The basics: Pressure gradient
Air in motion – the wind – arises from a difference in pressure over distance. Lines of constant pressure, called isobars, are used to visualize the pressure gradient on weather charts. The schematic below depicts an intense, wintertime cyclone in the mid-latitudes. The center of low pressure is shown by the red “L”, and an adjacent region of high pressure (to the west) is shown by the blue “H”. Isobars are the light gray lines in between the low and high centers. The closer these isobars are spaced, the more intense the pressure gradient, and the stronger the winds.
Some winter storms, especially those over the adjacent, warm ocean, become very intense – a consequence of the strong regional temperature differences during winter. Adjacent regions of high pressure, embedded within dense, arctic air masses, are also quite strong. So a tight pressure gradient, like that depicted in Figure 1, is really a wind-generating system resulting from the interaction of strong low and high pressure. This is also why winds usually blow most strongly once the storm’s cold front blows through; the high wind zone (hatched magenta region) is broadly located on the backside of the storm system. Sustained winds from the northwest are often in the 20-30 mph range, with gusts that top 40-45 mph. Such is the case today.
East Coastal lows that deepen rapidly, called bomb cyclones, strengthen the pressure gradient over the Mid Atlantic and especially New England. Strong, perhaps damaging, wind gusts coincide with the period of rapid deepening. For the Washington D.C. region, however, the storm often pulls away as it rapidly deepens, limiting the duration of our high winds.
Warm sector: Red sector alert!
The above image shows another windy location in a winter storm – its warm sector, the wedge south of the warm front and east of the cold front. Here, mild and humid southerly flow increases air mass instability. Lines of showers, and possibly thunderstorms, erupt in the warm sector (shown by the yellow and red scalloped regions, which mimic radar reflectivity). Additionally, a narrow, fast-flowing river of moist air often surges northward through the warm sector, called the low-level jet (also known as the warm conveyor belt). The jet is often strongest between 5,000-10,000 feet above the surface and may blow as strong as 60-70 mph.
When convective showers or thunderstorms erupt into the core of jet – as shown by the dotted, magenta oval – convective downdrafts can drag the jet’s momentum to the ground. Strong or damaging winds (50-60 mph) may gust within this region. The most intense winds tend to be localized and short-lived, rather than widespread and sustained (as on the backside of the storm). The National Weather Service (NWS) will issue a severe thunderstorm warning to cover the most intense cells, even though the convective clouds may not be deep enough to generate lightning.
Rarely, the pressure gradient within the storm’s entire warm sector may become super-intense. This happened over Washington on January 30, 2013; strong southerly winds generated mild gusts topping 45 mph, necessitating an unusual, warm-sector wind advisory during the middle of winter!
The isallobaric wind: Pressure that changes over time
Surface pressure that changes rapidly over time can create a gradient of pressure tendency, called the isallobaric gradient. If pressure drops rapidly in the core of a winter storm, while simultaneously rising in the adjacent high pressure cell, airflow accelerates between these regions of rapid change. The so-called isallobaric wind is distinct from the wind generated by the pressure gradient alone, but adds to the overall wind intensity.
The image below illustrates this idea for a case on November 12, 2007. The low pressure and adjacent high pressure regions are shown by the red “L” and blue “H”, respectively. Isobars are depicted every millibar, using solid gray lines. The very tight gradient over West Virginia and Virginia generated a stiff wind from the west, of its own accord.
The dashed blue contours show the region of pressure falling rapidly, near the center of the storm, at a rate of -2 to -3 mb per hour. Over Appalachia, red solid contours depict a simultaneous, strong pressure rise, approaching +4 to +5 mb per hour. The isallobaric gradient, spanning the region between these two tendencies, was quite strong, and created its own isallobaric wind, shown by the solid black arrow.
The foregoing discussion illustrates that the wind field is not a static entity; it is dynamic, changing literally by the hour. When a strong couplet of pressure fall/rise is superimposed on a strong pressure gradient, look out, the wind is going to howl!
Mixing it down: Fast winds at high altitudes
When it comes to predicting maximum surface winds, it’s important to distinguish between the sustained wind – which is typically a wind-minute average – and the more transient gusts. Gusts often exceed the sustained wind by a factor of 40-60 percent, and this value is larger over rough terrain, where turbulence becomes more extreme.
Gust prediction also requires us to think three-dimensionally. Much of the surface wind is derived from the pressure gradient or isallobaric component, but high momentum can also be brought down from fast flowing air aloft.
Winds invariably blow stronger with height, because the friction force (created by the rough surface) weakens with altitude. This creates a wind shear profile, or increase in speed with altitude, as shown below. This u-shaped (parabolic) profile of increasing wind speed is characteristic of the layer of the atmosphere just above the surface (known as the boundary layer), which extends about half a mile to a mile up.
The swirling winds feeding a strong storm really pack a punch a few thousand feet (meters) off the deck. It’s not uncommon to see westerly winds in the 70-80 mph range at these levels. If the air mass is unstable, convective currents churn within the layer. The overturning effectively mixes down some of the fast-flowing air to the surface. The process is shown in Figure 3, and is technically called “downward momentum transport”. Because the process is convective in nature, the fast flow arriving at the surface is localized and short-lived. The strongest wind gusts in the wake of a strong storm are often generated in this manner.
Weather models provide some guidance on the degree of momentum mix-down; rarely is the full intensity brought to the surface. Factors include the strength of winds aloft, depth of wind, and amount of instability. Most often, the instability is maximized during the hours of strongest solar heating, even in mid-winter. Cold air moving in from the north and west, behind the cold front, will warm sufficiently near the surface, creating a several thousand feet deep mixing layer. This is why wind advisories are sometimes timed to begin at sunrise and end at sunset. Stratus cloud cover, which sometimes wraps around the backside of lows, limits destabilization and thus mixing potential.
Briefly, it’s important to note that locations at high elevation, such as the Catoctins, Blue Ridge and Allegheny Front, are susceptible to higher surface wind – simply because the land surface projects upward into the profile of strong wind shear. This simple but critical concept is illustrated below along with some representative wind speed values.
So far, we’ve identified several factors leading to sustained and gusty winds in the wake of a strong winter storm: Pressure gradient, isallobaric wind, convective mixing (downward momentum transport), and elevation. All of these factors can push gustiness to the extreme for several hours, even under bright sun, sometimes approaching 70 mph. These winds are capable of widespread damage, including broken limbs, uprooted trees and utility outages. Highway travel becomes hazardous when gusts buffet cars and high-profile trucks – particularly over long spans such as the Bay Bridge. Air travel is delayed when strong cross-winds prevent aircraft from taking off or landing. And high winds fan wildfires. Let’s take a brief look at the criteria used by the NWS to issue advisories and warnings.
Wind Advisory and warning criteria
The NWS wind advisory criterion is precisely defined as sustained winds of 31-39 mph and/or gusts of 46-57 mph over land. A High Wind Watch will be issued if the following criteria are anticipated: (1) non-convective winds greater or equal to 40 mph lasting one hour or longer, or (2) winds greater than or equal to 58 mph for any duration. A high wind warning will be issued if winds of this magnitude are imminent or newly observed. It is useful to note that the 58 mph threshold is also used to define a severe thunderstorm.
Several wind advisories are often issued each year for our region, most often for the higher elevations west of the immediate D.C. metro. An example of the typical wind distribution during an advisory is shown in Figure 5, a case from November 23, 2011. This figure shows peak gusts in the wake of a coastal storm (shown in knots; multiply by 1.15 for mph). Peak gusts are confined to 30-35 mph over the Piedmont of Maryland and Virginia. However, note the increase to 45-50 mph along the Catoctins, Blue Ridge and Alleghenies. Only high-elevation counties were captured under a wind advisory that day. (Note, also, the enhanced band of winds aligned with the Chesapeake Bay, as westerly winds accelerated over the wide, flat water surface).
The image below portrays peak wind gusts for a stronger storm system on December 22, 2012, which created high winds everywhere down to sea level. Widespread wind advisory coverage ensued, for peak gusts in the 50-55 mph range – particularly along the ridge tops.
Low-end wind advisories typically do not incur utility disruptions, unless other factors come into play (discussed below). With high-end advisories, localized outages can occur. Wind warning situations – while rarely issued for the low elevations of Washington and immediate suburbs –can incur widespread utility damage. This was the case during twin, back-to-back wind events in February, 2011. Both of these events were “clear sky”, in the wake of powerful coastal storms. All four factors – pressure gradient, isallobaric wind, momentum transport and elevation – played key roles. Widespread gusts of 50-60 mph raked our entire region on February 19, fanning numerous wildfires and causing 80,000 utility outages. On February 25, an even stronger wind event propelled gusts area-wide into the 60-70 mph range, with over 150,000 outages, most triggered in the first hour of high winds.
A few final details
No one likes power outages, particularly in the dead of winter. When assessing outage likelihood, factors in addition to peak wind gust become important. Intense, late-fall cyclones, with foliage in partial abscission, lower the threshold for widespread outages. Recent heavy rains that saturate soil, or soils waterlogged from deep snowmelt, increase susceptibility to treefall. Tree limbs recently weakened by heavy, wet snow accumulation or thick ice accretion are more likely to fail at lower wind speeds. There are no hard-and-fast rules for any of this, so there are subjective margins of error when predicting the likely extent of wind damage.
I hope this article has shed some light on the challenges in forecasting high winds during winter storms. This “invisible” aspect of the storm can prove just as disruptive, and dangerous, as heavy precipitation and sudden temperature swings. Surprisingly little of this information can be found in any of the introductory meteorology textbooks, except the part about pressure gradients; this is the type of knowledge that truly distinguishes operational meteorology from pure, textbook theory.