Storm Forecast
Valid: Sun 26 Feb 2017 06:00 to Mon 27 Feb 2017 06:00 UTC
Issued: Sat 25 Feb 2017 22:10
Forecaster: TUSCHY

A level 1 was issued for an area NE of Scotland mainly for severe wind gusts.

A level 1 was issued for extreme S-Italy and offshore regions mainly for excessive rain and a few waterspout events.

SYNOPSIS

A very cold air mass with 500-1000 hPa thickness values of less than 490 gpdam and 850 hPa temperatures below -35 °C are currently analyzed over NE Canada. A piece of that air mass shifts to the SE and moves over increasingly warmer SSTs over the NE-Atlantic. In response to that cold air mass, an extensive and strengthening cyclonic vortex evolves south of Iceland and it gradually approaches Ireland, Scotland and UK. Numerous impulses circle that vortex and bring unsettled conditions to NW Europe.
During the evening hours, one impulse exits UK to the NE, later N. Numerical models agree in rapid development of that low once it moves offshore beneath a diffluent upper-level flow regime and beneath the left exit region of a strong upper-level jet. GFS, ICON and UKM agree in the development of a shallow warm-core low with a strengthening low-tropospheric wind field. Would not be surprised to see a well structured, compact low with a rather symmetric wind field between Scotland and the Faroe Islands during the end of the forecast.

Initially zonal flow pattern over CNTRL/N-Europe becomes more wavy as weak ridging develops downstream of the approaching trough. The progressive ridge crosses Germany during the day and shifts east towards Poland during the night. A strong cut-off low south of Italy remains more or less in place although a gradually eastbound motion is forecast which should bring the center of the cut-off towards the Ionian Sea during the end of the forecast.

DISCUSSION

... Sicily, Malta and east to the Ioanian Sea ...

Quasi-stationary upper low will be the focus for scattered thunderstorms. Before noon, thunderstorms are forecast to evolve somewhat more disorganized over onshore and offshore regions. Weak DLS precludes organized convection and heavy rainfall due to slow storm motion will be the main hazard. A few waterspout events are another possibility with augmented LL CAPE and weak BL flow. Thereafter, the center of the cut-off intensifies a bit and builds down to the lower troposphere. This results in a strengthening 850 hPa wind field of 20-30 kn east and north of the vortex. Responding enhanced BL flow and moisture advection (plus NW-ward expanding CAPE tongue) should cause a line of slow moving storms with excessive rain. Right now models keep most of that activity offshore. If numerical models take this vortex a bit more to the north, excessive rainfall may be possible over parts of far S-Italy. We added a small level 1 for the region with highest risk and best agreement of model data. An isolated waterspout event remains possible within the lightning areas.

... West of Ireland and SW-UK ...

Active postfrontal marine convection is forecast during the night as T500-SSTs spread increases. A few short-lived thunderstorms are possible - strong wind gusts and graupel remain the main hazard.

... NE of Scotland ...

Aforementioned shallow warm-core low will be another focus for isolated thunderstorm activity after midnight. Models like GFS indicate an overlap of dry mid-level air atop a tongue of more humid BL air mass, which advects towards the depression's center. We added a level 1 due to the risk of severe wind gusts as the 850 hPa flow strengthens to 25-30 m/s. We did not expand the northern part of the level 1 all the way to the end of the 15% lightning area as the BL air mass of the depression's inflow becomes disturbed betimes by S-Norway with lowering BL mixing ratios.