Hourly historical data is now being recorded at twitter.
 
Current Conditions:

At 2:00 PM, Phoenix, AZ conditions were mostly sunny skies at 60°F, wind was west at 3 mph. The relative humidity was 25%, and barometric pressure was falling from 30.06 in.

Forecast:

Today: Sunny. Highs 61 to 66. Northwest wind 5 to 10 mph.

Tonight: Clear. Lows 38 to 48. Northwest wind 5 to 10 mph in the evening, becoming light after midnight.

Friday: Sunny and warmer. Highs 67 to 72. East wind around 5 mph in the morning, becoming southwest in the afternoon.

Friday Night: Mostly clear. Lows 42 to 50. West wind 5 to 10 mph in the evening, becoming north around 5 mph after midnight.

Saturday: Mostly sunny. Highs 68 to 73. East wind around 5 mph in the morning, becoming south 5 to 10 mph in the afternoon.

Saturday Night: Partly cloudy. Lows in the lower 40s to lower 50s.

Sunday: Partly sunny with a 10 percent chance of showers. Highs in the mid 60s to lower 70s.

Sunday Night: Mostly clear. Lows in the lower 40s to lower 50s.

Monday And Monday Night: Clear. Lows in the lower 40s to lower 50s. Highs in the lower to mid 70s.

Tuesday Through Wednesday: Clear. Lows in the mid 40s to mid 50s. Highs in the mid to upper 70s.

(this data is updated every hour)
 
Details and Historical data can also be found at the National Weather Service.

Monsoon storm rolling in over the barns, September 4, 2006, Desert Hills Area, AZ (North Phoenix)