Storm Forecast

Storm Forecast
Valid: Mon 14 Apr 2014 06:00 to Tue 15 Apr 2014 06:00 UTC
Issued: Sun 13 Apr 2014 21:13
Forecaster: TUSCHY

A level 2 was issued for parts of Slovenia, E-Croatia and parts of Hungary mainly for severe wind gusts.

A level 1 was issued for parts of N-Italy, Slovenia, Hungary and Serbia mainly for large hail, severe wind gusts and an isolated tornado risk.

A level 1 was issued for E-Germany, parts of Poland, parts of the Czech Republic and Slovakia mainly for an isolated severe wind gust/large hail and tornado risk.

SYNOPSIS

A sturdy ridge over SW-Europe remains in place although global models indicate a gradual weakening of the mid-tropospheric thickness values from west to east. Along its eastern fringe, an extensive longwave trough becomes established over N/CNTRL Europe. This progessive feature amplifies SE-wards with significant height falls overspreading parts of Germany, the Czech Republic, the Alps and the N-Balkan States from NW to SE. Peak negative height tendencies are forecast over Austria, the Czech Republic, Slovakia, Slovenia and Hungary. This impulse is forecast to split into two pieces over the Alps with one strong impulse lifting to the NE towards the Ukraine, while another one gradually shifts to the N-Adriatic Sea and N-Italy.
In the lower troposphere, a pronounced pressure trough/cold front with a rise/fall pressure tendency couplet affects the SW Baltic Sea and Germany during the start of the forecast with a rapid progression to the SE. Thereafter however the area with pressure fall expands rapidly to the E and assists in a broadening and less defined surface trough structure. Conform to this scenario, a widening pressure couplet and weakening convergence along the front point to increasing frontolysis. It is not a surprise that a strong NW-flow, which interacts with the Alps assists in another area with enhanced surface pressure drop along/south of the Alps. The result will be that the SE-ward racing cold front over Slovenia and Hungary becomes held off for a few hours before moving to the SE during the late afternoon/early evening hours. The front is forecast to become quasi-stationary over the N-Adriatic Sea in response to the development of a weak Italian surface depression in the lee of the Alps.
Weak pressure gradients prevail over the E-Mediterranean and with cool mid-levels atop, unsettled conditions are expected.

DISCUSSION

... E/SE-Germany, Poland, the Czech Republic and Slovakia ...

Following up yesterday's thoughts, a broken line of partially deep convection along the SE-ward racing cold front will affect the area of interest mainly in the 06Z (E-Germany) to 18Z (Slovakia and SE-Poland) time frame. We probably will also see a diurnal increase of convection with additional prefrontal heating. Forecast soundings show thin/long CAPE profiles with low LCLs and 10-15 m/s LL directional shear. With the onset of some diabating heating LLCAPE also improves. E-Germany probably has the least severe risk during the cold front passage with gusty winds and graupel. Further to the SE however, better downward mixing of 15-20 m/s at 850 hPa should increase the risk for isolated severe wind gusts over the Czech Republic and SE-Poland. Isolated large hail is possible with strongest storms. Overall, the tornado risk remains low, although the broken convective structure of that front does not rule out storms which reveal transient deviant storm motions. This would result in SRH-1 and -3 of 100-200 m^2/s^2 and an augmented risk for rotating updrafts. We expanded the low-probability lightning area all the way to the Ukraine due to the strong forcing which accompanies the NE-ward lifting impulse and also crosses the eastbound moving front. Weak MUCAPE signals point to spotty and short-lived thunderstorm activity, but no severe risk is forecast with those storms.

The postfrontal airmass over the highlighted area experiences diabatic heating and with cold mid-layer temperatures atop, 200-500 J/kg weakly capped MLCAPE evolve. DLS remains weak, but with 100-150 J/kg LLCAPE and 15 m/s LL shear available the main concern will be organized and mainly shallow convection....last few WRF runs also indicate a few deeper udprafts. Embedded mid-level impulses in this brisk NW-erly postfrontal flow regime also enhance LL directional shear on a regional scale. Hence we expect scattered and diurnal driven thunderstorms with graupel and gusty winds the main hazards. However, an isolated tornado event and large hail can't be ruled out with the strongest activity. The maximized severe risk is anticipated over E-Germany and Poland (the Czech Republic might see an adverse timing of postfrontal subsidence during peak heating). Therefore a rather broad level 1 was issued, which combines the risks of the cold front passage and the postfrontal convection.

... N-Italy, Slovenia, Hungary, S-Austria and Croatia ...

The interaction of the cold front with the Alps creates a front with split-flow characteristics due to blocking of the LL flow. A belt of 500 to 800 J/kg MLCAPE evolves from N-Italy to Hungary. The cold front should start to push to the SE during the afternoon and early evening hours. Scattered deep updrafts evolve along the cold front and take advantage of 25-30 m/s 0-6 km bulk shear. CAPE/shear overlap indicates a good chance for organized multicells/isolated supercells with large hail and severe wind gusts. Confidence in initiation over N-Italy decreases from E to W as we move deeper beneath the anticyclonically sheared side of the mid/upper jet.

Upward motion associated with upper-level divergence will take place near the front along/south of the Alps. It will aid the development of a line of storms over SE Austria, that quickly moves to the southeast across E Slovenia, E Croatia and Hungary. 25-30 m/s winds at 850 hPa should mix down to the surface, so a concentrated swath of severe wind gusts could evolve. Any more discrete storm structure continues to pose a large hail risk. Although LL shear will be enhanced along the front, marginal LLCAPE/LL lapse rates should keep the tornado risk on the lower-end side. Also, strong CAA could help to undercut updrafts rather fast. Still, with shear seen in forecast soundings, an isolated tornado risk can't be ruled out. Beyond sunset, the main risk with those storms will be gusty winds/isolated severe wind gusts and marginal hail.

Isolated thunderstorms continue over the N-Adriatic Sea, N-Italy and the W-/CNTRL Alps all night long. Limited CAPE precludes organized/long-lived convection although isolated large hail and strong wind gusts remain possible...especially in case updrafts grow deep enough to take profit of 30 m/s DLS (e.g. N-Adriatic Sea).

... N-Portugal and NW-Spain ...

With lowering thickness values, rich BL moisture (GFS/EZ agree in BL dewpoints in excess of 15 C over N-Portugal) and weak forcing of an approaching upper trough from the SW, the stage is set for scattered thunderstorms. MLCAPE of rouhgly 500 J/kg, 15 m/s DLS and steepening mid-layer lapse rates create an environment supportive for a few strong to severe pulsating storms/isolated multicells with an isolated large hail and strong downburst risk. Isolated thunderstorms continue all night long, although this activity seems to be more elevated in nature. No severe risk is anticipated with the nocturnal convection.

... Other lightning areas ...

Limited CAPE precludes long-lived convection and this activity subsides after sunset. An isolated and short-lived storm can also evolve over the CNTRL Mediterranean, but coverage remains too isolated for a lightning area.

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