Oh, Rainy Day: Weathering the Weather!

It has rained a lot over the past few weeks here in southern California. While this native Virginian is enjoying the smell of rain, the wet shoes, and the sound of thunder rolling in, my fellow Californian’s just don’t seem as receptive. The month of January alone has produced over 150% more rain than the monthly average in parts of the LA Basin. Some areas of SoCal have even experienced over 200% more rain than the monthly average for January, like parts of Santa Barbara and Oxnard. Take a look at the precipitation maps below:

A recent precipitation map illustrating percent average monthly rainfall in southern California for January, 2019 (NOAA)

Below is the same map view showing the total rainfall in inches across the entire Peninsular and Transverse Ranges of southern California. A whopping 13.57 inches was measured at the San Marcos Pass in Santa Barbara; 280% above the monthly average! This shear amount of rain has positively resulted in greater snowpack in the mountains, which translates into more available groundwater during the spring and summer. However, the immediate impact of road closures, potential landslide hazards, and flooding is currently being felt by residents for days to come.

A recent precipitation map illustrating the total rainfall (inches) in southern California for the month of January, 2019 (NOAA)

Weather & Terrain

If you look closely at these maps, a pattern begins to stand out. Those areas which are on the windward side of a mountain range get a lot more rain than the leeward side. Whoa, okay let me back up. The windward side is the side of a mountain which faces the prevailing wind, hence – windward. The leeward side is that side of the mountain which is protected from the oncoming winds. These are old nautical terms which hardly get any use today outside of meteorology, flying, or sailing.

When a warm moist airmass is pushed towards a physical barrier, like a very tall mountain range, the airmass is forced upward in a process known as orographic uplift (Ahrens, C.D., 2003). At this higher altitude, the temperature and pressure is lower than at sea level. As a result of the lifting process, the warm, moist airmass cools and condenses, eventually producing precipitation. By the time the cooled airmass moves over the mountain range, most of the moisture has been released as precipitation, and the airmass descends as a warmer, drier, weaker version of it’s old self.

Orographic uplift results in a “rain shadow” on the windward side of a mountain and an arid region on the leeward side. (Macmillanhighered.com)

Under Pressure

Over the past 24 hours southern California has experienced the passage of multiple cold fronts (See the image below). These cold fronts typically bring continuous showers, high winds, a drop in temperature, and the occasional thunderstorm; all of which we (I) have thoroughly enjoyed over the past week. You’ll hear a lot about frontal systems from your local meteorologist, but what exactly is a front? A front is a transition zone between two air masses with different densities (Ahrens, C.D., 2003). Because temperature plays a major role in air density, typically fronts separate air masses with contrasting temperatures. Thus “cold” and “warm” fronts refer to the relative temperature of that airmass. There are actually four types of frontal systems including: cold, warm, stationary, and occluded fronts.

You’ll also notice on the surface chart below that all of the fronts are associated with low pressure systems (L represents an area of low pressure, H represents and area of high pressure). The solid black lines are called isobars, or lines of constant pressure. If you put your finger on one of the solid black lines and follow it, the atmospheric pressure will remain the same along that line. These isobars help represent atmospheric pressure gradients, and if we recall all systems want to find an equilibrium, such that areas of high pressure move towards areas of low pressure.

A recent surface analysis chart of the Continental United States or CONUS. Cold fronts are depicted by the solid blue lines with triangles. Warm fronts are depicted by solid red lines with half circles. Stationary fronts are represented by the alternating red and blue lines, and occluded fronts are represented by the purple line. Frontal movement is depicted by the direction of the triangle of half circle on the colored line. (NOAA)

Sweater Weather

Without getting too deep into the details just yet, we can summarize by saying that low pressure systems usually bring poor weather conditions, and the type of conditions experienced are absolutely contingent on the characteristics of an airmass behind a front. An airmass is a very large body of air with similar properties in any horizontal direction at any given altitude. These properties include moisture content (humidity), temperature, and pressure.

Currently southern California is experiencing the slightly chilling effects of a winter maritime polar airmass, which typically originates over Asia and the cooler polar regions (Ahrens, C.D., 2003). When this airmass moved across the Pacific Ocean to reach us, warmth and moisture was picked up along the way. Once this airmass arrived it was cool, moist, and relatively unstable; conditions perfect for the weather we are enjoying now!


Sources:

Ahrens, C.D., 2003, Meteorology Today, an introduction to weather, climate, and the environment, Thomson Learning, v. 7, 544 p.

Macmillan Learning, 2019, http://www.macmillanhighered.com/BrainHoney/Resource/6716/digital_first_content/trunk/test/hillis2e/asset/img_ch41/c41_fig08.html, accessed February, 2019

NOAA, 2019, National Weather Service, http://www.nws.noaa.gov/outlook_tab.php, accessed February, 2019

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