Storm Forecast
Valid: Tue 13 Jan 2015 06:00 to Wed 14 Jan 2015 06:00 UTC
Issued: Mon 12 Jan 2015 23:07
Forecaster: TUSCHY

A level 1 was issued for parts of UK, extreme N-France, most parts of Belgium and the Netherlands and parts of NW Germany mainly for a few severe wind gusts and an isolated large hail threat. An isolated tornado is possible mainly along the coasts.

A level 1 was issued for Crete and S-Turkey mainly for excessive rain and a few waterspout events.

SYNOPSIS

A train of Rossby waves continues to affect parts of Europe. A broad trough with very cold mid-level air affects NW Europe during most of the forecast and results in unsettled conditions for a broad area.
A pronounced cut-off low SE of Crete is forecast to drift a bit to the SE. It brings showers and thunderstorms to the E-Mediterranean.
Elsewhere, high geopotential heights with weak mid-level lapse rates and marginal BL moisture keep the overall risk for thunderstorms near zero.


DISCUSSION

... Ireland, Scotland, CNTRL/N-UK and the North Sea ...

The broad atmospheric picture showes limited risk for organized DMC. Nevertheless, SSTs next to Ireland, west of UK/Scotland and in the English Channel range between 10-12 °C with lower values in the North Sea (10 - 7 °C from west to east). Placing temperatures of less than -40°C at 500 hPa and less than -20 °C at 700 hPa atop the warm SSTs, very steep lapse rates of 8-9 K/km can be expected. Very low WBZ levels with elongated but thin CAPE profiles in offshore forecast soundings show a good chance for scattered thunderstorms. Somewhat drier air is seen especially around 800 hPa, but this should be compensated by the otherwise favorable conditions. Therefore a broad high probability lightning area was issued despite general concerns of lightning activity with marine convection and current (22Z at 12th January) lull in activity over the N-Atlantic.

BL moisture profiles indicate slim moisture to work with, which also decreases rapidly onshore. We therefore kept the coasts in the high probability lightning areas with decreasing values well onshore.
Thunderstorms will bring gusty winds due to 20 m/s LL flow in a well mixed air mass. Unusually cold air mass will also increase the risk of numerous marginal hail reports.

... S-UK, Belgium, the Netherlands, far N-France, NW Germany and SW-Denmark ...

The focus for isolated better organized convection exists over the highlighted area during the evening and overnight hours. At least 2 more significant mid-level troughs are embedded in this brisk westerly flow with the strongest one entering the area of interest during the night. Local area models show enhanced backing ahead of those vorticity maxima with SRH-2 in the 150-300 m^2/s^2 range. Enhanced BL convergence with locally enhanced moisture and therefore elevated pobabilities for deeper updrafts in an environment of 30-40 m/s DLS show a risk of organized convection. Severe wind gusts and isolated large hail will be the main risk, but an isolated tornado event cant be ruled out with low LCLs and strong LL shear. LLCAPE (inland) remains in the 50 J/kg range with 100-150 J/kg offshore, so the main interest will be onshore moving convection, before encountering an increasingly hostile environment. Amount of forcing and speed of steering winds could take the convection well inland with WRF showing convection even affecting NW Germany during the night. We therefore expanded the level 1 far inland. Over NW Germany, the overall risk should decrease but an isolated large hail and/or severe wind gust event can't be ruled out.

... Crete and S-Turkey ...

Gradually SE-ward shifting vortex just east of Crete creates a favorable long-lasting onshore flow towards S-Turkey. Combined with near orthogonal flow towards the mountainous area, excessive rain is likely. Latest data places the bullseye of QPF just to the east of our forecast area and hence only a level 1 was issued.

Crete also experiences a prolonged period of N-erly flow...turning more to the NE as the low drifts to the SE. This also enhances offshore fetch and supports richer BL moisture to be advected towards Crete. Again, the near orthogonal flow towards the mountains results in excessive rain and flash flood concerns. 100-150 J/kg LLCAPE and coastal convergence zones increase the chance for a few waterspout reports.