Storm Forecast
Valid: Mon 15 Jul 2013 06:00 to Tue 16 Jul 2013 06:00 UTC
Issued: Sun 14 Jul 2013 22:04
Forecaster: TUSCHY

A level 1 was issued for N Spain and far N Portugal mainly for large hail, severe downbursts and locally heavy rainfall amounts.

A level 1 was issued for Belarus, parts of the Ukraine and far W-Russia mainly for heavy to excessive rainfall amounts. A few large hail events are possible, mainly east of Belarus.

A level 1 was issued for parts of C-Sweden into W-Finland mainly for severe wind gusts and an isolated tornado risk.

SYNOPSIS

A more or less stagnant weather pattern continues over Europe. An extensive upper low over Poland into Belarus seems to undergo some re-organization with one vortex gaining strength over Belarus while moving ENE, whereas another one over E-Poland sharpens and moves to the SE as a potent short-wave trough. Other than that, no change in strength and position of the major upper trough is expected.
The long-lived upper low over N-Spain remains intact and keeps its wind field closed, although a gradual weakening is forecast until 06Z. Rising mid-level heights over the North Sea build east/southeast and cause a strengthening ridge over France into Benelux.
An intense cyclonic vortex approaches C-Norway during the day and affects also most of C/N Sweden and Finland during the forecast period. The accompanying cold front will be the main frontal surge during the forecast. This front crosses the N/C Baltic Sea during the night and enters Finland from the west during the end of the forecast.

DISCUSSION

... N-Spain and far N-Portugal...

No major change in the synoptic pattern is anticipated. However with a downward strength of the mid/upper level vortex forecast, mid-level lapse rates start to relax a bit. Despite that, constant onshore flow from the Bay of Biscay advects a moist air mass far inland. The cap remains strong along the coastal areas but expect another round of initiation along the mountains around noon onwards. Compared to yesterday, wind field at 700 to 500 hPa weakens by roughly 5 to 10 kt, so shear will be weak with 0-6 km bulk shear at or below 10 m/s. Forecast soundings reveal fat mid-level CAPE profiles, so despite missing shear, 1000 to 2000 J/kg MLCAPE should still assist in numerous intense pulsating thunderstorms, which grow upscale betimes into numerous erratic and cold pool driven thunderstorm clusters. Large hail and a few severe downbursts are forecast next to isolated heavy rainfall due to the expected slow storm motion.
During the night, thunderstorms continue with a gradual decrease in strength due to lowering CAPE. However, regions like Castile and Leon, La Rioja, Navarre and Aragon may see a few strong to isolated severe thunderstorms due to increasing SW-erly winds at 500 hPa and 10-15 m/s DLS. An isolated large hail event will be the main hazard.

The level 1 was expanded all the way to the E-central coast of Spain, where shear will be stronger, favoring a few mutlicell events during the afternoon/evening hours with large hail and strong to severe wind gusts.

... Rest of the lightning areas ...

Numerous 15-% lightning areas were added for south-central Europe, where weak shear (10 m/s or less) and patches of 500 to 1000 J/kg MLCAPE overlap. Given regional CAPE forecast up to 1000 J/kg CAPE, a few strong to temporarily severe pulse storms are possible, but expected limited coverage preclude a level 1. Marginal to isolated large hail and gusty winds will be the main hazard. Local upscale growth into a few loosely organized clusters might also induce a confined heavy rainfall risk. We issued a 50-% lightning area, where confidence in better lightning coverage is augmented ... this however does not increase any severe probabilities.

An extensive 50-% lightning area/level 1 was issued for NE Europe, where the mean-layer mixing ratio in the boundary layer exceeds 10 g/kg, resulting in widespread 800 - 1500 J/kg MLCAPE with local higher values well possible. Enough forcing is available given the healthy upper level vortex and numerous strong convergence zones in this area. Scattered to widespread CI is forecast with rapid upscale growth into numerous storm clusters forecast. The main risk will be heavy to locally excessive rainfall amounts, given deep moist profiles, weak shear, enhanced back-building probabilities along boundaries (where advection and propagation may work in the opposite direction), PW values in excess of 30 mm and weak capping. With local CAPE upt to 2000 J/kg, severe hail and downbursts are likely as well. Thunderstorm clusters continue well into the night with a gradual weakening trend, especially beyond midnight. Right now the highest probabilities for a more serious rainfall risk (convectively induced) are found east of Belarus, where effective PWs exceed 35 mm.

... C/N-Sweden, the N-Baltic Sea into Finland ...

A powerful upper level low moves ashore along the coasts of N-Norway with a rapid motion to the east. Not much activity is anticipated until noon, as the rough orography and unfavorable trajectories from the mountainous region to the SW keep moisture values ahead of the front quite low. This drying might be enhanced by downslope flow along the mountains.

This changes however during the late afternoon hours over C/E Sweden, where better BL moisture from the Baltic Sea will be in place and should increase CAPE-build up along/ahead of the cold front. Forecast soundings from E-Sweden into W-Finland reveal strongly veered profiles with 300 to 800 J/kg nearly uncapped MLCAPE. With a potent vorticity lobe pushing east, this cold front should speed east with an increasing degree of organization from C-Sweden to the east. Some higher resolved models show a split-front type structure crossing the level area from W to E with a confined CAPE line present along the cold front. Forecast hodographs ahead of the cold front indicate curved profiles with a sharp increase in length and curvature expected just ahead of the passage of the front. Hence, confidence increased that we might see a rapidly organizing forced line of convection over E-Sweden, crossing the N-Baltic Sea until midnight and affecting Finland thereafter. Regarding shear, there are some hints, that this cold front might move into an increasingly diffluent mid-level flow regime with some decrease of shear forecast while another and intense speed maximum remains displaced more to the south (e.g. over S/C Sweden). This might keep the wind gust risk in check. Nevertheless, expect a swath of strong to isolated severe wind gusts within our level 1 area. Given modest SRH-1 and low LCLs along the front itself, we would not be surprised to hear from an isolated tornado event as well.
Reduced BL moisture and more stable BL conditions ahead should induce a gradual weakening trend of the cold front over Finland beyond 00Z.

In case a better CAPE/shear overlap becomes more likely, an update may be needed for a concentrated swath of severe wind gusts.