Storm Forecast
Valid: Fri 05 May 2017 06:00 to Sat 06 May 2017 06:00 UTC
Issued: Thu 04 May 2017 16:54
Forecaster: DAFIS
A level 2 was issued for Poland and W Ukraine mainly for large hail,excessive rainfall, severe wind gusts and tornadoes.
A level 2 was issued for Bulgaria, S Romania and SE Serbia mainly for large hail,excessive rainfall, severe wind gusts.
A level 1 was issued for the largest part of the Balkans mainly for large hail, excessive rainfall, severe wind gusts and tornadoes.
A level 1 was issued for N Spain and SW France mainly for excessive rainfall, severe wind gusts and tornadoes.
SYNOPSIS
The mid-level cold pool over Central Europe is stretched towards the Balkans, as a ridge is formed on its SW side over France. A long-wave trough with a strongly negatively tilted axis is approaching Spain and France from the Atlantic Ocean and the jet stream is cyclonically curved over this are during the night of Friday. WAA in SW France will limit convective phenomena before noon, but the accompanied cold front that will cross Spain first, will provide a favorable environment for severe storms. Widespread thunderstorm activity is expected over East Europe, where very unstable air masses lie below steep lapse rates. Convective activity will also be significant in Turkey with local downpours during the afternoon.
DISCUSSION
.... Poland and Ukraine ....
The lack of strong QG forcing does not exclude the possibility of severe phenomena in the region. The ongoing WAA during Thursday brings moist air masses from the south and diabatic heating tomorrow is expected to produce widespread lift. Forecast soundings based on GFS show quite fat mid-level CAPE profiles, suggesting the potential of strong updrafts, thus large hail is expected. Several parameters replenish the absence of steep mid-level lapse rates and strong DLS, like the low-level strong shear with curved hodographs,1-1.6 kJ/kg MLCAPE and MUCAPE up to 2000 J/kg.Damaging winds and tornadoes are also possible during an isolated supercell near the borders of the countries in the afternoon. More isolated convection is expected in south from the level 2 area, where DLS is calculated near 10-12 m/s. The main threat will be excessive precipitation from slowly moving thunderstorms. Some storms may move NW towards the center of Poland during the night.
.... Bulgaria, Serbia and Romania ....
This is another hot-spot for severe thunderstorms during Friday, as very steep mid-level lapse rates form. The level 2 area will be quite far from the left exit of the jet stream that enters Albania and Greece in the afternoon, but DLS is expected to reach 15 m/s, overlapping with 1-1.6 kJ/kg MLCAPE. The morning cap will be destroyed rapidly from the diabatic heating. Abundant BL moisture increase the PW values up to 25 mm and flash floods are expected. Several multicells will form after 15z and perhaps one or two supercells will be able to produce large hail and severe wind gusts. NCAPE values also suggest strong updrafts during 12z-18z and elevated convection will feed storms also during the night of Friday.
.... Spain and France ....
As strong QG forcing is expected during the late afternoon from the sharp long-wave trough over the Atlantic Ocean, a cold front near the surface will ensure convective initiation firstly over Spain and then over SW France. Models agree on the overlap of some hundreds of CAPE (GFS shows MLCAPE up to 1.7 kJ/kg), with strong DLS oscillating between 20 and 25 m/s. A broken line of multicells or a squall line with bow segments may form near the shores of France, producing severe to damaging wind gusts and large hail. An additional risk will be excessive precipitation as PW reach 30 mm, but this risk is of less concern in the southern flank of the level 1 area, as storms there are expected to move fast.
