Extended Forecast
Valid: Fri 11 Oct 2024 06:00 to Sat 12 Oct 2024 06:00 UTC
Issued: Wed 09 Oct 2024 21:58
Forecaster: TUSCHY

A level 2 was issued across parts of far SW Spain mainly for excessive rain.

A level 1 surrounds that level 2 for similar hazards but also for a tornado threat. This level 1 also covers parts of Portugal.

SYNOPSIS

A stationary wave train with ongoing high-latitude blocking affects most of Europe. An extensive upper trough extends from the Greenland Sea into CNTRL Europe, although this trough turns more compact during this forecast with rising heights over CNTRL Europe.

An eastbound moving depression deep in the subtropics interacts with a SE-ward shifting upper PV+ anomaly, which results in a fairly rapid deepening rate of a depression just W of Portugal. This vortex encompasses numerous smaller-scale depression, which dive deep into the subtropics and cause an extensive atmospheric river event into the SW part of the Iberian Peninsula. Attendant active warm conveyor belt assists in rising and gradually blocking height anomalies over N Algeria/Tunisia into the CNTRL Mediterranean. As a net-result, this extensive trough stays offshore/W of Portugal for quite some time. The depression could acquire some shallow warm core signatures atop +1C positive SST anomalies.

An extensive cold front over E Europe runs into a broad depression over SE Europe, which becomes one focus for enhanced DMC activity. Otherwise the strong depression W of Portugal sends a warm front ashore with a very moist postfrontal warm sector airmass spreading into S Portugal/SW Spain (EFI IVT in excess of 0.8 with positive SOT values in the SW parts of the Iberian Peninsula). The cold front probably stalls just offhsore and supports an ongoing enhanced moisture transport towards Portugal/Spain.


DISCUSSION

... Portugal and Spain ...

A dangerous and potential life threatening and convectively driven (flash) flood event is possible especially in the level 2 area.

As the warm front lifts NE deep into S/CNTRL Portugal/Spain, the postfrontal warm sector/atmospheric river event features rich BL moisture with mixed BL mixing ratios in excess of 14 g/kg and effective PWs in the 30-45 mm range (increasing to the SW).

A strong S-erly BL flow just W of the Strait of Gibraltar meets more backed onshore flow over far S Spain. NWP guidance remains persistent in showing an extensive and long lasting low/mid-tropospheric convergence zone next to Seville into Cordoba. The final placement of any QPF maximum remains still uncertain that far out as subtle changes in the height/MSLP fields cause this convergence zone to drift N/S. Hence a rather broad level 2 area was added. Within this level area, a strong unidirectional flow regime with rather strong BL inflow assists in slow moving and temporarily training convection. MUCAPE in the 500-1000 J/kg range and deep warm cloud layers in the 3.5-4 km range are already supportive for efficient rainfall rates. In addition, a few semi-discrete supercells are possible in the WAA regime, as friction increases LL hodograph curvature a bit. Rotating storms would also be very efficient in offering impressive rainfall rates.

Expect 24h rainfall amounts in the 80-150 l/qm range with a few higher peaks certainly possible. Flash flooding and flooding in general is likely with that amount of rain next to very high hourly rainfall rates in training segments. The average rainfall amount for October in Sevilla is 57 l/qm with 2015 having a peak of 111 l/qm/month (AEMET). In addition, very dry conditions could induce enhanced surface runoff.
So to summarize, the general setup points to an augmented risk for numerous significant and potential life threatening flash flood events in the level 2 area. The risk continues beyond 06Z.

In addition a tornado event is possible with onshore moving convection.

The level 1 was expanded deep into Portugal, where slow moving convection poses an hazard for heavy rain with a few flash flood reports next to an isolated tornado threat.

... SE Europe ...

Prefrontal moisture advection ahead of the eastward shifting cold front assists in a belt of 500-1000 J/kg MUCAPE, which should remain capped beneath a plume of 850 hPa temperatures in execss of 15C. Hence the focus shifts more to the postfrontal part, where numerous thunderstorms erupt in a weak MUCAPE and 15 m/s DLS environment. AR-ChaMo is rather aggressive for hail but is more focused on the capped airmass. Nevertheless some hail in stronger updrafts is possible with pockets of 600 J/kg MUCAPE. For now a broad lightning area should cover that risk.