Sentence number 1 is correct because "likely to be" is being used like a prepositional phrase here. You can take it out and make a sentence that is still correct. There is (likely to be) rain. There is (likely to be) sun. In sentence number 2, if you take out the phrase "likely to be" the sentence is no longer correct. There is (likely to be) raining. .....X!.... This is incorrect. "There is raining?" No. We would not use a gerund here. Just the noun. To make sentence number 2 correct change "There" to "It" : "It is likely to be raining everywhere today except London" Now you can take out the phrase and the sentence is correct. It is (likely to be) raining.

This is an easy question: there is.... (+ noun) So, 'there is... rain.' (P.S. No, a 'gerund' (..ing form) is not the same as a noun) :)

Rain is a noun when used in the sentence "There is likely to be rain everywhere today except London."