Convective Outlooks 2017

Convective Outlook 01.08.2017

deutsche Outlook-Kurzfassung (aufklappen)
CategoricalHeavy RainHailWind GustsTornado

Risk Area (sq. km) Population Larger Cities in Risk Area
20 % 867.540 110.147.556
50 % 706.701 108.331.285
Severe Threat 1 367.968 55.902.317
Severe Threat 2 455.408 68.325.164
Severe Threat 3 84.562 13.356.492

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***** A major severe weather outbreak is likely from parts of France across Germany to N Poland *****

 

A Severe Threat 3 has been issued for NE Germany and NW Poland mainly for very large hail, significant severe wind gusts, (significant) severe convective precipitation and tornadoes

 

A Severe Threat 2 surrounds the Severe Threat 3 mainly for large to very large hail, severe to significant severe wind gusts, severe to significant severe convective precipitation and to a lesser degree for tornadoes

 

A Severe Threat 1 surrounds the Severe Threat 2 mainly for large hail, severe wind gusts and severe convective precipitation

 

Synoptic Setup

An extensive upper trough is anchored over the N Atlantic with its main axis running from the British Isles to the W Iberian coastline. Ahead of it, intense WAA regime spreads across central Europe yielding deep baroclinicity and an intensifying anticyclonic curved jet streak.
At lower levels, a wavy airmass boundary stretches from S France over Germany to N Poland, marking the interface between a polar-maritime airmass North of it and a highly unstable subtropical airmass to the south. As it slowly expands northwards, warm-frontogenesis takes place across Germany mainly in the evening and night to Tuesday.
Tomorrow, the aforementioned trough loses amplitude and experiences eastward propagation as a new deep Atlantic cyclone advances from the West. As a consequence, the trough-ridge pattern as well as the sfc boundary gets more progressive. Due to substantial overlap of high potential instability and unseasonably strong kinematics, a particular dangerous situation will be unfolding on Tuesday afternoon/evening across CRTL Europe.

Mesoscale Discussion

Today, isolated DMC along the northwards moving warm front mainly influenced parts of E France and S Germany. Severe risk with these storms was marginal due to missing BL rooting and high inflow layers, respectively.
Currently, the warm front is digging northwards and DMC has already developed along the accompanying surface boundary. Although most cells may be elevated, total severe potential will gradually increase, as the overlying jet intensifies (due to jetogenesis). As also sfc and ML frontogenesis is present in those regions, a top-of-BL LLJ may develop over night, working as a moisture conveyor belt. Well organized convection such as supercells or a MCS appear most probable in a curved line from Lorraine over SW to E/NE Germany during the early morning hours. As the storm propagation is forecast to be parallel to the boundary, severe or even significant severe convective precipitation will be the primary hazard, followed by severe wind gusts (at the leading edge of the MCS) and large hail up to 5 cm.
The overnights/early morning convection has a great impact on the situation on Tuesday in terms of debris cloudiness and/or outflow boundaries. Despite this uncertainty, we decided to issue a Severe Threat 3 for NE Germany to highlight the exceptional total severe potential in this region. Let’s get more precise:

– All models (even conservative ECMWF-IFS) agree in the fact, that moderate to high and well capped instability between 1500 and 3000 J/kg ML CAPE is superimposed by a highly kinematic environment with 0-6 km DLS of 20-35 m/s (even higher values are locally possible).
– Moderate 0-3 km SRH between 150 and 300 J/kg overlap with PLDs up to 2500 m will provide enough streamwise vorticity for updraft rotation
– As the airmass boundary gets cold front character due to increasing zonal momentum, it experiences frontogenesis (see n-component of Q-Vector) which is accompanied by enough lift to reduce CIN
– Furthermore, increasing QG forcing will be correlated with additional synoptic-scale upward motion
– Even if a morning-MCS reduces CAPE over some regions, insolation south to the boundary and moisture advection/ moisture pooling will result in quick airmass recovery
– CI is forecast rather late between 15-18 UTC, so that instability is able to build up steadily

Any discrete cell that initiates in such an environment will instantaneously achieve a mesocyclone and be capable of producing very large hail in excess of 5 cm in the early stage! Steep mid level lapse rates up to 8 K and CDI values of maximum 100 J/kg/km indicate a chance for locally giant hail of nearly 10 cm in diameter mainly in the Severe Threat 3 and the sern part of the Severe Threat 2.
As strong cold pools manage to form, the primary supercell type will be the HP type. Cold pool interaction with 0-3km bulk-shear between 15 and 25 m/s and a boundary-orthogonal storm motion vector will result in a forward building MCS/squall line with bowing segments. Even a (wide spread )damaging wind event cannot be ruled out given the increasing QG lift in the evening. Generally it has to be underscored, that the chance for linear clustering/bow echo development is maximized in the northern Severe Threat 2 / Severe Threat 3, whereas the cell mode in the srn Severe Threat 2 tends to be more discrete (supercells). As a Severe Threat 3 requires widespread severe/significant severe DMC, we go with a high-end Severe Threat 2 for srn Germany. An upgrade may be necessary tomorrow.
If CI actually takes place between 15 and 18 UTC, supercells have enough time to generate low level rotation before BL-stabilization sets it. We introduced a 5 % tornado line including the chance for a significant tornado event (> F2), where 0-1 shear is maximized while LCL is low.
Convection will stay severe to significant severe concerning all hazards while shifting to Poland and the Czech Republic in the evening/night.

 

Model consistency, additional remarks

—see text—

Convective Outlook 19.07.2017

deutsche Outlook-Kurzfassung (aufklappen)
CategoricalHeavy RainHailWind GustsTornado

Risk Area (sq. km) Population Larger Cities in Risk Area
20 % 1.013.101 185.593.751
50 % 415.076 6.638.875
Severe Threat 1 288.171 48.847.235
Severe Threat 2 176.309 29.820.069

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A Severe Threat 2 has been issued for parts of the Netherlands and NW Germany mainly for large hail, severe wind gusts, severe convective precipitation and tornadoes

 

A Severe Threat 2 has been issued for parts S and Mid France mainly for large hail, severe convective precipitation and to a lesser degree for severe wind gusts

 

A Severe Threat 1 surrounds the two Severe Threat 2 mainly for large hail, severe convective precipitation and to a lesser degree for severe wind gusts and a tornado event (in its northern part)

 

Synoptic Setup

Seasonably typical weak flow pattern influences the European continent. A cut off low over the north Atlantic slowly moves southwards while a weakening trough over E Scandinavia/NW Russia stays in place during the forecast period.
In between these features, ridging coincides with a strong meridional warm air current up towards the Polar Circle (10 °C Isotherm reaches Iceland on Wednesday).
At lower levels, a warm front that is associated with a poorly defined sfc low over GB, shifts northwards across France, inducing elevated convection over the British Isles in the next hours. The warm front denotes the leading edge of a spanish plume in which DMC gets likely tomorrow from S France to N Germany.

Mesoscale Discussion

…Netherlands, NW Germany…

South to the warm front, a sfc convergence is likely to devolop in the early afternoon hours over the southern Severe Threat 2 extending from NW to SE. Moisture pooling along the convergence and evapotranspiration will cause the specific moisture to increase up to 15 g/kg. With an EML above the moist sfc layer, ML CAPE is forecast to reach 2 kJ/kg with even higher values possible depending on the actual insolation. With northwards increasing DLS between 10 and 20 m/s and 0-3 km SRH of maximally 300 J/kg next to the warm front an explosive set-up is likely over the highlighted area. Nevertheless some uncertainties arise, as CI is forecasted differently by the NWP guidance. The strongest outlier is represented by the EURO 4 12 Z which puts the emphasis to mid North-Rhine-Westphalia (and eastwards) rather than to the NW Germany like other models (e.g. WRF4km, ECMWF). Current thinking is that a strong WAA – generated cap should suppress most DMC until the afternoon. A few earlier developing elevated or high based cells, however, cannot be excluded in association with the local WAA maximum (= synoptic scale lift) over W Germany/Be-Ne-Lux. We currently go with WRF 4km and expect CI over E NL between 12 and 15 Z slowly shifting eastwards to NW Germany. The more northern the cells initiate, the greater will be the chance for them to benefit from the aforementioned SRH and become supercells. WRF 4km for example predicts a right mover over north-German plains in the evening.
Other models show a more MCS-like storm mode, which is also an imaginable scenario as MCSs preferably develop south to a warm front. Excessive rain (notice the 20% area) and large hail up to 5 cm in diameter will be the main threats, followed by a severe wind gusts (especially if a MCS evolves). We also introduced a 5 % tornado line where the moisture convergence is most likely to overlap with the enhanced SRH values in the early evening.

…South to Mid-France…

East to the meridionally orientated flow-parallel cold front, a theta_e maximum is forecast to arise north to the Pyrenees beneath the northbound advected EML. Additional moisture pooling and slight lee cyclogenesis will result in late and high-based CI (around 18 Z). With 0-6 km DLS around 20 m/s and straight hodographs several split storms may form from Toulouse to the Massif Central. The main threats will be large hail and excessive convective precipitation. More to the north (E-CRTL France), storms may also initiate but due to weaker ML lapse rates and kinematics a Severe Threat 2 is not justified here.

…Alps…

Several weakly forced storms are forecast, but due to low PW-values (up to 25 kg/m²) and weak CAPE-shear a Severe Threat is not issued. Nevertheless an isolated severe event due to large hail / excessive convective precipitation cannot be excluded.

Model consistency, additional remarks

—see text—

Convective Outlook 22.06.2017

deutsche Outlook-Kurzfassung (aufklappen)
CategoricalHeavy RainHailWind GustsTornado

Risk Area (sq. km) Population Larger Cities in Risk Area
20 % 1.182.410 213.609.815
50 % 648.127 105.499.445
Severe Threat 1 891.134 96.186.955
Severe Threat 2 341.555 64.439.599

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***** An outbreak of severe thunderstorms is likely over parts of CRTL Europe including the chance for a significant straight-line-wind event. *****

 

A severe threat 2 has been issued for northern Germany, parts of W Poland and parts of the Czech Republic mainly for large to very large hail, severe to damaging wind gusts and to a lesser extend for heavy convective precipitation and tornadoes

 

A severe threat 1 surrounds the Severe Threat 2 mainly for large to isolated very large hail, severe wind gusts and to a lesser extend for heavy convective precipitation

 

A Severe Threat 1 has been issued for the Pyrenees mainly for large hail and excessive convective precipitation

 

Synoptic Setup

The main flow pattern is governed by a broad ridge that stretches form Morocco to western and CRTL Europe, which is sustained via thickness advection ahead of a quasi-stationary W-Atlantic trough. As a result of a cut-off, a well defined PV-lobe (which until now was vortex-trapped in the trough) gets catched by the strong mean flow south to an Iceland-low and travels eastward. The PV-anomaly can beautifully be detected by the help of WV satellite imagery. It manifests itself as a sharp secondary trough in the 500 hPa geopotential field and will play a major role in the today‘s setup.

Mesoscale Discussion

…Severe Threat 2 region, CTRL Europe…

The thermodynamical analysis of the PV feature shows no phase locking either with a lower theta- or a latent generated anomaly, so that it remains constant in magnitude. Thus, it can be interpreted as a decoupled upper level low. This information is important to understand the missing of a surface low in the vicinity of the upper PV anomaly.
However, NWP guidance shows the development of a sfc pressure trough originating from the Iceland low, which, in fact, is some kind of occlusion point cyclogenesis and completely independent from the aforementioned upper PV anomaly/secondary trough. In addition, reinforced LL CAA behind the sfc low which is maximized just below the upper level PV anomaly underlines the low cyclogenesis-potential of that feature. Summarising all the stuff, there are two nearly decoupled features: the sfc low and the upper PV anomaly. This dynamical constellation makes the overall setup quite complex.

##### The following thermodynamical analysis is based on the current 12 z NWP model guidance. We point out the uncertainty of the setup and do not speculate on alternative solutions #####

 

On the western flank of the ridge, an unstable airmass spreads NE-wards over western Europe during the night, reaching Germany in the early morning hours. As GFS strongly overestimated the BL moisture today over France (Trappes: dewpoint forecast: 20 °C vs. measurement: 15 °C ), we do not trust in the exceptional CAPE values of up to 3000 J/kg that GFS forecasts for tomorrow. We think that 1200-1500 with peaks up to 2000 J/kg (where moisture pooling occurs) are more realistic. We also refer to ECMWF and ARPEGE concerning this point.

According to the general NWP consensus, first DMC is likely in the morning hours over northern Germany which is linked to the occlusion point sfc low. In a small corridor (mainly over Mid Lower Saxony), WAA-generated 0-3 km SRH coincides with increasing CAPE around 1000 J/kg. Here, e.g. EURO 4 or WRF 4 km simulate a well defined MCS.
Although such a system might be elevated, BL rooting cannot be excluded. Then, the system may quickly become severe capable of producing severe to locally damaging wind gusts (if a strong cold pool develops) and heavy convective precipitation. Although the hail risk is reduced in the early daytime, we cannot exclude a large hail event as well. Regarding Corfidi vectors, the MCS will translate towards eastern Germany where further intensification is questionable due to missing CAPE. Nevertheless it has to be mentioned, that EURO 4 simulates a strengthening of the system.
Behind the MCS, broad subsidence is likely and will initially suppress convection over northern Germany. Additionally, LL advection of pseudopotential colder air behind the sfc trough will additively support CAPE reduction over NW Germany, what is shown by nearly all models. Here, the further convective development is most questionable.

To the south, over North Rhine Westphalia, S Lower Saxony, Hesse, Saxony Anhalt, Thuringia and Saxony insolation and strong capping will result in a loaded gun setup with the aforementioned CAPE values. 0-6 km DLS increases over the day with maxima of 22 m/s in afternoon. As the upper PV anomaly approaches in the afternoon/evening, QG forcing in terms of DCVA gets a subject and rapidly reduces CIN. A moisture return is also likely over NW Germany as a consequence of the anomaly’s teleconnection to the surface wind field. A line of DMC is forecast to arise somewhere between the German/Dutch border and the Sauerland/Teutoburg Forest, subsequently shifting very fast to eastern Germany. In NE Germany, CI is more questionable and depends on the amount of CAPE that may build up / re-build up after the morning-MCS.
Depending on the actual stability/CAPE values, shear vectors and local effects (old outflow boundaries, mountain ranges), storms may rapidly cluster into a squall line / linear MCS or stay longer in a single-cell mode over some time (before finally merge to a line, too). In both cases a severe weather outbreak is likely. Severe to damaging wind events (microbursts are likely due to delta-theta-e > 20 K), large to very large hail in excess of 5 cm (steep ML lapse rates up to 7 K) and local flash floodings are the main threats. Although tornadoes are possible as well, a generally high LCL hampers the development of cloud lowering/low level rotation.
We finally decided to issue a broad severe threat 2 for most parts of northern Germany and parts of extreme W Poland.
Following facts prevented us to issue a Severe threat 3:

1. The dynamical constellation with two vertically decoupled features, what results in a stage-by-stage CAPE-removal. For a Severe Threat 3, more unique and unambiguous lifting mechanism is required

2. The overestimation of BL moisture and CAPE by GFS

3. The overall uncertainties of the setup (will the morning-MCS over N Germany actually evolve?; How much CAPE will finally (re-)develop in NW/NE Germany? – How will be the exact spatial/temporal organization of the trough/PV anomaly? How strong is the cap, will CI take place too early?)

If there is a more distinct parameter overlap a wide spread damaging wind event is possible (derecho), but at the moment we do not think so.

Over the southern half of Germany, isolated multicell storms may initiate in the early evening hours (when CIN is minimized) primarily along mountain ranges. The main threats will be large hail up to 5 cm and isolated severe wind gusts.

…Pyrenees…

Isolated storms will fire up in phase with day time heating along the Pyrenees within a moderate CAPE/ low shear environment. Main threats will be heavy convective precipitation and large hail.

Model consistency, additional remarks

—see text—

Convective Outlook 28.05.2017

deutsche Outlook-Kurzfassung (aufklappen)
CategoricalHeavy RainHailWind GustsTornado

Risk Area (sq. km) Population Larger Cities in Risk Area
20 % 355.654 97.220.308
50 % 340.946 79.285.865
Severe Threat 1 174.373 46.013.453

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A Severe Threat 1 has been issued for N France, parts of Be-Ne-Lux and parts of Germany mainly for excessive precipiation, large hail and large hail accumulations and to a lesser degree for severe wind gusts

 

A Severe Threat 1 has been issed for wrn Alpine region mainly for excessive precipitation and large hail

 

— No detailed forecast text available —

 

Morgen ist primär die Südhälfte von NRW ab den Mittagsstunden von Gewittern mit lokalem Unwetterpotential
betroffen. Prädestiniert für die heftigsten Begleiterscheinungen in Form von Starkregen > 30 l/qm und Hagel bis 3 cm (auch große Hagelansammlungen möglich) sind dabei die Mittelgebirgsregionen. Aber auch weiter nördlich sind Gewitter möglich, die aber tendenziell ein geringeres Risiko für Unwetter aufweisen.

Für Unwetterwarnungen verfolgt morgen bitte die amtlichen Warnungen des Deutschen Wetterdienstes.

© Sturmjäger NRW 2011 - 2017


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