10 reasons it might not be wise to ride in the rain...

10. You still have not changed out the racing slicks on your bike so your tires have zero grip with the slightest bit of moisture on the road
9. Reduced visibility means that the dump trucks on Polk Street don't see you on their inside
8. Your niterider headlamp is not as waterproof as the packaging would claim
7. You don't have any wet lube
6. Your "pac-a-mac" clear plastic rain jacket resembles your 80 year old Grandmother's raincoat... (not that cycling is a sport with high-fashion stakes)
5. You detached the visor from your Pneumo helmet so you can't shield your eyes from the dirt and rain
4. You haven't had a bike with mudguards since grade school
3. You have yet to master wiping debris from the back tire without wiping out (and have the scars to prove it)
2. A rainy day is a great excuse to check out the indoors spin class your friend has been raving about
1. Hell, you live in California so what's ONE skipped workout in an entire season!

However, passing on a ride in gnarly conditions reminds me of an excerpt from the "Road to Paris", the documentary about the USPS team's preparation for TdF '01. One scene includes a training session/reconnaissance ride in the Alps in mid-February (maybe the Col du Galibier). It's late in the day, it's cold and the team is tired from doing hill repeats to exhaustion. Then it starts to snow. The team descends to the bottom of the hill where the support vehicles are waiting, the shivering riders clamber into the van and start wrapping themselves in blankets. Except, that is, Lance Armstrong. He calls over to Johann Bruyneel (USPS directeur sportif) and says he wants to check out the hill one more time. Bruyneel turns to the camera grinning and says in his thick Belgian accent: "Zeez is what nobody seez. Zeez is why he is great cyclist. Zeez is why he'll be champion again."