Friday, May 12, 2017

CAM Guidance in a Mixed-Mode Case

Yesterday, 11 May 2017, gave the participants in SFE 2017 many things to consider. A potent upper-level low pressure system was finally evolving eastward, after giving the Experiment interesting weather to forecast all week while sitting over the southwest. As the experiment began, ongoing elevated convection was already producing reports over northeastern Oklahoma, and the participants were eyeing the chance for some severe weather locally.

By 2000 UTC (3PM CDT, near the end of the SFE's daily activities), cellular convection was initiating all across northern Oklahoma, northern and western Arkansas, and northeast Texas. Many of these storms quickly began to rotate.

Wednesday, May 10, 2017

The Denver Hailstorm, 8 May 2017

If you have an interest in severe and unusual weather, you probably already know all about the hailstorm that struck Denver on Monday afternoon, shattering windows and damaging vehicles and roofs across the metro. Indeed, it made for quite the exciting Monday in the Spring Forecasting Experiment.

During the morning forecast discussion, participants noted that good forcing was present over Colorado, along with dewpoints considered sufficient for severe convection by Colorado standards (in the 50's). The moisture was modified Gulf moisture, arriving in CO by way of the Rio Grande thanks to the surface front that was the focus of most of last week's severe convection. Also noted was the unidirectional shear, as can be seen on this 1200 UTC (7:00AM CDT) hodograph from Albuquerque, which was upstream of Denver at 250mb and 500mb.

Sunday, May 07, 2017

Verification Determination

Verification is a huge part of the Spring Forecasting Experiment. Each day, we make multiple forecasts on different time scales (this year ranging from daylong outlooks to hourly probabilistic forecasts), and the first activity participants undertake on Tuesday-Friday is an evaluation of the previous day's forecasts. Additionally, in the afternoon, participants evaluate numerical guidance, by comparing model output to observations.

Selecting how to use observations for verifying some of the more nebulous aspects of severe convective weather is one of the challenges of designing the SFE. With some fields, it is easy enough to compare the simulated with the observed - take reflectivity, for example: