Update July 5 2010;
Hope for rain springs eternal in the desert and the mind of this writer. While humidity has crept up to near monsoon status at ground level on occasion, the pattern has been for upper winds with westerly components due to lows moving above the desert Southwest. We usually need southeasterly to south movement to bring the moisture up from Mexico.
The forecast for the area keeps moving chance of precipitation back and forth, but mainly back. The “heart” of the monsoon in AZ is from mid-July to mid-August. Late in the season we may sometimes get a boost from tropical storms and hurricanes moving up the Gulf of California or Pacific via the normally drying southwesterly end of monsoon period.
There are various attempts at truisms about early onset monsoon and late; usually early not so good, and late more robust, but I have seen questioning of these beliefs. But for now, it seems I hope the latter is correct.
Some years the monsoon period for AZ is early and others late or right on time. (Tucson’s first rains many years around July 4) Usually dew-points begin creeping up in the late June and early July heat. From Tucson you will see clouds beginning to show up to the Southeast, then over the mountains ringing the city. Here is a photo from yesterday showing the clouds southeast of Tucson.
This was taken in mid afternoon (June 24) with a temp around 107.
By evening, storm complexes in Mexico and far southeast AZ may surge moisture northwest to west as outflows, resulting in breeziness and increased cloud from the southeast. This is usually mid level cloudiness with virga–rain not making it to the ground through the dry lower atmosphere.
(June 24 Sunset To The Southeast)
This may go on sporadically for days, then suddenly it might become sunny and hot again for several days, or keep clouding up more day by day. For this area, nothing is exactly “how it always goes”, but usually the first best sign of the monsoons arrival seriously, is the wonderful smell of 1st rains in the desert, often late afternoon to around sunset. You know rain is making it to the ground, but it still may be days before you see rain around town in Tucson.
Dramatic late day cooling and dust may accompany these winds. Microburst conditions can occur locally. I think the monsoon period was declared a couple years ago to begin at June 15, to cover this atmospheric enrichment period, when dew-points may not be in the monsoon range, but hazardous weather could well be. The end of the thunderstorm season is now declared to be Sept 30.
Of recent monsoons, the drier ones were during El Niño-Southern Oscillation ocean conditions, the wetter during La Niña. Usually for rain; El Niño is good for winter rain and not for summer in AZ, while La Niña reverses that tendency. Where I am, the difference was between a 6 inch monsoon time and a 16 inch one. We are coming off of a wet El Nino winter, so a La Nina summer may be an unusually good time for the long drought stricken Southwest US.
The monsoon for the desert US is simply a kind of extension of the USA’s “Ring Of Fire”; the flow of moist air around high pressure in the Southern US and Bermuda High. Then the “Four Corners High” can steer storms off of the mountains in AZ and New Mexico to move off the mountains heading southwest across the lower desert areas.
During the monsoon, movements of the Four Corners High can dramatically affect storm direction and existence over a particular spot. Extension of the high over the Southern US can also dry out an heat up eastern AZ, sending the monsoon moist flow over to California or out into the ocean. Tropical systems can move from the Gulf of Mexico, Mexico, or the Gulf Of California into the flow to dramatically increase storminess and coverage of rains. Most days the heated valley air is drawn up mountain peaks to trigger the days first storms late morning to mid afternoon.
The Thermal Low, most often located over southwest AZ, usually sits and spins counterclockwise there, helping to draw up moist air from the south in Mexico. If this feature moves too far north, it may draw in drier air from the west. Today, the start of the monsoon flow is expected to diminish from a low pressure coming in from the west too close to the north of our region. The result is expected to be dryer air from the southwest–west, shunting the moist air into New Mexico.
Lows do not necessarily produce the same results, much depends upon location, and are they tropical or cold core types. A often “good” low during the season are the mesoscale convective vortex types created by thunderstorm complexes that are finishing up from yesterday’s organization in the morning hours. These can persist with counterclockwise rotation, lay down air boundaries that will come to focus later storm development. These can also be where most of the action is, so being near one, or it having passed close by may mean you will see a cloudy day and evening with no further storm developments for you.
Eastern Pacific features can play a big role in flow enrichment, and a storm complex over the Gulf Of California, and or in Mexico, can push very high dewpoints–into the 70’s, into southern AZ. This is called a “Gulf Surge”, it can happen almost without notice, resulting in dramatically increased storm conditions, as can many a low pressure feature embedded into the moist monsoon flow many from the east; Easterly Waves and Retrograde Lows.
No Chance Of Rain —- NOT!
The biggest rain event I encounter during the monsoon period was once in the 1990’s, when the monsoon’s flow was declared to have been driven east of Tucson Arizona. The weatherman announcing 0% chance of rain on that evenings news. At 2 in the morning I was jolted out of sleep by a big bang. Thunderstorms then persisted for nearly 4 hours, continually regenerating from flanking buildups from 2AM to 6AM. Nearly 4 inches resulted in my neighborhood, which was the worst hit. My entire yard was covered in rapidly moving water. The street corner over 2 feet deep with cars stalled.
What happened was a small upper air low bolted south from Canada, reaching us in 24 hours, where it stalled between the 2 mountain ranges to the north and east. The radar loop showed this low, which then seemed to suck up all the leftover moisture and wring it out right over Tucson’s east side before it exited to the east. A quite atypical event.
Each monsoon in the desert US is a natural work in progress. One thing is sure; I will be out photographing some of these dramatic conditions from home. If one were a storm chaser, most monsoon summer days are less that one hour from dramatic storms and astounding vistas. The higher cloud base and common hundred mile+ viewing, lends the eye to dramatic cloud feature sights and long lightening bolts.