From COSMO to ICON
Italy's high-resolution forecasts grew out of COSMO, the Consortium for Small-scale Modelling. It was formed in October 1998 by the German and Swiss weather services; the following spring the weather services of Germany, Switzerland, Italy and Greece signed a memorandum on non-hydrostatic modelling, replaced in 2001 by a formal agreement. Its common model, also called COSMO, was used by the members for their operational forecasts. In Italy, Arpae, the environmental agency of Emilia-Romagna, ran it through its hydro-meteorological service in Bologna in two configurations: COSMO-5M over the whole Mediterranean at 5 km, and COSMO-2I over Italy at 2.2 km.
COSMO-2I, as described by the consortium in 2020, had 576 × 701 points 0.02° apart and 65 levels. Its deterministic runs went out to 48 hours from 00 and 12 UTC, its edges were fed by COSMO-5M, and its starting state came from KENDA, an ensemble Kalman filter of 36 members plus a deterministic run. Meanwhile Germany's weather service, DWD, was scaling back COSMO's development in favour of ICON, its new model. From April 2018 to March 2022 a consortium project called C2I prepared the move to ICON's limited-area mode, ICON-LAM, which outperformed COSMO in its first tests; each member was left free to choose its own changeover date.
In 2020 Arpae planned to run ICON pre-operationally over Italy in 2021 and to move its data assimilation in 2022. The new chain, ICON-2I, became pre-operational in November 2023; in tests over August 2022 and May 2023, Arpae found its scores much better than COSMO-2I's for upper-air and near-surface variables, and similar or slightly better for rain. The assimilation of radar volumes became operational on 17 April 2024, and on 18 June 2024 ICON-2I was declared fully operational. It keeps the 2.2 km grid and the 65 levels, but forecasts 72 hours ahead instead of 48 and takes its boundaries directly from ECMWF's IFS.
Agenzia ItaliaMeteo, the national agency for meteorology and climatology, was created by Law 205 of 2017; its statute and regulations followed in 2020. It is described as Italy's national meteorological service, and since the 2026 Budget Law it has been directed and supervised by the Civil Protection Department, with its seat in Rome. ItaliaMeteo manages ICON-2I in collaboration with Arpae Emilia-Romagna, on CINECA's computing systems. The latest change came on 17 June 2026, when the chain moved to ICON 2025.10-2 and began nudging its soil moisture towards DWD's ICON-EU.
The science: ICON at 2.2 km
ICON-2I is the ICON model of Germany's weather service, with its triangular cells and its non-hydrostatic, fully compressible dynamical core, run over a limited area rather than over the whole globe. The equations are the same as in the global ICON and in ICON-D2; what changes is the size of the cells, about 2.2 km, and the fact that the model must be told what happens beyond its edges.
In limited-area mode, the outer rows of triangles are filled at regular intervals with a driving model's forecast, interpolated onto the ICON grid. Inside them lies a nudging zone, whose width can be set, where the model's state is pulled towards that of the driving model; only further in is it free to compute its own weather. For ICON-2I the driving model is ECMWF's IFS.
At 2.2 km deep convection is not parameterised: showers, thunderstorms, supercells and squall lines are computed by the model's own dynamics, which, Arpae notes, represents them more realistically than a 9 km model. Only shallow convection, the small cumulus of a fair-weather day, is described by a scheme. In the vertical, ICON-2I has 65 levels that follow the terrain near the ground and flatten with height.
A 2.2 km grid draws the coast in some detail: the Strait of Bonifacio, about 11 km wide, is several cells across. But Arpae points out that a model can only represent realistically features several times larger than its grid spacing, so the Strait of Messina, a few kilometres wide at its narrowest, is close to the limit of what it resolves. The model has no urban module and no soil analysis of its own, which is why its soil moisture is now nudged towards ICON-EU: both matter for temperatures over land more than for the wind at sea.
How a run is computed
ICON-2I's starting state comes from ICON-2I-KENDA, an assimilation cycle renewed every hour. An ensemble of 40 short forecasts plus a deterministic run is corrected each hour towards the latest observations by a Localized Ensemble Transform Kalman Filter (LETKF), the ensemble's spread telling the filter how far each forecast can be trusted. The corrections are fed in gradually rather than at once (incremental analysis update), and the analysis reaches up to 200 hPa, about 12 km.
The observations, as listed by ItaliaMeteo, are surface weather stations (SYNOP), radiosondes (TEMP) and reports from airliners (AMDAR, AIREP, ACARS), supplied by the Italian Air Force's weather service, together with the national radar network run by the Civil Protection Department: volumes of reflectivity and radial wind, and the rain estimated at the surface, which is brought in by latent heat nudging. In 2024 Arpae had only begun work on assimilating satellite data. Since 17 June 2026, the soil moisture of the 03 UTC analysis is also set halfway between ICON-2I's own value and that of DWD's ICON-EU.
From this analysis, the main forecast starts at 00 and 12 UTC and runs to 72 hours, its edges fed by ECMWF's IFS high-resolution forecast. Two companions start from the same cycle: ICON-2I-RUC, a rapid-update run every three hours to 24 hours, meant for the next 6 to 12 hours with the freshest observations, and ICON-2I-EPS, an ensemble of 20 forecasts from slightly different starting states, run once a day at 21 UTC to 51 hours.
ItaliaMeteo publishes the main forecast as open data on MeteoHub, its national platform built with CINECA, under a CC BY licence and without registration: one GRIB file per variable for the whole run, hourly from the start to 72 hours. A run's files appear about 1 hour 50 to 2 hours after its start time, and about three days are kept. ICON computes on triangles, but the open data come on a regular latitude-longitude grid of 761 × 761 points, 0.025° apart in longitude and 0.02° in latitude, about 2 km either way, from 33.7°N to 48.9°N and from 3°E to 22°E, with the wind given towards east and north.
Reading it at sea
ICON-2I is a model for the coasts and seas around Italy in the first three days. Its rectangle takes in the Gulf of Lion east of 3°E, the French Riviera, Liguria, Corsica, Sardinia, the Tyrrhenian Sea, Sicily, Malta, northern Tunisia, the whole Adriatic and the Ionian Sea as far as Zakynthos and the west of the Peloponnese. There it can draw the wind shaped by relief in more detail than ICON-EU's 6.5 km cells: the mistral and tramontane funnelled between mountains, the acceleration through the Strait of Bonifacio, the bora pouring down the Adriatic's eastern coast, the scirocco blowing up the Tyrrhenian and the Adriatic, and the sea breezes of a summer afternoon.
Where they overlap, it is one fine-mesh opinion among several: AROME over the Gulf of Lion, the Ligurian Sea, Corsica, Sardinia and the Tyrrhenian Sea west of 16°E; HARMONIE DINI from the Gulf of Lion to the Tyrrhenian Sea down to Calabria, and over the Adriatic; ICON-D2 in the northern Adriatic. When they agree, the forecast deserves confidence; when they disagree, the difference is a measure of what no model knows yet. At the large scale, though, ICON-2I is not independent: its boundaries come from the IFS, so a low that the IFS misplaces enters ICON-2I misplaced too, and near the edges of its domain the model is held close to the IFS. For an independent view of the weather systems, look at ICON or the GFS.
Arpae's verification is made against weather stations on land, and in June 2026 it found the 10 m wind of the operational configuration slightly too weak on average. Few of the observations it assimilates are made over the open sea, so there its starting state rests largely on its own previous forecast. In summer, under weak large-scale forcing, Arpae notes a moderate overestimate of the most intense rain cores: a thunderstorm drawn over your route is a warning, not a schedule, and its place and timing are uncertain.
The 10 m wind is a mean over each cell: the gusts under a shower, in a squall or in a bora jet are not in it. Its 72-hour range and two runs a day make ICON-2I a model for coastal passages and crossings: Corsica to the mainland, the Strait of Bonifacio, the Strait of Messina, an Adriatic crossing, or the last stretch of a longer route. Beyond three days, the global models take over.
ICON-2I in Saily WR
Saily WR downloads both runs of the day from ItaliaMeteo's MeteoHub as each is published, hourly from the start to 72 hours. ItaliaMeteo publishes one file per variable for the whole run, about 85 MB for each wind component; the app reads only the 10 m wind, fetching for each hourly step, out of each of the two files, the 16 bytes that give the length of its messages, then one byte range of about 1.2 MB, rather than the whole files.
The wind is used on the model's own published grid, 0.025° by 0.02°, with no regridding, and it already points east and north. The whole rectangle carries values, so there is no calm corner in it.
A route gets an ICON-2I version when its start and its end both lie in the model's rectangle, 33.7–48.9°N and 3–22°E. Menorca is inside; Mallorca is cut by the 3°E meridian, its western part outside; the Aegean is outside. The route is sailed from your boat's polar in that wind, interpolated in space and between the hourly steps, and drawn beside the routes of the other models covering the same water, where they overlap: AROME, ICON-D2, HARMONIE DINI, ICON-EU, ARPEGE-EU and the global models. The sea current and land avoidance you choose apply to it as to the others.
Every point of the rectangle carries a forecast, but the model is freest in its interior: towards the edges of its domain it is held ever closer to the IFS that drives it. The Data page shows live which ICON-2I run the app is on, how far ahead it reaches and the area it covers.
Sources
- Short-term forecasts, Agenzia ItaliaMeteo (2026)
- Previsioni meteorologiche modello ICON-2I, ItaliaMeteo open data catalogue (2025)
- About us, Agenzia ItaliaMeteo (2026)
- Status and developments in data assimilation for the ICON-2I model, Gastaldo, Poli et al., Agenzia ItaliaMeteo and Arpae, COSMO General Meeting (2024)
- Status of data assimilation for ICON-2I model at Arpae, Gastaldo and Poli, Arpae, COSMO KENDA meeting (2024)
- Aggiornamento configurazione ICON-2I: implementazione a partire dal 17/06/2026, Arpae Emilia-Romagna (2026)
- L'uso della modellistica a supporto della previsione, Gastaldo et al., Ecoscienza 5-6 (2024)
- High resolution data assimilation in the GLORI Project, Poli, Gastaldo et al., PrePEP conference (2025)
- COSMO model at ARPAE-SIMC, COSMO consortium (2020)
- Status of the transition from COSMO to ICON-LAM at ARPAE, Arpae, COSMO General Meeting (2020)
- Priority Project C2I: transition of COSMO to ICON-LAM, COSMO consortium
- The COSMO consortium, COSMO consortium (2025)
- ICON Tutorial – Working with the ICON Model, Deutscher Wetterdienst and Max Planck Institute for Meteorology (2023)