Day 65: Upcoming Soaker

Posted by on 6 Mar 2010 | 2 comments

Day 65 (a)

The Hydrometeorological Prediction Center (HPC) is forecasting quite a bit of rainfall over a large portion of the country during the next 5 days (120 hours).

Day 65 (b)

Examining today’s water vapor imagery, a couple of key features help explain why this is.  First, a very strong mid-level low is currently spinning to the west of the Baja Peninsula.  In addition to this, a fairly strong mid-to-upper-level jet streak is propagating around the southern portion of the jet streak.  (This is an oversimplification of what is really happening, but it gets the point across.)  In response to this mid-level low and strong jet streak, moisture from the equatorial Pacific is being drawn northward into the central United States.

Day 65 (c)
Day 65 (d)

Over the next several days, the mid-level low will track east from the Pacific Ocean into the central United States.  As it does, it will begin to take on what meteorologists call “a negative tilt”.  This means that the axis (center line) of the mid-level low (or trough) will become oriented more northwest to southeast.  This is important because it is a sign that the trough/low is strengthening.  Also, because of the negative tilt, the low/trough will move slower and draw up moisture from the Gulf of Mexico (in addition to the moisture already coming in from the Pacific!).  The strongest ascent (rising motion) tends to occur downstream from the trough axis. (This means, take the wind direction at the trough axis and pretend you are “blown” by the wind. Where you end up is where to expect the strongest rising motion.) The image above (and annotated below) display the positions of the various lows/troughs Monday at 18 UTC (12 PM CST) as predicted by the North American Model (NAM) from tonight’s 00 UTC (6 PM CST) run.

Day 65 (e)

Below is the NAM’s prediction of what the radar reflectivity might look like at the same time as above.  While the values displayed aren’t important, the fact that the highest values (yellows) encompass a large area is indicative of a heavy rain potential on Monday.

Day 65 (f)

Because this trough is negatively tilted (implying very strong forcing and strong ascent) and is moving into the central plains early in the day on Monday, it does not appear likely for a widespread severe weather event. However, isolated severe weather cannot be ruled out, especially across areas of northeast Texas, extreme southeast Oklahoma, southwest Arkansas, and northwest Louisiana.

  • Greg Blumberg

    I thought negatively tilted troughs were something we did want for severe weather, as they would be associated with stronger lows, leading to rapid moisture advection from the south, backed surface winds…am I missing something here in my knowledge?

  • http://www.patricktmarsh.com pmarsh

    If you have a negatively tilted trough, you tend to have all the things you mentioned, and as well as strongly backed winds a loft. You don’t necessarily want winds out of the south at 500mb for a big severe weather event (although it is possible).

    The key is that all of these things have to be in balance for a severe weather outbreak to occur. If you have a weak cap, and bring a negatively tilted trough into the area, you’ll end up with widespread heavy rain, but not a severe event. You need to balance the strength of the upper-level forcing with the strength of the cap, speed of moisture advection, temperature modification, etc.