Storm Forecast
Valid: Fri 07 Jul 2017 06:00 to Sat 08 Jul 2017 06:00 UTC
Issued: Thu 06 Jul 2017 15:01
Forecaster: TUSCHY
A level 2 was issued for CNTRL-/N-Spain and extreme SW-France mainly for excessive rain and large/very large hail.
A level 1 surrounds that level 2 for a similar risk with lower probabilities. In addition, a few severe wind gusts and an isolated tornado risk exist.
A level 2 was issued for E-/NE-Germany, W-/SW-Poland, the Czech Republic, SE-/E-Austria, Slovakia and parts of Hungary mainly for severe wind gusts, large/very large hail, excessive rain and a few tornado events.
A level 1 surrounds that level 2 for a similar risk with lower probabilities.
A level 1 was issued for SW-/S-Germany, the W-Alps, Switzerland and CNTRL-/E-France mainly for large hail, a few severe downburst events and locally excessive rain.
SYNOPSIS
A zonal flow regime persists, as the frontal zone extends from Ireland over Germany to the Ukraine. Embedded progressive but stable waves translate east during the forecast:
The first wave exits the Benelux area during the start of this forecast and exhibits its most pronounced cyclonic curvature for this forecast. Weakening/flattening of this wave occurs as it drifts to the east. It crosses Poland until 00Z and weakens to a very subtle wave later-on over the Ukraine. This wave is accompanied by a preceding broad thermal trough which weakens/warms as the day progresses. Interaction of this wave with a very unstable air mass cause this wave to become the main player for organized DMC activity. This wave is accompanied by a weakening and opening depression. Attendant warm front runs from NE Germany to W-Romania with a slow eastbound motion, whereas the cold front over CNTRL-/SW-Germany to CNTRL-France shifts leisurely E/SE. Both fronts merge over N-Germany into a quasi-stationary occlusion.
Another and much better defined diffluent wave affects NW-Europe (including Scotland, Ireland and UK) around noon and the S-North Sea/Denmark during the night. This wave is accompanied by a much stronger high-tropospheric flow and is more dynamic than the first wave. However while the first wave interacts with a moist and very unstable air mass, the second wave won't see substantial moisture recovery in the wake of the first wave, which limits the severe/thunderstorm risk for this forecast period.
Further south, a distinct cold-core low wobbles east over the Iberian Peninsula. This feature is accompanied by a broad/diffuse LL depression, which runs from Spain to the W-Mediterranean. A robust vortex down to the lower troposphere over CNTRL Spain results in prevailing SE-erly onshore flow for E/NE Spain despite diverging model guidance of various more or less pronounced surface vortices, which could weaken/enhance the onshore flow regime on a regional scale.
Intense WAA downstream of that low brings 850 hPa temperature to 24 °C or more over the Balearic Islands and further south, which impacts CI due to stout capping.
Elsewhere over the Mediterranean, very warm/hot conditions remain in place with low-end probabilities for isolated thunderstorm activity.
N/NE-Europe remains avoid of substantial thunderstorm chances as a NW-erly flow regime results in marginal low-tropospheric moisture beneath weak mid-level lapse rates, which is not favorable for CAPE build-up.
DISCUSSION
... Spain and extreme SE-Bay of Biscay/SW-France ...
A difficult task to pinpoint CI for the far W-Mediterranean including the Balearic Islands. Upper pattern shows a diffluent and also divergent flow regime, only increasing during the forecast with a consolidating upper jet core along the S-fringe of that vortex (over S-Spain). As this jet translates east during the forecast, the region between the Balearis Islands and E-Spain resides beneath the left exit with additional upward motion. In addition, models show weak/progressive disturbances in the PVU maps, which exit the base of that vortex and cross the area of interest from SW to NE. To summarize that, background conditions for CI improve during the forecast.
For stronger low-/mid-tropospheric lift, we expect a diffuse and N-S aligned surface low pressure channel E of Spain, which features numerous stronger 0-2 km convergence maxima. In addition, ongoing moisture pooling is this confluent flow regime increases low-tropospheric moisture with LL mixing ratios exceeding 12 g/kg and with surface dewpoints in the lower twenties. Aforementioned stout cap with 850 hPa temperature in excess of 20 °C is present during the day but weakens during the night from W to E.
Hence it is not of surprise to see a loaded gun situation in forecast soundings in that area, showing capped ICAPE of 1.5-3 kJ/kg. Dry conditions persist until sunset. Thereafter however, ingredients merge for somewhat higher probabilities for a few elevated thunderstorms, especially between the Balearic Islands and E-Spain, where outflow boundaries from onshore storms could also add enough LL lift for isolated CI. 20-30 m/s DLS and expected CAPE profile indicate a risk for large/very large hail. Hence the level 1 area was expanded east/offshore with a low probabilitiy lightning area to indicate a chance for severe with any sustained updraft.
Much more straightforward is the forecast for the onshore convection over CNTRL-/NE-Spain to SW-France and the SE-Bay of Biscay. Slow motion of the steering vortex results in long-lasting and abundant low-/mid-tropospheric forcing over NE-Spain. Persistent onshore flow advects the moist marine air mass beneath the EML with 1-2 kJ/kg MLCAPE forecast. 0-6 km bulk shear in excess of 25 m/s is as favorable as the rather impressive 20 m/s 0-3 km shear due to the healthy nature of that vortex.
The only uncertainty is the rather weak cap and ongoing shower/thunderstorm activity from the previous night regarding the final degree of CAPE ... at least over parts of NE-Spain. However, persistent moisture advection should replace any residual and stable outflow air from the previous night and not much diabatic heating is needed for intense updrafts during the day.
Therefore expect CI all day long with storm intensity increasing during the noon/afternoon hours. Initiating and more discrete storms pose a large/very large hail risk with a few strong to isolated severe wind gust events. The tornado risk is more enhanced along the E-coast of Spain, where BL moisture increases, but as capping strengthens at the same time, confidence in more than a low-end risk remains low. The main hazard seems to be excessive and flash-flood producing rain with slow moving clusters. Missing compact LL jet limits the training potential, but high moisture content and prolonged lift should result in a substantial flash flood risk.
Thunderstorm clusters with isolated large hail (augmented MUCAPE with 20 m/s DLS) and excessive rain build north during the night and affect the SE-Bay of Biscay and SW-France. Any discrete and rotating cell has the chance of producing even isolated very large hail. The risk gradually spreads north until 06 UTC.
... CNTRL Europe ...
The interaction of an eastward moving subtle mid-level wave with an extensive CAPE plume causes scattered to widespread thunderstorms over a broad area. The latest guidance places the area with the highest severe risk from E/NE-Germany to SW-Poland, Slovakia, the Czech Republic, E-Austria into N-Hungary. This wave is accompanied by a surface low, which moves from NE-Germany to N-Poland, opens and decouples from the SE-ward moving upper wave...hence the weakening trend. With constant surface pressure fall to the SE of this low, a NW-SE aligned surface pressure channel with an embedded warm front crosses those regions. This results in diffuse conditions for CI over a broad area.
Major uncertainties remain how the convection from the previous night unfolds. Latest guidance indicates a nocturnal MCS event over N-/CNTRL Germany, which pushes into either far SW-Poland or the Czech Republic around 06 UTC. Also, internal thunderstorm dynamics like cold pools are not yet handled well in numerical data and hence we could see storms further east compared to what QPF data currently advertises. Anyhow, ongoing and E/SE-ward moving storms should re-intensify during the day as diabatic heating pushes downstream MLCAPE in the 1-2 kJ/kg range with decreasing values towards E-Slovakia/far W-Ukraine/Romania. Rapid BL recovery is expected upstream over E-/NE-Germany and the W-Czech Republic with diabatic heating and residual warm/unstable air mass in place. Another round of CI is forecast over NE/E-Germany after 12 UTC with storms moving to the SE.
DLS in this area oscillates around 20 m/s with a gradual decrease during the afternoon/evening hours. LL/mid-level shear is enhanced from E-Germany to Hungary with strongest values along the warm front and in the warm sector over the Czech Republic, S-Poland and Slovakia. The same for the helicity values, which are maximized near/along the warm front.
With diffuse but enough forcing for CI in place, expect scattered to widespread (ongoing) storms along the warm front and scattered activity in the broad warm sector. CAPE/shear space supports organized multicells with all hazards including large/isolated very large hail, severe wind gusts and excessive rain. With a strengthening 850 hPa to 700 hPa flow during the afternoon/evening, the risk of bowing line segments with swaths of severe wind gusts increases especially from the Czech Republic, S-Poland, Slovakia to Hungary and far W-Ukraine (either due to the re-developing morning MCS or due to new development).The tornado risk is also enhanced with a moist BL and augmented LL shear both in the warm sector but also along the warm front itself.
New multicells/isolated supercells probably evolve later-on over NE-/E-Germany all the way to CNTRL-Denmark after 12 UTC, posing a threat for large/isolated very large hail, severe wind gusts, excessive rain and an isolated tornado event. Upscale growth into an MCS is possible over NE-Germany, which runs SE along the warm front. Nocturnal BL stabilization should add a gradual weakening trend over SW-Poland and the Czech Republic.
Further west, including S-Germany, parts of E-France and the W-/CNTRL Alps, the grazing wave adds questions regarding the strength of any forcing, which affects that area. However, the SE-ward trailing cold front should be strong enough for sparking numerous thunderstorms while progressing to the S/SE. MLCAPE is maximized along the cold front with peaks in excess of 2000 J/kg. DLS will be weaker but in general in the 10-15 m/s range with the highest values to the east (SE-Germany/CNTRL-/E-Alps). Developing thunderstorms will be multicell-type with a few supercells possible over SE-Germany and E-Austria. This acitivity should organize into cold-pool driven smaller clusters betimes while drifting to the S/SE. Large / isolated very large hail and excessive rain will be the main hazard, but isolated severe downburst events are also possible, given higher LCLs and drier subcloud layers. An isolated tornado event can't be excluded due to the complex orography and locally enhanced helical flow. We included SE-Germany and E-Austria in the level 2 due to the strong mid-level flow and high CAPE environment.
The extent of thunderstorm activity remains a bit questionable especially west of the Vosges as rising mid-level heights and increasing subsidence should lower the overall risk with westward extent ... this trend is in line with weaker frontogenesis along the cold front over France. Nevertheless a few evening and overnight storms along the orography of CNTRL-France remain possible. MLCAPE in excess of 1 kJ/kg with 15 m/s DLS support organized convection with large hail, severe wind gusts and excessive rain the main hazard.
Clusters over S-Germany/the Alps may persist well into the night, but overall a general downward trend of the activity is forecast as ridging builds in from the SW.
Beyond 00 UTC, the second wave, crossing the North Sea, will insert an isolated thunderstorm risk over the S-Baltic Sea and NW-Poland with 400-800 J/kg SBCAPE and 15 m/s DLS. Isolated large hail and gusty winds will be the main hazard.
