Storm Forecast
Valid: Tue 31 Dec 2013 06:00 to Wed 01 Jan 2014 06:00 UTC
Issued: Mon 30 Dec 2013 22:33
Forecaster: TUSCHY

A level 1 was issued for SW Greece and offshore areas mainly for heavy rainfall amounts.

A level 1 was issued for parts of Ireland, W Scotland/UK and offshore areas mainly for severe wind gusts. An isolated funnel/short-lived tornado event is possible.

SYNOPSIS and DISCUSSION

The retracted and exceptionally busy pattern over the N-Atlantic continues as another polar vortex approaches E-Canada. In fact, this vortex seems to be the most powerful vortex for this winter season, as 500 hPa heights drop to 481 gpdm and 850 hPa temperatures level out at -37°C! Despite an increasing ensemble spread in numerous oscillation indices, no serious change in the 'roaring 2013/14's' over the N-Atlantic is expected in the near future.

Latest OPC analysis places a weakening confluent trough with a filling 990 hPa surface vortex over the N-North Sea. A triple-barrel low with 966 hPa centers covers a broad swath from 45°W to 15°W and 40°N to 70°N. This feature will affect Ireland, UK and Scotland during the forecast with windy and wet conditions and there are a few features to talk about:

Extensive N-S aligned cold front already crosses Ireland at 06Z and affects UK/Scotland thereafter (probably leaving UK to the east at roughly 18 Z). Placed in the unfavorable part of that diffluent trough, not much activity is expected convective-wise. Even the most optimistic models show only patches of marginal CAPE along that front and like yesterday we expect some kind of narrow cold-frontal rainband to accompany the wind shift with gusty winds. Constant weakening of that front is forecast.
The post-frontal air mass is characterized by rather cold mid-levels with 500 hPa temperature falling to -30 °C or less. Active post-frontal sector with rather deep convection (already visible in latest WV/IR images) spreads east and affects the area of interest between 09 and 00Z. Embedded in this brisk westerly flow regime is a distinct trough axis, which affects Ireland around noon and Scotland thereafter. This time period will probably be the main one for more organized convection. Forecast soundings show 300-400 J/kg SBCAPE and 150-180 m^2/s^2 SRH-1 along that trough. Enhanced LL CAPE exists offshore/along the W-coast of Ireland, so next to enhanced downward mixing of 25 m/s winds at 850 hPa an isolated and short-lived funnel/tornado event seems possible. We would not be surprised to see a rather solid line of enhanced convection to affect W/NW and N-Irland around noon into the early afternoon hours. The level area was also expanded towards W-UK, where onshore moving thunderstorms could see a confined temporal overlap of modest (onshore) SBCAPE and increasing frictional BL shear. A short-lived funnel/tornado event is possible next to the wind risk.

During the overnight hours the risk of enhanced and deep convection gradually abates from W to E, as deep WAA sets in from the W ahead of another powerful depression. I don't want to spend too much time on that feature, as the system (or let's say, the most interesting part of that system for convection) remains west of Ireland. Nevertheless there are clear signals of a pronounced tropopause fold and intrusion of high/low tropospheric/stratospheric air, which could overspread the trailing and nearly E-W aligned cold front. Quite robust CAPE signals are forecast and this could result in deep and potentially electrified convection within an environment of 30 m/s near BL winds. Severe wind gusts will be the main hazard, but of interest is a not often seen offshore overlap of roughly 400-800 J/kg SBCAPE and 200 m^2/s^2 SRH-1. Most likely we will never know if a tornado/waterspout occurred with that set-up. Nevertheless, the level 1 was expanded to the SW especially for the enhanced and convectively induced severe wind gust risk.

A strong 1030 hPa (plus) high pressure area remains anchored over E-Europe. A small-scale cold-core low still affects Sicily and parts of the Ionian Sea. Slow moving showers/thunderstorms will be the main hazard with sleet, gusty winds and an isolated waterspout risk (although LL CAPE remains meager). The southern part of the warm conveyor belt (WCB) could see some improvement for convection during the overnight hours, which would affect far S-Greece. Some training convection is possible. The slowly eastward spreading WCB brings heavy (stratiform) rain to S-Italy and parts of W/S Greece ... repeatedly onshore moving showers probably cause high rainfall amounts over N-Sicily, too. Only SW Greece and offshore areas were highlighted by a level 1 as confidence in enhanced convection is more elevated compared to the other areas.

With that we close the chapter for 2013. We wish everyone a safe and eventful night of New Year's Eve and a Happy New Year.