Hi, I'm about to start a run through of Dragon of Icespire Peak with a couple of family members, one playing a Tiefling Warlock and the other a Dragonborn Paladin.

I'll be playing a vHuman Tempest Cleric and, I suspect, providing most of the healing for the party as the Paladin is a completely new player.

My question is, if I want to take a level or two of storm sorcerer (for some fun interactions and flavour, but also being extremely mobile on the front lines), how many levels can I take without it being heavily detrimental to my cleric progression?

Also, at what point should I take them?  I was thinking 1/1 to start with then going to lvl 4-5 cleric before taking any more levels of sorcerer.

Any help would be appreciated!

Edit: should also say they I don't imagine that we'll end up getting to a very high level, probably 7-10 at most.

(Having not played the module and not having read it through either to avoid spoilers in case I end up as a player)

I suppose the main questions you need to consider are:

• What abilities do you want to have? Assuming you hit 6 in either Class then Cleric 6 is pushback, and Sorc 6 is damage resistance and deal damage. Both are situational but useful.
• Are there particular spells you want to have?
• How many ASIs/Feats do you think you might need or do you have your eye on? (And what stats you have to back up casting on both Wis and Cha in case one is a lot lower as that may change the viability of the plans you make)

Once you've identified answers to those that you can work out what you might want to do levels wise.

Ultimately, 1 level of Cleric gets you access to some healing spells , and two levels gets you a use of the Channel Divinity (which recharges on a short rest unlike most other abilities from those classes). Most things after that are a preference on what you might want to do.

Thanks for the response!  The stats I've rolled are:

15+1, 12, 14, 10, 15+1 and 14

Alongside our Paladin I'll probably be mostly melee based but expect to also be picking up any major healing needs, so effectively a utility tank/healer.

My intention is to take mainly levels in cleric and take at most 2 levels in sorcerer.  That will give the character some more flavour, a few extra abilities and spells (thinking Booming Blade, Shield and some weather cantrips like gust) and I like the mobility that Storm Sorcerer provides, being able to manoeuvre around the battle field when casting spells. I plan on taking Warcaster at 1 and probably +2 Wis as soon as I can.

My intention was to start with a level of each (order undecided) and then stick with cleric until I have at least 5 levels in that for 3rd level spells, then potentially pick up a second level of sorcerer.

That sounds like a solid plan then.

One thing you might want to consider if you're playing with Tasha's enabled is the Transmuted Spell metamagic. That lets you change the damage type on certain spells when they're cast for 1 sorcery point, but you'd have limited uses of that ability due to the low level (3 times per long rest, or more if you sacrifice slots for sorcery points, but that kind of brings it in line usage wise with some other Cleric abilities like the Wrath of the Storm). Metamagic's a level 3 thing for Sorcerers though so you'd be sacrificing a level in Cleric for that but gaining some additional potential spell selections in return. Since it'd change a concentration spell like Flaming Sphere (an optional spell in Tasha's) at the time of casting, you could instead have a lightning/thunder sphere to throw around the battlefield.

Similarly, the Dragon's Breath is also useful with the push effect at Cleric 6 since you can push a few people back at a time, but being a Level 2 spell you'd need 3 levels of Sorcerer to get it.

As far as multiclassing slowing spell progression, you'll gain the slots since both classes are full caster, but won't have access to some of the higher level spells. Upcasting other things can still be effective though so it depends on what you want to be able to cast. Some spells like Spirit Guardians are great for melee based characters since it's more damage to enemies each of their turns, but you might drop concentration and it needs you to be at least Cleric 5 to access it. If you're adding a level or two of Sorcerer in there then it's not coming in until Character level 6 or 7.

Given your stats, the level at which the campaign will end, and wanting to go mainly cleric, take sorcerer for your first level and then go tempest the rest of the way. Or go cleric until 5th and then MC into sorcerer after that for one level, any time after 5th.

Two levels of sorcerer are not going to give you very much unless you decide to use one of your ASIs on the metamagic adept feat, but that's another can of worms that only makes sense in your campaign if you go into tier 3 play. 

If you start sorcerer, you will of course delay the big spike at 5th level by one, so there is that bit to consider. But you will gain con save proficieny, booming blade (BB), shield, absorb elements and whatever other cantrips you decide on. That first level is going to be rough though, since you won't be casting stuff like Bless, Healing Word, etc...and using tempestuous magic isn't really all that viable until you have enough spell slots to spam leveled spells every round. So prepare for a sucky first level ride. 

As for cantrips, Message is incredibly useful, mold earth is great, shape water is thematic, or green flame blade for another melee attack cantrip, take your pick. I would avoid the lightning based cantrips unless you really want to keep those in your back pocket for the occasional situation where a strength save (lightning lure) for a weak looking enemy is likely to fail or the enemy is armored and you think advantage is enough to hit (shocking grasp) since Charisma is only a14, and are worth keeping idle most of the time, except for those edge cases. 

Toll the dead and/or sacred flame plus booming blade should probably be your go to cantrips most of the time.

If you go sorcerer as your starting class, you could pick up war caster (WC) as your vhuman feat at first level and you are set with con saves for the career of your cleric. If you go cleric you might consider res con but not until 8th or even 12th level ASI because you'll need to use those to increase your wisdom stat at some point. Plus with booming blade AoO's (attacks of opportunity) become more useful and tactical when they come up if you have both BB and WC.

All in all, a one level dip will shore up your defenses, give you some extra mobility and not delay your spell progression too much or wait until sometime after 5th level cleric and decide then if you think it's really worth it then. 

Good luck and happy hunting! 

Thanks both, really helpful points and I did have in mind that I was unlikely to hit a high level so don't want to lose too many Cleric levels so may stop at just one in sorcerer.

Out of curiosity, if I were unable to take Booming Blade/GFB by the DM is it still worth taking the level of sorcerer?

Booming blade isn't necessary, but it does help to increase your effectiveness with melee weapons - clerics only get one attack per turn so no extra attack like other martials at 5th - and with WC it gives you a rider effect that will punish enemies for leaving your reach. And at 8th level tempest gains divine strike. So if you crit with booming blade and you have divine Strike (only on your turn and not AoO) that's 6d8, which you can max with your CD. It won't happen often but if you do it's going to do some serious damage. Add on a magic weapon that deals extra damage of some kind and you're looking at even more damage.

The thing is that after 5th level unless you're out of spell slots or trying to conserve spell slots you're not likely to use your weapon anywhere near as often as you did at early levels. It can still be an active part of your melee combat shtick, eg spirit guardians, spiritual weapon and a melee attack added into the mix is a very nice trio of damage, but you'll be using your weapon less and less as you gain levels. This will also depend on your DM and how many encounters you have in a day so ymmv and you might just need your weapon more often than I did. 

I don't think it's always optimal to delay 3rd level spells like dispel magic, aura of vitality (if using Tasha's added spells list), spirit guardians, and call lightning, just to name a few. They are really great and powerful spells that change the game at 5th. But if you feel like MCing at this point for narrative reasons or you just want to try out storm sorcerer or any other sorcerer, give it a whirl. 

I'm running a tempest 12/storm sorcerer 1 and I waited until 10th level to MC. I really wanted Destructive Wave at 9th level and didn't want to delay getting there. I will say that MCing into sorcerer has made my tempest into a beast of a tank though. I have a static AC of 22 (+1 plate and +1 shield) and being able to bump that with the shield spell to 27 is amazing. Absorb elements has been fantastic as well since my dex is only a +1 and I fail dex saves all the time. And at 10th level I have plenty of slots to really make use of tempestuous magic most of the time. Lots of added defense and extra mobility with the sorcerer MC, I love it.

Keep in mind that adding in sorcerer also adds in another spell focus (non costly material components only), so if your DM requires/tracks this mechanic, you'll need it for any spells that require a focus/material component. WC removes the need to have a free hand for somatic, so both shield and absorb elements are fine.

In the end, do whatever feels the most fun for you and your character. 

I will recommend making sorcerer your primary class, and taking 2 levels in Tempest Cleric.  Arcane Spellcasters can benefit more from Destructive Wrath, as they have better burst damage.  You can use it on lightning bolt or chain lightning.  Pick up Transmuted Spell, so you can transmute spells like cone of cold, fireball, otiluke's freezing sphere, and eventually meteor swarm.