Storm Forecast
Valid: Wed 12 Jul 2017 06:00 to Thu 13 Jul 2017 06:00 UTC
Issued: Tue 11 Jul 2017 23:02
Forecaster: PUCIK

A level 2 was issued for extreme SE Austria, NE Slovenia, SW Hungary and NE Croatia mainly for severe wind gusts and large hail.

A level 1 was issued for N Italy mainly for large hail and severe wind gusts.

A level 1 was issued for parts of Germany, N Austria, Czech Republic, S Poland and W Slovakia mainly for severe wind gusts.

A level 1 was issued for Romania, Moldova and Ukraine mainly for excessive precipitation, large hail and severe wind gusts.

SYNOPSIS

Synoptic scale pattern over Europe is dominated by a large cyclonic vortex at mid to upper troposphere over the Norwegian Sea. At its base, a pronounced short-wave trough is rapidly moving eastwards. As of 21 UTC Tuesday, short-wave is apparent in WV imagery approaching NW France. The trough is forecast to reach SE Poland by Thursday 06 UTC, with slight differences in timing and trough amplification among the global models. Accompanying this feature will be a strong cold front with polar airmass spreading from NW on the forward flank of the ridge that builds over W Europe, the Northern and the Norwegian Sea at the lower troposphere. The cold front that has resulted in widespread thunderstorm activity over parts of Central Europe in the previous days will slowly decay over W Russia and Ukraine, along with the trough at mid-troposphere. Most of the thunderstorm activity over Europe will be associated with these two cold fronts and their associated troughs.

DISCUSSION

... Germany, Czech Republic, N Austria, W Slovakia, S Poland ...

An impressive kinematic setup is in store for this region as the pronounced trough and cold front move rapidly eastwards. Ahead of the trough, PVA should provide large-scale upward motion, locally steeping the lapse rates and moistening the vertical profiles. Fast moving front could easily serve as an initiation mechanism. Along and ahead of the front, models simulate strengthening of lower tropospheric flow, reaching up to 25 m/s at 850 hPa, depending on the model and exact location. While forcing and vertical wind shear appear favourable, models do not agree on the degree of instability. Lower tropospheric moisture is forecast to increase slightly due to the advection from SW, but with mixing ratios increasing only to around or below 10 g/kg. Furthermore, lapse rates are forecast to stay less steep than 6.5 K/km. Thus, marginal instability at best is simulated with few hundreds J/kg in the more aggresive model runs. Situation may either produce a few low-topped, but well organised thunderstorms capable of severe wind gusts or no thunderstorms at all given absent instability. Complicating factor may also be abundance of cloudiness ahead of the trough. Despite highly questionable nature of the setup, Lvl 1 is introduced for a large area, where at least isolated thunderstorms may occur in the afternoon and evening hours.

... NE Italy, S Austria, Slovenia, N Croatia, SW Hungary ...

While it does not seem that abundant tstm activity during Tuesday would have depleted region of the lower tropospheric moisture, models simulate that lapse rates will get less steep over the area, decreasing CAPE in contrast to the previous days. Moderate to strong DLS will still be present, so that any storm that develops has a high chance of becoming supercellular. As the trough passes way north, widespread initiation can not be expected, especially not in the early afternoon hours. The cold front is forecast to quickly cross the eastern fringe of the Alps in the evening hours. In conjunction with that, more widespread storms may form with clusters capable of severe wind gusts. At the same time a small plume of steeper lapse rates will be advected from the Alps towards SE, slighly increasing the instability. Large hail will be concern especially early in the period and with more isolated development in the evening when cold front arrives. A small Lvl 2 is introduced for the region with highest likelihood of more widespread tstm coverage.

... Romania, Ukraine, Moldova ...

Surface observations reveal the presence of abundant lower tropospheric moisture. Together with around 6.5 K/km of mid-tropospheric lapse rates, moderate CAPE values, simulated in the range of 1000 to 2000 J/kg, seem well plausible. 10 to 15 m/s of DLS is forecast over the area, which suggests a potential for some well organised multicell clusters. Especially to the east of the area, hodographs reveal rather slow storm motion. Conditions seem to be sufficient for isolated severe storms that may be capable of large hail, severe wind gusts and also excessive precipitation. Initiation will be tied to the mountains with storms gradually moving over the lowlands by the late afternoon and evening hours.