Storm Forecast
Valid: Tue 24 Aug 2021 06:00 to Wed 25 Aug 2021 06:00 UTC
Issued: Mon 23 Aug 2021 23:45
Forecaster: BEYER
A level 1 was issued for parts of Central/Eastern Spain mainly due to large hail and severe wind gusts and to a lesser extent excessive precipitation.
A level 1 was issued for the Western and Maritime Alps mainly for excessive precipitation.
A level 1 was issued for parts of Central Italy mainly for excessive precipitation and to a lesser extent large hail. Waterspouts are possible along the Adriatic coast.
A level 1 was issued for parts of Croatia and Bosnia and Herzegovina into Serbia mainly for heavy precipitation, as well as locally large hail and severe wind gusts
A level 1 was issued for parts of Romania and Eastern Ukraine mainly for large hail and excessive precipitation.
A level 1 was issued for parts of NW Russia mainly for severe wind gusts. Tornados cannot be excluded.
A level 1 was issued for the Caucasus mainly for large hail and locally severe wind gusts.
SYNOPSIS
A strong upper-level high with corresponding surface high influences most parts of NW Europe into Central Europe.
Further downstream N and NE Europe into NW Russia are under the influence of cool airmasses with two shortwave troughs moving eastward during the forecast period. The further east one is accompanied by a 300 hPa jet maximum.
A persistent upper-level trough with several short wave features is present over Southern Europe with a rather zonal oriented trough axis. Warm, humid, and unstable airmasses are present in parts of the region.
DISCUSSION
... Central/ Eastern Spain ...
High CAPE values between 1000 and 2000 J/kg are forecasted for parts of Spain. They result from steep lapse rates and the inland advection of moist LL air from the Mediterranean Sea. Specific humidity locally reaches values of 14 g/kg, decreasing further inland.
Initially, this unstable airmass is capped with difficulties of initiation. With the mid-level trough axis that extends to the Iberian Peninsula, lift conditions are getting better during the afternoon. This is also supported by a weak maximum of IPV320K and a mid-level Q-vector maximum.
It is expected that storms will be initiated during the (late) afternoon and evening hours when capping gets weaker and lift conditions are better. Those storms do have an enhanced risk for severe wind gusts given inverted-V profiles up to 600 hPa, and rather high DCAPE values, preferentially inland. In addition, DLS shows high values in SE Spain due to the left exit region of a mid and upper-level jet. Thus also large hail can be expected.
Later in the evening and night hours models suggest that those storms may cluster. Storm motion vectors are forecasted to slow down. As a result, the chance of locally excessive precipitation is then enhanced.
...Western and Maritime Alps...
Convection is already ongoing from nighttime convection when the period of this outlook starts. The center of an upper to mid-level low is situated directly in the border region of Italy and France.
In the present moist and unstable airmasses more storms should develop during the day. Due to the described constellation, a low to mid-level convergence along the rim of the Maritime Alps is forecasted that supports ongoing and repeated convection.
The resulting storms will not move or only slowly due to their self-dynamics (local cold pools).
As a result, excessive precipitation can be expected with subsequent local flash floods
Due to the slowly eastward moving upper level low, the possibility of excessive precipitation moves also slowly eastward during the night then influencing the Western Alps and later also the Central Alps with decreasing risk.
... Parts of Central Italy ...
Steep lapse rates can overlap with moist airmasses. Specific humidity is forecasted with values between 12 and 14 g/kg. In addition, the area of interest is situated on the diffluent forward flank of the mid to upper-level trough that is initially positioned in the border region of Italy and France. This trough is expected to slowly move eastward. Thus main convective activity is expected in the afternoon and evening hours and should persist with a second maximum throughout the night when the trough axis crosses Central Italy. The trough axis is accompanied by a jet maximum and the area of interest is situated on its left exit region.
Given the high moisture and only moderate storm motion, excessive precipitation can be expected. Shear values are getting better later in the day (DLS: >15 m/s). Thus large hail is also possible.
Finally, low LCLs and LL convergent flow along the Adriatic coastline enhance the chance for waterspouts.
... Croatia and Bosnia and Herzegovina into Serbia ...
Similar conditions as in central Italy but with less moisture are present here. The upper-level trough is initially too far away, so that during the afternoon only a few storms may initiate over the mountains. Locally enhanced DLS shear values around 15 m/s give a certain chance of locally large hail.
During the late afternoon and night hours, the region of interest steadily gets on the forward flank of the approaching trough. Thus increasing convection is expected starting at the Adriatic coast and work its way further inland. Moist airmasses are advected inland coming from the Adriatic Sea. Thus the main hazard is excessive precipitation, especially close to the sea where repeated and stationary storms are fed by the moisture over the sea.
... Parts of Romania and Eastern Ukraine ...
This level 1 is only a marginal one. Along the cold front of a surface low over N Ukraine and Belarus, low-level moisture is enhanced in a narrow band and can overlap with steeper lapse rates. A few hundred J/kg of CAPE are forecasted. Deep layer shear values are only between 10 to 15 m/s but can be locally enhanced in Romania due to the orography. Locally large hail is possible with the first storms. With further storm initiation, the threat of excessive precipitation rises with PWAT in excess of 30 mm, moderate storm motion, and the possibility that storms may train in combination with orography.
...NW Russia ...
An active short wave trough is moving eastward during the forecast period. The area of interest is situated on its diffluent forwards flank and in the left exit region of an upper-level jet maximum.
Although moisture values are not very high the overlap with somewhat steeper lapse rates due to approaching upper-level cold air advection should result in a few hundred J/kg of CAPE in the afternoon hours.
Given high values of DLS, a few organized storms are possible that then have a chance to locally produce severe wind gusts.
Although low-level wind profiles do not look very promising, low LCLs are present and when a better-organized storm (supercell) may develop, a tornado cannot be excluded.
It should be mentioned that models differ in the development of CAPE. Some models only have marginal or nearly no CAPE reducing the severe weather threat. Therefore, this LVL1 is also on the marginal side.
... the Caucasus ...
Moist and humid air masses are present, whereas moisture and thus resulting CAPE is highest close to the Black Sea.
A short wave trough is approaching from the west. This will provide additional lift and enhance DLS. 15 to 20 m/s are possible. However, best shear conditions are present further east where CAPE is lower.
Storms should initiate around noon and large hail, as well as locally severe wind gusts (to the east), are possible. Storm activity will decrease in the late afternoon hours when the trough axis moves further east.
