Storm Forecast
Valid: Wed 27 Dec 2023 06:00 to Thu 28 Dec 2023 06:00 UTC
Issued: Tue 26 Dec 2023 22:17
Forecaster: TUSCHY

A level 2 was issued across parts of far SE-Ireland/NW-UK mainly for (extremely) severe wind gusts but also for a nocturnal tornado threat with a strong event possible.

A level 1 surrounds the level 2 with similar hazards but lower probabilities.

SYNOPSIS

An initially wavy streamline pattern over NW Europe turns more zonal already during this forecast as the steering vortex over NW Europe de-amplifies. Of interest is the breakup of this cyclone into numerous smaller scale vortices which cross Ireland/UK as filling but still healthy short waves.

Downstream ridging over CNTRL Europe and an amplifying wave over E Europe with CAA of a cold/continental air mass suppress any DMC probabilities.

An eastbound moving cut-off migrates from N Morocco to N Algeria. Cooling mid-levels and attendant lift may spark a few weak/short-lived thunderstorms.
The same just offshore of Portugal/NW Spain, where isolated WCB activity can't be ruled out.

DISCUSSION

... Ireland and UK/Scotland ...

The initial issue will be the warm jet event over UK, which weakens before noon. At the same time a new (wind)focus emerges with a rapidly intensifying cold jet over/just N of Scotland. This cold jet up to 800 hPa with top-weighted 80-90kt winds assist in hurricane force gusts just above the surface. This is not reflected in our warning sheme as this event is not accompanied by deep convection.

Of interest will be 2 time frames.

The first one evolves during the late afternoon hours from the Irish Sea into Scotland, as a NE-ward surging dry and IPV-rich mid/upper-level airmass overspreads an occluding surface (cold) front and results in a split-front structure. Forecast soundings indicate cloud tops in the -15/-20C range and an isolated thunderstorm is possible. Kinematics are not supportive for anything severe so expect mainly strong gusts.

The main time slot for severe evolves with a short-wave, which crosses Ireland between 18-21Z and affects N-UK/Scotland thereafter. This wave approaches with a reinforced surge of better LL moisture (occlusion) beneath cold mid-levels and an IPV-rich/dry mid/upper airmass. It's of no surprise to see rather healthy conditions for DMC activity with those background conditions.

The main uncertainty evolves with the wave's exact placement and its geometry as regionally up to 8 hPa intra-numerical MSLP difference is still present in latest NWP guidance.
Dependant on its geometry, expect one or two rounds of convection to pass those areas from SW to NE. The main focus resides along the occlusion but could also evolve next to the center of the wave.

Thermodynamics are marginal, but sufficient for DMC activity and reside in the 200-400 J/kg MUCAPE range. Kinematics are very good but differ in magnitude as GFS 12Z indicates pockets of 90kt plus winds at 850 hPa over the Saint George's Channel (21-00Z) with other models remaining more in the 65-80 kt range. Long and curved hodographs are forecast and expect severe to a few extremely severe gusts with any convection, which manages to build a strong/persistent downdraft into the well mixed BL. With a few NWP solutions one could even argue about a wind-driven level 3 due to a rather robust/closed convective line, but low confidence in final DMC coverage and uncertainty in BL gust parameterization of certain (and very aggressive) models preclude this step for now.

The other concern arises regarding abundant streamwise inflow into any updraft up to 2 km AGL within an unstable BL and with some 0-3 km CAPE. Hodograph appearance depends on the wave's structure but a few solutions show rather impressive curved/elongated hodograph signatures. Low LCLs and a semi-discrete storm mode both increase the concern of organized mesocyclones and an attendant tornado risk. Even a strong event cannot be ruled out. The combined wind/tornado risk sparked a confined level 2 upgrade for the area of most concern. Some modifications of the level 2 placement may become necessary due to the mentioned uncertainties with the short wave.