Storm Forecast
Valid: Fri 19 Apr 2019 06:00 to Sat 20 Apr 2019 06:00 UTC
Issued: Thu 18 Apr 2019 15:43
Forecaster: TUSCHY
A level 1 was issued across SE Spain mainly for excessive rain.
SYNOPSIS
Ongoing blocking pattern with 2+ sigma standardized geopotential height anomalies from Germany to N-Norway causes tranquil convective weather for most parts of Europe. Cut off from the westerlies numerous slow moving / quasi-stationary upper lows get into the focus for enhanced thunderstorm probabilities - a diffuse one over far NE France/Switzerland, a strong and compact one over the Iberian Peninsula and a broad one over SE Europe.
A wavy frontal boundary extends from N Algeria to Spain with not much net motion to the east due to constantly reconstituting LL depressions between the Sirrea Nevada/de Segura and the Atlas Tellien. A heavy rainfall risk evolves with this setup. Elsewhere the augmented vertical temperature gradient serves as focus for thunderstorm activity.
DISCUSSION
... Portugal and Spain ...
The upper low drifts south a bit and hence the coldest mid-levels reside over S Portugal/Spain. There diurnal driven thunderstorm activity is forecast with peak onshore activity anticipated between noon/evening and peak offshore activity occuring during the night. Best low-tropospheric moisture/mid-level lapse rate overlap exists between the Sierra Morean and Sierra Nevada, where weakly capped 400-800 J/kg SBCAPE exists. Weak shear causes slow moving thunderstorms, which bring locally large amounts of small hail and gusty winds. A local heavy rain threat is also possible. Rapid upscale growth into disorganized clusters occurs with a weakening trend beyond sunset.
Downstream of a quasi-stationary low pressure channel/embedded wavy front (SE Spain to NW Algeria), a hot and dry air mass over N Algeria spreads north and results in nice offshore EML soundings and capped conditions. Beneath the cap, a marine air mass with good moisture advects rapidly W due to impressive low-tropospheric wind profiles along this baroclinic zone. This moisture decouples from the EML and hence there is not much CAPE to work with. However intense low-tropospheric flow with near orthogonal orientation to the synoptic front results in a long duration nocturnal heavy rainfall event with orographic enhancement. Later on during the night models hint at an evolving weak MUCAPE plume along this front, so embedded convection could locally increase rainfall amounts. Only a level 1 with a low prob. lightning area was issued as likelihood for extreme rainfall amounts should be limited by weak CAPE.
... E France, Switzerland and SW Germany ...
Below the weak upper low with cold mid-levels, improving BL moisture and good diabatic heating result in a broad area with weakly capped 400-800 J/kg MLCAPE (regionally higher). Forecast soundings reveal decent mid-level CAPE and a moist troposphere with weak shear, so initiating storms pose a risk of hail (both isolated large hail and large amounts of small hail are possible). Gusty winds and locally heavy rain accompanies upscale growing and slow moving convection. Model discrepancies continue with the strength of the LL cap, but CI along the orography seems likely and given weak nature of capping, outward surging outflow boundaries probably also spark thunderstorms in the lowlands during the evening hours. We went the medium way of ICON/IFS and added a 50% lightning area where models constantly indicated CI but also issued a broad low prob. lightning area to account for potential CI beneath the whole upper low. This activity diminishes beyond sunset although isolated elevated thunderstorms continue well into the night due to existing MUCAPE and some synptic-scale lift.
... SE Europe ...
Numerous regions with augmented thunderstorm activity exist, but weak shear and modest CAPE preclude organized convection. Onshore activity weakens around sunset. Gusty winds and graupel/isolated hail will be the main hazard.
