Maddox type IV event

Another flash flood pattern occurs during the monsoon season when a shortwave, usually and easterly wave moves under or around the southern and southwestern edge of the subtropical ridge.  This type of event is strongly tied to the southwest monsoon season and is almost exclusively observed in Arizona, New Mexico, southern California, southern Utah and southern Nevada.  The surface pattern with these events is usually ill-defined.  One configuration of the general idealized 500-hPA (left) and 700-hPa (right) patterns is shown below. 

There is often a weak mid-level low near or just west of Baja that combines with ridge or 500-hPa low near or just east of the 4-corners to produce deep southeasterly low level flow.  A weak spoke of vorticity often rotates around the weak upper low with an extension of it that moves into the southwest U.S.  The threat area is usually located in advance of this vorticity lobe.  Often satellite imagery, especially water vapor imagery, provides the best clues of the existence of the weak 500 low and any vorticity lobes associated with it.  Also,  satellite images sometimes show very intense thunderstorm complexes  that develop first over Mexico or the Gulf of California and then subsequently propagate northward or northwestward into the U.S. during the late night or early evening hours. At 700-hPa there is often a thermal ridge located near the threat area with some warm advection implied across the threat area. 

Both figures are adapted from Chappell’s COMET notes.

The surface pattern is often indistinct but there is often a thermal low over mexico with a thermal trough extending across southern Arizona into California.   Precipitable water averages 1.32” but surges can lead to values much high than that.  The GOES sounder Precipitable water product and AMSU total precipitable water loops are great resources for monitoring moisture surges.  A site that will provide a loop of the latter is located at

Winds at all levels are very weak. 


The original Maddox et al. article offered a slightly different composite than Chappell for type IV events.  His composite has a easterly wave moving under the 500-hPa trough with any upper low farther south than the Chappell pattern.  The main similarity is that the threat area is in advance of the westerly moving impulse in an area of deep moisture and weak mid and upper level flow.  The area of low dewpoint depressions scalloped in the figure. 

Surface dewpoints above 60oF extend across Arizona into southern California. 

An example of a warm season western flash flood

500-hpa heights and water vapor imagery for 18 UTC 8 July 1999

12 UTC 8 Jul 1999 500-hPa height and absolute vorticity analysis

15 UTC 8 Jul 1999 500-hPa height and absolute vorticity analysis

12 UTC ETA 850-hPa winds, dewpoints superimposed on a precipitable water image

12 UTC surface analysis,  surface dewpoints are dashed

Now let’s look at an actual flash flood case in the west.  First look at the various archtypes.  Note that there is a heat low over depicted over southern California and to its east,  there is a large area of 60oF plus

Surface dewpoints extending across Arizona and southern California that is very similar to the area shown on the Maddox et al., type IV archtype.  There is also a weak vorticity lobe (the area labeled with the 9) extending across southern California at 500-hPa and an inverted looking trough suggesting that some type of impulse is working westward to the south of the upper level ridge.  The soundings at 1200 UTC for Desert Rock, Nevada (DRA) and San Diego (NKX) indicate there is a deep, moist unstable layer and that the precipitable water values are well above normal (1.68 for DRA (around 168% of normal) and 2.01 inches for NKX). 









15 UTC 8 Jul 1999 300-hPA, heights winds and temperatures.

The soundings suggest more moisture than shown on the composite type IV from the Chappell COMET notes.  The deep moisture and almost MAUL (moist, absolutely unstable layer) and weak winds aloft suggest that the cells may move slower than is typical for cells over the southwest.  The rather moist sounding suggests that potential downdrafts and outflow will be weaker than normal.  The weak flow suggests advection of the cells will be weak.  

The winds on the 15 UTC 300-hPa analysis are fairly typical of the pattern associated with this type of event.  The flow is often anticyclonic at upper-levels.  This ensures weak inertial stability which makes it easy for upper- level divergence to form with any cell that develops. 

Radar precipitation estimates for the event

Water vapor imagery (top left) indicates the convection had started to develop by 18 UTC.  The QPE for the date is shown above.  There were a number of areas of enhanced convection but that the serious flash flooding was located near Las Vegas rather than where the radar estimates were showing the heaviest rainfall accumulations farther to the northeast. The heavy rainfall amounts suggested by the radar imagery farther to the northeast may have been overdone because of the distance of the echoes from the radar site.  At such a distance,  the beam may be sampling a higher part of the cloud where hail contamination is more likely.  Another possibility is that there was very heavy rainfall across the area to the north but that few people live in the area so any flooding would go unnoticed.  The lack of good radar coverage is a forecast problem in the west and is a reason to combine radar information with satellite estimates when assessing rainfall over the intermountain region.


Floodwaters in the Las Vegas metropolitan area damaged or destroyed 369 homes with damage to public property amounting to $20,500,000 and caused two deaths.