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Looking ahead for the late May 2015 storm chase


Today (May 20, 2015) is a down day for us. Reason being is shown in the first image below which is the afternoon surface analysis. We can see that high pressure has invaded much of the central U.S. which has pushes the cold front way down into southern to southeaster Texas effectively riding the area of rich Gulf of Mexico moisture. A tentative target for today was Peco in southwestern Texas and the other was the Waco area in east central Texas. We chose to blow it off as the cold front surging southward is expected to undercut any of the convection reducing the severe threat and significantly reducing the tornado threat. So we're sitting in Amarillo right now where it is currently 12C. Not all, but much of Ontario as well as western Canada is warmer at the moment. Below I'll take a look at what our next potential chase days have in store. Quickly though it appears that the ECMWF is the model that is performing the best at this point in time. Through the medium range forecast the NAM appears to be trending toward the ECMWF pattern a little more than what the GFS is suggesting. Thus I'll use the NAM forecasts just for visualization and analysis since I cannot post the ECMWF forecasts.

Friday May 22, 2015

This looks to be the first of the next string of chase days. At 500 mb flow returns across the souther parts of the mid-west with an upper level low centered near the California / Nevada / Arizona borders. The NAM / ECWMF a relatively similar with position and timing of the features but the ECWMF does have 35+ knots of flow into the Lubbock to Wichita Falls areas through Texas that the NAM does not show. 

Moving down to 850 mb there is a persistent southerly to southeasterly low level jet that is forecast to be in place. The issue this arises is that as the upper level flow pushes in aloft large scale ascent combined with low level ascent up and over the warm front from the persistent low level jet will once again make for morning convection that will persist through the day. On the plus side the LLJ will generate ample low level shear and with the solid H5 winds in place should general good low to mid-level shear with veering hodographs.

A reflection of the potential morning and persistent convection is observed in the NAM temperature forecast through central Oklahoma, Kansas and Nebraska. The precip cooled air will also cause the surface warm front to stall out through northern and eastern Texas and through the Texas Panhandle and up into east central Colorado. The ECMWF does keep the low centered more toward the southeastern parts of Colorado near the border with New Mexico. Because of this the rich Gulf moisture is expected to get hung up through Texas and struggle to push into Colorado and Nebraska. Despite the NAM showing 55F dew points into Colorado the ECWMF keeps the dew point more toward the 50F range which is a little more realistic given the trend for the high pressure currently in place.

The lag in moisture is also reflected in the forecasted SBCAPE field as per the NAM below. There is a narrow window of 500-1000 j/kg into eastern Colorado that looks to build in between the dryline and warm front. Into Texas anywhere from 1000-3000 j/kg along and south of the warm front is possible.

The low level flow near the center of the low will make for better backed flow and low level helicity into eastern / southwestern Colorado. But the low level shear and veering winds near the warm front in north central Texas should prove equally and conducive and this setup has been the trend this May where the warm front through the Wichita Falls area has been productive. The ultimate evolution of the morning convection will also dictate whether the Colorado region can get enough daytime heating. Thus the Friday target could be from Childress to Wichita Falls and through Abilene to just east of the DFW area as outlined on the surface analysis with the southern area outlined by the green circle. The secondary target I believe will be southeastern Colorado pending moisture return and instability.

Saturday May 23, 2015

On Saturday the upper level low will drift northwards and be centered in eastern Utah near the Wyoming border. At the southern periphery of the upper low will be an H5 wind max of 25-45 knots pushing into western Texas, the Texas Panhandle, and into western Kansas. The ECMWF is a little more organize with the closed upper low and spreads the H5 jet a little farther east than what the NAM shows below.

At 850 mb a persistent southerly low level jet will be in place once again. The NAM shown below is showing a slightly veered regime but the ECWMF does keep the H8 winds more southerly to slightly east of south through the period. Again through the morning the persistent LLJ riding up and over the warm front, morning convection is expected which will keep things to the south.

Below the low analyzed in purple shows the NAM projected location and the one in black shows where the ECMWF forecasts the low to be. The frontal analysis reflects where the ECMWF has the frontal positions. With the morning convection expected again, the ECMWF again looks to be the more realistic solution. With the dryline expected to surge out through the central Texas Panhandle and the front to be draped through central Oklahoma into the eastern Texas Panhandle the environment along the I-27 corridor from Amarillo to Lubbock and into southwestern Oklahoma will be the target for now.