A Southwestern "Dryline"

A pattern change the next couple of days will result in substantial contrast in weather across the southwest United States.  A deep mid-latitude trough is developing off the Pacific Coast, while a ridge intensifies over the south-central United States.  This will lead to a confluent flow pattern over Utah and Arizona with significantly different airmass origins for the two states through Saturday morning.  A confluent flow pattern is one in which two different airstreams are brought together.

Most of Utah will be in the southwesterly flow ahead of the mid-latitude trough, resulting in hot, dry, windy conditions Thursday and Friday.  It's pretty much a worse-case scenario for fire-weather conditions, so let's be careful out there.

GFS 700-mb wind vectors and precipitable water (cool and warm colors
indicate low and high values, respectively)

In contrast, southern and eastern Arizona will be strongly influenced by easterly and southeasterly flow on the upstream side of the developing ridge.  While it is quite dry there today, moisture will be on the increase over the next few days, leading to an increasing thunderstorm threat.  They will need to worry about lightning in addition to accidental fire.

As can be seen at the end of the loop, this leads to a strong contrast in precipitable water between the two regions.  Precipitable water (a.k.a. integrated water vapor) is the depth of water one would have if all the water vapor in the atmosphere rained out.   By Saturday morning, values are less than 5 mm over central Utah, but more than 20 mm in portions of southern Arizona.  

Thus, the southwest will have it's own "dryline" separating relatively moist and dry air on Saturday.