Day 109: The Day Before The Day Before…

Posted by pmarsh on 19 Apr 2010 | 0 comments

In storm chasing circles “The Day” is known as the day in which all the numerical models forecast a severe weather outbreak.  Often “The Day” has been forecast several days in advance and storm chaser excitement builds as the the event draws closer.  ”The Day Before” is also known in storm chasing circles.  This is the day before “The Day” and is often characterized by high risk, high reward for viewing severe thunderstorms and tornadoes.  In other words, you aren’t guaranteed to see something (high risk), but if a thunderstorm can develop, there is a chance you could be the only “chaser” there to observe it (high reward).  Tonight, I’m going to highlight a potential “Day Before the Day Before” situation.

As I’ve alluded to in recent posts, numerical models are hinting at the possibility of severe thunderstorms on Friday across portions of the central United States.  This would be “The Day”, making Thursday the “Day Before”.  However, there is an extremely high risk, high reward situation setting up on Wednesday.

Above (annotated below) is a forecast from this evening’s 00 UTC (7 PM CDT) run North American Model (NAM) valid 18 UTC (1 PM CDT) on Wednesday.  It shows accumulating precipitation falling across portions of northeast Oklahoma.  This precipitation is forecast to develop late Tuesday in northwest Kansas and northeast Colorado, in the higher elevations.  (I would just like to point out that this is typical of late summer patterns, not late April!!)  This complex of showers and thunderstorms is then forecast to track southeast  overnight and dissipate during the afternoon over western Arkansas.

This is an important feature because the precipitation and associated cloud cover act to keep the temperatures cooler in the wake of the precipitation than the surrounding areas.  At the same time, warm, moist air is forecast to began moving northward in response to the developing low over the southwest United States.  As the warm, moist air advances far enough northward it will impinge upon the rain-cooled airmass forecast to be in place across northern Oklahoma and southern Kansas.  This increasing gradient in temperature is, by definition, a front.  In this case it is a warm, front.  Warm fronts are important in severe thunderstorm and tornado forecasting because they act as a source of low-level convergence which can help aid thunderstorm development.  Also, just north of the warm front winds at the surface tend to be out of the southeast or east and quickly switch to being from the southwest aloft.  This rapid change in wind direction with increasing height is a crucial component for thunderstorms to be able to sustain themselves.  This allows them to become stronger than than otherwise might.

The previous  images (first four) were all valid at 18 UTC (1 PM CDT) on Wednesday.  This last image is valid at 00 UTC Thursday (7 PM CDT Wednesday).  It displays the forecast Convective Available Potential Energy (CAPE) late Wednesday.  This is a measure of how much energy a thunderstorm would have to work with should one develop. Forecast values in the image above in far southwest Oklahoma and northwest Texas are more than sufficient for large updrafts to develop – a key ingredient of severe thunderstorms this time of year.  Couple this large amount of instability with a potential warm front in the area, Wednesday afternoon just might turn out to be “The Day Before The Day Before”.

Lastly, I want to say that this signal has been in the last several model forecasts of both the NAM and the Global Forecast System (GFS) model.  By no means is anything a guarantee, but it is certainly something to watch.  Thursday and Friday, almost certainly have a greater potential to produce severe thunderstorms.  Thus, don’t let your guard down if nothing happens on Wednesday.  There is a reason why it is known as a high risk, high reward kind of day!