Valid: Thu 10 Aug 2017 06:00 to Fri 11 Aug 2017 06:00 UTC
Issued: Wed 09 Aug 2017 00:01
*** This extended forecast was issued to highlight a possibility of severe weather outbreak including very large hail, damaging wind gusts and tornadoes in the corridor from Austria to CNTRL Poland ***
Please note that due to a lead time of 2 days, 50% lightning probability line was not drawn.
SYNOPSIS and DISCUSSION
On Thursday a warm and moist, highly unstable tropical airmass will be covering area between Italy and Poland. A low within much cooler polar airmass will be placed over France. On its W, E and S flank low will be surrounded by a moderate jet stream which will party cover unstable airmass over Poland, Czech Republic, Austria and N Italy. Exit region with a difluent flow is expected to be located over Czech Republic and Poland. NWP scenarios predict two shallow lows to develop over Poland and Austria. Such configuration provides that all ingredients for severe deep moist convection are avaiable. Thunderstorms developing in the convergence zones in a healthy environment of 1500-2500 J/kg ML CAPE will benefit from 20-25 m/s DLS. Convective cells developing in this conditions will be able of evolving quickly into multicell clusters and supercells with a potential of producing very large hail (steep lapse rates in mid-troposphere) and tornadoes (presence of low-level jet and a good vertical veering wind profiles, > 300 m2/s2 SRH). Persistent lift, decent instability and strong kinematic wind field (> 20 m/s at 700 hPa) may support linear systems with embedded bowing segments where damaging wind gusts will be likely, especially in the late afternoon and evening hours when shear will increase and convection will have time to organize. Given convective environment and synoptic scale setup, a derecho event cannot be ruled out. According to current mesoscale NWP scenarios, CI is the most likely in a corridor from Austria to CNTRL Poland. Global models predict a passage of the shortwave trough within the same area. A large MCS should form in the evening hours and move NEwardly through Poland during nighttime hours. Impressive values of PW (40-50mm) indicate that heavy precipitation causing local flash flooding should be also involved in the risk, especially over mountain areas. Main severe thunderstorm activity should develop in the wam sector of the boundary and remain active until nighttime hours, especially over Poland. Some of the NWP scenarios indicate developement of two severe thunderstorm clusters, one over Czech Republic and Poland, and second over Austria and NE Italy.
This forecast has few limitations that during the next 2 days may strongly modify the scenario. These concern:
1. Timing and the amplitude of the troughs which will be crucial in terms of CI, its placement and convective mode.
2. CIN values that may inhibit convective activity.
3. Cloud cover in the morning hours, which may limit diurnal heating, decrease instability and complicate CI. It may also shift the main activity more to the E.
Given aforementioned issues, it is possible that scenario in the final forecast may change and the area with the main risk (denoted by lvl. 2 polygon) may be shifted in space.