Since 13-14 June, in a large part of the peninsula, the thermometer in the Centre-North has no longer fallen below 30 degrees, and according to the projections of the European ECMWF model this threshold will remain without interruption at least until early July. This is the data with which Luca Lombroso, on the Meteored portal, lines up what could turn out to be a historic heat wave, not so much for the absolute peaks, which will certainly not be lacking, but for the duration.
Because it’s so hot
Multiple factors contribute to pushing the hot air towards the peninsula. A large anticyclone is consolidating over central-western Europe, with a maximum of 1025 hPa forecast for Sunday between the Alps and Northern Italy. At high altitude, the subtropical dome of the African anticyclone and a mild depression to the west of Spain act, while the subsidence further warms the air masses through compression. Astronomy is also involved, given that on Sunday 21st, the day of the solstice, the sun will reach its maximum height of the year in our latitudes.
Hot Friday, thunderstorms confined to the mountains
Already from Friday 19th the picture is that of a fully summer day throughout Italy. The cumulus clouds remain confined to mountainous areas, where orographic heat storms are triggered in the Alps and northern Apennines, at times strong but short-lived. The minimums do not drop: tropical night in almost all cities, above 20 degrees, with daytime highs between 36 and 38°C in the Centre-North and lower values, around 30-34, in the South.
The weekend of the solstice, between torrid and sultry
The solstice weekend brings a heat that is hard to call good weather. ECMWF estimates 36-38°C between Saturday and Sunday, even if the German ICON model does not exclude peaks of 40. Where it reaches 38-39 degrees, as in Turin and Bologna, the humidity will remain around 40% and, if anything, the stratifications at altitude and a slight contribution of desert dust will have an impact. Where the thermometer stops at 35-36, from Perugia to Verona, the higher humidity will transform the heat into real mugginess.
“Dry” thunderstorms and sudden gusts
There is a technical detail that explains the nature of these storms. The vertical profile of the atmosphere has a convective temperature around 36 degrees: beyond that threshold the convection is self-sustaining, but the dry and hot air keeps the base of the clouds quite high. The result is “dry” heat storms, short and isolated, with little rain and spectacular virga, the veils of precipitation that evaporate before touching the ground. The locally intense descending gusts of dry microbursts should not be underestimated. On Sunday, the great heat could favor small cumulonimbus clouds even on the boiling Po Valley.
The start of the week gives no respite
The start of the following week does not change the substance. The models agree on the intense and persistent heat; the uncertainty concerns tenths. ECMWF at times presents almost improbable highs, between 39 and 40 degrees for four or five days in a row in the Centre-North, only to then decrease slightly towards 36-38 from Monday to Wednesday, with a hypothetical decline on Thursday around 32-34. ICON, for its part, insists on 38-40°C at least until Wednesday. In both cases we are talking about four or five consecutive days with feverish temperatures.
How long will it last
How long it will last, then, also depends on how you define a heat wave. If the meter is the days with highs above 30 degrees, that threshold in the Centre-North has already been exceeded every day since the middle of the month, and according to ECMWF it will be continuously exceeded until the beginning of July. Real stormy or fresh bursts, which in summer should recur every six to eight days, will not be seen for at least twelve to fourteen days. The ensemble clusters also agree on the medium term, and no one is proposing changes. The doubt, writes Lombroso, is not whether it will be hot but how extreme it will be.
