Day 102: Potential Wet Pattern

Posted by pmarsh on 12 Apr 2010 | 0 comments

Anyone who might be a storm chaser might have been acting somewhat depressed of late.  Chances are this is because they have looked at the extended forecast and noticed that there are no “big chase days” forecast on the horizon.  In fact, it’s been a relatively quiet year for tornadoes (and severe thunderstorms in general).  And this pattern doesn’t show signs of changing any time soon…

The image at the top (and annotated below) is taken from today’s 12 UTC (7 AM CDT) run of the Global Forecast System (GFS).  It is a 132 hour forecast, valid 00 UTC Sunday, 18 April (7 PM CDT, Saturday, 17 April), of 500 millibar heights and winds.  As previously discussed, this is a good level of the atmosphere for meteorologists to evaluate when looking for troughs (cool and stormy) and ridges (warm and sunny).

The first thing that should jump out is the fact that most of the country is being influenced by a large ridge (the mountain looking thing in the black lines).  Areas that are underneath this ridge would expect (generally) sunny skies and warm conditions.  However, not everyone will get to experience the ridge in this manner.  The northeast United States is forecast to experience a shortwave trough (closed low) that should keep the weather relatively cool and unsettled with periodic rain and showers.

Also, to the south of the apex of the ridge discussed above, a shortwave trough is forecast to be present.  Essentially what is happening is that the shortwave trough is trying to move through the large CONUS wide ridge.  This typically leads to slow progression of the trough, with wind speeds within the trough being weaker (slower) than normal.  These weak mid-level winds often result in slow moving non-severe thunderstorms that produce heavy rain (assuming moisture is present). I’ve circled in green (below) the areas that would be most likely to experience this type of thunderstorms, based on this specific forecast.  Although not typically widespread, severe thunderstorms are possible in this kind of pattern with severe thunderstorms that do occur likely being of the short-lived variety with hail and wind the primary severe threats.

(Note, in the image below, trough axes are indicated by dashed black lines and ridge axes are indicated by solid black zigzag lines.)

Keeping in mind everything mentioned above, below is the Hydrometeorological Prediction Center (HPC) forecast of total precipitation for Thursday nigh through late Saturday afternoon.  Notice that the area of maximum precipitation in this forecast is roughly coincident with the area I circled above.  This would suggest a prolonged period of heavy rain for the western southern plains over the next week.  We’ll see.