Storm Forecast
Valid: Wed 06 May 2009 06:00 to Thu 07 May 2009 06:00 UTC
Issued: Tue 05 May 2009 23:08
Forecaster: PUCIK

A level 1 was issued for Poland mainly for severe wind gusts and tornadoes.


A level 1 was issued for Northern Russia mainly for large hail and severe wind gusts.


SYNOPSIS

A complex situation is anticipated over Europe during the forecast period, with strong northwesterly flow being established over the western part of the continent. Two embedded impulses will travel witin the flow, deepening with time. The first one will affect the weather across Northern and Central Europe, developing into a cut off low, moving rapidly SEwards across N Poland as of Thursday 00Z. The second one will have reached Ireland by this time. Between both features, a ridge will start to develop from the high pressure center over the SW Mediterranean. Otherwise, a broad trough will persist over E Europe, filled with cooler airmass. At the surface a ridge of high pressure will stretch from Atlantic into the SW part of Europe. Deep low pressure systems will be associated with the above-mentioned impulses and several frontal systems are noted by models in those regions. Another, persistent frontal zone lies across Western Russia, stretching into the Black Sea and adjacent Balkan states.

DISCUSSION

...Poland...

Very strong QG forcing is expected over N Poland between 15 - 18Z in association with the short wave trough that will deepen during its progress to the SE. As of Tuesday 18Z dry intrusion is already present with this feature in the left exit region of the strong jet stream ( with wind speeds over 60 m/s). Divergence at the upper levels will even strenghten the anticipated forcing as both GFS and ECMWF put the area under the "dry intrusion". Ahead of the cold front, models are simulating slight instability release, but the values of MLCAPE should stay well below 500 J/kg. Neverthless, strong lift could help with sustaining some deep moist convection. Strong deep layer wind shear is forecast, with more than 20 m/s , with 10-15 m/s in the lowest 3 km. Values for the 0-3 km are obviously not that impressive , but wind shear in the 0-1 km layer will be in range of 10 to 15 m/s. If thunderstorms manage to form, they will probably organise into linear segments and we expect that some marginally severe gusts can occur ( as the flow at both 850 and 700 hPa will approach 25 m/s) and also tornadoes ( albeit weak) are not ruled out due to the strong Low level shear and low LCLs predicted. Due to the combination of these threats, Level 1 is warranted. As GFS and ECMWF solutions differ in track of the short wave trough, a larger area is issued to count in the difference. This area will be closely monitored for a possible update during the day.


...Northern Russia...

Ahead of the cold front, a plume of higher Theta-E airmass will be advected. With the aid of surface heating, destabilization should result in the release of more than 500 J/kg of MLCAPE ( GFS is quite aggresive, showing more than 1000 J/kg in 12Z run and ECMWF staying in hundreds J/kg ). Strong southerly flow across the region will yield high DLS values, from 15-25 m/s. Moreover, more than 250 J/kg of SREH in the 0-3 km layer is forecast, aiding in the development of storm rotation. With this combination, well organised storms are expected, including supercells. A small linear segments might develop as suggested by the Corfidi method, showing forward building of the potential MCS. Thunderstorms, thanks to their high degree of organisation will be capable of producing large hail or even marginally severe wind gusts ( especially if the linearly organised convection will form). Towards the evening ( 15 - 18Z) a belt of stronger low level flow will increase the LLS values to more than 10 m/s along with enhanced SREH values in that layer, but still, as a surface based instability will be quickly dimnishing, chance of tornado remains questionable, but still, worth mentioning. Storm initiation and coverage is, however, questionable, due to the weak overall forcing and we will staly with Level 1, as only isolated severe weather is expected.