Extended Forecast
Valid: Sun 01 Aug 2021 06:00 to Mon 02 Aug 2021 06:00 UTC
Issued: Fri 30 Jul 2021 14:00
Forecaster: PUCIK

A level 2 was issued across N Italy through Slovenia to SE Austria mainly for excessive rainfall, large to very large hail, and severe wind gusts.

A level 2 was issued from N Hungary through Slovakia, E Poland for damaging wind gusts, large to very large hail, tornadoes, and excessive rainfall.

A level 1 was issued across Austria, E Czechia into Central Poland mainly for excessive rainfall and to the lesser degree for large hail.

A level 1 was issued across the high terrain of the Caucasus and Turkey mainly for excessive rainfall.

SYNOPSIS

Models have good agreement on the placement of a rather deep trough over W Europe with a strong SW-ly flow in the mid to upper troposphere from the W Mediterranean through central Europe towards N Russia. Several short-waves will be embedded in this flow, but their exact location and timing are more uncertain atm. In the lower troposphere, a wavy frontal boundary will stretch from the Alpine region through Czechia into SE Poland and N Russia. A low will develop along the frontal wave over NE Austria and is forecast to deepen as it moves towards E Poland during the afternoon and evening hours. Deepening of the low will be accompanied by strengthening flow in the lower troposphere. While ICON and ECMWF agree on the track and depth of the low, GFS has a more shallow solution with the frontal boundary shifted more to the E. Ahead of the boundary, steep mid-tropospheric lapse rates advected from Balkans will overlap with moist boundary layer, resulting in moderate to high CAPE values.  

DISCUSSION

... N Italy ...

Q-vector convergence at 850 hPa suggests upward synoptic-scale lift across the area during much of the forecast period. Scattered to widespread storm activity will likely occur already in the morning hours and the storm coverage may decrease towards the late afternoon. While there will be a lack of steep mid-tropospheric lapse rates over the region, abundant lower-tropospheric moisture will allow for 500 - 1500 J/kg of MLCAPE that will overlap with 0-6 km bulk shear of around 20 m/s. Well organised storms are forecast with threats of large hail, severe wind gusts. Heavy rainfall threat is likely with numerous rounds of storms on the southern flanks of the Alps.

... Alps to E Czechia, Slovakia, Poland, Belarus, and Ukraine ...

While many details of this situation remain unknown, there is a model agreement on:

1. Strong lower tropospheric synoptic-scale forcing along the frontal wave that will travel from NE Austria through E Czechia, Slovakia into Poland.

2. Abundant lower-tropospheric moisture ahead and along the boundary and steeper mid-tropospheric lapse rates being advected from Balkans.

3. Kinematically very impressive environment, featuring 0-3 km bulk shear exceeding 20 m/s, strongly curved hodographs in the lower troposphere, and 0-3 km SRH values exceeding 300 m2/s2 across a very wide area.

Numerous storms will likely be ongoing at the start of the forecast period, both along and on the cool side of the frontal boundary, where the strongest synoptic-scale forcing is forecast with a dominant threat of excessive rainfall. Storms will rapidly spread NE-ward during the day and surface-based initiation will likely occur ahead of the front in the afternoon hours (Slovakia / S Poland). Surface-based storms will rapidly organize into supercells or bow-echoes capable of all severe weather hazards. Given the combination of strong low-level forcing and most of the shear being confined in the bottom 3 km, it seems that the threat of long-lived convective windstorm stands out on top of other scenarios (such as isolated supercells with very large hail and tornado threat), but it may be too soon to pick the most likely evolution now. Isolated supercells will be more likely further E of the frontal boundary, in zones of weaker forcing and slightly enhanced CIN. As the frontal wave moves NE-ward towards Poland and Belarus, so will the attendant storms.

The corridor of the highest severe weather risk or the expected scenario may change in the future as the location of the frontal wave is shifted in the models or based on the evolution of the overnight and early morning convection. Nevertheless, this situation demands attention and has the potential to become a high-end severe weather event.

... Estonia to Russia ...

Straight and long hodographs combined with MLCAPE values btw. 500 and 1500 J/kg suggest an enhanced severe weather threat across the region. Currently, there is some disagreement among the NWP regarding the S-ward extent of the storms with ECMWF being the most aggressive one. A mix of line segments and splitting supercells is forecast with primary threats being severe wind gusts and large hail. Lack of clear signals in the low-level synoptic forcing in the individual models precludes indication of a more precise corridor of severe weather across the area.