CONVECTIVE FORECAST
VALID Sat 22 Oct 06:00 - Sun 23 Oct 06:00 2005 (UTC)
ISSUED: 21 Oct 16:43 (UTC)
FORECASTER: TUSCHY

SYNOPSIS

Strong depression west of Norway will shift northeastward along a SW-NE inclined baroclinic zone under the influence of a weakening short wave trough...Models agree in development of an elongated low-level pressure channel from Norway towards Finland during the latter part of the outlook...Weak upper-level trough will arrive over western Europe during the forecast period with accompanying weak frontal system, separating a warm and more humid airmass over the Mediterranean area from cooler and stable air, which arrives from the Atlantik...Isolated storms ( especially topographically forced ) should develop....Convection-wise the most interesting system will be a weak short wave trough over the central Mediterranean area, moving NE-ward during the early noon hours, providing enough energy for increasing convection development.

DISCUSSION

...central and southern Italy, Adriatic Sea, including coastal areas ...
Northeastward moving short wave trough will trail a warm and humid airmass over the Adriatic Sea... Storms will continue over Italy from the night hours, moving NEward... GFS forecasts pretty high instability values (SBCAPE of more than 1KJ/kg) but current thinking is that NMM with its more conservative values of about 500J/kg will be more reasonable...Weak cyclogenesis( with slightly higher shear values) will cross the Adriatic Sea during the early morning hours while pool of colder mid-level airmass should arrive a few hours later ( separation of best kinematic/thermodynamic areas)....Strong offshore flow along the eastern coastal areas (Adriatic Sea) will help to disequilibrate the atmosphere well inland...Despite the pretty weak background-flow, models show enhanced low-level shear values ( especially during the first part of the forecast period) along the coastal areas (SEE TEXT area)... This should locally enhance the severe weather threat ( isolated tornadoes / severe wind gusts ), although threat should be too marginal for issuing a categorical risk.

Most parts of central and southern Italy will be under a diffluent upper-level flow with 300hPa upper-level divergence chart showing broad area of positive values... This, combined with a moist and weakly capped environment should help for widespread TSTM development...Maturing storms will pose a risk for isolated severe wind gusts, otherwise kinematic parameter will be too weak for widespread storm organisation

...Gulf of Finland...
Short wave trough will cross the highlighted area with cooler mid-level airmass...in fact...forecast sounding of Helsinki shows pretty steep low-level lapse rate during the passage of the system...This combined with about 300J/kg sould be more than adequate for isolated to scattered TSTM development...Very strong signals in the LL shear ( up to 12m/s) present, so threat will exist for marginal hail/ isolated tornadoes.

...Eastern France and extreme SW Germany...
Prefrontal tongue of warm and humid airmass should be advected well northward ( reaching parts of western Germany )...Arriving frontal boundary ( arrival in the area of interest during the late afternoon/early evening hours), right rear entrance region of departing upper-level jet and topographically forcing should help for isolated to scattered TSTM development....Low level shear values up to 12m/s and strengthening DLS ( reaching 20m/s+ during the night hours) should yield a fine kinematic environment for some storm organisation...Current thinking is that low instability values and separation of decaying instability tongue/strengthening wind field would pose a risk for only a few strong-severe storms with a marginal hail/severe wind gust risk...however one or two tornadoes can't be ruled out, given the strong LL shear.

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