Where are all the tornadoes this season? Well the answer to that in the short term is the awful dual (both eastern and western) rex blocking pattern setting up with an eastern and western cutoff lows forecast to develop over the coming days. The first image below is this morning’s 500 mb analysis which has the two areas forecast to transition to cutoff lows in the next few days. The second image is the NAM 500 mb forecast for May 5 which shows the two cutoff lows (circled in red). This pattern is not conducive to tornadoes as the eastern low and recent cold front passage has essentially shut down the Gulf not allowing for any moisture to surge into the planes as well as placing a stagnant upper flow which does not produce the needed wind shear for supercell storms and tornadoes.

One of the main reasons for this blocking pattern and split flow regime can be attributed to the PNA index (Pacific North American Pattern). Simply but it describes the state of the Aleutian low which is a semi-permanent feature in the North Pacific. Ideally we would like to see this index in the negative which promotes western trough pattern, but is currently positive and is forecast to remain so into the middle parts of May, as seen in the graphic below from the climate prediction center. This positive phase usually means split flow pattern and western ridge (i.e. the current pattern).

There is however some relief in sight possibly in the May 8-10 time frame as the western cutoff low ejects eastward and brings along with it a descent amount of energy as seen by the area of enhanced winds through south central Texas and southern Oklahoma at 500 mb as shown below (GFS 500 mb forecast for evening of May 9)

Along with this pattern develops a weak surface low into southwest Kansas and promotes a good return of Gulf moisture with a dryline ejecting westward into TX panhandle along the OK border. On the plus side to, the upper level flow pattern will also promote a good amount shear thus making for a favorable supercell environment. The images on the left shows surface winds and moisture return to the planes, and the one on the right is the surface to 500 mb bulk shear. Unfortunately the ECMWF is not as optimistic with the setup and still continues to show no moisture return from the gulf and a lower amplitude upper trough. Also, we will not be able to make this setup as we still have work commitments and Dayna needs some dental work done.

After this period looks like things shut down once again as a ridge pattern takes hold again which is indicated by pretty much all key models here being the GFS/GEFS ensemble/NAEFS/ECMWF opp run and ensemble mean. But not all is lost here, and for time sake I will jump over a little bit of detail. But from May 15 onward things begin to look encouraging among the GFS operational model/GEFS and NAEFS ensemble. Below is the last panel of the GFS operational run form 12Z today and the area circled is of interested to me as it shows an area of substantial energy out over the Pacific and has been signaling so over the past couple days now. Although the current ridge over the west may be discouraging the energy pushing east I believe may be the turnaround come the latter part of May.

Even more encouraging about this trend is both the GEFS ensemble mean and NAEFS ensemble (shown below in respective order) have also continued with the increased energy over the Pacific tracking eastward. What I find a little more encouraging still is that both the ensembles show a persistent western trough and have continued to show increased Pacific energy tracking eastward. The one caveat is the western trough shown does not have a favorable orientation, but at this point I will take it.

Now the new Monthly ECMWF came out and through mid-May it looks rather disorganized and sloppy but after May 20 it gets pretty active. The one downside is that it looks to keep things in the mid to high planes region which would greatly have moisture issues and the other is into June it hints to want to really get some western trough. The new ECMWF ensemble control run on the other hand paints a very good picture from May 15 onward with a few good prospects and from May 18 onward begins to show a very deep trough begin to dig in near the 4 corners region. Now you will have to take my word on this since I used the Accuweather Pro site to look at the ECMWF data and they do not allow the images or links to be posted. So I can say with some amount of certainty that the early part of May will be pretty inactive, with a few obscure events possible, but from May 15 onward I believe things will take turn to a more active pattern. Here’s hoping.