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Don't forget that upper level trough

Many pilots tend to focus much of their weather analysis time on surface weather reports and surface forecasts. This includes the surface analysis chart, prog charts, surface observations and even terminal forecasts. But they don't take the time to look at the upper level or constant pressure charts. Much of the energy in the atmosphere is aloft and there are many scenarios that require you to examine the upper-level weather to completely understand what weather to expect along your proposed route of flight.

For example, this morning there was a strong cold front that was slowly moving southeast through the eastern U.S. bringing a solid line of showers and thunderstorms along and head of this cold front shown on this 15Z surface analysis above. There's an area of high pressure moving in behind this front, but would you expect convection in and around the area of Lakes Ontario and Erie well behind the cold front by simply looking at the surface chart?

Actually, this morning there were two convective SIGMETs just downwind of both lakes consisting of an area of showers and thunderstorms as shown above on the EZWxBrief EZMap display. What is causing this convection? Well, in order to understand this better, you need to look aloft at the 500 mb constant pressure chart.

On the 500 mb chart above valid at 15Z, there's a fairly high amplitude trough swinging down from Canada drawing in a massive amount of cold air aloft (the color depicted on the chart is called absolute vorticity). A trough is essentially a cold pool of air aloft. With a fairly warm ground and cold air aloft, this creates a fairly unstable atmosphere that can lead to areas of convection. Notice that the region where the convective SIGMETs were shown are essentially in the center of this upper level trough.

Most of this convection had pretty low tops (less than 30,000 feet) which is not uncommon in this situation. Given that, the likelihood of seeing any lightning is pretty low as well (by the way, lightning is not one of the criteria for convective SIGMET issuance). As shown above, even the simulated reflectivity forecast found in EZWxBrief's EZImagery issued the night before picked this up nicely with two areas of convection just to the east of the Lakes. Moreover, the EZForecast showed a distinct risk of rain and thunderstorms in the same area as shown below.

Even though the afternoon, this upper level trough continued to support very significant thunderstorms. This convection cannot be explained by the surface forecasts alone, but only by appreciating what an upper level trough can contribute.

KBUF 021954Z 20014G21KT 4SM +TSRA BR FEW008 BKN030 OVC070

Most pilots are weatherwise, but some are otherwise™

Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief™

CFI & former NWS meteorologist

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