Extended Forecast
Valid: Mon 16 Oct 2017 06:00 to Tue 17 Oct 2017 06:00 UTC
Issued: Sat 14 Oct 2017 18:59
Forecaster: TUSCHY

A level 1 was issued for parts of NE-Sweden und N-Finland mainly for severe wind gusts.

A level 1 was issued for parts of Ireland, UK and Scotland mainly for severe wind gusts.

SYNOPSIS

An extensive high pressure area with a pronounced positive 500 hPa geopotential height anomaly (compared to the 79-00 climatology) remains in place with a gradual drift to the east until 06 UTC. Weak waves circle that dome of high pressue but the lack of an overlap of ingredients precludes thunderstorm chances for most places.

DISCUSSION

... NE-Sweden and N-Finland ...

A vigorous depression crosses N-Scandinavia from W to E and sets the stage for a deep onshore cyclone. An intense zonal cross-barrier flow (over the Scandinavian mountains) with subsidence in the lee results in rapid drying of the post(cold)frontal air mass. Somewhat augmented LL moisture in the thinning warm sector won't be enough for CAPE development due to poor mid-level lapse rates and constantly lowering BL moisture. A very confined corridor with favorable upper dynamics beneath the left exit of a powerful 70 m/s upper-level jet and meager CAPE due to the ineraction of an eastward shifting dry slot atop the surface fronts (occlusion point) may result in a line of forced and shallow convection. Forecast soundings support elevated warm front convection near the triple point and slim chances for low-topped convection just along the intersection of the dry slot with the front(s). We therefore issued a small level 1 area with low lightning probs due to ongoing model discrepancies. In general postfrontal winds may approach the severe limit with favorable LL mixing and increasing LL lapse rates, but this risk is not accompanied by DMC activity.

... Ireland, W-UK and Scotland...

During the start, a powerful warm seclusion is placed west of the Bay of Biscay with an high-end near BL flow next to the center (850 hPa winds in excess of 50 m/s). Phase diagrams support the idea of an ongoing transformation from tropical to post-tropical during the forecast period although a deep warm-core could persist until this cyclone approaches Ireland during the day from the SW. Intense vertical shear likely results in an enhanced vertical tilt of the warm core betimes but baroclinic dynamics (including a coupled jet configuration) delay any rapid weakening. Numerical guidance still indicates the idea of extratropical transformation (e.g. development of a pronounced vorticiy advection couplet and rapidly increasing baroclinic gradients (fronts) in the surrounding area of the cyclone) but confidence in how fast this transformation occurs remains low.

Between 06UTC and 12 UTC, a surge of dry low-stratospheric air wraps cyclonically around the cyclone's center, probably ending the stage of a potential offshore sting jet event but with an ongoing cold conveyor belt jet event. This jet finally approaches Ireland beyond noon from the SW. This extreme wind field remains confined to the immediate region south of the center and probably avoid of deep moist convection. Hence no level assessment for this event. This scenario is dependant on how fast the extratropical transformation occurs but latest guidance continues to support the painted idea.

This surge continues during the day and the dry air at upper-levels circles the depression's center. Numerical guidance (especially limited area models) show the development of numerous vorticity filaments spreading N/NE. A rapid change of lift and subsidence accompanies this activity (similar to moats in tropical cyclones). In combination with the natural broadening of the depression's wind field during extratropical transformation, the overlap of dry air atop the warm/moist marine layer results in a broad area over E-Ireland, Scotland and W-UK with slim chances for enhanced convection (especially along the interface of the dry slot and the northward fanning warm conveyor belt) . A few convective-allowing models indicate an increase in convection during the noon/afternoon hours in the highlighted area.

Forecast soundings show low LCLs and intense LL shear (30 m/s 0-1 km shear and SRH-1 in excess of 400 m^2/s^2) . This is enough for rotating updrafts with an isolated tornado risk. The limited depth of the CAPE layer (up to 750 hPa) with the temperature in excess of 0°C probably keep the chance for lightning activity on the very low-end side due to lack of substantial graupel. Still a low probability lightning area was added for parts of the level 1, where best chances for sporadic lightning activity exist. Beside the tornado risk, downdrafts may bring severe wind gusts to the BL, which can occur well inland.

Right now the gradient wind risk will dominate the general severe risk and hence we will stay with a broad level 1 area for now. With respect to the further development of the hurricane and a potential delayed extratropical transformation (given current intensity boost to a major Cat.3 hurricane), a decrease or increase of convective probabilities is well possible. Please stay informed with your national weather agencies.

Elevated convection with a strong differential WAA regime occurs from the Bay of Biscay to UK and isolated thunderstorms are forecast. A strong inversion at 850 hPa should suppress downdraft momentum and hence limit the severe risk to near zero (excluding an isolated hail risk due to 400-600 J/kg MUCAPE and 20 m/s DLS). A few thunderstorms are also possible over Portugal, parts of Spain and N-Morocco. Heavy rainfall remains the main risk with this activity.