Storm Forecast
Valid: Sat 03 Feb 2018 06:00 to Sun 04 Feb 2018 06:00 UTC
Issued: Fri 02 Feb 2018 21:48
Forecaster: PISTOTNIK
SYNOPSIS
A pronounced long-wave trough at 500 hPa continues to move slowly eastward across Scandinavia, central Europe and the central Mediterranean. A new vorticity maximum ejects southward from Ireland to Spain on its back side and re-invigorates the main trough. By the end of this forecast period, a large trough with cool polar air and various cyclonic centers at the surface as well as at mid-level will fill much of the European continent. The strong and deep SW-erly flow at its forwards flank gradually recedes into southeastern Europe.
The main cyclone at the surface moves from north-central Italy (Sat 06 UTC) to the Belarus/Ukraine bordering area (Sun 06 UTC). It has formed in a strongly baroclinic environment and will probably deepen to a minimum pressure close to 990 hPa until Saturday night.
DISCUSSION
... S Italy, S and central Adriatic, W coast of Balkans ...
In the warm air advection regime ahead of the main mid-level trough, the previous two days' environment of limited CAPE (a few hundred J/kg), large-scale lift, strong vertical wind shear and pronounced veering is still in place. Friday 12 UTC soundings from Decimomannu, Pratica di Mare, Trapani and Brindisi confirm an overlap of CAPE up to 300 J/kg with impressive wind profiles with respect to convective organization.
Convection will likely go on into this forecast period. Similar to the day before, discrete, well-organized offshore storms may bring severe wind gusts, tornadoes and moderately large hail. When moving onshore in S Italy and along the W Balkans, excessive precipitation becomes the main risk. The main focus is on Montenegro and Albania, where another 100 to 200 mm may accumulate in the present forecast period on top of already extreme precipitation overnight. Embedded storms may occur anytime and flooding of extraordinary, possibly life-threatening dimensions must be expected! These areas are covered with a level 2. A level 1 are issued for Croatia and S Italy, where rainfall and thunderstorms will fade after the passage of the main trough axis, and for W Greece, where increasingly heavy rainfall is expected but an involvement of thunderstorms is uncertain.
... inland Balkans ...
An impressive low-level jet (20-30 m/s at 850 hPa) advects extremely mild and moist Mediterranean air far inland into the Balkans and even Ukraine in the warm sector of the vigorous cyclone. Starting from the night before, an overlap of patchy CAPE and pronounced synoptic lift may enable isolated to scattered thunderstorms near the cold front even in inland areas.
Maximum temperatures up to 20C and dewpoints around 8C are expected in Serbia, parts of Bulgaria and southern Romania. Storms may therefore become surface-based in the 09 to 15 UTC time frame and could then fully benefit from strongly sheared, veering wind profiles. Any storm that forms in this environment could easily mix down severe wind gusts to the surface, and a level 1 is issued for this risk. Marginally severe hail or a brief tornado are not ruled out in case a storm manages to stay discrete and turn supercellular. The severe weather risk will diminish as soon as storms turn elevated, but the low probability line is extended far northward to account for an advection of "convective debris" deeply into Romania.
... N Adriatic Sea ...
WRF and UM limited area models predict the quick evolution of a mesoscale low in the weakly sheared, postfrontal environment on Saturday morning that should linger until Saturday night and attain warm-core characteristics. Other models exhibit little to no signals for such a development and there are doubts that it could develop indeed that quickly, but a low probability lightning area and a level 1 are issued to be on the safe side. A close monitoring of the latest surface observations is recommended in order to avoid surprises.
