Storm Forecast
Valid: Wed 21 Jun 2017 06:00 to Thu 22 Jun 2017 06:00 UTC
Issued: Wed 21 Jun 2017 06:53
Forecaster: PUCIK / GROENEMEIJER

A level 2 was issued for Northern England mainly for (very) large hail, severe wind gusts and tornadoes.

A level 1 was issued for Central to Northern England, Southern Scotland and the adjacent Northern Sea mainly for the (very) large hail and severe wind gusts.

A level 1 was issued for Pyrenees mainly for excessive precipitation and severe wind gusts.

A level 1 was issued for Southeastern Alps mainly for large hail and excessive precipitation.

A level 1 was issued for SE Slovakia, E Hungary and NW Romania mainly for large hail and severe wind gusts, to the lesser degree for excessive precipitation.

SYNOPSIS

At 12 UTC Wednesday, at mid to upper troposphere, a ridge will stretch from Iberia towards France, S UK and W Germany. Embedded within the ridge, an ill defined low will slowly shift eastwards over the Balearic Sea and decay. To the west of the ridge, a pronounced short-wave trough will move from the Atlantic ocean towards UK. Timing and location of the trough differ from one model to another. While ICON places the trough over the UK by Thursday 06 UTC, ECMWF and GFS simulate the trough still west of UK at that point. Differences in the trough timing may prove important for the weather situation over the UK, but also for the development in the next forecast period. A deep trough will continue to reside over NE Europe with a strong flow on its flank.

Closer to the surface, a WAA regime is simulated over Iberia, W France and S to Central UK. West of the warm sector, a wavy frontal boundary will progress from the Atlantic towards E and will speed up its progress towards the end of the forecast period. A stagnant frontal boundary over the Czech Republic and Slovakia will turn into a slowly moving cold front across Ukraine and Russia.

Isolated to scattered DMC development is forecast south of the frontal boundaries, in the warm and moist airmass and along the mountain chains, such as the Alps. Several of these areas deserve a closer inspection below.


DISCUSSION

... UK ...

Models agree that low-level moisture will increase, with dewpoints above 16 deg C and lapse rates between 6.5 to 7 K/km overspreading the region, which should result in the MLCAPE values around 1500 J/kg. GFS simulates dewpoints well over 20 deg C over Central and S UK with associated CAPE values well above 2000 J/kg. Given the current dewpoint readings (as of 6 UTC Wednesday) well below 20 deg C, GFS seems too aggresive with the moisture. It is very likely that surface based parcels will remain strongly capped most of the day, given the presence of an EML aloft, with CAPE values maximizing towards the evening hours.

The flow will steadily increase at the lower and mid-troposphere. As a result, moderate to strong DLS is simulated for the late afternoon to evening hours over most of the UK. Models simulate the best shear profiles over N England, with considerable hodograph curvature in ECMWF and DLS reaching 25 m/s in all models. An environment like this is in principle conducive to well organized convection, including supercells, capable of (very) large hail and severe wind gusts. Over northern and central England, strong LLS, looping low-level hodographs, and a relatively low height of cloud base (below 1000 m) also suggest a potential for tornadoes.

The main limitation of the situation is the questionable extent and timing of initiation. Models suggest isolated initiation at best in the late afternoon and evening hours, while e.g. ECMWF suggests that widespread elevated convection occurs in the early morning to noon period over N England. Initiation of surface based storms will likely depend on the development of local convergence zones during the afternoon hours. Large scale upward motion from the approaching short-wave trough will likely be too late to considerably reduce the CIN. The presence of abundant cirrus ahead of the approaching trough may hinder surface heating and mixing of the boundary layer.

The latest model guidance nevertheless produces substantial amounts of covnective precipitation across the level 2across northern England. This is where the vertical wind profile suggest the highest potential a few long-lived supercells and where surface based initiation seems most likely.

... SE Slovakia, E Hungary, NW Romania ...

South of the stagnating / slowly moving frontal boundary, dew points around 14 deg C are observed. These values may increase slightly towards the afternoon hours thanks to the evapotranspiratio. This will yield moderate values of CAPE, around 1000 J/kg, towards the 15 and 18 UTC . At the same time, DLS will be around 15 m/s thanks to enhanced mid-tropospheric flow. Shear may be locally enhanced, especially on the southern flank of the Carpathians. Storms initiating in such conditions will organize into multicells with a transient supercell possible in the area with locally enhanced shear. Large hail and severe wind gusts will be the primary threats. Excessive precipitation is not ruled out over Romania, where moister vertical profiles may result in enhanced precipitation efficiency in the cells.

... SE Alps ...

Despite the fact that the whole Alpine range may experience thunderstorms during this day, a Lvl 1 is issued for this part of the mountain range, where models simulate DLS above 10 m/s. Thus, storms are more likely to become better organized here with enhanced threat of large hail given sufficient amount of CAPE. Heavy precipitation may occur as well with slower moving and/or larger clusters of storms.

... Pyrenees ...

Widespread initiation is forecast in the early afternoon hours over the mountain range with perhaps large clusters of storms developing. Given the slow storm motion, heavy precipitation will be the main threat. Over the foothills, deeper and relatively drier boundary layer may become conducive also for an isolated downburst event.