Ramblings of a Graduate Student

Whew!  A very busy today, which included an afternoon tour for the Trinity Junior High Science Bowl team and then an evening full of severe thunderstorms and possible tornadoes.

The image above is a radar image taken earlier this evening showing the severe weather event as it unfolded across the state of Arkansas.  It’s a busy image, so here is what everything means:

• Yellow shaded areas = Active Tornado Watch
• Reddish-Purple shaded areas (NW AR and SW MO) = Active Severe Thunderstorm Watch
• Yellow solid lines = National Weather Service Forecast Office boundaries
• Large area with solid red line and light red color fill (north-south oriented thing from AR into LA) = Active Storm Prediction Center Mesoscale Discussion
• Solid orange lines = Outlines of Active Severe Thunderstorm Warnings
• Red solid lines (no color fill) = Outlines of Active Tornado Warnings

As you can see, it’s a lot to absorb – just look at the number of warnings in effect!  Also, you can see quite a large area was (still is as I write this) under the threat for possible tornadoes overnight.  While the threat of tornadoes has decreased from earlier this evening, it is still an appreciable threat and shouldn’t be taken lightly.

Today was a very busy day gearing up for various NWC activities over the coming weeks.  (I even managed to squeeze in a quick The Weather Channel interview.)  Because of all these things, I didn’t have the ability to spend as much time analyzing tomorrow’s potential severe weather threat as I would have liked.  However, the image above (and annotated below) include show the current (as of 10 PM CST) of Mondays severe weather producing mid-level low (yellow), Wednesday’s potential severe weather producing mid-level low (red), and the area most likely to see severe thunderstorms tomorrow (green).

I hope to provide more discussion about this tomorrow.

The image above was taken approximately the same time a tornado was hitting Hammon, OK.  The tornado occurred with the convection associated with the brighter colors in far western Oklahoma.  It was a classic cold core setup, meaning a lot of instability was generated along the eastern edge of the cold-pool aloft associated with the mid-level low.  It so happened that the mid-to-upper level jet streak (cyan color fill below) associated with the mid-level low was positioned such that an area of large scale lift was juxtaposed over the mid-level cold pool.  These two factors, combined with surface convergence along subtle surface boundaries evident on KFDR radar (not attached) enabled thunderstorms to develop in an environment with a lot of environmental vorticity.  At least one thunderstorm this afternoon was able to tap into this environmental vorticity and produce several tornadoes.

Storm season is officially “here”.

The mid-level low discussed in last night’s post has moved into far southeastern California and far western Arizona tonight.  Downstream (east in this instance) strong ascent (rising motion) in the atmosphere has resulted in widespread showers and thunderstorms across portions of the desert southwest.  In fact, severe supercell thunderstorms (with severe thunderstorm warnings issued for them) developed this evening across portions of far eastern New Mexico.

This mid-level low will continue to move east overnight and will be bringing widespread showers and thunderstorms to a large portion of the south-central United States tomorrow.

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).

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.

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.

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.

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.

This morning I gave a short weather briefing at the National Severe Weather Workshop.  The image above is one of the graphics that I showed.  Not a lot of active weather, however portions or northern Kansas and southern Nebraska did have thunderstorms this evening.  It won’t be long until this blog is discussing various severe thunderstorm threats.

Tomorrow I hope to discuss the late weekend / early week storm.  A lot of heavy rain across portions of the central United States is looking like an almost sure bet.

Tomorrow I am giving a weather briefing at the National Severe Weather Workshop.  The map above is essentially what I’ll have to talk about…a closed 500mb low over Colorado.  After the active pattern we’ve had the past few weeks and the active pattern we’ll probably have next week, it’s kind of boring.  Oh well, at least I get to do something!

PS…The image above is from the North American Model (NAM) and was creating from this evening’s model simulation.  It is valid at 18 UTC tomorrow, or 12 PM CST – about an hour and a half after the briefing.