Storm Forecast
Valid: Wed 31 Jul 2013 06:00 to Thu 01 Aug 2013 06:00 UTC
Issued: Wed 31 Jul 2013 05:02
Forecaster: TUSCHY

A level 1 was issued for parts of Ireland mainly for heavy rainfall amounts.

SYNOPSIS and DISCUSSION

Behind a NE-ward moving upper trough, which exits the North Sea to the E, strong ridging builds in from the WSW. An extensive surface high pressure area then covers most of C/SW Europe.

The breakup of any shear/CAPE overlap continues during the forecast which also lowers organized DMC probabilities. A few spots still see sufficient CAPE for a few stronger pulse storms with marginal to isolated large hail, including the Oslo area (SE Norway) and Romania into Bulgaria. The latter area might see a more enhanced risk due to better mid-level dynamics and DLS of 10 to 15 m/s. However, the main activity stays beneath the SE-ward dropping cold-core low, where weak shear prevails.

C-UK into Ireland might see the best probabilities for an isolated severe event. A sharp warm front moves ashore during the daytime hours from SW to NE. The postfrontal air mass features subtropical characteristics with MIMIC-TPW confirming that by anomalous high PW values. The main issue will be weak mid-level lapse rates with this kind of air mass, also indicated by POES soundings over the N-Atlantic. EZ and GFS disagree in CAPE build-up and this time I would prefer to work with GFS BL moisture for that outlook (surface dewpoints ~2 K higher compared to EZ with readings in the upper tens). Yesterday's synop reports from NW France already had dewpoints in the upper tens and a similar air mass should cover S/C-UK and Ireland today. Therefore, ~500 J/kg MLCAPE might be a realistic scenario, with highest values along the surface warm front itself, where deep BL moisture exists and offsets diurnal mixing. Strong veering of the wind field is forecast with strong directional and speed shear ( 150-200 m^2/s^2 and 20 m/s DLS). Forecast soundings and model data show LCLs below 800 m with some LL buoyancy present, so any existing thunderstorm will be able to take profit of a helical inflow. An isolated tornado event may occur with isolated thunderstorm development. Main uncertainty right now, if thin CAPE profiles and warm EL temperatures really support electrified convection. For now a 15 % lightning area was issued. Despite a low-end tornado risk, no level 1 for that hazard was issued. However, tornado chances increase in case DMC probabilities do the same. Any existing thunderstorm decays rapidly after sunset.

Another issue will be heavy rain over Ireland, as the surface low gradually approaches from the SW during the night and a low-end level 1 was issued for parts of Ireland. Showers and isolated thunderstorms along the warm front itself will also be effective rainfall producers (PWs up to 40 mm), but progressive nature of those convective elements should keep the rainfall risk below a level 1.