June 18th, 2025: When Will Our Monsoon Get Going?

Synopsis

The typical late June extreme heat will continue through Friday as high pressure aloft dominates the Desert Southwest. This weekend into early next week an anomalous synoptic scale pattern is expected with a deep mid latitude trough along the West Coast and a high amplitude subtropical ridge east of the Mississippi River. This will allow high temperatures to cool by a few degrees and below excessive heat criteria, but will delay the onset of the monsoon in Arizona. By late next week, ensembles show moisture gradually moving northward toward Southeast Arizona. At this time, the onset of our monsoon could occur as early as the last few days of June.

The Pre-Monsoon Heat

Tis the season to sit in a pool or in the AC all day. The typical June heat has arrived which is usually a sign the monsoon is knocking on the door of Arizona. As of 2:45PM MST this afternoon, surface observations show temperatures between 105 and 110 degF across the lower deserts with a few locations reporting temps a degree or two above 110.

Surface observations as of 2:45PM MST courtesy of the NWS.

It is also evidently dry with dew points in the 20s to 30s degF across the region. 

The 500mb subtropical anticyclone ("The Monsoon High") is in the vicinity and currently centered near Northern Baja California.

18z GFS 500mb analysis courtesy of Tropical Tidbits.

Tomorrow will be the hottest day as the 500mb anticyclone moves eastward over the Continental Divide in Southern New Mexico in response to a deepening mid latitude trough in the Northeast Pacific. 

18z GFS 500mb cyclonic vorticity valid for 11AM MST tomorrow courtesy of Tropical Tidbits.

High temperatures in the lower deserts will likely reach between 110 and 115 degF, so please stay hydrated, limit time outside, and check on people you know who are exceptionally sensitive to extreme heat!

Enemy of the Monsoon Onset: Troughs

By this weekend into early next week, a high amplitude synoptic scale pattern is expected across CONUS with a deep mid latitude trough along the West Coast and a brutal 594-597 dm high east of the Mississippi River.

12z GEFS 500mb cyclonic vorticity loop from 00z Saturday through 00z Thursday courtesy of Tropical Tidbits.

Mid latitude troughs like this are bad news for the onset of our monsoon as they disrupt the northward propagation of the subtropical anticyclone or "Monsoon High". 

However, 1000-500mb thickness will decrease allowing high temperatures to remain below excessive heat criteria through this period. Unfortunately, dry southwest flow will prevent any rainfall chances for Arizona through at least mid next week.

So...When Can We Expect the Onset?

The first thing to look at when forecasting the onset of the monsoon is the 500mb map. As shown above, the pattern through at least the middle of next week is far from conducive for an onset as the prevailing wind flow needs to become more southeasterly/easterly. The GEFS as well as the EPS (will only show GEFS) have been hinting at a more favorable pattern late next week.

12z GEFS 500mb cyclonic vorticity valid for Friday June 27th courtesy of Tropical Tidbits.

In addition, ensembles show moisture gradually moving northward into Southern Arizona as early as the middle of next week.

12z GEFS total precipitable water loop from 00z Tuesday through 00z Sunday of next week courtesy of Tropical Tidbits.

However, this is 7-10 days out, so there's still quite a bit of uncertainty. 

Another thing to keep an eye on besides model output is to our south, particularly Northern Mexico and the Gulf of California. Sea surface temperatures in the Gulf of California are starting to get toasty with CDAS showing temps in the upper 70s to low 80s.

12z CDAS sea surface temperatures courtesy of Tropical Tidbits.

 

The Gulf of California is a significant source of primarily low level moisture, so always important to keep an eye on those SSTs. 

Northern Mexico can also play an important role as moist outflows in Sonora can transport low level moisture into Southern Arizona. In addition, Sonoran MCCs (Mesoscale Convective Complexes) play a critical role in Gulf of California moisture surges as they can initiate the thermally direct circulation responsible for moisture surges. 

Overall, I anticipate at least high terrain convection in Southeast Arizona by the last few days of June, and hopefully a complete onset of our monsoon to start July. Fingers crossed!