Storm Forecast
Valid: Sat 19 Jul 2008 06:00 to Sun 20 Jul 2008 06:00 UTC
Issued: Fri 18 Jul 2008 18:19
Forecaster: TUSCHY

SYNOPSIS

Intense upper trough over the North Sea swings eastwards with strong CAA on its upstream side, overspreading Scotland, Ireland and UK during the next 24 hours. Another upper trough is parked over parts of eastern Europe with a strong surface depression moving slowly northwards. Hot and stable conditions prevail over the Mediterranean.


DISCUSSION

... Western Russia ...

Another round of severe weather develops during the day as a broad low pressure area at the surface slowly lifts north / northwestwards. The N - S aligned frontal boundary alters into a warm front, which progresses slowly NW - wards. At upper levels, a weak and flat upper trough still covers the area and numerous disturbances rotate around this feature, so enough forcing at LL and mid - / upper - levels is present for scattered initiation. Even if forcing would be too weak, strong diabatic heating should overcome capping so confidence is high regarding high storm coverage beginning already during the midday hours.

A combination of up to 2000 J/kg MLCAPE in the warm sector, a strongly backed surface flow in the northern quadrant of the surface depression and 15 - 20 m/s DLS all point to a significant severe thunderstorm risk with long lived supercells possible. Very strong LL shear just north of the surface low and along the warm front with up to 15m/s and SRH - 1 values in excess of 300 m^2 / s^2 point to an enhanced tornado risk and even a strong tornado will be possible. This is particulary the case during the evening hours, as LCLs come down. Otherwise, severe wind gusts and large hail can occur with each severe thunderstorm.

Betimes, thunderstorms will cluster and a more widespread severe wind gust risk could evolve if the latest GFS runs are correct, increasing the winds at 850 hPa to 25m/s. We therefore issued a broad level - 2 area.

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

The weather over the Iberian Peninsula can be described in two words: Very hot ! Temperatures at 850 hPa climb to widespread 24 - 28 °C , pushing the surface temperatures well above 30 °C with readings just shy below 40°C in far S - Spain. An impressive EML with very steep mid - level lapse rates overspreads the forecast area from the south so a potential unstable but well capped atmosphere will be present during the day. The focus for a few thunderstorms finally arises during the night hours over N - Spain and N - Portugal, where mid - levels stay somewhat more humid and cap is weaker. In addition, BL moisture is somewhat better with dewpoints well above 10 °C. Finally a slow moving but strong vort max crosses the area from the SW during the evening and night hours so we also have to consider elevated development in the northern part of the capped area. LCLs will decrease from 4 - 5km to below 2km over the highlighted area and even below 1km over extreme N - Spain. The main risk will be large hail and even an isolated signficant hail report is possible as SRH at the lowest 3km is very strong ( 200 - 400 m^2/s^2) but also strong downbursts are likely, especially for the elevated storms further inland. We decided to issue a level area as confidence is high enough regarding initiation.

... Parts of Belgium and Germany ...

Generally speaking the environment is prime for scattered thunderstorms over Germany as an intense mid - level streak pushes eastwards and cold mid- levels overspread the area from the west but we know that the devil is in the details and determines the final degree of storm organisation so we have to split the whole area in three subareas:

... S - Germany ...

GFS did a fine job today regarding dewpoints and we therefore stick with its forecast to increase those values a few degrees tomorrow afternoon ahead of an eastward progressing cold front. The front itself is not very strong but timing is good as it will cross S - Germany from the west during the afternoon and evening hours. If indeed that much insolation is possible what models indicate, we could get some low - end instability release. Main upper - level forcing is still far west and this area is placed in the right exit region of the mid - level streak but atmosphere is only weakly capped and surface front should help to initiate a few thunderstorms. LL shear is strong but SRH-1 values are weak so the main risk will be a strong to isolated severe wind gust risk. Rapidly decreasing LCLs along the front could result in an isolated tornado report, too. Confidence in better storm coverage is too low to go with a level area right now.

... N - Germany ...

Ongoing CAA at mid - / upper - levels atop a still moist and warm BL result in moderately steepened lapse rates in the 2 -4km layer and widespread MLCAPE release of up to 500 J/kg are likely. Diffluent upper streamline pattern and embedded disturbances but also the approaching left exit region of the intense mid - level streak over W-central Germany should result in a prolonged period of lift with scattered thunderstorm activity during the day. Although shear is not impressive we have 10 - 15 m/s DLS and a broad area of 5-10 m/s SRH-1. LCLs stay low and we can't exclude an isolated tornado report. Otherwise, marginal hail and strong wind gusts accompany stronger storms. The highest thunderstorm coverage will likely occur over NW Germany with more scattered nature further to the east.

... Parts of Belgium and west - central Germany ...

This area will see the strongest forcing, best shear but only limited instability release. We will go with a marginal level - 1 for this area as intense forcing and another SE - ward moving cold front could help to sustain a forced line of storms. Shear at all levels would be adequate for a few organized storms with a isolated large hail / tornado and strong wind gust threat.


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