The decaying remnants of a tropical depression will move through the desert southwest over the next couple of days, dragging with it enough tropical air that residents might think they woke up in southern Florida. This excess moisture will lead to very heavy rain that could easily produce flash flooding in vulnerable areas.

It is unbelievably gross right now in places like Arizona and southern California. The “it’s a dry heat” joke is just a dream on a day like today. Take a look at this dew point map—cooler colors show drier air, while warmer colors show air you can swim through.

The 11:00 AM temperature in Phoenix was a manageable 85°F, but the dew point was 73°F. To put that in perspective, the dew point in Key West, Florida, at the same time was also 73°F, and the dew point in Honolulu was 74°F. Dew point values in the 70s are gross. Very gross. We’re talking about moisture that makes you sweat within a minute of stepping outside.

The dew point is a much better measure of the moisture in the air than relative humidity, since the relative humidity is...well, relative. Air temperature goes up, relative humidity goes down. Air temperature goes down, relative humidity goes up. That’s why the relative humidity can be 35% on a hot day and it still feels like death.

Normally when you think of a desert, you think of air so dry it can crack your skin in half at the first gust of wind. Most of the southwest is as humid as the Gulf Coast and Hawaii right now because of Tropical Depression Sixteen-E, a system that just made landfall on the eastern shore of the Gulf of California (or the Sea of Cortez, depending on your preferred name for it).

On its current path, what’s left of the system will trek right over Arizona and New Mexico through the day today and tomorrow, bringing with it oodles (technical term) of deep, tropical moisture from the Pacific. In addition to the dew point, one of the ways we can judge how much moisture is present in the atmosphere is a metric called “precipitable water,” often abbreviated as PWAT.

Precipitable water, measured in inches, is the amount of rain that would fall if you condensed all of the moisture in a small column of the atmosphere. For example, if the PWAT is 1.35” in Little Rock, it means that if you were to wring out all of the moisture in the atmosphere over Arkansas’ capital city, it would produce 1.35” of rain.

The notability of these values is relative to where they were measured—a PWAT of 0.60” on a late September day is normal in northern Maine, but it would be well below normal—nearing record lows—down in southeastern Louisiana.

This morning’s weather balloon launch in Tucson, Arizona, found a PWAT over the southeastern part of the state of 1.69”. For the desert, that’s significant. Above is a chart from the Storm Prediction Center showing PWAT averages from tens of thousands of early morning (1200z) weather balloon soundings in Tucson since the 1950s.

The thick black line in the middle shows you the average PWAT values in Tucson on any given day, while the thin red line and the thin blue line show you the record highest and record lowest PWAT values on each day.

The PWAT of 1.69” recorded by this morning’s weather balloon is a record for the morning of September 21, and it’s well above the average of about 0.89” you’d typically see on the first day of astronomical fall and the tail-end of monsoon season.

All of this is delightfully geeky, sure, but what does it mean in practical use? Higher PWAT values mean that showers and thunderstorms have more moisture to work with; when storms can tap into deeper moisture reserves, it means there’s a real chance that areas will see heavier, more prolific rainfall, and that’s not something you want in a desert.

This morning’s forecast from the Weather Prediction Center shows several inches of rain falling over the desert southwest—mainly over Arizona—with some areas seeing two or maybe even three or more inches of rain if they get caught under the heaviest thunderstorms. Flash flood watches are in effect from southern California through western New Mexico, and as of this post, there’s already a flash flood warning up for Nogales, Arizona, due to 1.50” to 2.00” of rain falling in just a couple of hours.

If you’re going to be in the area at any point through mid-week, keep an eye on arroyos (washes) and areas that traditionally flood when it rains heavily. Don’t drive through a flooded roadway—no matter what’s on the other side of that flood, fording a water-covered roadway isn’t worth risking your life and the lives of your rescuers. It doesn’t take much water to lift a vehicle and hurl it downstream. Even if you survive, Arizona has a “Stupid Motorists Law” on the books that makes you pay for your water rescue if you drive through a flood.

[Images: AP, GREarth, author, SPC, author]

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