Dry Slot-A Brief Explanation.


Tuesday, February 4th 2014, 4:47 pm
By: News On 6


As mentioned in yesterday's blog, the wintry weather for today had several issues to deal with that made for a very interesting forecast scenario. One of the things I mentioned was the potential for a dry slot to move over us during the day and will now attempt to explain what is meant by that.

The first graphic on the right is a simplified version of the vertical profile of temperature and moisture for Tulsa as of Noon today. This is what we refer to in weather circles as a Skew-T log P diagram. Won't get into the specifics of all that, but for the purposes of this discussion, I have labeled the features of interest. The red and green lines represent the vertical distribution of temperature and moisture respectively and notice that they are both to the left of the blue line which means they are both below freezing from the surface all the way up. Notice also that they are basically on top of one another up to an elevation of 2397 meters or 7864 feet above the surface. In other words, the atmosphere was basically saturated to that level which accounts for the overcast skies and precipitation. However, notice how rapidly the air dries out above that level which corresponds to a temperature between -10 to -20 degrees Celsius. I have labeled that level the snow level because that temperature is the most favorable for the big snow flakes to develop, assuming there is adequate moisture. This is known as the dendritic growth zone in case anyone wants to know. Obviously, the moisture is lacking at those temperatures which means the wintry precipitation formed in an environment more favorable for snow grains or very small snow flakes or other forms of wintry precipitation.

We were anticipating this dry slot would impact our snow fall totals but the system moved in much more quickly than thought so that the opportunity for at least a couple of hours of modest snowfall such as occurred this past Sunday just did not materialize here in Tulsa. Often the heaviest snowfall will be in the comma head pattern that develops just NW of that dry slot and we just missed out on that.

Anyway, that was then; now what about the future. Now that the storm system aloft is quickly moving on eastward much colder air at the surface will be pouring in behind it for the next few days. In fact, temperatures will at times be about 30 degrees BELOW normal through this forecast cycle. Not only that, but gusty northerly winds will bring wind chill values to near if not below zero for Wed morning and perhaps Thu mornings as well. Keep in mind, today's storm system has left behind a significant snow pack in KS and with northerly winds that will insure very cold air continuing to drain over us for several days at least.

With the cold air in place, another in a series of systems aloft will be moving over the state for later this week and into the coming weekend. The latest and greatest guidance today has come in much drier although some additional light snow will be possible for Thu/Fri and perhaps over the weekend. Of course, this is subject to change as those systems are a long ways off and have not yet been sampled well by our observational network.

One thing that is pretty certain, below normal temperatures will likely persist well into next week. Notice the 8-14 day graphics on the left. We are still in a signal suggesting temperatures averaging below normal as we go through the middle of the month but at least some moderation will be taking place during that period. Also, the precipitation signal is not very robust for that period either.

So, stay tuned and check back for updates.

Dick Faurot