Storm Forecast
Valid: Sun 30 Sep 2018 06:00 to Mon 01 Oct 2018 06:00 UTC
Issued: Sat 29 Sep 2018 19:04
Forecaster: TUSCHY
A level 2 was issued across the CNTRL/N Aegean Sea and surrounding regions mainly for excessive rain.
A level 1 surrounds the level 2 mainly for excessive rain and a few tornadoes.
A level 1 was issued for S Turkey mainly for excessive rain (SW Turkey) and a combination of excessive rain and large hail (S-CNTRL Turkey).
SYNOPSIS
For most of Europe a convective-wise quiet forecast period will be in store. A progressive high-index flow regime over N-Europe turns more wavy in response to a deepening trough over NW Europe. The attendant trough will be placed over the North Sea-Benelux-N France during the end of the forecast. A cut-off over S-France gets caught up by this trough, lifts to the NE and dissolves until 06Z.
Yet another upper short wave approaches the S Adriatic Sea and the Ionian Sea from the N and evolves into a broad upper trough over Greece during the night.
A progressive ridge moves from CNTRL Europe rapidly to the east and will be analyzed over the E Ukraine during the end of the forecast. This pattern results in dry and cool conditions for most areas north of the Mediterranean excluding a tongue of better BL moisture ahead of the deepening trough, which affects the Baltic Sea and regions further east. Meager mid-level lapse rates however only cause marginal SBCAPE and low-end lightning probabilities.
The warm-core cyclone over far E Greece is forecast to drift N over the CNTRL and N Aegean Sea. The aforementioned consolidating upper trough over Greece will push this vortex gradually to the north although a diminishing vertical extent of that vortex also results in decoupling from the steering mid-level flow and a potential drift more to the NE during the night. In addition the eastward moving anticyclone to its north may decrease the northward movement of the cyclone during the day but should not be able to suppress the general lifting trend towards NE Greece. Models diverge regarding the vertical extent of that vortex and how intact it re-emerges over the Aegean Sea. Hence substantial differences regarding track speed and direction exist.
***For detailed information on the track and intensity of the synoptic-scale wind field of 2018M02, please refer to specialized product (Mesoscale discussion) as long as they will be issued***
DISCUSSION
... E/NE Greece and W-Turkey ...
As the depression moves offshore over the Aegean Sea, drier and cooler air is forecast to advect offshore from N/CNTRL Greece and into the southern periphery of the vortex. This decrease in low-tropospheric moisture supports a general decoupling from the plume with warm/humid and unstable air to the south and a general decrease of the LL moisture content in the SW/S quadrant of the vortex. Despite the intrusion of the drier air, the rest of the vortex (NW/N/NE and E quadrant including the center) still reveals deep and very moist profiles. Forecast soundings in these quadrants show a deep layer with high moisture content, elongated but marginal CAPE profiles and still enhanced shear especially in the lowest 3 km in response to the weakening warm-core cyclone. Hence despite meager CAPE, the ingredients for an effective warm rain process continue with a large warm cloud layer, the already mentioned moisture and the favorable LL wind field for training convection. Hence the risk exists for flash flooding - potential significant on a regional scale. In addition elongated and curved hodographs in the NE quadrant continue to indicate an augmented tornado risk especially over NE Greece and NW Turkey.
Given ongoing model uncertainties, we decided for another day with a rather large level 2 area to highlight the enhanced flash flood threat.
Further south (between Crete and SW Turkey) the plume of warm/moist and unstable air resides. Weak CIN and mesoscale convergence zones support numerous thunderstorms. CI gets enhanced during the forecast by a passing short-wave. SBCAPE up to 1kJ/kg and weak shear favor slow moving thunderstorms with heavy rain. Repeatedly onshore moving thunderstorms may result in local flash flooding over SW Turkey. In addition, slightly enhanced LL shear along the coasts and better LLCAPE indicate a waterspout threat for this area. Hence the level 1 was expanded to this region.
...S Turkey ...
A mainly capped but very unstable air mass resides over the E Mediterranean with MLCAPE in the 1-2 kJ/kg range. The main trigger for isolated convection will be the passing short-wave and some models show CI as far east as Cyprus and SE Turkey. DLS in the 20 m/s range, robust CAPE and deep moist profiles in the lowest 3 km favor organized multicells/isolated supercells with large hail and excessive rain. Due to the unertainty of coverage and model discrepancies regarding CIN we expanded the level 1 far east but did not upgrade any area to a level 2 for now.
...S France ...
Beneath the decaying upper low, southerly low-tropospheric winds advect better moisture north beneath cold mid-levels and hence steeper mid-level lapse rates. 100-400 J/kg SBCAPE evolve and may support a few thundery showers with isolated graupel and gusty winds. This activity grows upscale into a disorganized cluster with heavy rain including a few embedded thunderstorms. This activity lifts NE over Switzerland towards far S Germany during the night with a gradually weakening trend due to decreasing CAPE. Nothing severe is forecast.
