Forecasting Difficulties of the Rainshadow


Mount San Jacinto stands 10,834 feet above sea level, and is the second highest mountain range in Southern California. ("Mount San Jacinto SP"). It is a majestic sight, located directly northwest of a vast array of flat desert lands known as the Coachella Valley. In particular, the city of Palm Springs, a popular resort city that lies at the base of the mountain's leeward side, experiences weather that is very different from that of its southern California counterparts. Palm Springs sees sunshine, while a nearby city sees snow. It pours rain just a mile away from Palm Springs, with not a drop falling on the sunny city. The San Jacinto Mountain range can be credited with bringing numerous dry and sunny days to Palm Springs, but it is also this mountain range that causes uncertainty among weather forecasters in the area, because of its ability to alter air masses, and often change weather phenomenon as it passes over the region, bringing surprises in the form of the occasional desert washout, or the inevitable big bust.

The glamorous city of Palm Springs boasts 350 days of sunshine a year. ("Vacation Palm Springs"). For this reason, it is popular among golfers, outdoor enthusiasts and tourists. With so many consecutive days of sunshine, forecasting weather in the region must be a simple chore, right? Not necessarily. Sure, for the most part, forecasting sunshine and mostly clear skies is a safe bet among weather personalities in the area. But there are those days - the 10 to 15 days a year featuring significant weather events - that tend to throw these weather forecasters a curve ball. Be it unexpected rainfall, a ruthless wind event or even deadly heat, there is often a degree of surprise about the real weather in Palm Springs, due mostly in part to its massive westward mountain range. Palm Springs literally, does not see it coming. And when they do see it coming, it's hard to tell how much the city will get, or how long it will last.

Climatologically speaking, desert climates get their name because they experience irregular rainfall of less than 250 mm (9.75 in.) per year, very high evaporation rates, and low relative humidity and cloud cover. ("Rain shadow Deserts"). Specifically, Palm Springs is shielded from large amounts of precipitation, by the San Jacinto Mountains. This mountain range (situated almost parallel to the coast) separates seaboard cities from inland ones, and acts as a barrier amongst moisture-laden clouds. The range creates a classic case of the rainshadow effect.

A rainshadow occurs when prevailing winds carry moisture from the ocean, eastward over dry land, dropping precipitation on the windward side of a mountain range. Clouds are forced to rise over the range, causing the air to cool and drop its moisture. Very little precipitation falls on the leeward side of the mountain, creating a dry desert climate directly east of the range. The resultant desert is said to be in the rainshadow. ("A Rain Shadow"). Palm Springs is a prime example of a southern California rainshadow. The city sees, on average, 5.8" of rain per year. Compare this to its westward (windward) neighbor, Beaumont, which receives an average 19.3". ("Palm Springs").

There are small-scale difficulties associated with forecasting weather in the rainshadow that occur on a daily basis. Grey skies on the windward side of the mountain become bright blue over the Coachella Valley. Light showers crossing the mountain peaks meet dry skies over the desert. But there are larger problems to forecasting as well, that are less frequent, but more serious and also a direct result of the rainshadow effect. In particular, forecasting large precipitation events (chances, amounts and timing of a storm) in the desert is a sketchy task, at best. It seems to be just a little too easy to get it wrong in the rainshadow.

Simple topography of a mountain range, with its varying elevations, makes forecasting on the leeward side of the range difficult. The summit of Mount San Jacinto lies at 10,834 feet above sea level. Along the same range is Valley Station 2,643 feet, and Mountain Station, 8, 816 feet. ("Mount San Jacinto SP"). Air masses ascending and descending the mountain take on different characteristics at different points up and down the range. For example, a moist air mass approaching from the north may provide the bulk of its precipitation on the northern most region of the range, leaving light showers in the northwestern portion of the Coachella Valley (Palm Springs), while the southeastern portion of the desert (perhaps the city of Thermal) receives drizzle (or none at all). Conversely, a tropical air mass approaching from the south (or shortest end of the range) may leave abundant rain near Thermal, with the same large rainfall near Palm Springs. It's difficult (almost impossible) to tell how much rain will fall on the desert floor, with a lack of uniformity concerning heights on the mountain range. In fact, it is not uncommon for the National Weather Service to give preliminary rainfall estimates of less than a tenth of an inch of rain for all desert cities, with much more than that actually falling on a few of them (due in part to the uneven mountain topography.) Or they predict moderate rain at times, with none at all making it to the floor. (This is often the case.)

Being that there are so many pleasant weather days in the area, the anticipation of a big weather event tends to draw a lot of attention. That attention translates into front page newspaper articles, souped up television newscasts and even more air time for local weathercasters. It is hyped and over-hyped. Forecasters that tend to be taken for granted close to 350 days a year are now in the spotlight, forced to answer the inevitable question. The question is not "Is it going to rain?" The question, almost always sarcastic in nature, is, "Is it really going to rain this time." This alludes to the fact that there are many instances when rain is possible in the forecast area (usually a 20 percent to 40 percent chance) and it fails to occur. This predicament is all too common throughout the Coachella Valley, and more often than not, the sky doesn't fall.

There are consequences to forecasting a significant event incorrectly in Palm Springs. The reputation you have as a weather forecaster 350 days of the year doesn't matter, if the 10 times you are put to the test, you fail. And failing (or the appearance of it) when forecasting on the leeward side of the San Jacintos is inevitable. It is quite possible to forecast light showers, and end up sending breaking news vans to the scene of major flooding and road closures. That's because the leeward side of the range, with very dry conditions, encompasses very dry, hard surface - conducive to runoff. Even a correct forecast of small amounts of rain could appear incorrect, if runoff and poor drainage leads to road closures. (It happens.) This runoff adds to the impression that precipitation amounts were underestimated, when in fact, they may not have been. In many cases, it's a lose-lose situation for forecasters in the region. Forecast a chance of showers and fail to see rain - you're wrong in the eyes of your viewers who watch an over-hyped newscast. Forecast rain and experience road closures - it appears you underestimated the storm.

Although there are many books and websites that discuss the subject of the rainshadow, "one problem in studying the rain shadow effect is that information on this topic is very difficult to obtain. I have an entire bookshelf full of weather books, and none of them have more than a glossary definition," said Matt Haugland, a weather professional who hosts a website about the rainshadow. ("The Rainshadow Effect"). Furthermore, my complete search of the web turned up little else. It seems that aside from the meteorologists at the National Weather Service, no expert advice for forecasting weather in a rainshadow exists. Even the National Weather Service located west of the San Jacintos, depends heavily on weather spotters for information on storms that cross the range. I know, I am one. And this dependency does little to help with advance notice of storm potential.

In my three years of experience forecasting the weather in Palm Springs, I have found that the most effective way to improve forecasting in a desert climate, is to experience the seasons over a lengthy period of time; perhaps years. Learn to recognize patters, particular setups, and keep journals of the results. Pay close attention to extreme events, and the atmospheric conditions that caused them, when they do catch you off guard. The best forecasters in the desert are those that have been in the region the longest.

Mount San Jacinto may create a forecasting problem for weathercasters living in Palm Springs, with its varying elevations. But it is also this range that can be credited with creating the popular desert city so many people travel to see every year. It is this range that lends to the 350 plus days of sunshine in the area, and most likely one of the main reasons forecasters (and anyone else, for that matter) choose to live in the sunny city.

Works Cited

"A Rain Shadow". (
"Mount San Jacinto SP". (
"Palm Springs". (
"Rain Shadow Deserts". (
"The Rainshadow Effect". (
"Vacation Palm Springs". (
Photo of Mount San Jacinto. (
Map of Southern California. (