CROMETEO FORUM

Storm Forecast
Valid: Mon 20 Aug 2007 06:00 to Tue 21 Aug 2007 06:00 UTC
Issued: Sun 19 Aug 2007 21:57
Forecaster: GROENEMEIJER

SYNOPSIS

Monday at 06 UTC... the major feature on the forecast maps is a trough that has its axis over the British Isles and western France.
This trough is filled with relatively unstable air and should move eastward very slowly during
the next days while evolving into a slack cut-off low over southeastern France on Monday and Tuesday. At the surface, a low-pressure system should develop in the Gulf of Genua on Monday and later the low should "jump over" the Alps on Tuesday when cyclogenesis is expected over southwestern Germany.
An intense jet streak over Ireland at 06 UTC is expected to round the base of the trough on Monday evening. Downstream, to the east of the trough, the temperature and height gradients and hence the flow is not very strong but nevertheless advecting moist, warm air northward.

DISCUSSION

Western and southern France...

In the left exit region of the intense jet forecast to cross the region during the evening, strong vertical motion is expected. This should lead to the formation of a stratiform cloud shield with unstable air in its wake in which a number of convective showers are expected. Given the very strong wind shear, convection will likely be organized into a few lines. The passage of these lines may be accompaied by gusts around the severe limit of 25 m/s. Additionally, one or two tornadoes may develop with the first convective lines that may profit from sufficiently helical inflow to support storm rotation.

Italy, Austria, north-western Balkans, Hungary, Czech and Slovak Republics, Poland, extreme eastern Germany...

Within the large warm-air advection regime over the level 1 area, moderate amounts of weakly-capped CAPE around 1000 - 1500 J/kg should be able to form in many places.
A weak shortwave trough that is initally located over north-eastern Italy and moves north-north-eastward reaching northern Poland on Tuesday morning.
Rather widespread storms should form ahead of the trough and somewhat lower coverage is expected elsewhere. The kinematic environment looks moderately favourale for severe storm development with deep-layer wind shear of about 10-15 m/s. A number of strong multicells should be possible locally causing a few large hail and strong/severe wind events. Storms should diminish after sunset expect for northern Poland where the trough will likely keep storms continuing overnight.

Storm Forecast
Valid: Tue 21 Aug 2007 06:00 to Wed 22 Aug 2007 06:00 UTC
Issued: Tue 21 Aug 2007 02:00
Forecaster: VAN DER VELDE

SYNOPSIS

A large low pressure system has settled over western Europe, with the 500 hPa core over France and a surface low over western Germany at 15Z. Warm low-level airmass advects to the west around the north side of the low, while flow over central Europe is northward towards the warm front stretching from Denmark/northern Germany to the Baltic States. A weak old cold front may be found at mid levels and low level moisture from Poland into western Romania and Bulgaria, at altitude bending over southern Italy and Tunesia.
The jetstream is found at the southeastern side of the upper low, over Spain, the western Mediterranean Sea, Italy and the western Balkan.


DISCUSSION

...W Germany, Netherlands, Belgium...

Within the moist airmass north of the center of the surface low, slightly electrified showers may develop in a low CAPE environment... very marginal instability. Combined with very low LCL in the moist air, low level shear values reaching over 12 m/s, deep layer shear over 15 m/s and SREH >200-400 m2/s2 create an environment that is potentially capable of producing a tornado, if only organized convection would manage to develop. However, the massive cloudiness seems likely to inhibit convective chances.

...NE Germany, Poland, Baltic States, Slovakia...

This is the area where largest instability is found (1000-2500 J/kg), fair amount of shear (10 m/s) and reasonable triggering are expected. Clustered storms are likely to occur, with chance of large hail, especially where more helical inflow is present (SREH enhancements in GFS, mostly along the warm front over western Baltic States). High LCL over 2000 m and too weak deep layer shear do not suggest much chance for tornadoes even with 0-1 km shear >10 m/s and considerable CAPE. Severe gusts do seem possible at isolated scale.

...Romania, Bulgaria...

15 m/s deep layer shear and significant CAPE may organize persistent storms with chance of large hail and/or a severe gust.

...Italy, Balkan coast...

A large area of deep layer shear 20-30 m/s and storm-relative helicity locally in the 200-300 m2/s2 range create possibilities for supercells to develop, despite somewhat weak GFS CAPE signals. It is expected that with cold mid level air flowing over the mountains more instability is created on the lee side than the model produces, as frequently seen before. During the evening, low level shear increases to 15 m/s over Croatia (max), and tornado chances are present along the Balkan Adriatic coast. Large hail and isolated severe gusts may occur as well.

IZVJEŠĆE POMORSKOG METEOROLOŠKOG CENTRA SPLIT DRŽAVNOG HIDROMETEOROLOŠKOG ZAVODA
DAN 22.08.2007. U 12 SATI

UPOZORENJE
Mjestimice neverin, uglavnom na sjevernom dijelu Jadrana. U noći su mogući udari juga do 35 čvorova.

STANJE
Ciklona iznad zapadne i srednje Europe pruža dolinu nad Jadran.

VREMENSKA PROGNOZA ZA JADRAN ZA PRVIH 12 SATI
Jugo 5-15, potkraj dana i u noći do 20, samo je mjestimice moguće do 24 čvora. More 2-3, u noći je prema otvorenom moguće 4. Vidljivost 10-20 km, mjestimice oko 4 km zbog sumaglice. Promjenljivo oblačno. Mjestimice kiša, pljusak i grmljavina, uglavnom na sjevernom dijelu Jadrana.

VREMENSKA PROGNOZA ZA DALJNJIH 12 SATI
Promjenljivo oblačno s mjestimičnim pljuskovima i grmljavinom, uglavnom samo na sjevernom i dijelu srednjeg Jadrana. Na sjevernom, a zatim i na dijelu srednjeg Jadrana umjeren sjeverozapadni vjetar. Drugdje umjereno do umjereno jako jugo, postupno će slabiti. More umjereno valovito, još u početku na južnom i srednjem Jadranu mjestimice valovito. Temperatura zraka bez veće promjene.

PMC - Split

VREMENSKO IZVJEŠĆE, dan 22.08.2007 u 12:00 sati

UPOZORENJE
Mjestimice neverin, uglavnom na sjevernom dijelu Jadrana. U noći su mogući udari SE vjetra do 35 čvorova.

SITUACIJA
Ciklona iznad zapadne i srednje Europe pruža dolinu nad Jadran.

VREMENSKA PROGNOZA ZA JADRAN ZA 24 SATA
SE vjetar 5-15, potkraj dana i u noći do 20, samo je mjestimice moguće do 24 čvora, sutra će slabiti i na sjevernom Jadranu okrenuti na NW 5-15 čvorova. More 2-3, u noći je prema otvorenom moguće 4. Vidljivost 10-20 km, mjestimice oko 4 km zbog sumaglice. Promjenljivo oblačno. Mjestimice kiša, pljusak i grmljavina, uglavnom na sjevernom dijelu Jadrana. Temperatura zraka bez veće promjene.

Storm Forecast
Valid: Wed 22 Aug 2007 06:00 to Thu 23 Aug 2007 06:00 UTC
Issued: Tue 21 Aug 2007 23:50
Forecaster: GATZEN

SYNOPSIS

An intense cut-off low is centered over France. A strong upper jet curves around it from southern Scandinavia to southern British Isles, around the trough axis over Iberian Peninsula, western Mediterranean, Alps, Poland, and southern Baltic Sea. A very strong jet streak is expected from southwestern Mediterranean to Austria/Poland during the period, where 300 hPa winds are forecast to reach more than 60 m/s. To the north of the cut-off, a sharp ridge connects an Atlantic high with high geopotential over east Europe.

While very warm air mass spreads into east Europe ahead of the cut-off, maritime air mass over France advects into western Mediterranean. In the range of the broad frontal zone ahead of the cut-off low, and along the northern boundary of east European warm air mass, as well as in the range of the cut-off center, rich moisture is present especially in low levels. This leads to widespread convective potential. Especially in the range of the strong upper jet, potential for a party dangerous situation is forecast during the period.

DISCUSSION

Central/northern Mediterranean, Balkans

Ahead of the cut-off low, a deep southwesterly flow advects very warm air originating from northern Africa into southern Italy, and southern Balkans. Latest soundings indicate 850 hPa temperatures of 25°C and a well-developed EML. Actually, soundings over northern Mediterranean and Balkans indicate steep low-level lapse rates and weak capping. On Wednesday, rather strong capping inversion is expected, but given very moist low-level air, instability will likely increase. Latest GFS indicates 1000 J/kg CAPE as well as LI below -5K.

East of a quasi-stationary frontal boundary from Sardinia to central Italy and northern Balkans, a rather heterogeneous moisture distribution is expected in the WAA regime, and embedded low-level boundaries like outflow boundaries of old convection, sea-breeze fronts, and orographic convergences will likely be the focus of enhanced low-level buoyancy. Given the strong upper jet streak from southwestern Mediterranean to northern Italy, and northern Balkans, QG forcing in the range of short-wave troughs and speed maxima is also likely. All together, initiation of storms is not excluded. Thunderstorms that form will likely become supercells near the frontal boundary, where vertical wind shear is very favorable given 30 m/s DLS, 10 m/s LLS, and 100-300 J/kg 0-3km SRH. Supercells are expected to produce the whole range of severe weather, including large hail, severe wind gusts, and tornadoes. Even strong tornadoes are not ruled out, although there is limited 0-1 km vertical wind shear. Further east, vertical wind shear and veering profiles are weaker, and chance for initiation seems also be weaker. Storms that may form will likely be isolated and will profit from quite high instability. Very large hail, severe wind gusts and maybe isolated tornadoes are not ruled out. Limiting factor is the capping inversion that may be quite strong, suppressing any convection in the southern regions. Thunderstorms are forecast to merge into mesoscale convective systems in the evening hours, moving to the northeast into northern/central Balkans. During the night, latest GFS shows a weak ridge spreading across the region, and thunderstorms may weaken in the southern and western portions of the risk area.

Southeastern Germany, Austria, western Alps, Czech, Slovenia, Poland

Rich low-level moisture is present that will likely remain on Wednesday given warm air advection in the range of the main frontal boundary. Latest soundings indicate steep mid-level lapse rates, and instability is likely to develop. Uncertainty exists about the capping inversion. However, current thinking is that QG forcing at the cyclonic flank of the jet streak as well as low-level forcing in the range of frontal boundaries will likely lead to initiation during the day. First storms are forecast to move across Poland in the range of a short-wave trough ahead of the jet streak, while additional thunderstorms are forecast to develop in the afternoon/evening hours further south. Favorable veering profiles and strong vertical wind shear will likely lead to rotating updrafts, and supercells are forecast, capable of producing large or very large hail. Limiting factor for high winds and tornadoes is weak low-level vertical wind shear, but a few events are not ruled out given 20 m/s 0-3 km vertical wind shear. Will exclude parts of northern Italy, and central/western Alps, where latest model output indicates cool air mass that will likely be stable. Convective activity is expected to go on during the night hours given persistent forcing. Severe potential will only slightly weaken due to slowly decreasing low-level temperature.

Northern Balkans/southern Finland to western Russia

At the northern boundary of east European warm air mass, rich low-level moisture has developed as indicated by latest ascends. Additionally, steep low- to mid-level lapse rates are present in the warm air mass. GFS model shows that CAPE will develop in the range of the frontal boundary. Current thinking is that CIN won't be too strong. Thunderstorms are expected to develop in the range and south of the frontal boundary during the day. Given a short-wave trough that separates from the cut-off low and accelerates eastward, QG forcing and increasing vertical wind shear is expected. In the afternoon/evening hours, GFS calculates up to 20 m/s 0-3 km vertical wind shear and 150 J/kg 0-3 km SRH. Given weak CIN, high boundary-layer moisture, and favorable veering profiles, thunderstorms that form are forecast to evolve mesocyclones with the whole range of severe weather. With 20 m/s 0-3 km vertical wind shear, severe wind gusts and tornadoes are expected, while large hail can also be possible with every rotating updraft that forms. Convective activity is forecast to spread eastward during the evening/night, as upper short-wave trough continues eastward, too. Persisting QG forcing is expected, and a well-developed MCS may develop, capable of producing high winds in association with bowing lines. Rather widespread severe winds may be possible if this scenario comes true.

Southwestern Iberian Peninsula, northern Africa

Although low-level moisture is limited and capping inversion is rather strong, QG forcing in the range of the propagating strong upper jet as well as low-level forcing may be strong enough to initiate thunderstorms that will benefit from strong DLS and will rapidly become supercells. Large hail and severe wind gusts seem to be possible. Limiting factor is that instability and forcing will be relatively weak. Thunderstorms are forecast to weaken after sunset.