Extended Forecast
Valid: Thu 27 Sep 2018 06:00 to Fri 28 Sep 2018 06:00 UTC
Issued: Tue 25 Sep 2018 19:50
Forecaster: TUSCHY

SYNOPSIS and DISCUSSION

This extended outlook was mainly issued due to the augmented probabilities for a gradually developing warm-core cyclone south of the Ionian Sea.

Anticyclonic Rossby-wave breaking is en route over SE Europe with the main vorticity lobe departing east over E Europe, leaving behind an elongated and gradually constricting PV streamer over the Ionian Sea and Greece. This cut-off process is accompanied by rather chilly mid-levels, which overspread the warm Ionian Sea (latest readings of the Advanced Very High Resolution Radiometer indicating SST values in the 26-28 C range with increasing values to the south). The main source for LL cyclogenesis is found along a tightening baroclinic zone in the SE quadrant of that mid/upper vortex.
IFS had persistent signals of this scenario during its past few runs and latest model suite including GFS and ICON joined that scenario with ongoing zonal variations of where LL cyclogenesis may occur. However all models place the evolving LL vortex in the same synoptic environment, which
would indeed support vigorous intensification as proposed f.ex. by IFS due to

- a tightening/deep temperature gradient
- persistent QG forcing along the E quadrant of the cut-off
- improving upper divergence over the baroclinic zone especially during the night
- a northward advected pool of warm/moist air with SBCAPE in the 500-1000 J/kg range, interacting with the evolving vortex.

Following past hints on phase diagrams, a developing shallow warm-core cyclone is possible. Still ongoing augmented but gradually weakening DLS next to the PV streamer and probably pulsating convection (also displaced from the vortex' center) would support a gradually structuring shallow warm-core structure until 06Z. This idea is supported by most models with GFS the less aggressive and IFS the most aggressive one (excluding fine-mesh models). But even the IFS has a tilted LL/mid-level vortex with warm-core signatures mainly below 700 hPa at 06 UTC, so confidence increases that until 06 UTC nothing more than a shallow warm core structure is the most likely outcome - if at all (probabilities for a deeper warm-core cyclone increase beyond 06 UTC).

The vortex will push N/NW, east of the southbound moving cut-off. The final track remains uncertain with ongoing 500 hPa ensemble spread regarding the path of the cut-off but also due to uncertainties how deep/structured the vortex will be until 06 UTC. For now no level area was issued with mentioned uncertainties, a weakening CAPE/shear overlap betimes and a rainfall threat, which stays mainly offshore until 06 Z. Crete will be monitored for an upgrade if the vortex shifts more east (which is constantly proposed by GFS).

Beside that vortex a zonal/progressive flow pattern affects N Europe. Embedded in this flow regime is a mid-level wave, which crosses S Norway/Sweden and results in an eastbound moving LL cyclone over S Sweden. The depression's intensity still differs in global models with good agreement with respect to its track. The evolving warm sector is characterized by a modified marine air mass from the North Atlantic. Rather warm mid-levels keep mid-level lapse rates on the weak side and hence no substantial CAPE build-up is currently anticipated in an otherwise favorable kinematic environment. Further model runs will be evaluated regarding the magnitude of overlap of forcing/low-end CAPE and shear along the cold front for potential enhanced probabilities of severe wind gusts in a dynamic/forced setup. For now only low-end lightning probabilities were added, including also the Baltic States.

Another low-end lightning area was issued for parts of NW Russia, where isolated short-lived and non-severe thunderstorms evolve beneath cold mid-levels and due to ongoing but weakening/ eastward departing QG forcing. This threat diminishes beyond noon.

A 50% lightning area was issued for Portugal, where a retrograde moving cold-core vortex interacts with a plume of subtropical air over the far W Iberian Peninsula and further offshore. The weakening vortex adds only marginal synoptic-scale lift to this setup and surface based CI seems difficult with augmented CIN. However active mountain convection is expected. Weak DLS and 500-1000 J/kg MLCAPE support slow moving convection with excessive rain the main risk. GFS remains reluctant with the onshore moisture whereas IFS and ICON continue to show coherent signatures of robust CAPE and weak shear. N-Portugal may see a confined upgrade to a level 1 in the SWODY 1.
Another high probability lightning area for non-severe convection was issued over S Spain and N Morocco.