Storm Forecast
Valid: Fri 18 Aug 2017 06:00 to Sat 19 Aug 2017 06:00 UTC
Issued: Thu 17 Aug 2017 21:34
Forecaster: TUSCHY

A level 2 was issued for parts of Germany, W-Poland and the W-Czech Republic mainly for large/very large hail and severe wind gusts.

A level 1 surrounds the level 2 for similar risks with lower probabilities. This also includes an isolated tornado risk over NE-Germany/W-Poland and an excessive rainfall risk for E-Switzerland, W-Austria and far NW-Italy.

A level 1 was issued for NE-Spain mainly for large hail and severe wind gusts.

SYNOPSIS

A vigorous trough rolls up from the west and spreads east towards CNTRL-Europe during the day. Two strong short-waves circle its base and lift NE affecting a broad area from S-Sweden to N-Italy. Both waves seem to merge until 06Z, offering an extensive filament of DPVA along the eastern fringe of the longwave trough.
A strong subtropical ridge resides over SW- and S-CNTRL- Europe with ongoing hot and dry conditions. A weakening upper low is situated over E-/SE-Europe, as a high-over-low pattern begins to unfold.

At lower levels, an extensive LL vortex between Iceland and Scotland faces a 1025 plus hPa high pressure area over far W-Russia. A broad baroclinic zone with numerous fronts runs from Scandinavia to CNTRL- and W-Europe. The main focus for DMC activity will be an eastward rushing cold front and a potential prefrontal convergence zone around and north of the Alps. This front slows down along the Alps and becomes the focus for an heavy rainfall event during the night.

DISCUSSION

... SE-France to the Alps to Germany and parts of the Czech Republic and far W-Poland ...

A modified plume of subtropical air advects into the area ahead of an eastward sliding cold front. Given the nature of that airmass, lapse rates at mid-levels remain marginal over most regions with better values along and N of the Alps as an Alpine EML plume fans to the northeast. As a result, one can expect only several hundreds of J/kg MLCAPE with peaks of 1 kJ/kg over S-/SE-Bavaria.

Regarding forcing, the approaching short-waves and the main longwave trough already add abundant input. However as the main trough sharpens and acquires a negative tilt with an intensifying 50 m/s plus upper-level jet, the area with highest CAPE (from N-Switzerland to Bavaria) resides beneath a pattern with strong upper divergence, which also affects the cold front further north over CNTRL- and NE-Germany. The main question will be how long storms can stay discrete. In general, this pattern supports a growing and extensive cluster during the evening and overnight hours especially over the southern half of the level 2.

Before upscale growth however occurs, a time slot between roughly 10-18 Z exists, where more discrete convection evolves. Regions of interest are:

- an ana-like cold front with an extensive shield of stratiform rain, which stretches across E-CNTRL France and NE-/N-Germany. With differential diabatic heating due to a sharp gradient in cloud cover along the front, strengthening frontogenesis occurs simultaneously with an improving upper-level streamline pattern regarding upper divergence. Therefore, expect the leading edge of the straiform rain/cold front to become more active in respect to thunderstorm development over W-CNTRL Germany to N-Germany. A 20-25 m/s 700 hPa jet streak speeds to the NE parallel to the cold front with constantly increasing 0-6 km bulk shear (up to 25-30 m/s) and near front-parallel shear vectors. All this indicates the chance for organized convection along the cold front with small NE-ward racing line segments/bow echoes. Isolated tail-end supercells over E-/NE-Germany are also possible as the activity gradually builds east into the more CAPE-rich air mass. Small line segments and more discrete structures probably merge into an extensive line of thunderstorms over NE-Germany. Despite the moderate CAPE forecast, intense shear results in a rather broad level 2 area due to the risk of numerous severe events. In general, severe wind gusts and large hail will be the main hazard. However, latest model runs constantly sharpened a prefrontal surface trough over E-Germany, which adds more LL shear and improves hodograph structures regarding an isolated tornado threat especially over E-/NE-Germany and W-Poland during the afternoon and evening hours.

- S-Germany to SE-France, where CAPE/shear space supports organized multicells and a few supercells with the latter risk being maximized over far S-Bavaria and from the Swabian Alb east. Timing is still a bit uncertain and the main activity could shift a bit into the late evening and early night hours. This would limit the overall severe risk a bit. We kept the level 2 for now due to a looming risk of a few evening multicells/supercells with an enhanced severe risk.
Another potential area of isolated supercell development extends all the way down to Switzerland. Strong subsidence ahead of the mid-level impulse and a plume of drier mid-level air (sampled well by 12 Z soundings over S-France) should assist in some hours with diabatic heating before noon so the risk of too much cloud cover seems to be low right now. Large to isolated very large hail and severe wind gusts will be the main risk although a few flash flood incidents can't be excluded. Rapid upscale growth into an extensive cluster occurs betimes with the overall risk changing to excessive rain with local flash flood problems. A combination of upslope flow along the mountainous regions, slow storm motion and high effective PWATs (30 mm or more) likley produce a swath of high rainfall amounts. Decreasing MUCAPE and thunderstorm activity precludes a rainfall level 2 for those regions and hence we excluded parts of E-Switzerland from the level 2.

The risk of organized multicells extends all the way to the Pyrenées and NE-Spain with large hail and severe wind gusts the main hazard. The severe risk decreases towards S-CNTRL France but increases again over NE-Spain, where MLCAPE approaches 1 kJ/kg. Strong mid-/deep layer shear may assist in an isolated very large hail event over NE-Spain but lack of forcing may limit the overall activity. With ongoing uncertainties regarding CI, a rather broad level 1 was added for NE-Spain.

... Belarus and E-Baltic States to W-Russia ...

A plume of 400-800 J/kg MLCAPE with roughly 10 m/s DLS results in scattered pulsating storms with isolated multicells. In general sub-severe thunderstorm activity is forecast, but we can't exclude a few large hail events with stronger updrafts. Repeatedly developing convection over the same region may also produce a few excessive rainfall events. No focus for higher risk probabilities exists and we did not upgrade this area. This activity vanishes while shifting east during the night.

... UK ...

A brisk westerly flow regime with a well mixed postfrontal air mass and 15-20 m/s 850 hPa winds in place cause a risk of strong wind gusts with graupel or small hail. Could see an isolated severe wind gust over CNTRL-UK during late afternoon/evening in response to a 25 m/s LL speed max, but rapidly decreasing CAPE precludes a level 1. The activity exits UK east after sunset.