It's been awhile since I've had a reason to post! Most of July has been devoid of anything but hot weather. Typically, the summer monsoon (which is a switch in seasonal upper level winds from the west to the southeast), provides for periods of wet weather, with afternoon and evening thunderstorms a fairly typical part of July weather for our area.
However, the best we've been able to do most of the past month is endure high humidity at times from gulf surges coming up from the southeast. The monsoon flow has stayed just far enough east of us to keep precipitation mostly to the east of the Colorado River thus far. It appears that may change in the next few days.
WILL WE GET RAIN?
Beginning as early as Friday afternoon, but more likely Saturday afternoon, we will see increasing cumulus buildups that should lead to scattered afternoon and evening showers and thunderstorms.
An increasingly humid airmass will invade the entire area over the weekend, possibly lasting through next Monday or Tuesday. This will likely create several days of the following weather conditions:
- Scattered showers and thunderstorms, especially over the nearby mountains and near the higher terrain near Joshua Tree.
- Increased humidity, especially Sunday and Monday, with dew points likely 65-75 degrees
- Possible heavy rain, hail and flash floods accompanying any storms.
- Possibility of localized high winds with storms, as well as a haboob/dust storm if large storm complexes form to our southeast.
- Lowering of high temperatures, possibly down to 100-108.
- Gulf Surges will likely intermittently create higher periods of humidity, accompanied by breezy southeast winds to 25 mph, dew points reaching as high as 75-78 degrees at times, and hazy skies with reduced visibility.
UPPER AIR PATTERN
WHAT IS A "GULF SURGE"?
Gulf surges are the most common source of our summer humidity here in the Coachella Valley. Weather patterns far to our east and southeast can set up in a particular manner, usually when a high pressure system is centered over the Four Corners region, that allows a southeast wind pattern to set up.
During the summer months, this particular setup allows for almost daily rounds of heavy thunderstorms to form along the mountain ranges of Northwest Mexico. Here's a recent satellite image, showing large thunderstorms over Northwest Mexico during the evening hours.
These southeast winds, when combined with the thunderstorm complexes in Northwest Mexico, interact in such a way to create the perfect "gulf surge." Here's a little more about how that occurs.
Thunderstorms form in a variety of weather conditions across the globe. Their structures allow these clouds to soar far into the atmosphere - sometimes over 55,000 feet high! They rely on "inflows" of air being inhaled into their bases, and due to rising air currents, the moisture condenses and forms into the cumulonimbus clouds that produce thunderstorms.
The portion of this process that many people are not as familiar with, are the "outflows" of air that descend out of the same cumulonimbus clouds. Often time the "inflow" portion under a thunderstorm cloud is dry, whereas the "outflow" portion under a cumulonimbus cloud is where the heavy rain, hail or wind is found. These outflows often flow out and downward, ahead of the thunderstorm, and are even stronger if they follow the direction that the storm is traveling.
Outflows often have much cooler, humid and dense air than the air flowing up into the same clouds (since the air is traveling downward through rain-cooled air). This slightly cooler and humid air often picks up considerable speed, sometimes gusting to 50-60 mph or more, and will flow out well ahead of the thunderstorm. Due to the southeast wind pattern, it helps steer these outflows northwestward, along the Gulf of California. There is less drag as air moves over water (compared to moving over land), so the humid air descending out of the clouds combines with the thin layer of humid air found over the Gulf of California during the summer months, and can move northwest from 20-40 mph or more, carrying the sticky airmass quickly our direction.
The dew points will reach 80 degrees or more along the Gulf, often cloaking the immediate coastlines near San Felipe, Puerto Penasco or Guaymas, Mexico with intense humidity during this type of weather pattern.
As the winds continue to blow northwest, the shallow, humid airmass is able to modify slightly, becoming slightly less humid (but still sultry) as it moves up the Colorado River Valley, and also moves up into the Imperial Valley, the Salton Sea, and up into the Coachella Valley. Dew points often reach 65-75 degrees, and occasionally, can go higher under extremely humid conditions. We will sometimes smell the hydrogen sulfide from the Salton Sea if these winds are strong enough to stir up the water, as the air blows towards the populated areas.
The effects of this pattern can be dramatic when the weather shift overtakes the Coachella Valley. The wind will shift to the east/southeast, (sometimes gusty and at other times, the winds may only be 10mph or less) and dew points may shoot up from the 40's to the 70's within 10-15 minutes. Skies often become hazy, and if this surge is a particularly strong one, it may be combined with a full-fledged "haboob", which is a wall of dust several thousand feet deep, accompanied by a severe wind shift and strong winds over 40 mph. Occasionally, thunderstorms may even follow in the hours to come... but most of the time, it just becomes sticky, hot, humid and oppressive.
Gulf surges often arrive here in the early morning hours, due to thunderstorms the night before, in the "source region" of Northwest Mexico. It takes about 6-12 hours for the weather systems forming that far away to arrive in our local area. As is often the case, the gulf surge will last 2-6 hours, then the humidity levels will begin dropping, dew points will retreat back into the 50's and 60's, and winds will eventually turn calm, or return back to blowing from the northwest... unless distant storms reform over the same area, and cause the same process to begin yet again the next afternoon and evening.
LONG TERM OUTLOOK
Enough about gulf surges - and back to how things look for next week. At this stage, it appears this moist pattern will likely dry out sometime around next Tuesday or Wednesday, and high temperatures will climb back up closer to 110 degrees again by the end of next week. However, this is a low to moderate confidence outlook.
In the meantime, take note if the National Weather Service issues severe weather or flash flood warnings this weekend or early next week. Moving water is dangerous! This pattern has the potential to cause damaging flash floods in localized areas.