Storm Forecast
Valid: Sat 08 Sep 2018 06:00 to Sun 09 Sep 2018 06:00 UTC
Issued: Fri 07 Sep 2018 12:26
Forecaster: TUSCHY
A level 2 was issued across E-CNTRL Spain mainly for excessive rain.
A level 1 surrounds the level 2 mainly for the similar risk with lower probabilities. In addition an isolated large hail and tornado risk exist.
A level 2 was issued across parts of the NE Black Sea mainly for excessive rain.
A level 1 surrounds the level 2 mainly for the similar risk with lower probabilities. In addition an isolated tornado risk exists.
A level 1 was issued across parts of SE Finland and areas further east/southeast mainly for isolated large hail, severe wind gusts and locally excessive rain.
SYNOPSIS
On the synoptic-scale not much will change compared to yesterday. A cold-core low over N-Denmark lifts N and later on to the NW over SW Norway with any still existing surface depression beneath the upper low filling up during the day.
Further south a quasi-stationary wave train still impacts most of the Mediterranean with a filling trough over the Iberian Peninsula and a gradually consolidating cyclonic vortex over SE Europe. In between both features a strengthening and northward building ridge allows subsidence to cover most of the W Mediterranean with resulting lower CI probabilities.
A cold and dry northerly flow regime impacts far NE Europe and suppresses DMC activity.
A cold and warm front stretch a warm sector over S Finland and over areas to the SE. This thinning warm sector is characterized by warm and moist air and hence serves as focus for thunderstorm activity.
A symmetric but shallow warm-core depression over the NE Black Sea is forecast to move ashore just east of the Crimean Peninsula and towards the Sea of Azov. The latest track forecast takes the depression's center over a region with SSTs around 20-21 C before increasing towards the coast to 23-25 C. Surface dewpoints in the upper tens and a deep column of moist air should sustain that structure during landfall and in case of the center crossing the Sea of Azov, only a slow rate of weakening is likely until 06 UTC. Weak shear at mid/upper levels but only modest upper divergence and temporal influx of drier air from the NW should keep the intensity of this feature in check which seems to be shy below subtropical storm strength (850 hPa winds of 40 kn and near BL winds around 30 kn / subtropical in case of a temporal deeper warm core structure just pior to landfall). Regardless of the final intensity and term, the organized vortex supports structured rain bands with an attendant severe risk.
DISCUSSION
... Crimean Peninsula and the Sea of Azov ...
Landfall of a potential subtropical depression (probably shy below subtropical storm strength) is forecast during the outlook. Forecast soundings show a deep moist air mass in place with elongated thin CAPE profiles. Hodographs enforce the idea of a well structured shallow warm-core low and show strongly curved signatures next to the onshore moving vortex. Combined with very low LCLs the risk of a few tornadoes/waterspouts continues. Beside that threat, excessive rain once again prompted us to issue a level 2 for this area. Given the history of heavy rain during the past days, the overall flash flood threat is substantial!
The wind gust risk depends on the final strength of the vortex and is more a gradient wind event. Still strong gusts are forecast with 40 kn not that far off the BL.
... Iberian Peninsula and the W Mediterranean ...
With the upper trough placed over Portugal and W-Spain a prolonged period of onshore flow along the E coast of Spain is forecast. This advects a moist marine air mass ashore and beneath meager mid-level lapse rates (most likely lowered due to ongoing convective overturning).
CI is early and widespread over E Spain due to favorable synoptic-scale lift and orographic lift along the mountains. Forecast soundings show elongated and thin CAPE profiles which become larger in areas with better diabatic heating (up to 1000 J/kg MUCAPE). The weak CIN/dynamic setup however favors clustering convection with excessive rain the main risk. Along the coast, stronger LL directional shear may also support an isolated tornado/waterspout event. In case of updrafts taking profit of regionally better CAPE, an isolated large hail risk is also possible.
The general thunderstorm activity diminishes over far SE Spain during the afternoon with ongoing activity further north. During the night a flare-up of thunderstorms occurs offshore as the main upper trough approaches from the W. This activity resides in an environment with better CAPE/shear (1kJ/kg and 20 m/s DLS), so large hail is also a potential risk. These thunderstorms could cluster and grow upscale into an MCS, which continues to the NE (Balearic Islands) at 06 UTC onwards.
We upgraded a small area to a level 2 for excessive rain over E-CNTRL Spain due to signals of persistent convergent BL flow and training convection. Otherwise a broad level 1 should cover the mixed severe threat.
... S-Finland and areas to the E/SE ...
A thinning warm sector is characterized by high BL moisture (maximized along the synoptic-scale fronts) and by mid-level lapse rates aoa 7 K/km. This assists in MUCAPE aoa 1 kJ/kg in an overall weakly sheared environment. Only the warm front over far SE Finland offers stronger deep-layer flow (25-30 kn), which could result in a few longer-lived thunderstorms. The main risk however will be isolated large hail and a few severe downbursts (with rather dry sub-700 hPa layer) next to excessive rain with clustering and slow moving convection. The main limiting ingredient is weak forcing with upper ridging still placed aloft. The warm front and crossing weak waves should support isolated to scattered CI and a level 1 was issued.
... Denmark and SW Norway ...
The departing upper low and residual cool mid-levels support ongoing thunderstorms with gusty winds, graupel and heavy rain. Until the afteroon hours a low-end waterspout risk continues with enhanced LLCAPE but a gradually strengthening low-tropospheric wind field should finish that risk from Denmark to SW Norway.
