dimanche 21 septembre 2014
European Storm Forecast Experiment
Storm Forecast
Storm Forecast
Valid: Sun 21 Sep 2014 06:00 to Mon 22 Sep 2014 06:00 UTC
Issued: Sat 20 Sep 2014 23:02
Forecaster: PISTOTNIK
Valid: Sun 21 Sep 2014 06:00 to Mon 22 Sep 2014 06:00 UTC
Issued: Sat 20 Sep 2014 23:02
Forecaster: PISTOTNIK
A level 1 was issued for N Italy, S Austria, Slovenia, Croatia, Bosnia-Hercegovina, Montenegro, Serbia and W Bulgaria for excessive precipitation, large hail and severe wind gusts.
A level 1 was issued for E France, Switzerland, S Germany, the W Czech Republic and W Austria mainly for excessive precipitation.
A level 1 was issued for S Portugal and S Spain mainly for large hail.
SYNOPSIS
An upper-level trough rapidly amplifies from Scandinavia into central Europe. It induces a strong surface cyclogenesis over the Baltic Sea and various weak ones in the triangle between Poland, Northern Italy and Romania. A tongue of polar air erupts southward into central Europe on their rear flank and replaces the unseasonably warm and moist air of the past few days.
An anticyclone is placed over the British Isles, to its south flanked by a stationary cut-off low off the Portuguese coast. A strong zonal flow (up to 30 m/s at 500 hPa) stretches from northwestern Africa into the central and eastern Mediterranean region.
DISCUSSION
... southern Alps and central Mediterranean into the Balkans ...
Still plenty of CAPE (700 - 2200 J/kg) was present across the central Mediterranean region according to the Saturday 12 UTC soundings. However, apart from Trapani (3100 J/kg), not the extreme values of the previous days were reached any more. Low-level cooling with increasing westerly winds will likely continue to quietly reduce CAPE and reinforce the capping inversion on Sunday. Central and southern Italy will therefore only see isolated, struggling storms or no activity at all, similar to the previous days.
A higher coverage of storms is expected from the southern Alpine region into the Balkans, where CAPE will be lower (probably below 1000 J/kg) but only weakly capped, and where a series of travelling vorticity maxima facilitate convective initiation. Especially over orographic features of the Balkans, quite many storms may form with a maximum in the afternoon and evening hours. Coverage in the southern Alpine and northern Adriatic region is somewhat more uncertain, but the arrival of the main trough may keep storms going well into the night there.
With still plenty of moisture and a possibility of repeated rounds of convection, excessive rainfall is again the main hazard. In addition, deep-layer shear between 15 and 25 m/s and regionally enhanced helicity may also promote better storm organization with a risk of large hail and severe wind gusts. Distinct upper-level cooling overnight could support the formation of one or two tornadoes (either supercellular or "spout-type") with offshore convection over the Gulf of Genoa and the northern Adriatic Sea late in the forecast period.
... France into Poland and east-central Europe ...
Ahead of the cold front, the same warm and moist air will again allow CAPE up to 1000 J/kg under 10 m/s deep-layer shear. Scattered to widespread, mostly disorganized thunderstorms will form. Due to slow storm motion and precipitable water around 30 mm, once more a few flash floods are possible.
Vertical wind shear will gradually increase from the northwest with the amplification of the main upper-level trough, but pronounced cold air advection and associated subsidence in the range of the southward slipping cold front will probably kill any deep convection before it can attain any better organization.
... Denmark, S Scandinavia, Baltic Sea ...
Showers and low-topped thunderstorms under limited CAPE and mostly weak shear in the polar air mass should not pose any severe weather risk.
... Iberia ...
Warm air advection and positive vorticity advection provide increasing lift ahead of the cut-off low. Rich Atlantic moisture will be advected well inland with southwesterly winds, and CAPE is expected to build to ~500 J/kg inland and probably to higher values near the east coast. Scattered to widespread, diurnally driven storms are expected.
Deep-layer shear increases from 10 m/s in the northwest to 20 m/s in the southeast, with slightly veering profiles. This may be sufficient for multicells and a few supercells with a primary risk of large hail. In case of supercells, severe wind gusts and an isolated tornado are not ruled out, either.
Activity may propagate offshore and spread towards the Balearic Islands overnight with an ongoing severe weather risk.
A level 1 was issued for E France, Switzerland, S Germany, the W Czech Republic and W Austria mainly for excessive precipitation.
A level 1 was issued for S Portugal and S Spain mainly for large hail.
SYNOPSIS
An upper-level trough rapidly amplifies from Scandinavia into central Europe. It induces a strong surface cyclogenesis over the Baltic Sea and various weak ones in the triangle between Poland, Northern Italy and Romania. A tongue of polar air erupts southward into central Europe on their rear flank and replaces the unseasonably warm and moist air of the past few days.
An anticyclone is placed over the British Isles, to its south flanked by a stationary cut-off low off the Portuguese coast. A strong zonal flow (up to 30 m/s at 500 hPa) stretches from northwestern Africa into the central and eastern Mediterranean region.
DISCUSSION
... southern Alps and central Mediterranean into the Balkans ...
Still plenty of CAPE (700 - 2200 J/kg) was present across the central Mediterranean region according to the Saturday 12 UTC soundings. However, apart from Trapani (3100 J/kg), not the extreme values of the previous days were reached any more. Low-level cooling with increasing westerly winds will likely continue to quietly reduce CAPE and reinforce the capping inversion on Sunday. Central and southern Italy will therefore only see isolated, struggling storms or no activity at all, similar to the previous days.
A higher coverage of storms is expected from the southern Alpine region into the Balkans, where CAPE will be lower (probably below 1000 J/kg) but only weakly capped, and where a series of travelling vorticity maxima facilitate convective initiation. Especially over orographic features of the Balkans, quite many storms may form with a maximum in the afternoon and evening hours. Coverage in the southern Alpine and northern Adriatic region is somewhat more uncertain, but the arrival of the main trough may keep storms going well into the night there.
With still plenty of moisture and a possibility of repeated rounds of convection, excessive rainfall is again the main hazard. In addition, deep-layer shear between 15 and 25 m/s and regionally enhanced helicity may also promote better storm organization with a risk of large hail and severe wind gusts. Distinct upper-level cooling overnight could support the formation of one or two tornadoes (either supercellular or "spout-type") with offshore convection over the Gulf of Genoa and the northern Adriatic Sea late in the forecast period.
... France into Poland and east-central Europe ...
Ahead of the cold front, the same warm and moist air will again allow CAPE up to 1000 J/kg under 10 m/s deep-layer shear. Scattered to widespread, mostly disorganized thunderstorms will form. Due to slow storm motion and precipitable water around 30 mm, once more a few flash floods are possible.
Vertical wind shear will gradually increase from the northwest with the amplification of the main upper-level trough, but pronounced cold air advection and associated subsidence in the range of the southward slipping cold front will probably kill any deep convection before it can attain any better organization.
... Denmark, S Scandinavia, Baltic Sea ...
Showers and low-topped thunderstorms under limited CAPE and mostly weak shear in the polar air mass should not pose any severe weather risk.
... Iberia ...
Warm air advection and positive vorticity advection provide increasing lift ahead of the cut-off low. Rich Atlantic moisture will be advected well inland with southwesterly winds, and CAPE is expected to build to ~500 J/kg inland and probably to higher values near the east coast. Scattered to widespread, diurnally driven storms are expected.
Deep-layer shear increases from 10 m/s in the northwest to 20 m/s in the southeast, with slightly veering profiles. This may be sufficient for multicells and a few supercells with a primary risk of large hail. In case of supercells, severe wind gusts and an isolated tornado are not ruled out, either.
Activity may propagate offshore and spread towards the Balearic Islands overnight with an ongoing severe weather risk.
samedi 20 septembre 2014
Sep 20, 2014 0600 UTC Day 1 Convective Outlook
DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 1229 AM CDT SAT SEP 20 2014 VALID 201200Z - 211200Z ...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE UPPER MS VALLEY... ...THERE IS A SLGT RISK OF SVR TSTMS FROM LOWER MI TO NRN MO... ...SUMMARY... SCATTERED SEVERE THUNDERSTORMS ARE EXPECTED SATURDAY FROM SOUTHWESTERN LOWER MICHIGAN TO NORTHERN MISSOURI. LARGE HAIL AND STRONG WINDS WILL BE POSSIBLE. SEVERE WIND AND MARGINALLY SEVERE HAIL ARE ALSO LIKELY ACROSS MUCH OF NORTHERN MINNESOTA AND VICINITY. A LOWER THREAT OF SEVERE WEATHER EXISTS ACROSS NEVADA AND WESTERN UTAH. ...UPPER MS VALLEY... EARLY MORNING WV IMAGERY CLEARLY DEPICTS A STRONG SHORT-WAVE TROUGH OVER CNTRL AB...DIGGING SEWD IN LINE WITH LATEST MODEL GUIDANCE. 00Z GUIDANCE SUGGEST 80KT+ 500MB SPEED MAX WILL TRANSLATE INTO WRN ND BY 18Z...THEN INTO SWRN MN BY EARLY EVENING. EXIT REGION OF THIS FEATURE WILL ENHANCE UVV ACROSS THE RED RIVER VALLEY REGION AND A STRONGLY FORCED SECONDARY COLD FRONT SHOULD ADVANCE TO A POSITION OVER ERN ND BY 18Z. DESPITE THE SOMEWHAT MARGINAL MOISTURE IN PLACE AHEAD OF THIS SYSTEM...COOLING PROFILES AND STRONG FORCING SHOULD ENCOURAGE A BAND OF CONVECTION TO DEVELOP ALONG WIND SHIFT BY EARLY AFTERNOON. THIS ACTIVITY WILL THEN SPREAD SEWD ACROSS MN AS EXIT REGION OF JET ADVANCES DOWNSTREAM. DAMAGING WINDS ARE POSSIBLE WITH THIS ACTIVITY...ESPECIALLY IF LINEAR MCS EVOLVES AS A FEW HIGH RES MODELS SUGGEST. MARGINALLY SEVERE HAIL COULD ALSO ACCOMPANY THE MORE ROBUST UPDRAFTS. ...MID MS VALLEY TO LOWER MI... BAND OF STRONG THUNDERSTORMS HAS EVOLVED FROM NERN NEB INTO SERN MN AT 05Z. THIS ACTIVITY APPEARS TO BE AIDED IN PART BY ENTRANCE REGION OF EJECTING SPEED MAX THAT WILL TRANSLATE INTO SERN CANADA DURING THE DAY. WHILE CONVECTION MAY BE ONGOING AT THE BEGINNING OF THE PERIOD OVER PORTIONS OF NRN LOWER MI...MORE SIGNIFICANT CONVECTION IS POSSIBLE LATER IN THE DAY AS BOUNDARY LAYER WARMS. AS MID-LEVEL SPEED MAX EJECTS INTO SERN CANADA LOW-LEVEL FLOW SHOULD VEER ACROSS THE MIDWEST AND A CONFLUENCE ZONE SHOULD ESTABLISH ITSELF FROM EXTREME SERN WI...SWWD INTO NRN MO. GREATEST BUOYANCY IS EXPECTED ACROSS THE MID MS VALLEY WHERE SBCAPE COULD APPROACH 3000 J/KG. AS TEMPERATURES WARM INTO THE MID 80S CINH SHOULD WEAKEN SUFFICIENTLY FOR SCT TSTMS...LIKELY BY 22Z. THIS ACTIVITY IS EXPECTED TO DEVELOP WITHIN INCREASINGLY SHEARED NWLY FLOW REGIME. LARGE HAIL AND DAMAGING WINDS ARE POSSIBLE WITH ORGANIZED MULTI-CELL CLUSTERS AND ISOLATED SUPERCELLS. ...GREAT BASIN... UPPER LOW OFF THE SRN CA COAST IS EXPECTED TO EJECT NEWD ACROSS CA INTO NV BY THE END OF THE PERIOD. MID-LEVEL FLOW WILL INCREASE ACROSS THE LOWER CO RIVER VALLEY INTO THE GREAT BASIN BY AFTERNOON AND HIGH-BASED CONVECTION THAT EVOLVES ACROSS THIS REGION MAY ORGANIZE WITHIN DEEP SELY FLOW. FORECAST SOUNDINGS SUGGEST ORGANIZED ROTATING UPDRAFTS ARE POSSIBLE BUT PW VALUES ARE NOT PARTICULARLY HIGH. GIVEN THE WEAK INSTABILITY WILL MAINTAIN 5 PERCENT SEVERE FOR ISOLATED SEVERE WIND GUSTS AND MARGINALLY SEVERE HAIL. ..DARROW/COHEN.. 09/20/2014
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