Musings and forecasts of Andalusian weather and climate, from a veteran meteorologist.
By: esteban9, 12:47 PM GMT on August 27, 2010
I hope you're able to escape the heat...looks like even hotter temps are on the way this weekend as an upper level ridge of high pressure approaches from the west.
Since I've made a few posts about the extraordinary and deadly thunderstorms and flash flooding in Córdoba province (on 16 August), I want to call your attention to the following. Here is a meteorological post-analysis of the event by the Spanish met service AEMET (see the link to the PDF file at the bottom of the page). Warning - the document is in Spanish and is intended for true "weather geeks" like me. Nevertheless, there are lessons therein for all of us, some that might help save lives in the future. Further, this analysis, like mine, was done with the benefit of hindsight. The old adage about 100% hindsight therefore applies to both. That said...
The AEMET report states that the culprit was a "supercell" thunderstorm of tremendous rain intensity, which remained nearly stationary for several hours over the same area (the southern part of the province). The supercell was very relatively small in areal coverage (isolated), so nearby parts of the province received no rain. The storm was fostered by very warm moist air ahead of a surface cyclone, which combined with an unusually cold air pocket at altitude. This combination generated the extreme instability and moisture (actually, moisture and instability augment each other from a thermodynamic perspective) that I noted in my blog posts, as evinced by the Gibraltar sounding.
AEMET hoisted a yellow alert for thunderstorms and rain (greater than 20 mm per hour) at 2200 hours on the evening of the 16th, for the "Subbética Cordobesa" y "Sierra y Pedroches" areas of the province. This alert was based on observations of already-occurring phenomena (heavy rain and subsequent flooding occurred between 2100 and 0100, with over 200 mm falling near Aguilar de la Frontera, in less than four hours). To gain an appreciation of how unusual this volume of rain is, consider that there hasn't been 200 mm falling IN ONE DAY in all of the province since 1971! Truly a rare event. This rarity constitutes a major challenge for meteorologists looking to warn the public in advance - I'm sure many of the agency's meteorologists weren't even alive in 1971.
To wit, the AEMET analysis concludes:
The prediction of such convective phenomena is presently a challenge for operational activities, as the available numerical weather prediction models are far from able to predict with sufficient accuracy the location and timing of their occurrence. That is why the activities of monitoring and now-casting, a few hours in advance of the development of such events, is the only realistic way to address them. The combined use of all means of observation available, satellite data, radar, lightning, conventional surface observations, etc., together with the numerical forecast models and appropriate conceptual models of different convective phenomena, is the only way to anticipate with sufficient accuracy both the occurrence and the location of the same, and to estimate their potential intensity.
I couldn't agree more with the last paragraph. The challenge is great, but it is not impossible to meet. During my service as as a National Weather Service lead forecaster who was responsible for severe weather warnings, I have seen it accomplished, though not always with 100% accuracy and ample lead time (that is impossible with our current state of knowledge and technology). I also know that such severe weather warnings have saved lives and property. Therefore our challenge is to constantly improve our knowledge, technology, and vigilance, to make such warnings more often than not.
By: esteban9, 9:58 AM GMT on August 18, 2010
The prognosis for more flash flooding was borne out yesterday in the western suburbs of Seville last evening, although nowhere near the intensity and damage occurred as in Córdoba the previous evening.
Looking back briefly at the Córdoba storms, take a look at the Gibraltar sounding (Skew-T) diagram about the time of the events. While it's not necessary to understand the diagram, I call attention to two derived parameters from the sounding (shown in the table at upper right of the figure). The first is the precipitable water, or "Water" in the table. This is the new champion for atmospheric moisture, at 4.19 cm (circled in magenta) available in the vertical atmospheric column for precipitation. This value is akin to those in the tropics! The other parameter is Warm Cloud Depth, or "WCD" in the table. This value shows how much depth of the clouds existed at above-freezing temperatures (greater than 0 degrees Celsius). Since the amount of moisture increases with temperature, deep warm clouds have the potential for heavy rain production. And indeed, 3.4 km WCD is well into the "danger" range. So one can see that the sounding has a wealth of information for assessing the potential for flooding and severe weather; moreover, these data are from actual measurements and not from computer models. Thankfully in Andalucía we have the Gibraltar site launching balloons twice a day!
The upper low that has triggered the last two days' thunderstorms will weaken and move slightly east today, so the threat of heavy rain will diminish somewhat. Nevertheless, last night's sounding is almost as moist as the one 24 hours earlier, and upper winds are very light. This means that any storms that do form will be slow-moving and dump a lot of rain on one spot. So the situation warrants vigilance, especially for those in the eastern half of Andalucía and in the various sierras.
By: esteban9, 2:40 PM GMT on August 17, 2010
This story from El Pais describes (in Spanish) the circumstances of these deaths, which occurred after heavy thunderstorm rains began at 2200 local time last evening.
Two people were killed when their all-terrain vehicle was swept away by flood waters in Aguilar de la Frontera, about 50 km south of Córdoba. The other perished from a wall collapse at his home in Bujalance, 35 km east of Córdoba. These were caused by flash floods, which arise and subside very quickly from thunderstorms, both of which can be very isolated in coverage. This rapid development is often very hard to predict and warn. Contrast this type of flooding with the riverine flooding along vast stretches of the large Guadalquivir river last winter, which incidentally also devastated areas around Córdoba. Riverine flooding develops and persists over long periods and affects main stem rivers and their regulating systems (dams and reservoirs).
Here is a map of cloud-to-ground lightning flashes for the hour between 2200 and 2300 in southern Iberia. Córdoba province is outlined by the thick black line. The locations of Bujalance and Aguilar de la Frontera are shown by the small blue and red dots, respectively. The lightning flashes were not long lived, but they were very frequent and densely packed over the two cities with the fatalities. This confirms that powerful thunderstorms were to blame for the flash flooding and resulting deaths and injuries.
The thunderstorm threat persists the remainder of today and evening, as related in earlier posts here. Be aware. Last but not least - if you are driving and see a rapid flow of deep water in the road ahead - do not cross!!!
By: esteban9, 11:05 AM GMT on August 17, 2010
The possibility of thunderstorms broached yesterday has become a reality. A closed upper low centered over the Strait of Gibraltar will provide more than enough trigger for said storms, since the atmosphere is already very unstable (and unseasonably cool and humid). The activity is presently focused on Sevilla, Cadiz, and northern Malaga provinces, but look for this to expand to other areas as we progress toward afternoon. Heavy rains will occur with the strongest thunderstorms, possibly creating local flooding. A pretty vigorous storm for mid-August!
Coverage and intensity of the storms will wane tomorrow, but they will still be about.
By: esteban9, 10:15 AM GMT on August 16, 2010
Hello again all,
There's the possibility that eastern Andalucia may see some thunderstorms and heavy showers tomorrow...AEMET has a yellow alert hoisted. There is certainly a lot of moisture around...it's humid! Stay tuned.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.