For several years, I've used the rough metric that really bad weather can knock off up to 1/3 of your range. 2/3 of 265 is 177 miles. I haven't tested this in the S yet, but let's run some rough numbers to see if that might hold.
From previous discussions, in good weather at ~60mph you should burn about the rated range of 308Wh/mile (one of Tesla's linked charts shows a smaller number than that; but as the comment from Rod and Barbara notes, it doesn't match the other graph. I think 308 is a pretty good number that fits in with most calculations). 308 * 265 miles = 81.62kWh. 85kWh may be a nominal value, with some in reserve.
Now let's assume some incredibly bad weather. Pushing water out of the way in heavy rain may add 5%, which would put it up to 323.4wh/mile. I'm guessing on the 5%, so let's be conservative and make the math easy, and just say 3 miles per kWh to push the car (even though that's more like 8%).
If you drive for 3 hours or 180 miles, you'll use 60hWh pushing the car. Cinergi turned the HVAC controls up full blast and said it looked like it was pulling about 7.5kW. Over three hours, that would be an additional 22.5kWh. Add that to the 60kWh to push the car, and it looks like you just passed the limit.
So worst case, 177 miles could well be the limit (at 60mph). Note that that is REALLY bad weather; I can't imagine keeping the HVAC at full blast for three hours, although I live in a pretty temperate area. These are very rough calculations and I haven't driven in really bad weather in the Model S yet. In slightly-cold-with-just-a-little-rain weather at 68mph, I've typically been on track to get about 240 miles of range, so I've been a lot closer to best case than worst case.
Here's a rough chart to illustrate the point. I took Tesla's wh/mile numbers, bumped them up 8% because they didn't match other numbers we know (including the other chart in the blog linked above), and that's the green line on the top. The I applied Cinergi's roughly-measured 7.5kW pull for all HVAC on high, that's the shaded part. I didn't add the ~5% drag for rain. But depending on how high the HVAC is turned up (and remember there may be some automatic usage for the battery even if you have climate control turned off in the cabin), your expected range should fall somewhere in the shaded area.
At slower speeds around town, HVAC can exact a very heavy toll. But man, you'd have to be driving for many hours to run in to trouble - no need to worry about that end of the graph. At freeway speeds, how fast you're driving still matters more than HVAC.
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Note: in all of the above, I assume the driver is using the throttle carefully and TRYING to get good range. The above calculations show a constant draw to push the car at 60mph in rain of 20kW. But if you mash the throttle to accelerate, you can pull over 300kW; it won't take much of that to skew these numbers. This is one of the reasons why some people driving around in town (and doing a lot of accelerating, which is a big deal in a car this heavy) are seeing low projected ranges despite low speeds. Another reason is the start-of-trip HVAC overhead of getting the car (including the powertrain; not just the interior) to the right temperature is a much bigger deal over short trips than long trips.
