Storm Forecast

Storm Forecast
Valid: Sat 03 Aug 2013 06:00 to Sun 04 Aug 2013 06:00 UTC
Issued: Sat 03 Aug 2013 01:09
Forecaster: PISTOTNIK

A level 1 was issued for S Norway and S Sweden for severe wind gusts, tornadoes, large hail and excessive precipitation.

A level 1 was issued for a belt from W Poland and E Germany to E Spain for large hail, severe wind gusts and to a lesser extent for excessive precipitation.

A level 1 was issued for the Western Alpine region for excessive precipitation and severe wind gusts.

SYNOPSIS

An extensive low-pressure system to the Northwest of the British Isles strengthens its status as the dominant steering center. Its frontal system pushes across Norway, Sweden and Denmark during the forecast period along with an intense short-wave trough, whereas the long tail of the cold front - bereft of any forcing - gets more or less stationary and inactive across Germany, France and Spain.
Ahead of the cold front, a deep and moderately strong Southwesterly flow continues to advect very hot air from NW Africa into Central Europe. This area will also be the focus for organized thunderstorms on Saturday.
Otherwise, poorly organized convection in an environment of limited CAPE and weak vertical wind shear is expected over the British Isles, where another short-wave trough reactivates the maritime post-frontal air, and in the vicinity of an old upper-level low centered over the Ukraine.

DISCUSSION

... Norway, Sweden ...

Pronounced warm air advection and the powerful short-wave trough provide strong QG lift from the Southwest. Especially towards Norway, the flow in the warm sector turns more and more helical (0-3 km storm-relative helicity up to 300 m^2/s^2) and impressively sheared (0-6 km shear up to 30 m/s, 0-1 km shear up to 20 m/s). Steepening lapse rates and fairly good low-level moisture will likely result in some hundred J/kg of CAPE, though they may initially be strongly capped (refer to Fri 12 UTC's Stavanger sounding). Convection in this environment will probably start from mid-levels and may need quite some time of superimposed lift and/or diurnal heating to root down into the boundary layer. However, as soon as these storms do so, they can quickly turn into multicells, supercells or bowing lines. The plentiful precipitation signals in all forecast models hint at quite widespread thunderstorm development by the afternoon hours. However, due to the remaining uncertainty whether the storms will indeed be able to become surface-based, it was decided to stay with a level 1 for a risk of severe wind gusts, moderate hail (2-3 cm) and possibly a few tornadoes. Excessive precipitation may become another issue in case of growth into one or more larger storm clusters. Convection will continue to move northeastward overnight, while it gradually becomes elevated again and starts to weaken.

... Poland, Germany, France, Spain ...

The hot air mass ahead of the slowly eastward moving cold front creates another make-or-break scenario on Saturday. It is characterized by steep lapse rates, but also by a strong cap between 850 and 800 hPa. On Friday, the first storms did not form before 16 UTC in Eastern Spain and 17 UTC in France, but the latter turned very intense within a few more hours. Further North, convective initiation even completely failed to happen despite dew points in excess of 20C in the Netherlands.
A similarly sluggish start but possible explosive developments can be expected again on Saturday, and once more it critically depends on an amalgam of synoptic-scale forcing, intense diurnal heating and gradual moisture accumulation along the front to weaken and finally break the cap. The day will begin with convective debris from the French storms overnight which will move northeastward across Western Germany and Denmark, and an outflow boundary which will proceed more deeply into Germany until it becomes stationary at some time. Some hours with little or no thunderstorm activity and more or less unimpeded insolation are expected to follow around noon.
Isolated to scattered thunderstorms will become more and more likely again the longer the afternoon advances. Some of these storms will probably struggle and fall victim to the strong cap again before they manage to obtain sustainable updrafts, but those which succeed to break the deadlock have good chances to become very well-organized in an environment of 1000-1500 J/kg CAPE, 20-25 m/s deep-layer shear and regionally augmented storm-relative helicity. In particular, this arduous initiation process may prolong the time frame with only isolated, unrivalled supercell storms which are capable of producing large to very large hail and severe wind gusts. Later a growth into one or two MCSes with a persistent severe weather threat into the night hours is possible, including excessive rain as well.
The consistency of the model pool with respect to the strongest precipitation signals is very low, regarding both their timing and their placement. Due to the high degree of uncertainty, it was decided to issue a broad level 1 belt all the way from Germany and Western Poland into Eastern Spain, even though confined areas within this belt may well see developments worth of a level 2 later on. In particular this applies to Eastern Germany and Western Poland, the only region which will see noteworthy synoptic-scale forcing in the right entrance of a jet streak: here the formation of a convective line with a higher coverage of severe wind events becomes a realistic option towards evening, but the confidence is just not high enough for a level 2.

... Western Alps ...

Intense diurnal heating in a weakly sheared environment will result in at least scattered afternoon storms, which will move very slowly and are anchored over orographic features. A low-end level 1 was issued for a possibility of localized flash floods or downbursts

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