*Blog written Wednesday, May 15, 2013*
Because May 18 is still 3 days away, a lot of data and information can change between now and then.
As of this writing, the NAM model is just now starting to pick up on Saturday’s outlook and I will therefore not be including it in this blog. Instead I will be referencing the GFS, CMC, and ECMWF models.
SATURDAY, MAY 18
It appears that severe weather is likely on Saturday, May 18, Sunday, May 19, and Monday, May 20, and possibly on Tuesday, May 21. All of the reliable forecast models are picking up on a trough, that is currently off the Pacific Northwest coast, and digging into the western CONUS by early Saturday morning. It looks as if a smaller short-wave trough will push a head of the main trough and will help support thunderstorms in western and northwestern KS on Saturday evening. Return flow from the Gulf of Mexico is currently in place and will continue to transport moisture for this weekend’s events. Both the GFS and ECMWF agree that dewpoint temperatures will reach 60F+ degrees as far north as North Dakota by Saturday afternoon. Some places in TX, OK, AR, KS and MO could even reach 70F+ degrees.
On Saturday, a dryline will be set up in western OK, western-north TX, central KS, western NE, SD & ND. The focal point of severe weather will occur in the central and high Plains. A strong mid-level ridge will likely keep thunderstorms from developing south of Nebraska, at least prevent anything severe. However, if the cap can break, severe weather will be likely in TX, KS and OK Saturday afternoon. 00Z CMC and 00Z ECMWF have two surface lows developing by Saturday morning, one in SD/ND and one in western KS. 06Z GFS has only one very broad surface low in SD/ND, but does eventually develop the second surface low by Saturday evening. The biggest severe weather threat will be in the warm sector, just south of the warm front and a head of the cold front. By Saturday evening, the warm front should be draped across SD, though IA, and south IL. The greatest forcing will be from west-central SD, down through central NE, and into west-central KS. The secondary surface low in western KS will help kick up thunderstorms, perhaps quickly becoming severe late Saturday afternoon. It is still unclear just how the thunderstorms will evolve over time. Large hail and damaging winds WILL likely be the main threats on Saturday. If cells become isolated and discrete, then there is enough shear for isolated tornadoes, especially in western/northwestern KS, where the short-wave trough will be located.
SUNDAY, MAY 19
On Sunday morning, showers and thunderstorms will be ongoing from Saturday. Forecast models are suggesting that the severe threat will be more scattered by Sunday afternoon and evening. The short-wave trough positioned over western KS on Saturday will round the base of the main mid-level trough/low. This will cause a messy scene for the Midwest and northern Plains. Post-frontal severe thunderstorms will be possible in SD and NE. Hail and wind will be the main threats for these areas.
The biggest threat on Sunday for severe weather will be from central OK, through eastern KS, to northern MO and southern IA. The tornado threat also ramps up. Forecast soundings and hodographs support strong supercells, capable of very large hail and a few tornadoes.
MONDAY, MAY 20
Still lots of questions for Monday. Models are in disagreement with location of the surface low(s) and frontal position by Monday evening. However, each model agrees that this has the potential to be the best day for tornadoes, out of the three days. GFS has bulk shear values of 60+ knots in northern OK and southern KS by Monday evening. However, I do believe the cold front will be through the area by that time. The corridor between south-central OK through southern IA has the best severe potential, including tornadoes. Below is a graphic from the Storm Prediction Center for these three days, shading in the areas with the best severe potential for the respectable days. I do agree with the graphic, for the most part.