Estofex have issued a Level 1 warning for parts of the UK: A level 1 was issued for S-UK mainly for an isolated tornado risk. SYNOPSIS A deep trough approaches the Iberian Peninsula during the morning and slows down while moving east. A break-up into numerous troughs with shorter wavelengths is forecast, which affect an area from Spain and Portugal to Great Britain. Downstream ridge over the CNTRL Mediterranean features very unstable but capped air, as EML spreads north over the moist marine air mass. Interface of both synoptic features will be the area of most concern for organized severe. A quasi-stationary cold-core low is centered over W-Turkey and the Aegean Sea with another round of active onshore thunderstorm activity forecast. Wintry air mass covers far N-Europe. Cold profiles and meager low-tropospheric moisture suppress convection, excluding offshore regions, where warmer SSTs (9-14 °C) offer some fuel for initiation. The same for SE Sweden, where better BL moisture is present. DISCUSSION ... CNTRL, N-France, Benelux, W/NW Germany and S UK ... A couplet jet configuration over NW France allows cyclogenesis of moderate strength over the N Bay of Biscay (e.g. 5-10 hPa pressure drop/24 h). It sends a warm front over France to the north and will be found over SE UK and the Netherlands during the end of the forecast (already occluding). An eastbound moving cold front comes on its heels and should affect CNTRL France during the night. In addition, numerous mid-level vort maxima will cross the warm sector during the forecast and insert additional lift for scattered CI. All models agree in rapid and extensive moisture return due to the dynamic nature of this event. Moisture will be advected northbound around the Massif Central with orographic channeling between the Alps/Jura and the Massif Central. Most models bring an healthy theta-e ridge all the way towards CNTRL France, which increases confidence in adequate low-tropospheric moisture content. Warm sector should see some post-frontal clearing as somewhat drier mid/high tropospheric air filters in from the west, but not sure how cirrus clouds from ongoing thunderstorms over S-France (see next paragraph) will affect the warm sector. However we should see at least a few breaks in the clouds, so with mid-layer lapse rates in excess of 7 K/km, roughly 800 J/kg MLCAPE should be realistic...mainly west and north of the Massif Central. Initiation is forecast as eastbound moving cold front interacts with available CAPE tongue during the afternoon hours onwards...e.g. W/CNTRL France. 20 m/s DLS, augmented LL directional/speed shear and LCLs around 1 km allow initiating storms to become well organized with large hail, strong to severe wind gusts and an isolated tornado risk. If storms can manage to form next to the warm front, an isolated strong tornado cant be excluded as LL shear maxes out. However, rapid upscale growth into a N-S aligned MCS is forecast due to the near front-parallel low/mid-tropospheric flow and excessive rain due to temporal training is possible. After sunset, a gradual decrease in thunderstorm activity occurs although a few elevated storms remain possible all night long. Bound to the northward surging warm front/occlusion, elevated thunderstorms spread N/NE during the night and affect W/NW Germany and Benelux. Elevated nature should keep this activity non-severe. S-UK however is placed next to the still deepening cyclone with strong S/SE-erly flow. Models differ in degree of CAPE build-up, but there are some hints that even an isolated near-surface based thunderstorm can occur during the night. With maximized LL shear along/just south of the synoptic front an isolated tornado event is possible. Marginal hail and gusty winds accompany this activity.
Posted on: Sun, 12 Oct 2014 10:18:51 +0000