Extended Forecast
Valid: Thu 22 Jun 2017 06:00 to Fri 23 Jun 2017 06:00 UTC
Issued: Wed 21 Jun 2017 06:02
Forecaster: TASZAREK

This extended forecast was issued to highlight a risk for a possible severe weather outbreak over CNTRL Europe including tornadoes, very large hail, damaging wind gusts and excessive precipitation.

An upgrade to level 3 is eventual, but is strongly dependant on the following model updates.

SYNOPSIS

Broad low is located over NW Russia, while a ridge with a highly unstable airmass cover W and SW Europe. A jet streak extends from British Isles, through Denmark, Poland up to Ukraine and borders these two large features. An increased airflow in the 850-500 hPa layer overlaps with an unstable airmass over Benelux, N Germany and NW Poland. Current NWP scenarios indicate a passage of the shallow trough with a low-level jet around 15-21 UTC over Benelux and Germany.

DISCUSSION

A very steep lapse rates (> 7 K/km) originating from NW Africa overlap with a rich boundary layer's moisture (~ 12-14 g/kg) that results from an intense evapotranspiration and moisture pooling taking place over W Europe since Monday. The aforementioned combination of lapse rate and low-level moisture will create on Thursday a healthy 2500-3500 J/kg ML CAPE stretching from France up to CNTRL Germany, with a local maxima exceeding even 4000 J/kg (France, SW Germany). Current NWP scenarios indicate that this whole area will be overlapped by an enhanced DLS values (> 15 m/s) indicating that developing convective cells will have a potential of organizing into multicell clusters and supercells. The best "kinematics" are expected over Benelux and N Germany, where a low-level jet will increase LLS to 10 m/s, MLS to 10-15 m/s and DLS to 0-6 km resulting in a high values of a CAPESHEAR composite.

In the opposition the previous extended forecast, the position of the trough by GFS was considerably shifted to the west. It is also predicted to be more shallow than previously expected. These issues slightly drop severe weather potential, especially when it comes to CI. Although convergence boundaries will be still available, a weaker forcing will put more importance to the magnitude of the cap. Current mesoscale NWP scenarios indicate very late CI, around 18-21 UTC over N-CNTRL Germany, with storms moving in the late evening hours to Poland, therefore currently this area is considered to have the best potential for severe thunderstorms. However, it must be highlighted that most of the models are not very consistent in simulating the location and timing of CI, mostly due to different scenarios regarding position of the trough.

If storms will fire-up over Germany in a such thermodynamic and kinematic environment, they will quickly become severe. According to mesoscale NWP scenarios, a likely discreet supercells may be capable of producing large to very large hail and damaging wind gusts. Vertical wind profile will be not very supportive for tornadoes (low helicity, straigth hodographs), but given enhanced low-level shear with high instability and isolated character of the supercells a tornado event cannot be ruled out.

Later, when the storms will organize along the outflow, a QLCS with a damaging wind potential may form and enter Poland in the late evening hours. GFS predicts a healthy ~ 1000 J/kg ML CAPE overlapped by a strong stright-line hodograph flow in 850-500 hPa layer. A late evening derecho event cannot be ruled out, however it is less likely compared to previous NWP updates.

The most important factor that should be monitored in the following model updates is the position and timing of the trough, small changes to its position may strongly influence severe weather potential and thus final forecast. Another limitation refers also to the magnitude of the cap which with not very strong forcing may inhibit CI. Recent surface observations over France indicate also that GFS seems to overestimate dew points by 1-2 degrees and thus instability may be not as high as predicted by GFS.