Back to the "Basics"

Each day in the Spring Forecasting Experiment, before we consider any numerical weather prediction, participants hand analyze surface and upper air maps. While hand analysis is less common in the digital age, it's an important aspect of our daily routine. Why, you might ask? Well, what do you do when two model runs give you something like this:

This is the ARW control member of the CLUE (left) and the NMMB control member of the CLUE (right). Initial and lateral boundary conditions were the same for these two models, and they were far from the only ones providing radically different solutions for today's forecast scenario. The huge spread showed the importance of the mesoscale setup on this day, requiring a thorough understanding of the current situation. So, after the orientation, we set the participants to their maps. When we discuss the maps, we start from the upper levels and work our way to the surface. The first level is 250 mb:

Participants noted that the persistent east coast trough is trying to shift east, with a split flow across the Great Plains. The flow at 250mb was about 30kts over Oklahoma, with a 90kt jet sampled in Flagstaff, Arizona and Las Vegas. We did have divergent flow across Oklahoma and Texas, which probably helped the MCS that moved across the area of interest last night.

At 500mb, there is a disturbance in the flow from the morning MCS over Oklahoma. However, the flow over our area of interest was about 30-40kts, sufficient for supercells with the instability present. Though the morning 500mb chart was complex, it suggested the potential for a sharp dryline to set up:

The opportunity for each participant to contour a map means that with the number of participants each week, we get multiple takes on some maps, such as our 700mb map. At 700mb, a trough was present across the region of interest. We didn't expect this trough to shift much, but instead for a series of small waves to come through the trough. Flow was 20-30kts across Texas:

At 850mb, there was a large plume of moisture from Houston all the way to Minnesota, with dewpoints over 10C! After dealing with weak moisture return for most of the previous weeks of SFE 2016, this was quite a change, and indicated that we would have a broader area of severe potential. There was also a 30-35kt jet this morning over the Texas Panhandle into Kansas. The temperature field at 850mb appeared to be affected by the MCS, with cooler air over northern Texas and Oklahoma, causing a thermal ridge that would require recovery to become unstable once more. One of the major questions we had for today was if and where we might get recovery behind this system.

The next level we contour is 925 mb. Though it's below ground in much of the west (hence the lack of observations!), it provides valuable information east of the Rockies. We had two participants contour 925 maps:

These detailed maps showed good flow ahead of the MCS, on the order of 40kts, along with a ribbon of high dewpoints. Overall, the maps showed warm temperatures aloft, and southerly winds advecting moisture northward from the Gulf of Mexico. We also noted a bit of backing around where the 925mb surface meets the ground, suggesting that upslope flow would be moist and wind profiles could be more conducive to tornadogenesis should storms initiate. There was also a note of the cold, dry air west of the Appalachians, reaching all the way into the deep south. The western extent of this air (roughly Arkansas) marked a boundary beyond which we expected all convection to decay rapidly.

By far the most frequent map done each day is the surface map. As we have many more observations at the surface than we do aloft, often the maps zoom on the region of interest. These maps also help us get a handle on mesoscale features of import: key to today's forecast. The location of the boundaries and the progression of the morning convection shaped our understanding of how the day should progress, and helped us determine which models would weigh more heavily on our forecast.

A focus on the observations affected by the ongoing morning convection

These maps showed that ample low level moisture, in the form of 60's dewpoints, stretched all the way to Kansas. The dryline was diffuse but still identifiable, and we thought that it would sharpen throughout the day with heating. Participants also noted a mesohigh associated with the MCS, and indicated that the outflow boundary would be something to watch.

As the day's weather is playing out, the clearing we anticipated across central Oklahoma didn't quite work out. Rather, clouds and stable air kept us in Norman relatively cool and contained the worst storms (as of this writing) further west than we anticipated initially. We began to pick up on this trend for our afternoon forecast updates, but tomorrow's verification should be an enlightening one. The mesoscale details are difficult for the CAMs to grasp and present a large forecast challenge. This week looks as though it will be full of these mesoscale challenges. Stay tuned.