© 2017 by  Fernando Caracena

The Arriving Monsoon

Every year around the Summer solstice the Desert Southwest in the United States has a heatwave. The newspapers are full of news about the record temperatures being recoded daily in the inland West. The temperature recorded in Phoenix, Arizona goes above 40 degrees Celcius (104 Fahrenheit) everyday and it edges up to 49 C! By the 4th of July, the days are hotter than a firecracker! Suddenly the heat wave ends in thunderstorms. After days of hot, dry weather, fireworks displays may be cancelled on account of rain! Since there are many new arrivals in the area there is always a lot of discussion about the crazy weather; but the old timers will tell you otherwise. This is not unusual weather. It fits a pattern that repeats at about the same time every year. The break in the heat is produced by desert rains that arrive with the Arizona Monsoon (see 2nd Ref, here).

The transition from extreme heat to summer rains and cooler weather in the desert (still hot) comes with a shift in the winds at upper and lower atmospheric levels. That time is an exciting time for storm chasers. Although living in a climate that is a bit more comfortable in the Summer (Boulder, Colorado), I am still an enthusiastic watcher of the North America Monsoon through various sources of weather data, primarily the deep atmospheric sounding. Over the next few days, I intend to post some of my analysis of these data using computer routines that I have developed.

The flow patters over the full troposphere still indicate that the extra-tropical cyclone (ETC) track is still far south enough to maintain a dynamic sinking of air over the Desert South West, which is keeping the atmosphere over that region stable against deep moist convection. The atmosphere along the eastern slopes of the central Rocky Mountains is strongly affected by ETCs passing over the northern United States: over Montana, Wyoming and the Dakotas. Alternate lifting and sinking of air in the atmospheric column is strongly modulating the weather along the eastern slopes of the Front Range and adjoining plains. The Continental Divide shows up as a boundary between dry air masses to the west and air masses bearing variable amounts of water vapor to the east. The hot desert winds blowing into the Western Slope are putting a big stress on the pine forests and are rapidly the snow pack on the high mountain ridges. The stream are swollen with cold snow-melt water.

I remember as a boy growing up in El Paso, Texas how the monsoon appeared in that area toward the end of June and early July. After too long a period of very hot, dry weather, under westerly wind, distant lightning would appear at night in the eastern sky. The old timers called that "heat lightning."  Days later, a line of towering cumulus clouds could be seen growing in the eastern sky in the afternoon. The hot dry wind continued to blow from the west. Day after day, the deep moist convection advanced closer to the area against the surface wind. Then one day, the heavy storms would be over the Hueco Mountains in New Mexico just east of El Paso. That evening there was a rush of rain cooled air from the east, and at night, a dramatic lighting display appeared in the eastern sky, almost overhead. The next day, thunderstorms and rain showers would develop in the area, which brought in much cooler and more moist air. The area of deep moist convection continued its advance toward the west until El Paso was left in the clear, where the easterly wind continued to be moist but the temperature recovered into the 90s (F) and it became as uncomfortable as when the area was under the pre-monsoon air mass.

Now that I have the tools and leisure, I plan to do a study of the the changes in deep tropospheric circulation patterns associated with the appearance of the North American Monsoon.

6-30-14 MDT, Boulder, Colorado:
This Friday in Boulder, CO at 11:30 AM the temperature is a cool 65 F and it is forecast to warm up to 76 F later this PM. The cold front passing over us last night brought in cooler air and low clouds. Yesterday, we had a high of about 83 F, and there were thunder showers over the area. We had 0.10 inches (2.54 l) last night. The end of June rain brought the rainfall total for the month up to 0.58 inches (15 l) compared to 5.72 inches (145 l) in May. June generally has erratic rainfall totals in Colorado, because the transition between Spring and Summer rains generally happens during a hot dry interlude between the seasons. South of Denver along the Front Range area, the Summer peak in rainfall is greater than that of Spring. This pattern is reversed for the area north of Denver. For example, Boulder has its peak, mean precipitation total in the month of May.

7-02-17 13:31 MDT, Boulder, Colorado:

A late June cold front brought us a bit of rain (0.10 inches, or 2.54 l) and cool, maximum temperatures in the 60s F (15-21 C). Currently we are at 86F(30 C) under a sky full of cumulus cloud debris. Although Denver is quite dry, low level moist air is curving into the Foothills of the Rocky Mountains north of Denver.

Afternoon thunderstorms (we were hit by one) brought only a trace of rain as sprinkles. We were hit by a weak microburst.

7-03-17 11:00 MDT, Boulder, Colorado:

The temperature is 76 F after a low reading this morning of about 62 F. The forecast high temperature for the afternoon is 89 F. We are likely to have a repeat of yesterday' s weather, but today we have a bit more low level moisture in the air, which has a tight, increasing gradient across NE Colorado. We could get some measurable precipitation today is we get a direct hit by a shower. It is still dry enough that we could have some microbursts.

12:20-12:25 MDT. A patch of alto cumulus clouds passed overhead, bringing us a light sprinkling of rain. There was no thunder and no wind. I noticed that these clouds are drifting in from the north west (NW). We are downwind from Rocky Mountain National Park. We have a very good chance of having measurable rain today.

The Inter Tropical Convergence Zone (ITCZ), which is mirrored in the lines of tropical storms is propagating north. The GOES-West, infrared satellite images indicate the the ITCZ, a belt oriented along an east-west axis, is now located over Mexico, the northen extent of which is Manzanillo, Mexico. A surge of low-level moist air has come up from the south over the Gulf of California and is moving over the desert along the Arizona-California border. Here 100+ F temperatures combined with high dew point temperatures of 50 to 60 F must make that environment just awful.  Unfortuneately, very hot dry air at the mid atmospheric level (500 mb) renders the environment convectively dead.  Farther east over Aizona, there may be some hope for showers, but there the dew point temperatures are lower.

04 July 2017 10:15 MDT:

The surface observations reported by NOAA are available at the METAR site., which for Denver, Co at 1600 UTC are the following:

METAR for:

Note that the observation time was actually 1553 UTC, where UTC is the new name for Greenwich Mean Time (GMT). UTC and GMT are almost the same but not quite, the former being recommended for scientific records involving time. In meteorological reports, UTC time is abbreviated to Z for Zulu, or military, time.

The western coast of Mexico, facing the Gulf of California, has been generating thunderstorm complexes every afternoon now, in response to a surge of tropical moisture up the narrow gulf that was prompted by a tropical disturbance passing over Mexico. The surge of moisture yesterday brought a line of thunderstorms in Arizona along the Mogollon Rim into SW New Mexico, and mid level moisture from this surge reached West Texas spawning a thunderstorm complex over the Davis Mountains.

05 July 2017, 13:12 MDT, Boulder CO:

The following. weather observation was made in Denver Colorado at 1853 z (12:53 MDT):

07 July 2017, 17:20 MDT, Boulder CO:

The temperature is 76 F (24C) after a light shower (0.02 inches or 0.5 l) brought the temperature down from about 89 F (32C). Yesterday, the official temperature at Denver reached a high of 100 F (38C). The corresponding temperature for my house in Boulder county was about 93F (34C). Boulder is not higher in elevation than Denver, but we are closer to the Foothills that initiate showers around noon. These showers often bring down or level off the afternoon temperature so that it is a few degrees cooler than that over the plains east of highway I-25.

The monsoon is not yet well established in NW Mexico. A few days ago a tropical disturbance to the south, moving westward, brought a surge of moisture that reached southern Arizona at the surface. This low-level moisture fed a line of thunderstorms that followed the Mogollon Rim. The deep moisture however, turned across northern Mexico and into W Texas where a thunderstorm complex formed over the Davis Mountains. Today, a patch of subsidence marked by dry air moved over the extreme northern portion of the Mexican monsoon belt, temporarily shutting off convection there. However, the effects of other disturbances arriving from the east are triggering a convective complex in the mountains west of Los Mochis, Mexico.

At present, I am conducting a number of analysis of the evolving tropospheric structure  over NW Mexico and SW United States to get an idea of how the summer rainy season become organized over those areas. I intend to write something about this in a later post.