Day 77: Upcoming Weekend Storm

Posted by pmarsh on 18 Mar 2010 | 0 comments

It has been a very hectic day, and the numerical weather prediction models for this weekend have helped make it even more hectic than it would have been otherwise.  Last night, I mentioned how almost all the numerical guidance (at least those available to me when I blogged) indicated that the upcoming southern plains storm would be relatively minor, with most of the precipitation being generated in the quickly passing frontal zone.   This was mainly the result of the numerical guidance keeping the mid-level (short) wave (trough) “open” and preventing the development of what is known as a closed low.  Beginning with today’s 12 UTC (7 AM CDT) numerical models, the forecast of an “open wave” during the weekend has dramatically changed.  However, before I jump into all of this, let’s first take a look at what I mentioned a couple of days ago - the chance of severe thunderstorms on Friday.

There are several things working against the chance of a widespread severe weather outbreak tomorrow.  However, the biggest one is the lack of deep, rich atmospheric moisture.  (Moisture is an essential ingredient in the development of any thunderstorm.)  The image above depicts the amount of moisture currently throughout the troposphere.  Notice how the Gulf of Mexico has relatively low moisture values.  This is because a storm that moved through the Gulf of Mexico during the last 36 to 48 hours has “cleared out” all the available moisture and shoved it south and east out of the Gulf.  This means that it will take some time for the moisture to come back into the southern plains – most likely longer than we have.

Another ingredient necessary for severe thunderstorm development is a source of lift (or rising motion).  The image above is from this evening’s 00 UTC (7 PM CDT) North American Model (NAM) and depicts a forecast of rising motion (lift) at 00 UTC (7 PM CDT) tomorrow evening.  What is important about this forecast is that the rising motion is found in a skinny band located along the cold front and not in a widespread area ahead of the cold front.  This leads me to believe that the warm sector will most likely be void of enough lift to develop thunderstorms, so we’ll be forced to rely on the cold front to generate enough lift for severe thunderstorms.  This means there will really only be one shot for severe thunderstorms – not repeated chances – however thunderstorms will certainly be possible.

Lastly, severe thunderstorms need to have a lot of instability.  One measure of instability is known as Convective Available Potential Energy (CAPE).  The image above depicts the amount of CAPE at the same time as the previous image’s vertical velocity (lift) depiction.  Notice how there is hardly anything there!  This means that the entire line of thunderstorms that might develop along the cold front would have to share what little instability is depicted above.  This will also act to limit the severe threat.

And, lastly, the snow potential.  It’s a bit late, so I won’t go into all the details that I had previously planned to cover.  The image above from the National Weather Service Forecast Office in Norman, OK, pretty much says it all.  A good portion of Oklahoma will see the potential for a significant snowfall over the weekend.  Keep in mind that these are “accumulation” forecasts, or in other words, the amount of snow you’ll see on the ground.  This is impressive when you consider that a lot of the area will see temperatures in the upper 60s to lower 70s tomorrow.  Ground temperatures will be incredibly warm at the start of the event, meaning that a lot of snow will melt initially.  A lot of snow would have to fall in order to get accumulations forecast above.  Actually, if we took some of the models at face value, portions of Oklahoma could see 1-2 FEET of snow…

With all this said, referring back to the first image, I have concerns about moisture into this storm.  This could be one potential failure mode.  This certainly bears watching…