This scenario assumes that Russia plans to encircle the Ukrainian capital Kyiv with a massive attack from the north, using troops stationed in Belarus under cover of a maneuver. In this scenario, preparations for an attact on the land borders of Crimea and the Donbass serve as distraction.
A rapid advance would be enabled by the good state and availability of roads leading south from the Belarusian border. The Ukrainian military is in its majority positioned against the Donbass border and Crimea, and against the border with Russia. On the Belarus border, the number of installations does seem to be limited, although there are several air bases and small airports in the region.
Attack from Donbass
An attack from Donbass, using the Russian proxy forces with the help of regular troops, is possible and gives a certain level of deniability. This level, in the current atmosphere, would not be of much use, though. Also, the warning time would be relatively long as regular troops would take one to two days to reach the front with heavy equipment. We expect such an attack to take place only after a decapitation attack has taken place or was at least attempted.
Attack from the Russian border north of Donbass
This scenario is in some ways similar to the "Attack from Donbass" scenario and probably would be combined with it. Cities like Charkiv and Sumy would be surrounded and "put under Russian protection". Any unilateral occupation and annexation would be difficult due to popular resistance and the continued existence of a free Ukrainian government.
Black Sea coast landing operation
A landing operation could take place west of Mariupol, south of Kherson or in the vicinity of Odessa. In the latter case, troops from Transnistria could support the operation. Such an operation would be high risk, though, as the preparation and coastal approach of landing ships would be detectable. Russia, as a land power, has no practical experience with such landing operations, and Ukraine owns a number of modern anti ship missiles. Such a landing operation would, to our estimate, be attempted only in a later phase of the war. Landing ships from the Baltic have entered the Mediterranean some days ago and would be expected in the Black sea around 2022.02.05, but would probably need to be refuelled and staffed in Sevastopol.
Air based infrastructure attacks
A large scale attack on Ukrainian defense installations and infrastructure could be made without land based forces entering the country. The high damage to the civil population and the risk of destroying one of the nuclear power plants of Ukraine makes this a very risky approach. Ukrainian air defense is limited, but the country is large and Ukrainian armed forces would disperse and prepare for resistance. Also, the international reaction would be very strong, and the chance that Russian political goals were accepted even after a prolonged attack are low.
As an initial scenario, we hold the "Rapid Decapitation" scenario as the most probable. This scenario would probably be followed by a combination of the other scenarios if it is not completely successful in regards to the Russian political goals.
Addendum - 2022.02.24
As of today, Russia has attacked Ukraine. The agressor selected to combine all of the scenarios, attacking from Donbas and the region north of it, from Belarus and from Crimea, while also air attacks try to destroy Ukrainian infrastructure. Information about Black Sea landing operations are still not confirmed. The extremely fast escalation came as a surprise to JCOT. Further analysis will show if Ukraine did prepare to counter these scenarios.
Addendum - 2022.02.24-17:00
The fast progress of Russian troops from Crimea probably signifies that Ukraine was not prepared for an attack from the peninsula. JCOT held this scenario for improbable because of the possibilities to block access to the mainland by mine fields. It seems this was not done sufficiently, and neither were the bridges over the Dnepr secured or destroyed. As expected, there was not yet any landing operation. The "Fast Decapitation" scenario seems to have priority, with a successful airborne landing operation at the Antonov airport west of Kyiv and land forces slowly progressing from the north on both sides of the Dnepr and also from the east towards the city.