Monday, January 28, 2013

Nasty Day Tomorrow as Significant Severe Weather Threatens

**I am declaring a Storm Action Day tomorrow, meaning that most resources will be devoted to analyzing and tracking the severe weather situation.**

The Storm Prediction Center has issued a Moderate Risk of severe weather for tomorrow, January 29th, in the face of a possibly significant severe weather threat across the outlined area.

The NAM model is shown above with the forecast for 500mb vorticity values. The term 'vorticity' defines a spinning motion in the air. This spinning motion is synoptic, or on a large scale, so it is not a producer of tornadoes. High areas of vorticity indicate low pressure is in the area, and this is how we find low pressure systems. Looking at the forecast above, which is valid for the late evening hours of January 29th, shows our storm system pressing down into Texas and Oklahoma, depicted by the deep bending of the contour lines. Something I want you to look at is how these vorticity areas are directed. For instance, if you look closely, you will see that the base of the storm system in Texas has the highest vorticity areas tilting a little towards the Southeast (if you cannot see it, I marked it in). This is a classic example of the infamous negatively-tilted storm system. The infamy of it is that, in a negatively-tilted storm system, cold air aloft is encouraged to flow over the storm system. You weather enthusiasts know that thunderstorms rise and strengthen faster in cold air aloft, so negatively-tilted storms have a tendency to produce stronger thunderstorms than their positively-tilted storm counterparts (highest vorticity values point towards the southwest).

An analysis of the upper air for roughly the same timeframe as the image above reveals a very strained jet stream. You are viewing the forecasted wind speeds for the jet stream, which can also help us identify areas of high and low pressure. If we look near the Front Range of Colorado, we see an empty space pushing down. That is our low pressure system. Again, we are seeing higher wind speeds pointing towards the southeast than winds pointing to the southwest at the base of this empty space, affirming my suspicions of a negatively-tilted storm system. The winds on this forecast are projected to max out above 150 knots in central Texas in this forecast, and winds between 130 and 150 knots in the Central Plains. These unusually high wind values are more typical of spring! Nonetheless, the strength of these upper level winds shows that any storms that do develop will have a strong backing, at least in the jet stream.

Now we get into the forecasted simulated reflectivity, which is a forecast of what the radar could show in the future. This short range model is valid for the late morning hours of January 29th. Already, we can see some moderate rain and thunderstorms surfacing in eastern Oklahoma, and this is where I anticipate the show to start. Strong upper level winds will support eventual strengthening of these storms. However, in these late morning hours, I do not expect any big severe weather threat other than some gusty winds and hail. It's about what happens later in the day that concerns me.

We move now to the afternoon of January 29th. We now see that our storm system has blossomed, with heavy showers and elevated convection (thunderstorms that are stronger than normal, but not yet severe) ongoing in much of the eastern Plains into the Midwest. Some severe thunderstorms are ongoing in the leading edge of this squall line formation. Going by this graphic, I would expect the strongest storms to be in Arkansas and Missouri at this point, with weaker (but still heavy) storms in the Midwest.

This image, valid for the early morning hours of January 30th, shows just how big this severe weather event could be. Should this short range model's forecast verify, we could see storms stretching from border to border- Canada to Mexico (if you count the Gulf of Mexico as a border, or if you use latitudes  from northwest Mexico)! Taking a look at this image, definite strengthening has occurred in this severe weather event, with very heavy convection now plaguing the Midwest and Great Lakes. We could even see some fairly intense convection happening over Michigan. Regardless, these storms could continue through the night, if the nocturnal lower level jet stream strengthens overnight. For those unfamiliar with the term, the lower level jet stream is just like the jet stream, except weaker and located lower in the atmosphere. It is nocturnal.

I think the maps above displayed my view on the situation well, and I see no need to make a map at this moment. I do suggest, however, that all those in the Plains and southern Midwest prepare for a rough ride tomorrow, especially in the evening/overnight hours.

For you storm chasers, here's my Chase Spot: Jonesboro, AR to Jackson, TN in the evening, Memphis, TN and east after dark.