Tuesday, April 1, 2008
The Ugly Side Of Spring
Winter hasn’t released its icy grip yet
|Whether Punxsutawney Phil sees his own shadow or not, winter is losing its death grip. But it isn’t dead yet. Widespread icing still exists during the transition months of March and April. Gulf moisture, warmer temperatures and an overactive jet stream guarantees that convective SIGMETs will begin to spring out of hibernation. With temperatures slowly on the rise, you need to tailor your briefings to focus on key weather products that track the vernal transition.|
|Having XM Satellite Weather information, whether you get it on your G1000 or on a portable GPS, greatly benefits decision making and flight planning before and during a flight.|
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.
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.