Storm Forecast

Storm Forecast
Valid: Sat 11 Jan 2014 06:00 to Sun 12 Jan 2014 06:00 UTC
Issued: Fri 10 Jan 2014 23:07
Forecaster: TUSCHY

A level 1 was issued for W-Norway mainly for severe wind gusts.

A level 1 was issued for extreme N-Germany, S-Denmark, the SW Baltic Sea and parts of NW and N Poland mainly for isolated severe wind gusts.

SYNOPSIS

The rather zonal mid / upper streamline pattern over most of Europe gradually transforms into a wave train of troughs and ridges with increasing amplitudes ... especially over NW Europe. A powerful 970 hPa depression over the NE Atlantic and attendant WAA support a northward advancing ridge over Ireland, UK and Scotland and attendant downstream development over N/C Europe with a deepening trough. A stable upper low over the Iberian Peninsula drifts to the east towards Tunisia during the end of the forecast.

Regarding synoptic fronts, a progressive cold front will be the most striking feature for thunderstorm potential. During the start of the forecast this front is located over the SE North Sea with a rapid motion to the SE. It crosses N-Germany and Denmark during parts of the daytime hours and affects Poland during the night.

DISCUSSION

... S-Spain, N-Morocco/Algeria and parts of the W-Mediterranean ...

BL moisture will be the main problem with somewhat higher mixing ratios offshore and along the coasts. 500 hPa temperatures in the core of that vortex drop to -20 C or less, but the main forcing remains displaced from the Mediterranean Sea and affects areas like N-Morocco and Algeria. Hence, only isolated thunderstorm development is forecast beneath the colder mid-layer temperatures where steeper lapse rates exist. Onshore flow of better BL moisture over far NW Algeria results in some thunderstorm options well onshore and therefore the thunderstorm area was drawn far to the south. Next to sleet and gusty winds, nothing severe is anticipated.

... Denmark, N-Germany and N-Poland ...

This could be a rather substantial severe risk, if everything would be in place. A pronounced diffluent trough pushes east/southeast. A 45 m/s mid-layer jet streak comes along with that trough with its left-exit region remaining anchored atop a SE-ward racing cold front. QG forcing is also quite impressive and so is the influx of lower stratosphere air within the tropopause fold. No question, forcing and shear will be very good but not the prefrontal moisture advection. Models agree well in lowering the content of moisture (e.g. PWs decrease from short below 20 mm to just above 10 mm with similar signatures seen in mixing ratios).
Confidence in a narrow cold-frontal rainband structure along that cold front is the highest over Denmark, parts of N-Germany (especially NW Germany and the far NE part), the SW Baltic Sea and parts of N-Poland. All those areas see onshore flow of well mixed marine air with faint SBCAPE signals, before mixing brings moisture values down. 850 hPa winds oscillate around 20 m/s, so strong to isolated severe wind gusts will be the main hazard. I do not yet trust the rapid decay of onshore convection with so much background forcing. Hence the lightning area was expanded well inland with a dent over NE Germany due to a longer onshore fetch of BL flow and enhanced mixing.
In case the prefrontal moisture will be more robust than currently expected, both the lightning area and the level 1 area need to be adjusted to the S/SE.

Very cold mid-levels result in active marine convection west of SW-Norway. With 25 m/s winds at 850 hPa, severe gusts are possible and a level 1 area was added.

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