Storm Forecast

Storm Forecast
Valid: Fri 18 Nov 2016 06:00 to Sat 19 Nov 2016 06:00 UTC
Issued: Thu 17 Nov 2016 21:06
Forecaster: TUSCHY

A level 1 was issued for areas over and south of Sicily mainly for excessive rain and isolated hail.


A Rossby-wave is placed over the North and Norwegian Sea. It is associated with an extensive and deep cyclonic vortex. Brisk
flow surrounds that vortex and affects parts of CNTRL/NW Europe. A weak upper low over NE Algeria/N-Tunisia feels the influence of that intense vortex and moves slowly to the E towards the S-Mediterranean. E-Europe experiences calm conditions with dry/stable BL air mass and rising surface pressure.

... CNTRL Mediterranean ...

The upper low shifts east during the forecast and approaches Malta from the W/SW. Accompanied cooling at mid-levels overspreads SSTs well into the 20s, so expect 400-800 J/kg MLCAPE over the CNTRL Mediterranean and 1 kJ/kg plus south of Sicily. Shear remains weak, so scattered disorganized thunderstorms are forecast with a risk of heavy rain and graupel. A level 1 was issued for areas over and south of Sicily, as rich CAPE environment may support a few longer-lived thunderstorm clusters with excessive rain and isolated large
hail. Flash flooding is possible.

... NW and CNTRL Europe ...

First round of stronger convection exists over E-CNTRL France and CNTRL Germany along eastward moving cold front (06-15 UTC). Setup for numerous short narrow cold-frontal rainbands (NCFR) remains supportive from 06 UTC onwards and deeper updrafts/downdrafts may help to bring stronger winds down to the surface. LL shear (speed and directional) is very strong, so an isolated tornado event can't be ruled out especially with activity before 12 UTC. Low confidence in lightning activity, long-lived nature of NCFR events but also ongoing model uncertainties preclude the issuance of any level 1 area. A low probability lightning area was added, where confidence in limited lightning activity exists...all the way to the E-Baltic Sea.
W/SW France will see weak CAPE and intense LL shear overlap before noon beneath the anticyclonically sheared flank of a mid/upper-level jet. Expected marginal CAPE precludes organized thunderstorm probabilities for now.

A somewhat more elevated thunderstorm risk exists over parts of Belgium and the Netherlands during the daytime hours, as steep mid-level lapse rates overspread adequate rich BL moisture. 300-600 J/kg MLCAPE is forecast with 20 m/s DLS. This overlap assists in stronger updrafts with enhanced wind gusts and graupel or isolated hail. In case the passage of a weak low/mid-tropospheric wave during the afternoon hours materializes, the severe wind gust risk may increase with augmented ageostropic flow in a well mixed air mass. Model uncertainties with strength and timing of that wave remain large and even far NW Germany could be affected. Forecast soundings show low-end risk for an isolated tornado event, as SRH 1 km increases before sunset with lingering MLCAPE. A sub-severe thunderstorm risk over offshore regions persists all night long.

... S-Finland ...

Progressive mid-tropospheric wave crosses S-Finland with some elevated convection possible. Low-tropospheric flow starts to relax and convection should not root into the BL, so confidence in severe convection is low. Forecast soundings support that idea with rather stable near BL stratification and a good chance for some elevated instability. GFS is the only model with tendencies for signals of near surface based convection, which would tend to increase the severe wind gust threat with 25 m/s flow at 850 hPa. Bad timing (over night) and GFS as an outlier kept us from upgrading. Gusty winds and graupel will be possible.

... Rest of the lightning areas ...

Marine convection with scattered showers/thunderstorms is forecast over NW Europe. Gusty winds and graupel will be the main hazard.

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