Storm Forecast
Valid: Sat 09 Sep 2023 06:00 to Sun 10 Sep 2023 06:00 UTC
Issued: Fri 08 Sep 2023 09:06
Forecaster: TUSCHY

A level 1 was issued across parts of Portugal and Spain mainly for a few hail events and a low-end tornado risk over Portugal.


SYNOPSIS

The stable European blocking pattern is about the break up as the longwave trough just W of Europe weakens, but this pattern remains still intact to be classified as "ongoing blocking" for this forecast period (e.g. IFS-ENS with full member suite for blocking). Numerous impulses circle that trough and affect a broad area from Portugal to the Bay of Biscay. Further N towards UK/Scotland and Ireland, mid/upper flow features some weak mid-level cyclonic curvature, too. This is caused by another upper trough over Ireland, interacting whith the W-European trough. This configuration maintains a SW-erly flow regime over the area of interest with numerous subtle short waves passing to the NE.

Ridging extends from the W-Mediterranean to Scandinavia with the highest geopotential height anomalies still centered over CNTRL Europe. Warm mid-levels and a rather dry BL air mass (emenating from the surface anticyclone over far E Europe) keeps CI slim at best (even along the orography).

Convective-wise the most interesting feature will be the upper trough SW of Greece/SE of Sicily with an attendant weak surface cyclone (international name is DANIEL). DANIEL remains atop SSTs of 26-28C (e.g. roughly +1 K anomaly) in a rather moist environment with an area-averaged sounding of up to 60%-70 RH (300-850 hPa layer) in the N/W quadrant while decreasing to more hostile 40% in the SE quadrant. During the past few days, the main driver convective-wise was persistent LL convergence N of its center. HVIS data indicated numerous weak LL vortices emerging from this cluster while rotating S around the main center. Satellite derived shear gives a 30-40 kt magnitude with NWP area averaged data showing around 40 kt 200-850 hPa shear, which pushes drier air towards the center. A weakening PV streamer atop with a broad area of moderate vorticity and mentioned dry air was not enough to induce substantial intensification of DANIEL until now.

NWP guidance however agrees in a rather potent short wave to approach DANIEL from the NW during the day, either passing just to the S or merging with DANIEL. This wave is well captured in WV data and pronounced drying supports the NWP idea of a rather potent wave. A following second impulse is about to cross Malta durig the forecast with a rather impressive imprint in PV fields. Finally a wedge of moist air currently circles the W quadrant of the upper trough and extends from Sicily to NW Libya. This airmass could finally weaken the advection of aforemention dry air and help to support deeper and longer-lived updrafts in the vicinity of DANIEL.

Accompanied enhanced PV gradient (approaching short waves) with an evolving weak coupled jet configuration increase upper divergence over DANIEL as seen by enlarged irrotational wind vectors. Models agree in some deepening during this forecast and it has to be seen if more persistent banding close to its center sparks any subtropical classification (especially during the more favorable diurnal maximum in combination with the mentioned dynamics).
Phase diagrams indicate a shallow warm core anomaly which seems reasonable given proximity to land and a closing window for strengthening. Given improving dynamics and moisture, some more rapid organization of DANIEL's core can't be ruled out, pushing it into the strong "subtropical storm" range.

Uncertainties persist how this short wave finally interacts with DANIEL and support a spin-up of this depression. It is possible to see a more active W/N quadrant (convective-, wind- and rain-wise) with struggeling convection along its SE quadrant. Nevertheless, conditions along the Libyan coast detoriate during the day with strong to severe wind gusts the main risk until 06Z as rain bands should stay offshore durig this forecast.

DISCUSSION

... Portugal, parts of Spain into the Bay of Biscay ...

Warm conveyor belt convection is forecast, partially elevated until diabatic heating assists in more surface based conditions during the afternoon. A sharp W-E shear gradient exists but placement of this high shear zone remains uncertain (IFS W of GFS). With warmer 700-800 hPa layer over E Spain, more elevated convection with isolated large hail is forecast. With no major forcing mechanism passing by, CI should be confined to the mountains - isolated to scattered in nature.

Further W, over Portugal and NW to W-CNTRL Spain, CAPE/shear overlap seems supportive for multicell/isolated supercell convection, especially with IFS. Diurnal driven more surface based CI with some LL hodograph curvature indicates an isolated tornado risk. First round of clustering convection occurs before noon with the passage of a stronger mid-level wave but even thereafter, cyclonic SW-erly flow regime with ongoing 500 hPa Q vector convergence and onshore flow should support more strong to severe thunderstorms. For S-Portugal slight mid-level warming behind the morning wave could reduce CI.
A few strong to isolated severe storms are possible during the night, mainly with an isolated hail risk.

.. Ireland, UK ...

A plume of moist/unstable air crosses this area during the day. WAA assists in clustering morning convection with some hail and heavy rain before noon. During the afternoon hours, scattered storms develop once again in the vicinity of a diffuse frontal boundary, which acquires (at least temporarily) more cold front characteristica. CAPE/shear space supports a few storms - not enough for a level area.

In case the more bullish IFS verifies (e.g. better BL moisture), a few large hail events are possible over SW UK. Hence BL moisture has to be monitored closely during the day. Uncertainties for this remain too large for a level 1, especially as most models stay drier at BL conditions.

... N-Libya ...

Current expectation is that rain bands stay offshore until 06Z as drier S-erly inflow diminishes convection along the S/E qudrant of the depression. Nevertheless an enhanced wind gust risk exists over the Bay of Sidre and just inland with gale to hurricane force wind gusts possible during the day. ICON-EPS gives 15% for Bft 12 gusts along its W fringe (offshore). This region is out of our area of responsibility so no level was added.

... Coastal area of S Turkey ...

Weak onshore flow advects a marine BL airmass ashore beneath moderate mid-level lapse rates. A confined belt of 1-2 kJ/kg MUCAPE is possible along the coasts. Models disagree in final cap strength but most models show at least isolated CI. Any storm would pose a large hail, heavy rain and severe wind gust risk but confidence in how far inland the marine layer advects is too low for a level area.