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

At 8:00 AM, Phoenix, AZ conditions were partly sunny skies at 61°F, wind was east at 3 mph. The relative humidity was 37%, and barometric pressure was rising from 29.82 in.

Forecast:

Today: Partly sunny and cooler. Slight chance of showers and thunderstorms. Highs 72 to 77. Southeast wind around 5 mph in the morning, becoming west 10 to 15 mph in the afternoon. Chance of measurable rain 10 percent.

Tonight: Clear. Lows 45 to 55. West wind 5 to 10 mph in the evening, becoming north after midnight.

Saturday: Sunny. Highs 70 to 75. Northeast wind 5 to 10 mph.

Saturday Night: Clear. Lows 42 to 52. Northwest wind around 5 mph in the evening, becoming northeast 5 to 10 mph after midnight.

Sunday: Sunny and warmer. Highs 75 to 80. East wind 5 to 10 mph.

Sunday Night Through Monday Night: Mostly clear. Lows in the mid 40s to mid 50s. Highs in the upper 70s to lower 80s.

Tuesday: Partly sunny and cooler. Highs in the lower to mid 70s.

Tuesday Night And Wednesday: Partly cloudy. A 10 percent chance of showers. Lows in the lower 40s to lower 50s. Highs in the upper 60s to lower 70s.

Wednesday Night: Partly cloudy. Lows 40 to 50.

Thursday: Mostly sunny. 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)