Tuesday, May 17, 2011

Today's lessons: 'Backwards' rain, trends

The upper-level low pressure system centered near the Grand Strand this morning is a good opportunity to look at a pair of weather phenomena affecting the Charlotte area today.

And they are conditions we see at other times in the year.

The rain showers moving slowly across the region today are mostly drifting from east to west. As most of you know, that's backwards.

Normally, precipitation in our region comes from west to east -- or southwest to northeast. Occasionally, when a "back door" cold front is dropping from the north, we'll see precipitation move from north to south. But largely, it's more of a west-to-east movement.

The counter-clockwise flow around the upper-level low today is responsible for the "backward" rain. With Charlotte to the west of the low's center, the flow around the system is bringing precipitation inland from the coast. That's something we also see later in the summer, during hurricane season.

When a tropical system is making landfall on the South Carolina coast, or even moving northward along the coastal plain, the counter-clockwise flow brings precipitation toward us from the coast.

In 2004, when the remnants of Hurricane Frances moved northward from Georgia into the western Carolinas, the Charlotte area was on the northeast side of the low. That day, showers and thunderstorms moved south to north into our region.

Some people who plan outdoors activities (attention, joggers) have learned to check the radar before starting. Today, watch to the east of Charlotte. That's where our precipitation is coming from.

Today also is a good time to talk about trends. Meteorologists have a saying, "The trend is your friend." That means when computer models or changing forecasts are following a trend, it often provides a strong hint about what is to come.

The trend in recent days has been to lower the anticipated daytime temperatures today. On Sunday, forecasts called for highs in the upper 60s today. On Monday, those forecast highs for today were dropped to the lower and mid 60s. This morning, I've seen some forecasts of a 60-degree high. This seems to happen frequently in cold-air damming events, when computer models initially underestimate how chilly it actually will be.

That said, don't be surprised if temperatures today -- which fell into the lower and mid 50s before daybreak -- don't reach 60 degrees this afternoon.


discourser said...

Funny to see this article today. I hollered at Spouse to check the radar station on TV while I was moving some plants around yesterday evening. When Spouse reported rain coming from the east, I said "no way" and had to come in to check it out myself.

Anonymous said...