20210119

 20210119

Finally, something to talk about!  A "Type 2" Atmospheric River event will impact southern Arizona over the next few days.  A "cut off" low is currently forming southwest of San Diego and as the low is "cut off" from the main jet stream, it doesn't move much for 24-36 hours which results in a significant transport of subtropical moisture northward into the southwestern US. 

This moisture will interact with both the dynamics from the low and also the mountainous terrain of southern Arizona resulting in a fairly long precipitation event (~24 hours).   Below are the most recent 4 model runs which generally agree on the amounts and locations.  In far SW Arizona, nearer to the low, where conditions are more favorable, very heavy rain is likely as well as a few thunderstorms.  Some areas in/near Yuma may see over 2-3 inches of rain.  Far south central Arizona is another area that could see over 2 inches as the wet air is lifted by the higher terrain as it moves northward.

As mentioned above, this is a fairly long duration event.  As seen below, the plume diagram for the UofA has the precipitation falling at a moderate rate over 24 hours, so flash flooding doesn't look likely.  Urban areas may still see some flows in dips and washes though.

The one exception for flooding risk is the Catalina Mountains Bighorn burn scar.  As seen below, there is a high risk of >1.5" over the area.  Precipitation is likely to be all rain, even at the top of Mt Lemmon.

Below is the 0-5cm soil moisture forecast which indicates it exceeds 25% during the event.  Thus, some runoff is likely for much southern/eastern Arizona.  This is just a guess as I'm no hydrologist!