Storm Forecast
Valid: Wed 22 Jul 2009 06:00 to Thu 23 Jul 2009 06:00 UTC
Issued: Wed 22 Jul 2009 00:43
Forecaster: PUCIK

A level 2 was issued for Central / Eastern France and Western Germany mainly for large hail, severe wind gusts and tornadoes.

A level 1 was issued for Southern France and Northern Italy mainly for large hail and severe wind gusts

A level 1 was issued for Northern Portugal and Northwestern Spain mainly for severe wind gusts and tornadoes.

A level 1 was issued for Central / Northeastern Germany, Western Czech republic and Poland mainly for large hail and severe wind gusts.

SYNOPSIS

A broad cyclonic vortex with center over the Eastern Atlantic is forecast to persist also during this forecast period. A strong, southwesterly flow at all levels of troposphere is observed on the fringes of the vortex. A jet-streak at 300 hPa is forecast to emmerge from the west and its axis should reach France by 00Z. Windspeeds in this feature should exceed 50 m/s. To the east, a ridging process is underway and by Thursday 06Z a significant ridge will cover an area from the Central Mediterranean into Eastern Scandinavia, moving slowly eastwards. Trough lying over Eastern Europe will slowly fill with scattered showers or thunderstorms in that area.

DISCUSSION

... Central to Eastern France / Western Germany ...


A potentially dangerous situation is forecast to evolve in this area as strong, southwesterly winds will create a favourable environment for well organised storms, including supercells. DLS over 25 m/s with more than 20 m/s of windshear in 0-3 km layer is simulated by models. Moreover, high SREH values should form in the range of frontal trough and a shallow surface cyclone, locally above 300 J/kg. Ahead of the slow moving frontal zone, EML is being advected northeastward. Both GFS and ECMWF agree on developing a belt of instability in the area, though GFS is being more optimistic with MLCAPE values. From both perspectives, moderate values of 500 - 1500 J/kg seem plausible.

Forcing is more questionable in this situation, though models keep on simulating synoptic-scale upward motion. Storms will probably firstly evolve close to the frontal zone around 15Z. Then, an approaching jet-streak might initiate convection in the broader area, as is suggested by enhanced fields of divergence over the region. One of the detrimental factors might be a fact, that storms might quickly cluster into MCS and an isolated development of supercells would soon end after the initiation. Convection from the previous forecast period might also play its role, but the exact effect is difficult to forecast at the moment.

With very good overlap of thermodynamic and kinematic parameters, supercells that form will be capable of producing large to very large hail and damaging wind gusts. Strong LLS (over 10, locally even up to 15 m/s) and SREH in 0-1 km are suggestive of tornadic potential in persistent, rotating storms. Later on, storms will likely merge into MCS, which should track to the northeast. Due to the very strong shear in lower levels, bowing segments might form within MCS with scattered severe gusts and extremely severe gusts not ruled out.

This area will be closely monitored for a possible update but at this time, a high end Level 2 is warranted.

...Central Germany / Western Czech republic...

Passage of nocturnal MCS is quite possible in this region in the belt of strong shear in 0-3 km, so that severe wind gusts might occur. The exact track of MCS is different in consequent model runs, so it is difficult to make exact conclusions at the moment. GFS 18Z even initiates widespread convection in the evening hours, which points to the possiblity, that storms might form before the MCS itself passes. In case that isolated storms fire, moderate MUCAPE values and DLS over 0-6 km suggest, that large hail can occur, especially if supercells manage to form. In western half of German Level 1, an isolated tornado can not be ruled out either, with more than 10 m/s of LLS and low LCLs toward the evening hours.

...Northern Germany / Poland...

Combination of moderate instability (MLCAPE values over 1000 J/kg) and moderate wind shear in 0-6 km (15 - 20 m/s) could result in the formation of well organised storms with a threat of large hail so that Level 1 is warranted. Storms will be most widespread close to the northern extent of high Theta-E airmass, with decreasing coverage, but increasing intensity (stronger instability) to the south.

...Southern France...

Storms might initiate also in this area, especially towards the evening and night hours (as suggested by GFS and ECMWF ). Despite the fact that instability will be decreasing, very high DLS (over 30 m/s) and high SREH values suggest that well organised storms might form, including supercells. In such case, isolated large hail event can not be ruled out, as well as severe wind gust.

... Northern Portugal / Northwestern Spain ...

Models keep on simulating low-end instability release over the region, with MLCAPEs mostly under 500 J/kg (except for the western coast). However, good forcing from the approaching trough should aid in the convective initiation. Storms that form, will have a very high potential to become supercells, given more than 35 m/s of DLS and high SREH values ( locally above 250 J/kg in 0-3 km layer) should be available. Shear will be very strong also in the lowest km, yielding values up to 15 m/s. Therefore, a tornado and severe wind gust threats are forecast and in case of supercells, marginally severe hail can not be ruled out either. This area will be closely monitored for a possible update - if predicted instability or forcing strenghtens over the area a level 2 will be necessary. One of the questionable factor is also the rate of storm clustering as low level boundary will be oriented parallel to the prevailing flow.