 850hPa wind field is well above 15 m/s and strong downdrafts will accelerate due to evaporative cooling. Strong QG forcing and 700hPa winds exceeding 30 m/s should be adequate for a severe squall line that develops during the afternoon / evening, capable of producing widespread severe gusts with some gusts exceeding 34 m/s. The line will translate northward across the level-two area and should reach southern UK, Belgium and the Netherlands in the late evening.

The airmass over western Germany is expected to be strongly capped but wind speeds near 15 m/s at lower levels could be strong enough for orographic lift, supported by some QG forcing as signalized by 500hPa Q-vector convergence. Moderate instability and up to 20 m/s deep layer shear will be adequate for organized multicells or supercells if indeed some convective cells develop. Other NWP models like NMM show some weak precip signals. Even though convective initiation is uncertain, western Germany is included in the level-one area.

As some uncertainties still remain, an update may follow later on.

...Southwestern France, northern Spain...

In the early morning, a convective cluster / MCS is expected to form over southwestern / southern France in an environment with some hundred J/kg of CAPE and 25 m/s deep layer shear. Some severe gusts may occur in an environment with 850hPa winds exceeding 25 m/s and isolated large hail is not ruled out. GFS suggests that storms will likely persist for almost the whole day. Torrential rainfall will lead to an enhanced flash flood threat.

...Northern Italy, Switzerland...

On the eastern flank of the jet streak, some hundred J/kg of CAPE should be created during the day. Deep layer shear will increase to about 35 m/s and strong southeasterly low-level winds are present under the southerly upper jet streak. Although CIN up to -100 J/kg is forecast, isolated supercells may develop as SRH values around 300 J/kg will be in place. Storms should be capable of producing isolated large hail and severe gusts. Convection will likely undergo substantial weakening after sunset as the major part of CAPE is surface-based.

...Southern Poland, Slovakia, southwestern Ukraine, western Romania...

In the right entrance region of the jet streak over the Ukraine, a few hundred J/kg of CAPE should be created during the day. Forcing is expected to be rather weak but some organized multicells may develop. Isolated large hail and / or severe gusts are not ruled out but a level-one seems not warranted as instability is limited and the overlap of instability and shear covers only a very small area.
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