Storm Forecast
Valid: Fri 27 Sep 2013 06:00 to Sat 28 Sep 2013 06:00 UTC
Issued: Thu 26 Sep 2013 20:18
Forecaster: TUSCHY
A level 1 was issued for Portugal and parts of W-Spain mainly for excessive rain, a few tornado events and isolated large hail/strong to severe wind gusts.
SYNOPSIS
A deep cold core low over NE Europe remains in place as numerous impulses circle the main vortex. Dry BL air is present beneath that feature, so deeper updrafts remain confined to the offshore/coastal areas, where vertical temperature gradient increases/helps to boost SBCAPE values a bit. Another cold core low is situated west of the Iberian Peninsula with a slow motion to the east/southeast. This will bring the trough onshore over N-Portugal/Spain during the overnight hours. Ridging in-between weakens with a gradual decrease of mid-level heights expected. Warm and mostly stable conditions prevail over most of the Mediterranean.
DISCUSSION
... Portugal and W/NW Spain ...
An healthy looking atmospheric river signal is forecast to approach SW Portugal from the SW with TPWs in the mid 40s (mm) , comparable with a TWP anomaly of near 200 %. Forecast soundings reflect that moist environment with deeply saturated profiles and a warm low to mid-level (temperature) nose (equal to the characteristics of a subtropical air mass). Mid-level lapse rates however improve a bit as a mid-level thermal trough approaches Portugal from the W (beyond 12Z). For most of Portugal, MLCAPE will be in the range of 300-700 J/kg with higher values towards the southern part of Portugal where abundant BL moisture spreads ashore.
Final shear magnitude (especially in the lowest 2 km AGL) heavily depends on timing and strength of short-waves which emerge of the base of the vortex. Most models however show some LL backing and therefore enlarged/curved LL hodographs. LCLs aob 700 m and SRH-1 up to 250 m^2 s^-2 would create a favorable set-up for tornado activity, but the main concern will be how long any storm will stay discrete. A prolonged period of lift of a very moist and weakly capped air mass should favor clusters of showers/thunderstorms during the forecast. However, any break in the clouds/precipitation could temporarily/regionally boost CAPE values and probabilities for a few more discrete storms. Even without discrete activity, complex interaction of storms and orography could assist in a few spin-ups. 15-25 m/s DLS also favors organized mutlicells/isolated supercells, so we would expect a few tornado events with that set-up. Isolated large hail and strong to isolated severe wind gusts will be possible, too. Finally, excessive rain is likely with effective warm rain process profiles seen in forecast soundings. Training or repeatedly onshore moving showers/thunderstorm should enhanced the flash flood risk over all of Portugal.
During the night, showers and isolated thunderstorms still affect the highlighted area, but low CAPE values should keep the severe risk low. We don't want to rule out an isolated tornado event along the W-coast of Portugal with slighty better LLCAPE profiles and adequate shear.
Also during the night, elevated thunderstorm activity occurs over W/NW France and probably also over SW UK and parts of Ireland. Despite marginal hail and heavy rain, no severe risk is anticipated. Still, storm motions of 5 kt and high TPWs point to an isolated excessive rainfall risk especially over far NW France.
...E-Baltic Sea ...
Organized lines/streets of showers and isolated thunderstorms could evolve with cyclonic flow atop. High LL CAPE values and signals of healthy mesoscale convergence zones increase confidence for some organized lake effect bands. However, temporal backing/veering and slight mid-level warming should keep the stage of organization on the moderate side. A few waterspout events are well possible, but this risk remains too diffuse for issuing any level area for now.
