Storm Forecast

Storm Forecast
Valid: Tue 19 Nov 2013 06:00 to Wed 20 Nov 2013 06:00 UTC
Issued: Tue 19 Nov 2013 01:25
Forecaster: TUSCHY

A level 1 was issued for Scotland and N-UK mainly for strong to severe wind gusts.

A level 2 area was issued for the N-Adriatic Sea, SE Italy, the E part of the Adriatic Sea and the E-coast of the Ionian Sea mainly for excessive rain and an isolated tornado/large hail event.

A level 1 surrounds the level 2 areas mainly for an isolated tornado risk, excessive rain and strong to severe wind gusts.

A level 1 was issued for S Sardinia mainly for an isolated tornado risk.

SYNOPSIS

A strong upper trough over NW Europe continues to amplify south. A decaying upper low over the C-Mediterranean gets caught by this approaching trough and a constant NE-ward motion of this decaying upper low is expected during the forecast. A strong cyclogenesis occurs N of Scotland and a confined but healthy depression exists over the W-Mediterranean. Both features are described more in detail below. Strong high pressure over E-Europe keeps the weather calm.

DISCUSSION

Regarding the strong depression over the far W-Mediterranean:

00Z data confirms that the surface pressure is down to 998 hPa over NE Spain, 997 hPa over the Balearic Islands and 995 hPa just south of France (offshore data). With westerly boundary layer winds still present over the Balearic Islands, the depression's center is still located to the north, which is supported by latest IR loops (although thick cloud shield lowers confidence regarding the exact position). Latest phase diagrams now agree in a shallow warm-core structure with that feature, so we would not be surprised if the expected pressure of some high resolved models verifies (just shy above 990 hPa). Expected forecast track of this vortex takes it over increasingly warmer SSTs of 19-20 C next to the Balearic Islands and up to 22 C further to the SE. Hence I would not be surprised if this vortex bottoms out below 990 hPa while crossing the Islands. Despite aforementioned supportive ingredients, much needed cold air at mid-levels seems to be a bit diplaced to the north and east of the depression's core, and a marginal warm-up trend continues at mid-layers through the forecast. However, it would be a surprise if this feature won't keep its strength (at least) until it approaches Sardinia from the west during the afternoon hours. A gradual weakening trend is then forecast due to a rapid drop of SSTs west of that Island and also during the passage over Sardinia before regaining some strength over the Tyrrhenian Sea, where SSTs increase once again. Another landfall is anticipated over S-C Italy around midnight before entering the cooler C-Adriatic Sea during the end of the forecast period.

The main risk with that feature will be a confined area with strong to severe wind gusts, which runs from south of Sardinia to the Tyrrhenian Sea to S-C Italy. Strength of LL winds, LCLs below 800 m and some SBCAPE/LLCAPE also points to a somewhat enhanced tornado risk with onshore moving showers/thunderstorms. The tornado risk seems to be maximized over S-Sardinia (noon and afternoon) and around Napoli (Italy, 00-06Z). An isolated strong event can't be ruled out. For now a level 1 was issued due to ongoing uncertainties regarding strength and final track of that depression.


... S-Italy, Adriatic Sea, the Ionian Sea and adjacent areas ...

An eastward creeping cold front will be the focus for scattered to widespread DMC activity mainly during the daytime hours. Ongoing extensive cluster of storms keeps going from the overnight hours and approaches the E coast of the Adriatic Sea during the afternoon hours and the E coast of the Ionian Sea during the evening hours. Embedded in a 20 m/s southerly LLJ, a prolonged period of moist inflow from the south is forecast with ML mixing ratios in excess of 10 g/kg and effecitve PWs in the upper 20s and lower 30s (mm). With strong convergence along that front and ongoing diffluence aloft, the stage is set for slow storm motions with some back-building tendencies anticipated. How fast the front pushes east remains a bit unclear in model fields, which determines the final amount of precipitation. Right now we think we could see one precipitation maximum over the N-Adriatic Sea with a gradual shift towards W-Slovenia during the day, another one over S-Italy and a third along the E-coast of the Adriatic Sea (e.g. Montenegro and Albania). Despite those uncertainties, we upgraded all areas to a broad level 2, as more than 100 mm/24 h could be a distinct possibility ... especially along the westward facing slopes of N-Albania and Montenegro. The least confidence exists with the part over the N-Adriatic Sea, given limited time before the cold front arrives.

We also expanded the level 2 area towards W-Greece, where back-building storms could also result in excessive and flash flood producing storms. A tornado risk also exists from the N-Adriatic Sea all the way down to W-Greece (E-coasts of the Adriatic and Ionian Sea), as onshore moving storms will have a temporal overlap of decreasing CAPE and increasing frictional LL shear. In addition, isolated large hail is possible, especially with tail-end storms.

During the end of the forecast, DMC probabilities also increase over the Aegean Sea, where 400-800 J/kg MLCAPE and 15-20 m/s DLS overlap. A few organized multicells could bring gusty winds and heavy rain to NE Greece and W-Turkey. Timing of CI remains unclear and hence only a 50-% lightning area was added for now. The main activity should start after 06Z.

... Scottland and parts of UK, 00-06 Z...

Rapid cyclogenesis is forecast SE of Iceland and N of Scotland during the overnight hours. This deepening rate is driven by impressive dynamics (e.g. left exit of a 55 m/s jet stream at 500 hPa). Despite rapid advection of higher theta-e air towards the center, no warm seclusion process is yet anticipated ( which will affect this forecast area during the following day ! ). Therefore, our main concern will be the southbound racing cold front. Warm ELs and meager prefrontal moisture lower confidence in more than isolated storms. However, intense forcing and some low-end SBCAPE should assist in the development of a forced line of shallow convection, which races to the south. Postfrontal pressure tendencies don't look that strong mainly due to the rapidly developing back-bent occlusion to the north. Nevertheless we expect strong to severe wind gusts as postfrontal winds at 850 hPa increase to 25-30 m/s. A level 1 should cover that event due to a rapid weakening trend onshore and deteriorating environmental conditions for a more severe and widespread convectively induced wind event. Also, confidence in more than spotty lightning activity is quite low.

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