Heavy rains likely later this week

Published 7:00 am Tuesday, March 8, 2016

Heavy rainfall is possible as the chance of showers and thunderstorms increases on Wednesday and becomes even higher on Thursday. There’s a possibility that showers could persist into Friday and Saturday.

The major cause will be a slow moving low-pressure system in the middle and upper atmosphere. The strength, track, and slow movement of this low will all work together to give us a chance of rainfall totals exceeding four inches over the course of the event.

The low is already showing up on upper air weather charts as an unusually energetic counterclockwise swirl over northwestern Mexico.

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All the numerical computer models show the low moving very slowly eastward over the next few days. The models predict that the center of the low will remain far to the south near the Texas-Mexico border before beginning to slowly lift north on Friday. It’s unusual to see an upper low that far south in winter. This location will keep us in a very moist air mass with plenty of upward motion in the atmosphere on Wednesday and Thursday, and possibly Friday and Saturday too.

Last week forecasters began tracking energy over the Pacific Ocean that eventually fueled the formation of this system. Winds at 30,000 feet altitude exceeding 175 miles per hour were estimated from satellite data. That’s not unusual for late winter or early spring. What was unusual was the fact that these high wind speeds covered an enormous area over the Pacific–at one point, over one million square miles.

As the high wind area approached North America, it encountered the strong temperature contrasts associated with the differences between land and ocean. This caused the flow to bend into a deep U-shaped trough over the western United States. Inside the “U” was lower pressure than outside the “U.” The closed upper low now situated over northwestern Mexico developed at the base of the trough.

Normally we would be on the wet, eastern side of the low pressure for only a day or so before it scooted to our east. But, a strong ridge of high pressure over the east coast is blocking the low’s movement. Until that ridge breaks down the low will move very slowly.

There is considerable disagreement between two of the best-performing computer weather models regarding when the rain starts and ends.

Both models indicate that our rain chances begin increasing on Wednesday and continue to increase through Thursday. There is more uncertainty about when the rain exits our region. At the latest, the rain should end by Saturday night and we could dry out with only widely scattered showers as early as Friday.

There is also the threat of severe thunderstorms at the front end of this event on Wednesday and Wednesday night. High winds, hail, and even a tornado are possible if any storms do become severe.

Temperatures should remain mild with highs in the 70s and lows in the 60s. The models are hinting that we could see our first 80 degree temperatures of the spring early next week.

By Skip Rigney