Storm Forecast
Valid: Wed 14 Sep 2016 10:00 to Wed 14 Sep 2016 06:00 UTC
Issued: Wed 14 Sep 2016 10:35
Forecaster: TUSCHY

A level 2 was issued for far SE France mainly for excessive rain.

A level 1 was issued for the W-Mediterranean mainly for excessive rain, isolated tornadoes and isolated large hail.

A level 1 was issued for parts of UK mainly for excessive rain and isolated large hail.

A level 1 was issued for the W-Balkan States mainly for excessive rain and isolated large hail.

SYNOPSIS

Blocked large-scale pattern persists over Europe. Healthy upper low spins over the S-Bay of Biscay / N-Iberian Peninsula. Strong impulse rounds the base of the upper low and ensures an increasingly negatively titled appearance, as it overspreads the far NW Mediterranean and swings north into extreme SW France. WAA downstream still supports stout ridging over most of CNTRL/N-Europe, where unseasonably hot conditions for this time of year continue. Numerous synoptic-scale troughs circle that high pressure area and bring unsettled conditions along its fringes with isolated lighning options f.ex. over S-CNTRL Norway. A large-scale positive tilted trough extends from W-Russia all the way to the CNTRL Mediterranean. Especially next to and over the Mediterranean, accompanied cold mid-levels result in scattered thunderstorms especially during the day.

As a side-note for the Bay of Biscay:

Latest satellite data show a rather elongated depression's center over the N-CNTRL Bay of Biscay. In addition, the center features numerous smaller-scale swirls. It now tends to move to the SW and later-on more to the S along the back of the upper low, which would take the surface low beneath somewhat cooler mid-levels and atop higher SSTs.

Latest uptick of convective activity over the CNTRL Bay of Biscay may indicate a reforming center of that depression, which would then be placed more in proximity to the deeper convection and beneath colder mid-levels. Improving thermodynamic conditions should then ensure an increasingly more conducive environment for a strengthening LL vortex over the S-Bay of Biscay during the night. There are some hints that DMC may develop along that vortex and wrap (at least partly) around its center, transforming that low into a tropical-like system. Cyclone phase diagrams support that idea for numerous runs now with more or less pronounced warm-core structures.
Currently analyzed more than 40 kt deep-layer shear also abates a bit, although shear can remain enhanced with such features without inducing a weakening trend.
This low will approach the N-CNTRL coast of Spain during the end of this forecast and it should make landfall beyond 06 UTC. Strong winds and heavy rain would accompany such a system, which is not covered by our forecast procedure! Issued lightning areas over the far S-Bay of Biscay cover thunderstorms ahead of that feature.

DISCUSSION

... NW-Mediterranean including S-France, Corsica and Sardinia ...

Main focus for widespread DMC activity resides along a leisurely eastbound moving cold front, which already sparks numerous healthy looking thunderstorms over offshore areas and west of Marseille.

This activity remains beneath a diffluent but only weakly divergent mid-/upper flow pattern, although upper divergence gradually improves, as negative tilted trough draws near from the W. Undisturbed LL inflow - currently established - becomes increasingly disturbed by Sardinia and Corsica, which should lower the inflow's BL moisture for SW-France a bit during the rest of the forecast. Hence, peak of CAPE most likely can be expected until the afternoon hours, before decreasing a bit during the evening and overnight hours. 15-20 m/s DLS along that front promote enough shear for organized convection, although front-parallel flow should support rapid line-up of storms - already seen in current remote sensing data.

The main risk will be excessive rain especially along the coasts of SE France, as 850 hPa winds increase to 15 m/s and enhance the odds of training. Also, augmented LL shear especially along the coast and in the proximity of the front indicate a chance for an isolated tornado. The heavy/excessive rainfall and isolated tornado risk extends further to the S/SE and encompasses all of Corsica and Sardinia. There, weaker forcing and an enhanced angle of the front/flow increase the chance for more discrete storm structures with an attendant large hail threat - in addition to the aforementioned risks. This activity spreads towards the W-Tyrrhenian Sea until 06 UTC.

... E-France to Belgium ...

During the afternoon hours, a few thunderstorms probably erupt over NW Switzerland and later-on (during the night) also over NE France and Belgium, as surface based parcels become increasingly capped. However, enough MUCAPE persists during the night with ongoing WAA, which should support ongoing storms. 10-15 m/s DLS is enough for an isolated large hail threat.


... Catalonia and NE-Spain ...

First round of storms occurs as cold-mid-levels (500 hPa temperatures of less than -20 °C) spread east atop some residual moisture behind the cold front. Steep lapse rates should support weakly capped 300-600 J/kg SBCAPE. Weak DLS precludes organized convection, but cold profiles indicate a risk for graupel/small hail and gusty winds with stronger storms.
Coastal areas may see a temporal severe risk during the afternoon/evening hours during the passage of a weak mid-level wave. Responding BL winds ahead of that wave advect somewhat more humid air inland with MLCAPE forecast to increase to more than 500 J/kg just onshore and to more than 800 J/kg over offshore regions. 20 m/s 0-6 km shear is enough for a few organized storms with an isolated large hail risk.
This acitivity diminishes after sunset or shifts north into SW France - although sub-severe.

... CNTRL UK ...

Latest surface data indicates dewpoints in excess of 15 °C, also visible on satellite data with broad areas of stratus cloudiness. 00 UTC soundings indicate pronounced capping beneath 800 hPa and moderately steepened mid-level lapse rates atop, as fading EML spreads north. With good diabatic heating, up to .5-1 kJ/kg capped MLCAPE seems possible. Regarding CI, models diverge with some showing robust initiation and upscale growth into a northward moving cluster, whereas others remain more reluctant - mainly due to varying thermodynamic conditions in the models. With upper flow becoming increasingly divergent and analyzed numerous more or less pronounced convergence lines, isolated to scattered CI seems plausible. Very weak DLS precludes organized DMS, but magnitude of CAPE should support numerous strong updrafts. Large hail and excessive rain will be the main hazard - the latter risk especially, if storms grow upscale into a cluster. This activity weakens during the night with loss of daytime heating, but isolated storms could continue well into the night with enough MUCAPE forecast until 06 UTC.


... W-Balkan States ...

The far W-Balkan States could see a few stronger storms with heavy rain (isolated flash flooding possible) and isolated large hail. A low-end level 1 was added to account of strong CAPE build-up and hence the aforementioned severe risk. This activity diminishes after sunset.