The Ugly Side Of Spring

While adverse weather, such as dense fog, low ceilings and turbulence, play a role in the decision to go or stay, icing and convection are the heavy hitters this time of year. As wintertime stratus morphs into springtime cumulus, the freezing level and NEXRAD image tend to compete for attention. It's common to be worried about thunderstorms one day and icing the next. Obviously, thunderstorm and icing forecasts are critical. Specifically, you'll need to pay close attention to the freezing level as well as convective outlooks.

Freezing Level
The lowest freezing level has the most impact of any single meteorological factor during the early spring months (http://adds.aviationweather.gov/icing/frzg_nav.php). Day to day, it changes more during early spring than at any other time of year. It's not unusual to see the freezing level drop 5,000 feet or more within a 24-hour period at any one location. A round-robin instrument flight on Monday might not be possible on Tuesday because the lowered freezing level now places you into possible icing conditions. Even though the freezing level is something every pilot needs to know all year long, pay special attention as spring emerges.

On your preflight analysis, get comfortable with the big picture first. That's what meteorologists refer to as the synoptic view. Start with the mean sea-level-pressure surface analysis and forecast. Get a sense of where the adverse weather is located and where it may be moving. Take note of all of the surface low-pressure areas and surface frontal zones, especially those with occluded fronts. When a low-pressure system begins to occlude, it's near its peak intensity.

While it's important to identify the location and movement of frontal systems, the mean sea-level chart provides a dearth of information; it tells only part of the synoptic picture. Often the story at 500 mb (18,000 feet) provides many more clues about the weather you might expect to encounter, including how the freezing level might change along your route.

Here's a perfect example. As you fly south, the freezing level typically rises. If the freezing level is forecast to be about 12,000 feet in Portland, Ore., a flight to Los Angeles, Calif., should be a no-brainer. Worst case, the freezing level while en route to Southern California should be at least 12,000 feet or higher. You certainly wouldn't expect to find the freezing level in Los Angeles to be at 6,000 feet.

But it happens, and a constant-pressure chart will help illuminate this fact. It's a common belief that the freezing level will always rise as you head south. That can be a dangerous assumption. The freezing level doesn't always rise as you head south. Being aware of the synoptic picture is the key. Be sure to take a look at the 500 mb constant-pressure chart. If a significant upper-level trough exists at 500 mb over Southern California, then you should definitely expect a lower freezing level as you approach the center of the trough.

The trough doesn't provide you with a quantitative measure of the freezing level, but should be a red flag to dig deeper before you depart. The lower the height of the 500 mb surface, the colder the temperatures are in the column of air below. This equates to a lower freezing level below the center of the trough. The 500 mb constant-pressure forecast is useful when you're looking at the weather one or two days in advance.

Convection
Despite the fact that most thunderstorms in the United States are just getting started, some of the deadliest outbreaks occur during this time. If your flight plan takes you anywhere near a cold front, it's imperative to identify the convective threat ahead of and along the cold front. The NEXRAD loop is a fantastic tool, but doesn't tell you much about the future. Check the convective outlooks, terminal aerodrome forecasts (TAFs) and area forecasts (FAs) before you depart.

In the early spring, most of the convective SIGMETs will be limited to the southeastern quarter of the United States based on convective SIGMET climatology compiled at the Aviation Weather Center (AWC). Convective SIGMETs are issued only for areas or lines of thunderstorms that meet the convective SIGMET criteria. In other words, these thunderstorms have to be significant to aviation. Of course, all thunderstorms are significant to pilots, but when they occur with such high density in a particular area or along a line, they will be a greater risk, and the AWC will likely issue a convective SIGMET to prepare pilots for the inevitable.

At this time of the year, a good percentage of the thunderstorms in the southeastern states are associated with a rapidly deepening area of low pressure and a strong cold front. Many of the thunderstorms develop in the late afternoon in a solid line well ahead of the cold front. They often are severe and likely contain large hail and tornadoes.

In the morning, you'll want to be sure to check the convective outlook issued by the AWC (http://adds.aviationweather.gov/data/airmets/airmets_CB.gif). This is a forecast showing broad regions that are likely to see the issuance of convective SIGMETs in the next two to six hours, whether or not convective SIGMETs are currently active.

Given the widespread nature of these thunderstorms, both FAs and TAFs depict the likelihood of these thunderstorms quite well. Time of onset is the key factor from a planning perspective. Keep an eye on amendments to these forecasts. TAFs and FAs are amended on an as-needed basis.

In addition to these official advisories, check out the experimental simulated reflectivity (http://www.emc.ncep.noaa.gov/mmb/mmbpll/cent4km/v2/nmmwrf.refd1000_animate_1h.html), which does a remarkable job of depicting the location and time of onset of these thunderstorms hour by hour. It does so in a way familiar to pilots by depicting what the NEXRAD image might look like in the future. Looping the simulated reflectivity images will provide a good indication of the direction and speed of movement of the line or area of thunderstorms. Keep in mind that this precipitation weather product is an experimental forecast and shouldn't replace the official forecasts from the NWS. And, no, it doesn't cover the western states at the moment.

Springtime thunder has a strong signature in the forecast tools used by meteorologists. As a result, it's relatively easy to predict these major events. A day or two in advance of your planned departure, the Storm Prediction Center (SPC) provides two useful thunderstorm outlook products. While the SPC isn't aviation-centric, it issues one-, two- and three-day convective outlooks as well as an experimental enhanced resolution outlook. The one- and two-day outlooks graphically depict a forecast for general thunderstorm activity as well as the potential for severe storms over a 24-hour period (www.spc.noaa.gov/products/outlook). Each outlook has a textual discussion that provides details that are often technical, but may further quantify the timing and location of the convective event.

On the morning of your flight, check out the experimental enhanced resolution thunderstorm outlook (www.spc.noaa.gov/products/exper/enhtstm). This graphic provides a narrower forecast time window than the regular outlooks. It doesn't, however, address the severity of the thunderstorms.

From a weather perspective, early spring is by far the most challenging time of the year to fly. Altitude selection takes a back seat to route selection around convection especially when flying into parts of the southern tier. Fortunately, most of the weather systems do move through quickly, so keep your schedule flexible. You've probably heard the old saying, "In like a lion and out like a lamb." From an aviation weather perspective, you can certainly count on the lion, but don't be complacent; that lamb can show a bit of a mean streak now and then.