"Winter thunderstorm" is not an oxymoron. Indeed, even in central Pennsylvania, thunder-snow squalls occasionally accompany the core of a very cold 500-mb low (like the fitful squall during the winter of 2002 - see image at left). My point here is that just because it's winter doesn't mean that thunderstorms - even severe thunderstorms - can't occur. In this section, I'll try my best to give you an overview of where and, to a lesser extent, how winter thunderstorms occur (my goal is to impart sufficient information that students can carry on a meaningful discussion).
Let's start with the Pacific Coast. During winter, very cold, high-altitude pools of air associated with upper-level low-pressure systems routinely swing southeastward from the Gulf of Alaska, destabilizing the troposphere and setting the stage for thunderstorms (usually weak to modest instability compared to the highly unstable environments associated with classic outbreaks of severe weather over the Great Plains).
California can "rock'n'roll" much more than Washington and Oregon. Indeed, the Storm Prediction Center occasionally issues severe-thunderstorm and tornado watches for the Central Valley of California when conditions are favorable for supercells to form (supercells are thunderstorms with rotating updrafts - they often produce severe weather). As a general rule, California supercells erupt behind cold fronts associated with strong, occluded mid-latitude cyclones (in concert with the trailing, cold 500-mb low moving inland). Moreover, research has shown that a strong jet streak typically accompanies the outbreak of tornadic thunderstorms in California. The schematic below shows the classic synoptic set-up for severe thunderstorms in the Central Valley during the cold season.
Let's examine this pattern favorable for severe weather more closely. when a strong low-pressure system approaches the California Coast during winter, the cold front can sweep well inland well east of the longitude of the surface low. The west-southwesterly flow behind the advancing cold front then paves the way for a lee trough to form in California's Central Valley (click here for a primer on lee troughs). Meanwhile, with the syrface low still lingering offshore (northwest of San Francisco, for example), Califirnia's Central Valley channels the low-level flow of air, causing expected southwesterly post-frontal winds to blow from the southeast instead. When a Pacific cold front sweeps relatively far east of the parent surface low, the trailing 500-mb trough often deepens (intensifies), producing robust southwesterly or westerly winds in the middle troposphere.
Such patterns are conducive to small thunderstorms erupting in the Central Valley because the lower troposphere often becomes unstable (sunshine in the wake of the cold front heats the recently-moistened ground beneath the approaching cold 500-mb trough) and 300-mb divergence associated with the jet streak enhances lift. In such an environment of dynamic lift and vertical wind shear, these storms can acquire rotation fairly quickly and become supercells. Lesson learned: Most of California's infrequent but recognizable regional severe weather events typically occur in concert with low-level southeasterly (and post frontal) winds. I'll add that southeasterly winds ahead of the lee trough also advect moisture northward (fuel for thunderstorms -- I should note that severe thunderstorms that form closer to the coast draw moisture from relatively humid air over the Pacific).
For the rare cases of small-scale tornado outbreaks in the Los Angeles metropolitan area, a modified version of this pattern sometimes mirrors the more common scenario in the Central Valley. Near Los Angeles, the surrounding mountains can cause otherwise southwesterly low-level winds to blow from the southeast or south-southeast whenever a low-pressure system approaches from the northwest. A case in point ... Around midnight (local time) on December 28, 2004, a 500-mb low pressed eastward from the Pacific (check out the 08Z analysis and note the southwesterly 500-mb flow over southern California). Meanwhile, mountains channeled the low-level flow, creating southeasterlies (check out the 08Z analysis of 1000-mb streamlines below). With the judiciously placed left-exit region of a 300-mb jet streak arriving overhead, the stage was set for small supercells to erupt (check out the 08Z radar reflectivity). These storms produced a couple of small tornadoes that damaged parts of Los Angeles (see the storm reports).
As it turns out, the low-level southeasterlies sometimes form a low-level jet stream, which is a narrow channel of relatively fast winds that sometimes develops in the lower troposphere. For the record, I typically use 850 mb (approximately 1.5 kilometers) as a proxy for the altitude of low-level jet streams. Besides rapidly transporting moisture, southerly low-level jet streams increase vertical wind shear (in the lower troposphere) and thus can raise the stakes for tornadoes (you will learn more about the role of low-level wind shear and tornadogenesis in Lesson 7). I note that upper-level jet streams are often coupled with low-level jet streams, so you now have a better sense of why the classic synoptic pattern over California can favor tornadoes during winter. In such situations, meteorologists say that the upper-level jet stream and low-level jet stream are coupled (for the record, coupled jet streams also occur east of the Rockies).
To see coupled jet streams in action, check out the 250-mb and 850-mb analyses at 00Z on May 5, 2001. Note that a low-level jet set-up over north Texas and Oklahoma in the left-exit region of the 250-mb jet streak over eastern Texas.
To gain scientific insight into coupled upper-level and low-level jet streams, check out the schematic above (keep in mind the northerly ageostrophic component to the wind and the corresponding divergence that occur in the left-exit region of the 300-mb jet streak). In response, a region of negative pressure tendencies develops in the lower troposhere beneath the area of upper-level divergence. The pocket of pressure falls causes low-level southerly winds to accelerate, which often paves the way for a low-level jet stream. With the idea of coupled jet streams in mind, a low-level, southeasterly jet stream can develop over California during winter in concert with an arriving 300-mb jet streak. Such a low-level jet stream rapidly transports moisture northward and increases the low-level vertical wind shear (and thus heightens the risk of California tornadoes).
In case you're wondering, there were indeed coupled low-level and upper-level jet streams associated mini-tornado outbreak near Los Angeles on December 28, 2004. Check out (below) the 850-mb (left) and 300-mb (right) analyses of vector winds ... arrows depict wind direction and wind speeds are color-coded in meters per second. Folks, this is a classic configuration of coupled jet streams. Again, the vertical wind shear associated with the low-level jet stream and southwesterly flow at 500 mb heightens the risk that thunderstorms can attain rotation and spawn small tornadoes in California during the winter and early spring.
Lest I leave you with the impression that a cold, upper-level 500-mb low is the only way to trigger winter thunderstorms in California, I point out the prolonged spate of heavy rain (with isolated tornadic thunderstorms on the 9th and 10th) that inundated Southern California between January 7-11, 2005. You probably will remember this horrifically wet and stormy period if I remind you of the tragic landslide that buried a subdivision of homes in the small coastal town of La Conchita (where ten people were killed) photo #1, photo #2. Nearly 30 inches of rain fell in the mountains west of Los Angeles (obviously, orographic lift was at work). Flooding across the region took a huge toll on property (millions of dollars in damage - see these images: photo #1, photo #2).
In this case, there wasn't any cold 500-mb low pressing inland overhead. Rather, a positively tilted 500-mb trough hovered offshore (the corresponding 500-mb low was centered west of Washington State - check out the chart of 500-mb heights at 12Z on January 8, 2005). In turn the positively tilted trough drew the relatively moist subtropical jet stream northward (check out the chart of 250-mb vector winds at 12Z on January 8, 2005, below). The bottom line was that this configuration was a perfect set-up for thunderstorms and heavy rain, even though the cold core of a 500-mb low was really not directly involved.
Winter thunderstorms along the West Coast can also produce hail when the cold core of upper-level lows press inland. Although we'll study hail later in greater detail, it should make sense to you that it needs to be chilly aloft for hailstones to survive their trek to the ground. By way of preview, forecasters routinely use a maximum altitude of 11,000 feet for the wet-bulb zero as one guideline for predicting hail. The wet-bulb zero (WBZ), in case you're interested, is the altitude where the wet-bulb temperature is zero degrees Celsius (the wet-bulb temperature is the temperature to which the air cools by evaporating water into it while keeping the pressure constant). If you want to experiment with wet-bulb temperature, which depends on relative humidity, check out The Weather Calculator. You might also play with this interactive tool that shows you how to calculate wet-bulb temperature on a skew-T diagram.
At any rate, the WBZ constitutes the level at which hailstones would start to melt. So, in order to have hail reach the ground, the wet-bulb zero, as a general rule, must be lower than 11,000 feet. Thus, the idea that pools of cold air associated with upper-level lows provide a favorable environment for hailstones that form in thunderstorms over the West Coast should not seem far-fetched. To drive home my point, hail fell at Arcadia near Los Angeles on October 17, 2005, when an unseasonably cold 500-mb low (here are the corresponding 500-mb temperatures) swung over Southern California.
Upper-level cold pools promote thunderstorms along the coast of the Pacific Northwest. On February 25, 2004, for example, a cold 500-mb low (500-mb heights, 500-mb temperatures) swung southeastward from the Gulf of Alaska and caused thunderstorms (as seen in this multi-spectral satellite image) to develop over Pacific waters west of Oregon and northern California (learn about multi-spectral satellite imagery). Thunderstorms with small hail later moved onshore.
Generally, lower surface temperatures and dew points limit severe thunderstorms over the coast of the Pacific Northwest during winter. Typically, severe thunderstorms associated with upper-level cold pools must wait until spring. On April 29, 2003, for example, thunderstorms associated with a 500-mb low (500-mb heights and 500-mb temperatures at 12Z) hit the Portland, Oregon, region (left, below). Funnel clouds were sighted (right, below), but, without a jet streak over the Pacific Northwest to promote strong vertical shear, severe weather was rather limited.
During the first six weeks or so of the spring semester, keep your eye on the West Coast for synoptic patterns conducive to thunderstorms. During these times, please consult the Web site at the Storm Prediction Center to help you formulate your discussion.
Of course, winter thunderstorms occur elsewhere in the country - such as the Gulf Coast States (to which I alluded in the Introduction). Sometimes, snow accompanies convection, as it did at Salt Lake City around 00Z on November 28, 2006 (technically, just before the official start of meteorological winter on December 1, but clearly within the confines of the cold season). For starters, check out the lightning data from 0000 UTC to 0059 UTC and the timely METAR at 0032 UTC ... here's a primer that will refresh your memory on how to decode METARS. The earlier 12Z model runs predicted a strong jet streak to dive southeastward in concert with a digging trough associated with an arctic air mass invading the Rocky Mountains from Canada (below, please note the 12-hour NAM forecast for 300-mb heights and wind speeds, valid at 00Z on November 28). The strong uplift in the left-exit region of the jet streak worked in tandem with a conditionally unstable sounding to set the stage for thunderstorms.
The 12-hour forecast for MSLP and 1000-500-mb thickness from the 12Z NAM run on November 27 (valid at 00Z on November 28) showed thicknesses dropping below 546 dekameters at Salt Lake City. Hold your horses, Grenci ... I thought that 540 dekameters was the universal critical thickness. Not so fast, my friend. Based on statistical studies of observations of thickness and precipitation type, the 50-50 chance of frozen precipitation (in other words, the critical thickness) increases with increasing elevation. The basis for this relationship between critical thickness and elevation is that the "warm" boundary layer (where snowflakes can melt when conditions are marginal) tends to shallow (become thinner) as elevation increases. At Denver, Colorado (elevation of 5,333 feet), for example, the critical thickness is approximately 552 dekameters.
The thunderstorms at Salt Lake City were surface-based, meaning that buoyant air parcels rose from the ground to form the updrafts of storms. That's not always the case during winter ... thunderstorms can also be elevated. Read on.
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