Storm Forecast

Storm Forecast
Valid: Tue 29 May 2018 06:00 to Wed 30 May 2018 06:00 UTC
Issued: Mon 28 May 2018 17:34
Forecaster: TUSCHY

A level 2 was issued across NE-France and Benelux to Germany and the W-Czech Republic mainly for excessive rain.

A level 1 surrounds the level 2 for lower/more localized excessive rainfall instances. Otherwise, isolated large hail, an isolated tornado event and a few severe wind gusts area possible.

A level 2 was issued across NE-Italy to Slovenia to Croatia mainly for excessive rain.

A level 1 surrounds the level 2 for lower/more localized excessive rainfall instances. Otherwise, isolated large hail is possible.

A level 1 was issued across most of Turkey mainly for excessive rain. Otherwise, isolated large hail is possible.

A level 1 was issued for parts of France and Spain mainly for excessive rain and a localized severe wind event.

A level 1 was issued for N-Finland to the White Sea mainly for severe wind gusts.


The general pattern features persistent blocking for all of Europe, excluding the far NE part over Finland to W-Russia, where a mobile mid-level wave races to the SE.
Otherwise blocking persists over S-Scandinavia to the Ukraine in terms of positive height anomalies. Lower geopotential heights remain in place over W-Europe, whereat this broad trough is composed of one large mid-level vortex over the Iberian Peninsula and a retrograde moving cold-core low, which shifts from the CNTRL-North Sea to Scotland. Another tiny cold-core low over E-Hungary moves into SW Poland during the end of the forecast but does not add lots of lift to the general setup.
Downstream ridging over the CNTRL-Mediterranean builds N/NE and links up to the N-/E European positive geopotential height anomaly. Finally a temporarily strengthening upper trough over Turkey will stay put and results in another active thunderstorm day.

Once again, no synoptic-scale fronts are forecast (excluding the warm and cold front passage over far NE-Europe) and hence a combination of diurnal heating, orography, weak mid-level waves and numerous convergence zones dictate the final thunderstorm coverage. The main area of concern arises from a low pressure channel, which extends from Germany to France, where it ends into a weak LL vortex, which relocates from W-France to the NE-Bay of Biscay. This broad area experiences a persistent LL confluence regime and abundant LL forcing for widespread CI.


... NE-France to Benelux and Germany and the far W-Czech Republic ...

An active severe thunderstorm day is in store with excessive rain being the main focus.

Forcing arrives during the day in form of a broad mid-/upper level trough, which swings from Switzerland/E-France to far SE-UK during the night. In addition, the already mentioned LL pressure channel adds additional LL forcing for CI. Models disagree in smaller-scale vortices, embedded in this low pressure channel, which could result in regionally augmented lift. The most likely path of such a vortex seems to be from W-Germany to Belgium. Beside that synoptic-scale input, the mesoscale/orography will once again dominante in where and when CI occurs.

A prolonged confluence regime allows a gradually deepening BL with LL mixing ratios in excess of 10 g/kg over a broad swath from Benelux to Germany. Aloft, mid-level lapse rates, although not that steep, were able to recover a bit due to missing widespread convective overturnig from the previous day and ongoing weak differential temperature advection. Therefore all models agree in widespread 1-1.5 kJ/kg MLCAPE whith substantially higher values highlighted by some CAMs along pronounced LL mesoscale convergence zones. PWATs oscillate between 25-35 mm.

It's not a col situation, but nevertheless a complete breakdown of the steering flow is anticipated with forecast storm motion vectors of 5 kt or less. In addition, 1-2 km AGL flow along the east side of the low pressure channel is enhanced which appears in forecast hodographs in form of small and looped hodograph structures (indicative of potential training situations). Given the amount of CAPE, robust updrafts are forecast despite missing shear with slow moving pulsating cells/multicells.

CI is forecast around noon with constant increase of thunderstorms thereafter. The quasi-stationary nature of those thunderstorms results either due to the absent steering flow or due to regionally enhanced LL flow (favoring training/back-building). Storms also remain anchored to the mountainous areas for some time. Obviously, excessive rain is a substantial threat. Similar to past events, a few thunderstorms may deviate (e.g. along outflow boundaries), ingesting enhanced streamwise vorticity and coupled with augmented LL CAPE/low LCLs will pose a temporal tornado threat. Beside that, isolated large hail is possible with initiating storms due to robust updraft strength and also swaths of strong to isolated severe wind gusts can occur as cold pools conglomerate into temporarily forward propagating small-scale MCS events.

Of interest could be the potential spin up of the vortex, which shifts from far W-Germany to Belgium. This additional forcing and its progressive nature could modify the overall convective mode to a more linear and progressive one over far W-Germany, Luxembourg and Belgium as indicated by a few model outliers. This would lessen the rainfall risk a bit and increase probabilities for severe wind gusts. Therefore the evolution of this potential vortex has to be monitored closely.

Despite those uncertainties, a broad level 2 was issued and was expanded far north over Germany to cover more isolated extreme events. This also includes an eastward expansion towards the W-Czech Republic, where CI is possible in an high CAPE/weak shear setup (despite stronger capping). The level 2 highlights the current state of knowledge with the highest threat of excessive rain, where potential significant flash flood events can occur. In case extreme rainfall amounts impact complex orography or urban areas, devestating and life-threatening flash floods can occur!

... N-Italy to far S-Austria and Slovenia and south to Croatia and Bosnia and Herzegovina ...

A similar thermodynamic and shear setup to the aforementioned one exists S of the Alps. Weak upper divergence occurs in the exit region of an upper jet, which assists in the widespread nature of CI (next to the orography). Initiating storms pose an isolated large hail threat but the main risk shifts more to an excessive rainfall risk with slow moving convection. In addition, upper divergence favors upscale growth into numerous slow moving clusters. Gusty winds accompany the strongest activity (approaching severe criterion on a local scale).

Far NW-Italy and S-Switzerland experiences a weakening overnight cluster, which could produce heavy rain up to around noon. This area was added to the level 1 despite the more destructive nature of this event with respect to diurnal heating (residual cloudiness). Some air mass recovery is anticipated during the late afternoon hours for a few strong to severe storms.

... Spain to CNTRL-France and Turkey ...

A weak/moderate cyclonic mid-level flow affects Spain/France, where MLCAPE of 0.5 to 1 kJ/kg and DLS of less than 10 m/s overlap. A weakly capped environment supports early CI (with probably ongoing activity from the previous night) and scattered to widespread pulsating thunderstorms are forecast. An isolated large hail event is possible although the main risk will be heavy to locally excessive rain. Cold pool driven clusters my pose a strong to isolated severe wind gust risk as the activity grows upscale. The thunderstorms gradually weaken during the night.

The S-Massif Central could see a prolonged period of slow moving thunderstorms, but the model guidance does not justify an upgrade as the convection weakens beyond noon as the southerly LL flow diminishes.

E-France was downgraded with lots of clouds and stratiform rain before noon, which limits diurnal CAPE build-up. A few non-severe storms are possible during late afternoon/evening.

Parked beneath an upper trough, a similar thermodynamic/kineamtic setup exists over Turkey with scattered to widespread slow moving convection. Isolated large hail and excessive rain will be the main hazard before the activity diminishes during the night. The widespread nature of storms should limit overall CAPE availability and hence no level 2 was issued. An intensifying LL vortex over the E-Black Sea serves as focus for an overnight cluster, which remains mainly offshore but could bring heavy rain to parts of the coastal regions of N-Turkey.

... N-Finland to the White Sea ...

A forced line of DMC activity is forecast with the trough/cold front passage beneath the left exit of a powerful mid-/upper-level jet. Forecast hodographs show rather straight signatures with forecast soundings indicating well mixed subcloud layers, so severe wind gusts will be the main hazard. Missing intense LLJ (only 30-35 kn at 850 hPa) preclude a more widespread severe threat/potential upgrade. The line races to the SE and weakens over the White Sea due to a stabilizing BL (also offshore). The level 1 was drawn far SE due to the strongly forced nature of this event.

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