Storm Forecast
Valid: Fri 19 May 2017 06:00 to Sat 20 May 2017 06:00 UTC
Issued: Thu 18 May 2017 20:12
Forecaster: TUSCHY

A level 2 was issued for parts of Germany mainly for large hail and severe wind gusts. An isolated tornado event is possible.

A level 1 surrounds the level 2 for a similar risk with lower probabilities including excessive rain.

A level 1 was issued for N-Italy and SE-Switzerland mainly for excessive rain, large hail and an isolated tornado/severe wind gust threat.

A level 1 was issued for parts of the Ukraine mainly for large hail and severe wind gusts and to a lesser extent for an isolated tornado event.

A level 1 was issued for SW-UK and NW-France mainly for an isolated large hail and tornado event.

A level 1 was issued for SW-Turkey mainly for isolated large hail and excessive rain.

SYNOPSIS

An amplified streamline pattern is ongoing with two Rossby-waves affecting the western and eastern parts of Europe.

The western trough is well visible in moisture-channel imagery and extends from Scotland over France to the far NW Mediterranean. A basal mid-/upper jet streak is captured in latest sounding reports over Spain. This wind maximum translates east during the day and causes an increasingly negative tilt of the upper trough. Strong mid-level geopotential height falls occur over N-Italy and a diffuse mid- to low-level vortex evolves over NW Italy. This scenario is recognizable in all models.

The eastern trough features less thermal contrasts especially along its southern part. Hence the surrounding wind field is less pronounced compared to what we see further west (excluding the eastern quadrant with a lifting subtropical jet, which occurs to the east of our forecast area). Embedded in this trough however is a well structured cold-core low over the Ukraine, which gradually sags south towards the W-Black Sea.

In-between both troughs, WAA continues to support a deep ridge from the W-Balkan States to S-Scandinavia. Not much E-W shift is forecast although substantial geometric changes occur during the forecast, as thickness decreases rapidly along its southern part (here: the W-Balkan State).

This general pattern results in a confined channel with enhanced low-/mid-tropospheric moisture, which runs from the CNTRL-Mediterranean over CNTRL-Europe to S-Scandinavia. Embedded in this low-pressure channel is a wavy front with at least three more distinct LL vortices: one evolves over NW-Italy, another one shifts north from France to the S- North Sea and a third one moves east over Finland. Hence, the front pushes east as a cold front over the W-CNTRL Mediterranean and most parts of Germany with more quasi-stationary segments in-between. Preceding convergence zones also play a role in CI with one prominent convergence zone forecast to affect CNTRL/E-Germany.


DISCUSSION

... Ligurian Sea, N-Italy and the W-Alps ...

Lowering mid-levels in course of an eastbound swinging upper trough, a structuring LL vortex and a leisurely eastward trailing cold front add enough forcing/lift for widespread CI from W to E ... probably ongoing along the French/Italian border from the night before. Favorable upper ventilation occurs with evolving mid-/upper-level coupled jet structures and strong/persistent divergence. Forecast sounding over NW-Italy show veer/back profiles as mid-level CAA overspreads ongoing low-tropospheric WAA regime. Strong south/southeasterly winds advect a moist and unstable air mass from the Ligurian/Adriatic Sea to SE-Switzerland and with mid-level cooling/steepening lapse rates, 400-800 J/kg MLCAPE will become available during the day. DLS remains augmented along the E/S fringes of the CAPE plume with peak values of 15-20 m/s. Of interest is the hint on augmented LL shear / SRH in a rather moist air mass with low LCLs ... especially valid for N-CNTRL Italy.

Expect a growing cluster of thunderstorms over Switzerland and NW-Italy to gradually expand east during the forecast. Excessive rain threat is maximized along the Swiss/Italian border, where upslope flow and slow moving convection enhances rainfall rates and duration. From the Ligurian Sea to N-CNTRL and NE Italy, the risk of organized multicells/isolated supercells increases with more favorable CAPE-shear spacing. Confidence in long-lasting discrete storms is not high as rapid upscale growth into clusters is expected. However, aforementioned directional low-tropospheric shear may result in an isolated tornado threat beside the more prominent large hail risk. The wind gust threat increases with maturing convection (cold pool driven). Coverage and intensity of the DMC decreases during the night in the eastern part of the level 1 area, although substantial offshore CAPE over the Adriatic Sea supports an ongoing severe risk with any storm until 06 UTC.

... Germany ...

Although a favorable ingredient overlap for organized DMC is likely, numerous impacts on the mesoscale and synoptic scale cast uncertainties on the exact course of that event. One uncertainty f.ex. will be how the convective debris from the Italian activity may impact storms over S-Germany including foehn conditions along the Alps.

A northward lifting depression over Benelux pushes a cold front over W-Germany slowy east. In addition, a prefrontal convergence zone re-develops during the day as differential heating sharpens moisture and temperature gradients over CNTRL/E Germany. BL mixing ratios in excess of 10 g/kg offset less impressive mid-level lapse rates with 0.5 - 1 kJ/kg MLCAPE possible...decreaseing from S to N.

A tightening mid-level geopotential gradient results in a strengthening mid-level flow and DLS of 20-30 m/s. Much weaker shear is expected at lower levels, although 0-1 km shear may be locally enhanced (e.g. along the convergence zone). Combined with the aforementioned thermodynamic profile, well organized multicells and a few supercells are possible.

Main question remains when and where thunderstorms evolve, as convection from the previous night, a weakly capped air mass, upper divergence and strengthening forcing at low and mid-levels result in potential CI all day long within the highlighted area. In general the risk of organized severe convection should increase during the day with diabatic heating and increasing CAPE/shear (e.g. more elevated morning convection may root into the BL betimes).
We kept the level areas rather broad due to uncertainties with the exact position of the front and convergence zone. In addition, the front over CNTRL-N-Germany may be pushed back to the west a bit during the day in response to falling pressure over SE-Germany, which would broaden the sector with warm/unstable air.

Organized convection poses a risk of large hail. Weaker mid-level lapse rates and accordingly smaller CAPE profiles in the hail growth layer may preclude significant hail ( > 5 cm), although an isolated event in maturing supercells can't be ruled out. Heavy rain occurs and clustering may increase the excessive rainfall risk betimes. Despite modest low-level flow, cold pool driven swaths of severe wind gusts are possible with bowing convection. The severe wind gust risk may also be maximized over SE-Germany, where a dry and deeply mixed subcloud layer and strong pressure rises/isallobaric gradient from Switzerland favor an higher wind gust threat (not necessarily tied to DMC). SE-Bavaria may see an upgrade if BL moisture remains more elevated than currently anticipated. An isolated tornado event can't be ruled out especially with DMC rooting into a cooler and moist BL west of the convergence zone.

The activity diminishes gradually during the night with a decreasing severe risk as CAPE diminishes.

... E-Denmark and S-Sweden ...

Northward surging moisture and diabatic heating cause modest MLCAPE of 400-800 J/kg during the daytime hours. DLS of 15-20 m/s exists and a few organized multicells are possible. Forecast soundings show enough mid-level CAPE for an isolated large hail event. Heavy rain may pose a local flash-flood threat, especially as storms cluster betimes. This activity decays during the overnight hours.

... SW-UK and France ...

Placed beneath the upper trough, cold mid-levels overspread a moist BL. With diurnal heating, 400-800 J/kg SBCAPE is forecast. DLS remains weak, so pulsating and clustering storms are forecast with gusty winds, marginal hail and heavy rain. Somewhat steeper mid-level lapse rates over SW-UK and NW-France may result in an isolated large hail event. In addition, forecast soundings indicate a moist and weakly capped BL air mass with 100-200 J/kg LLCAPE build-up during the afternoon hours. A few funnels/isolated tornadoes are possible and a level 1 was issued to cover the enhanced tornado and hail threat in those regions. The activity vanishes after sunset.

... Ukraine ...

In the range of the cold-core low, an overlap of 400-800 J/kg SBCAPE and 15-25 m/s DLS exists. Diurnal driven convection may organize into multicells and isolated supercells with large hail being the main risk. BL moisture varies substantially with deeply mixed and very dry subcloud layers along the western fringe of the low (W-CNTRL Ukraine) and moist LL beneath the low. Hence, isolated severe wind gusts to the west and an isolated tornado threat over the CNTRL-Ukraine exist. The convection gradually decreases during the night with loss of diabatic heating.

... S-Turkey ...

Weak mid-level gradients beneath the upper trough cause weakly sheared conditions (DSL less than 10 m/s). A short wave travels along the upper trough to the S and crosses SW-Turkey during the day. Accopmanied cool-down of the mid-levels atop a moist BL air mass results in MLCAPE of 400-800 J/kg. Slow moving storms pose an excessive rainfall and isolated large hail risk. This activity gradually diminishes during the day from W to E as drier air filters in from the NW.