Convective Outlooks 2015

Convective Outlook 13.08.2015

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Validity Date:

  valid from: 13.08.2015, 12:30 UTC
          to: 14.08.2015, 06 UTC

issued : 13.08.2015, 12:30 UTC

  Forecaster: Armin K., Sebastian B.

 

A Severe Threat 2 has been issued for S France mainly for large hail, isolated severe wind gusts und to a lesser extend for tornadoes.

A Severe Threat 1 has been issued for N France, Be-Ne-Lux and W Germany mainly for excessive precipitation including flash floodings, small to medium size hail and to a lesser extend for strong wind gusts and tornadoes.

 

Synoptic Setup

Ahead of a long-wave trough sfc cyclogenisis takes place over northern France in response to day time heating and slight QG forcing. With the maximum theta e values, a tight convergence travels from France to Germany whereas the original cold front remains rather ill defined over wrn France with a weak theta e-gradient.

Mesoscale Discussion

…Southern France…

An active MCS has formed along a upr short wave over S France. With ML CAPE around 1500 J/kg and enhanced 0-6 km DLS of 20 m/s well organized tail-end storms currently affect the region north of Avignon . These storms may include supercells capable of producing large hail and isolated severe wind gusts. Later on, after airmass recovery with a well defined EML, a second wave of severe tsms will influence soutehrn France with a total severe potential around 12 %. Thus we intruduced a Severe Threat 2 for that area.

…Northern France, Be-Ne-Lux, Western Germany…

To the North of the forecast area there are similar CAPE values (1000-1500 J/kg) but a less kinematic environment with DLS of 10-15 m/s. DMC that initiates at the aforementioned convergence will thus be of pulse storm-nature, but also a few multicells are possible. According to High res model data an organization into a broken line is likely. Due to backward and sideward storm builds, training effects with heavy rainfall and flash floodings are forecast. Also a few hail events (1-3 cm in diameter) will occur. As ll flow strengthens in the evening and SRH strengthens in the evening a tornado event cannot be ruled out especially if storms remain sfc based for a longer time.
Later on the storms will get a more westerly propagation component and effect wrn German during the night hours. With no insolation the severe potential will gradually weaken.


 

 

Model consistency, additional remarks

Except for the exact position of the convergence and the location of CI, the situation is simulated consistently.

Convective Outlook 07.08.2015

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Validity Date:

  valid from: 07.08.2015, 09:30 UTC
          to: 08.08.2015, 06 UTC

issued : 07.08.2015, 09:30 UTC

  Forecaster: Armin K., Mike W.

 

A Severe Threat 1 has been issued for N France SW Belgium and W Germany mainly for large hail isolated flash floodings and to a lesser degree for strong wind gusts.

A Severe Threat 1 has been issued for S France mainly for isolated large hail.

 

Synoptic Setup

A well amplified trough initially ranges from Greenland to the British Isles. While shortening its amplitude during the forecast period, it slowly shifts eastwards. Upstream, a generally subtropical airmass as advected NE wards with 20 °C isotherm affecting most parts of Mid Europe, especially Germany.

Mesoscale Discussion

At lower levels a poorly pronounced cold front, which orientates along the European continental west coast, remains quasi stationary due to parallel upr flow. The prefrontal situation is rather complicated with several convergence zones and thermal meso-lows residing in the warm sector. One of that thermal lows currently affects NW Germany with weakly organized elevated morning convection. Severe potential is quite low with these storms and isolated heavy rainfall is the main threat. Another meso low has been detected over Mid France with help of moisture transport vectors. It will move to the north and arrive over N France at ~18 UTC.

….. Northern Severe Threat 1…..
Intraday, a plume of indifferent mid level lapse rates (~ 7K) emanating from Spain will be shifted NE-wards increasing overall potential instability in the highlighted area. As a consequence CAPE is forecast to reach 1000 -1700 J/kg with highest values over srn France, east Belgium and Luxemburg.
Main CI is expected over N France at the second meso-low in the evening hours. But also further east isolated DMC could occur as remnant outflow boundaries/convergences of the first low interact with secondary mountains such as Eifel or Rothaargebirge.
Strongest (multi)cells will develop over N France due to best CAPE shear overlap (0-6 km DLS ~20 m/s) and move over SE Belgium to swrn Germany during the early night hours. As 0-3 km SRH increases up to 200 J/kg, even one or two supercell storms are not unlikely. In the updraft dominant stage cells may easily produce large which is also the main threat. Thanks to PW values > 40 kg/m² local flash floodings could occur, too. Due to generally high LCLs (dry boundary layer) and weak 0-1 km LLS the tornado potential is below 2 %
During the night hours storms may cluster and form a (linear) MCS as shown by WRF 4km. Then the main thread shifts to isolated severe wind gusts.

….. Southern Severe Threat 1…..
With strong day time heating and steep mid level lapse rates isolated storms will form along the Pyrenees and move to the northeast later on. Large hail will be the main hazard.


 

 

Model consistency, additional remarks

GFS has backpedaled the set-up in the last model runs significantly mainly over Belgium and wrn Germany. Therefore we go with a high end Severe Threat 1 in that area at the moment with an optional nowcast -upgrade if the situation becomes more explosive.

Convective Outlook 24.07.2015

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Validity Date:

  valid from: 24.07.2015, 09 UTC
          to: 25.07.2015, 06 UTC

issued : 24.07.2015, 09:00 UTC

  Forecaster: Armin K., Mike W.

 

A Severe Threat 1 has been issued for southern Germany and parts of the Alps mainly for medium size to isolated large hail and excessive precipitation.

A Severe Treat 1 has been issued for the northern half of Germany, Be-Ne-Lux and extreme east France mainly for strong to severe wind gusts and to a lesser degree for small to medium-size hail and excessive precipitation. An isolated tornado event cannot be ruled out.

A Severe Threat 1 has been issued for southern France and Genoa mainly for medium-size to isolated large hail and excessive precipitation.

 

Synoptic Setup

Today and tomorrow parts of Europe face a synoptically interesting and unusual weather situation. A sharpening short wave/ IPV Max, initially located over the European Atlantic coast rapidly translates NE wards. At lower levels, a phase-shifted Theta-Anomaly starts to interact via tele-connection with the IPV Anomaly resulting in a baroclinic development with a deepening sfc low. That cyclone is forecast to evolve well defined Shapiro Keyser characteristics with a back-bend shaped double-warm front and a warm sfc core. Until Saturday noon that core experiences a pressure drop of more than 10 hPa to a pressure of 993 hPa (12 UTC). As a consequence a notable pressure gradient will form south the the sfc low.
Convective-wise, the main focus lies on the warm-sector of the evolving depression which encompasses wide parts of Mid Europe as well as on the approaching cold front which basically has some kind of complex double structure.

Mesoscale Discussion

…Southern Germany, Parts of Switzerland…

In the WAA regime of the Shapiro Keyser Cyclone, instability will slowly arrange due to increasing BL moisture and steepening mid level lapse rates. The highest ML-CAPE of around 2000 J/kg is forecast over southern Germany where Foehn effects may significantly contribute to higher vertical temperature gradients. Here, one should keep in mind that such orographically induced instability often goes along with a very dry boundary layer so that moisture supply is quite low.
Nevertheless we expect high-based CI during the afternoon in the vicinity of (secondary) mountains. With relatively weak DLS values around 10 m/s storms will not be well organized,but medium-size to large hail and ample hail accumulations as well as local flash floodings could likely occur.

…Mid- and Northern Germany, Be-Ne-Lux, extreme east France…

As the sfc low approaches in the evening hours kinematics will become more favourable for organized convection. The whole forecast pattern gets very complex in matters of the frontal structure of the cyclone. Therefore one should distinguish between three foci:

1. If the convection over southern Germany clusters and shifts to the North it experiences increasing DLS up to 20 m/s and enhanced 0-3 km SRH values. As a consequence it may organize into an MCS capable of producing local flash floodings and isolated severe wind gusts.
2. During the early night hours a LL convergence is forecast to travel trough the warm sector affecting extreme E France, Be-Ne-Lux, NW Germany and later on Mid and N Germany. That convergence will take over the role of the former cold front that follows at the end of the forecast period. Despite relatively low instability of 200-600 J/kg CAPE, linearly organized convection is likely due to strong LL flow up to 20 m/s in 850 hPa and notable QG lift. Strong to severe wind gusts will be the primary hazards, especially if bowing segments form as shown by WRF 4 km.
3. As usual in case of Shapiro Keyser cyclones the main cold front is overwhelmed by the dry intrusion so that weather activity is rather low. Anyway according to WRF, some convective storms could also initiate at the original cold front and the back-bend occlusion, respectively. This will be the case over Be-Ne-Lux and extreme west Germany during the morning hours. Again isolated severe wind gusts are likely.

…southern France and Gulf of Genoa….

CAPE amounts of 1000-2000 J/kg and high BL moisture result in tropical soundings. According to EURO 4 DMC will initiate along the coastline in the evening hours. Due to high PW values flash floodings will be the main threat. An isolated large hail event cannot be ruled out, too.


 

 

Model consistency, additional remarks

In general the models agree in the fact that a Shapiro-Keyser cyclogenesis will affect Mid-Europe. Some uncertainties arise on the mesoscale concerning the convection over southern Germany and a possible northbound-moving MCS. As mentioned in the main text the frontal structure of the cyclone is very complex due to the interaction with the warm sector convection and the dry intrusion. A MD will be issued in the evening hours.

Convective Outlook 07.07.2015

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Validity Date:

  valid from: 07.07.2015, 00 UTC
          to: 08.07.2015, 00 UTC

issued : 06.07.2015, 22:30 UTC

  Forecaster: Armin K.

 

A Severe Threat 2 has been issued for parts of Germany, eastern France, Switzerland, extreme north-west Italy and Austria mainly for large to very large hail, severe to damaging wind gusts and to a lesser degree for tornadoes.

A Severe Threat 1 which surrounds the Severe Threat 2 was issued mainly for medium-size to isolated large hail, strong to severe wind gusts and to a lesser degree for tornadoes.

 

Synoptic Setup

On Tuesday an upper trough that moves to the British Isles supports WAA over Central Europe. As a consequence subtropical airmass returns in the warm sector of the corresponding sfc low, followed by an increase of convective activity.

Mesoscale Discussion

The main focus lies on a well defined thermal low in the area of highest theta-e in the warm sector . A prefrontal convergence is expected to form within that LL trough as it moves from France to Germany. Besides this feature a well pronounced mid level jet will shift into the forecast area, yielding clearly enhanced DLS between 25-30 m/s. The main question arises in matters of instability. At 15 UTC GFS simulates 1000-3000 J/kg SB CAPE (ML CAPE 1000-2000) with the highest values in southern Germany. In contrast ECMWF only provides 600 -1000 J/kg with some extreme maxima near the Alps. These striking discrepancies may arise from different BL moisture estimations.

In comparison with other mesoscale models, we think that the instability of GFS should be more realistic. Anyway it is rather questionable how far the plume is advected to the north and whether WAA cloudiness inhibits stronger insolation.

CI is forecast over extreme est France /swrn Germany in the early afternoon. Any cell that develops in such an environment may rapidly generate a mesocylone. This will preferably be the case over the southern half of Germany where CAPE reaches the highest values. Then large to very large hail in excess of 5 cm may occur. Due to the strong upper jet and dry mid level conditions straight line winds should also be considered. Towards the night DMC will cluster and probably form an linear organised MCS.

Further north storms may generally have a linear character, as shear vectors are more perpendicular to any intensifying cell. Less hail- but higher wind risk is expected. Even 1 or 2 bow-echos with damaging wind gust are quite likely, particularly if CAPE is as high as GFS simulates. Despite of strong LLS the tornado risk is forecast with „only“ 5% due to relatively high LCL heights . The highest tornado risk is forecast in the eastern part of the severe threat to where LCL lowering and strengthening LL flow will occur in the evening.


 

Model consistency, additional remarks

– discussed above-

Convective Outlook 05.07.2015

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*****A widespread severe weather outbreak seems to be likely especially over East-Belgium/Netherlands and northern Germany on Sunday afternoon / evening*****

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Validity Date:

  valid from: 05.07.2015, 00Uhr UTC
          to: 06.07.2015, 22Uhr UTC

Release Date: 04.07.2015, 23Uhr UTC

  Forecaster: Armin K., Sebastian B.

 

A Severe Threat 3 has been issued for most parts of Northern Germany as well as for parts of Belgium and the Netherlands mainly for swaths of damaging wind gusts (possible Derecho ?), large to very large hail and the chance for a strong tornado.

 

Synoptic Setup

The gradual amplitude-reduction of the Mid-European ridge introduces a zonalization that will interrupt the hot conditions over the forecast area. In the vicinity of a deep cyclone over the eastern Atlantic a secondary trough rapidly translates to GB. On its forward flank WAA will temporary strengthen before the associated sfc cold front finally replaces the subtropical airmass by maritime air. Then, a particular dangerous convective situation will evolve.

 

Mesoscale Discussion

The formation of the cold front along the French Atlantic coast line can be well observed by satellite data (band of alto/cirrus clouds). In the broad warm sector over Mid Europe severe convective storms developed mainly over nern Germany. For some details, see our out-running CO. The main question will be whether additional convection will initiate during the night over nrn France and Belgium. Although some models (WRF) indicate cluster development, we do not expect stronger cells at the moment (missing lift). Nevertheless one should keep in mind that any over-night convection could influence the today’s set up.
According to general model consensus we think that insolation will be nearly undisturbed. Therefore, strong sfc heating in combination with high BL moisture and steep mid level lapse rates should lead to rapid CAPE buildup with maximum values of 2500 J/kg over sw-Germany. Although the timing of the approaching cold front is quite uncertain, it will be late enough to benefit from high potential instability. As frontal theta gradient sharpens, baroclinity will result in a strong LL flow with maximum wind speeds around 17 m/s in 850 hPa. Additional lifting ahead of the frontogenetic front in combination with pressure falls will lead to the formation of a prefrontal convergence. In contrast to the last days some lifting is also generated by QG forcing (DCVA) ahead of the approaching trough. With stronger T-gradients also in higher levels thermal wind will result in a strong mid level jet which may have a little component perpendicular to the sfc front. If jet crossing takes place, progressive front propagation is likely. Enhanced mid level flow also causes DLS up to 25
CI is forecast in the early noon over Mid-France near to the cold front. Storms may rapidly become severe (supercellular) and produce large to very large hail in their initial state. With delta theta e of about 20 K and the strong upr lvl flow, severe wind gusts will soon be the primary hazards. Since the shear vectors are quite perpendicular to the front / convergence we expect that storms cluster into a quasi linear system with embedded bow echos. Even a derecho cannot be ruled out as the parameter-overlap is really astonishing. Then there will be a great chance for swaths of damaging wind gusts leading to a severe threat 3. Due to LL veering with 0-1 km SRH of 100 – 200 J/kg (even higher over nrn NL with less CAPE) and lowering LCL in the evening we introduced a 10 % tornado line. Especially if cells remain more discrete and possible overnight convection enhances BL moisture, there will be a certain chance for a strong tornado in the northern half of Germany.

 

Model consistency, additional remarks

In general NWP models show a particular dangerous situation with high total severe potential. Anyway there are some uncertainties:


1. Timing of the cold front. If the front arrives earlier, greatest severe potential will shift to the east.
2. Speed of the cold front. If the cold front, as mentioned in the main text, propagates fast, the prefrontal convection can benefit from the frontal dynamics. If the cold front moves more slowly the convergence / Squall line could easily veer away from the frontal interface and hence run into a less favourable environment
3. Debris- Cloudiness / stratiform rain from overnight convection should always be considered.

Despite those limiting factors we introduced a Severe Threat 3 because of the high damaging wind potential . Please observe the situation with particular attention!

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