Storm Forecast
Valid: Fri 05 Jul 2019 06:00 to Sat 06 Jul 2019 06:00 UTC
Issued: Thu 04 Jul 2019 19:06
Forecaster: PISTOTNIK
A level 1 is issued for the Russian and Georgian Caucasus region mainly for excessive convective precipitation and large hail, and to a lesser degree for severe convective wind gusts.
A level 1 is issued for parts of the Ukraine, Moldova, Romania and Bulgaria mainly for excessive convective precipitation and large hail, and to a lesser degree for severe convective wind gusts.
A level 1 is issued for NW Spain for large hail and severe convective wind gusts.
A level 1 is issued for parts of Lithuania and Belarus mainly for severe convective wind gusts and to a lesser degree for tornadoes.
A level 1 is issued for far N Russia mainly for excessive convective precipitation and large hail, and to a lesser degree for severe convective wind gusts.
SYNOPSIS
Low 500 hPa geopotential has nested from Iceland to S Scandinavia and the Baltic States. At its southern flank, an unseasonably strong frontal zone runs from the British Isles to the Ukraine. An embedded short-wave troughs crosses Poland, the Baltic States and Belarus. Another one follows over the northern North Sea.
Subtropic high pressure dominates to the south.
DISCUSSION
... south of ca. 48N and from Italy eastward ...
On the warm side of the main frontal zone, rich low-level moisture beneath moderately steep lapse rates allows some hundred to ca. 1000 J/kg of CAPE across wide areas. In the absence of noteworthy synoptic forcing, isolated to scattered afternoon storms will form over some orographic features (mainly over the Balkans, N Turkey and the Caucasus). Convective activity will also increase along a sea breeze front from SE Romania into the Ukraine in the evening.
Vertical wind shear is very weak in the lower troposphere before it increases to 10-15 m/s across the 0-6 km layer. Expected storm mode is therefore single- or at best multicellular. Isolated flash floods, large hail events and perhaps downbursts are possible. Overall, not much severe weather is expected.
... SW Europe ...
Warm air advection increases ahead of a cut-off low west of Portugal. Whereas hot and dry conditions are present in the interior of the Iberian Peninsula, robust but strongly capped CAPE develops over the Mediterranean Sea and the Bay of Biscay. There is a limited chance for isolated convective initiation in the afternoon at the sea breeze fronts over N and E Spain, where elevated terrain helps to heat the onshore and upslope flow enough to break the cap. 0-6 km shear between 15 and 20 m/s could promote storm organization with a risk of large hail and isolated (dry) downbursts.
Elevated and non-severe convection is also expected over the Bay of Biscay, where synoptic lift is stronger. It can spread into W France overnight.
... NE Poland, Belarus, Baltic States ...
A pronounced surface cyclone forms in the left exit region of the mid-level jet and reaches is deepest core pressure around 990 hPa around Friday 06 UTC just off Estonia, before it starts to fill while it slowly moves east. Synoptic lift and stretching ahead of the short-wave trough steepen the mid-level lapse rates to 7 K/km, and a few hundred J/kg of CAPE can form along the cold front and in the postfrontal maritime air.
Convection at the cold front may grow deep enough to benefit from strong shear (0-3 km: up to 15 m/s, 0-6 km: up to 30 m/s) in the late morning and afternoon. It can therefore organize into multicells, temporary supercells or small bow echoes with a risk of isolated to scattered severe wind gusts and perhaps one or two tornadoes.
Behind the cold front, a mesoscale storm field with maximum wind gusts between 25 and 30 m/s will move onshore in Latvia, but deep convection is not expected anymore then.
... Slovakia into central Ukraine ...
Scenarios in the stronly sheared environment ahead of the surface cold front and just south of the mid-level jet axis differ remarkably. Whereas GFS and WRF predict CAPE up to 800 J/kg and remarkable precipitation in a corridor from Slovakia into the Ukraine, ECMWF and ICON simulate hardly any CAPE and precipitation signals. Our outlook leans towards the more conservative solution and includes only a low probability lightning area. However, if storms form, they could organize well under 0-6 km shear around 20 m/s and pose a risk of severe downbursts and marginally large hail.
... far N Russia ...
Near the NE corner of our domain, warm and moist air is advected far NW-ward and wraps around the Scandinavian upper-level low. Up to 1000 J/kg of CAPE are predicted in response to daytime heating. The northern half of the CAPE reservoir is overspread by strong mid-level flow, yielding 0-6 km shear up to 20 m/s. Scattered to widespread, mostly daytime-driven storms are expected. In addition to a risk of excessive rain, especially well-organized storms to the north may also bring large hail and severe wind gusts.
