Storm Forecast

Storm Forecast
Valid: Fri 10 Apr 2015 06:00 to Sat 11 Apr 2015 06:00 UTC
Issued: Thu 09 Apr 2015 18:42
Forecaster: TUSCHY

SYNOPSIS

A ridge remains entrenched over CNTRL Europe with little change in thickness but with some eastward adjustement of its axis. It will tilt from a N/S (Germany-Tunisia) direction over to a NE/SW (Tunisia-northern Balkan States) orientation. Despite its healthy looking appearance on the maps, mid-levels feature rather cool/cold temperatures with 500 hPa readings below -20 C along its fringes. Steepness of mid-level lapse rates is another approach, which offers quite high values over a broad area. Numerous more or less pronounced disturbances gradually gain ground over W-Europe due to the shift of the ridge axis and an slow increase of convection over parts of W-Europe is forecast.
A pronounced and progressive mid/upper wave rapidly approaches Ireland/UK and Scotland from the west and encroaches on this region during the overnight hours. Postfrontal marine convection will be the focus for deeper updrafts.
Finally, a quasi-stationary and gradually weakening cold-core low over SE Europe will be another region where spotty convection will be possible.
However for many European regions, hostile conditions continue in respect of organized DMC probabilities.

DISCUSSION

... E-Mediterranean / Turkey ...

Despite cold mid-levels, the BL air mass quality worsens during the forecast as increasingly dry air, which emerges from the high pressure over the Balkan States, advects south and spreads beneath the upper low. Once again, enhanced baroclinity exists between the cold landmass of Turkey and the warmer SSTs of the E-Mediterranean. A shallow vortex is forecast to strengthen a bit offshore, but this just increases the advection of the cool/stable continental airmass over the offshore areas. We therefore expect a very low chance for storms to develop in our forecast area (mainly to the east). One or two short-lived storms could occur over CNTRL/N-Turkey, where locally better BL moisture beneath cold mid-levels may provide enough instability for deeper updrafts. We therefore added a low-probability lightning area.

... N-Ireland ...

Overnight conditions feature a classic postfrontal marine convection event as 500 hPa temperatures temporarily drop below -35 C. Gusty winds and graupel accompany strongest storms. The lightning area was added, where coldest mid-level temperatures and strongest parcel layer depth signals were found.

... Portugal, parts of Spain, SW France and the W-Alps...

Yesterdays' soundings offered weak lapse rates and thin BL moisture, which was prone to mixing. Today, somewhat steeper lapse rates are forecast and with veering winds, an advection of a moist marine air mass is expected. If models are on the right track, this moisture also deepens a bit compared to yesterday. PVU maps clearly show a positive tilted trough which gradually spreads east and this should improve atmospheric (background) conditions for CI. The air mass will be weakly capped and current thinking is that main CI occurs, where strongest PVA, best BL moisture and favorable timing exist and overlap. This should be the case over Portugal and NW-Spain. Orographic enhancement should result in a few cluster of showers/thunderstorms, which gradually spread E/NE until sunset. Nocturnal stabilization of the boundary layer should induce a rapid decline of the thunderstorm activity. 15 m/s DLS may support a temporarily better organized updraft with hail being the main threat (probably non-severe).

The moisture plume also affects W-France. In addition, falling pressure over the W-Mediterranean ensures southerly winds with weak moisture advection towards the W-Alps. Given cold nature of the mid-atmosphere and locally good diabatic heating with meager CAPE and eastbound spreading synoptic-scale forcing, I don't want to rule out an isolated, short-lived and non-severe thunderstorm over W-France. Another focus will be the Swiss Alps, where some models give persistent signals for overnight CI (mainly due to orographic lift) . Again, enhanced convection with showers/isolated thunderstorms may occur and spread NE during the night, affecting parts of Switzerland and far SW/S Germany. Despite being so early in the convective season, confidence increased enough for adding a low probability lightning area, which could be too defensive if forecast soundings verify, which show adequate conditions for elevated convection. Nothing severe occurs with that activity.

Another focus for overnight convection will be far S-Spain, as the eastbound moving trough interacts with rich BL moisture. Expect numerous thunderstorms but they should remain sub-severe.

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