Tuesday, May 23, 2017

Evaluating the Forecast Evolution

Every year in the SFE, a fundamental problem arises when evaluating the long-range full-period forecasts: how to rate the long-range full period forecasts. On the Innovation Desk, participants are given the chance to issue Day 3 forecasts in addition to their day 1 forecast, if time and the potential warrants. Due to the weekly structure of the SFE (which runs M-F), at best two of these forecasts can be evaluated each week - those issued on Monday for Wednesday, and those issued on Tuesday for Thursday. Luckily, with a relatively active period of severe weather CONUS-wide, the three weeks of the experiment so far have yielded evaluations for four out of the potential six days that have Day 3 forecasts. Two of these forecasts give examples of how the long-range forecasts can change as the day of the event draws nearer, and more guidance becomes available: 10 May 2017 and 18 May 2017.

Saturday, May 20, 2017

Mind the Gap

Continuing on the last post's theme of choosing the proper forecasting domain when we have multiple areas of convection to contend with, today's discussion will focus on the boldest of forecasting moves:

The gap.

The full period forecasts issued by each desk are a group effort, with input from participants guiding the placement of the lines. Prior to issuing the lines, participants consider observations, coarse-scale operational models such as the GFS and the NAM, and fine-scale operational and experimental models, such as the HRRR, FVGFS, and the members of the CLUE ensemble. Convection-allowing models, with grid spacing of ~3 km, provide very realistic-looking radar signatures that can give confidence in specific areas of threat beyond those of the GFS. For a quick example, see the GFS forecast for 18 May 2017 at 0000 UTC:

The echos from the HRRR suggest that these storms would be supercells, given the strong tracks of  hourly updraft helicity (as indicated by the black contours) and the individual reflectivity echoes. Images such as these can give forecasters more confidence in the location(s) of convection, particularly when compared to the larger-scale QPF precipitation products that current coarse-resolution models can provide. 

So what does this have to do with gapping the forecasts? And what does gapping the forecasts even mean?

Tuesday, May 16, 2017

Picking Areas during an Active Week

This week is gearing up to be the most active week thus far in the SFE, with every day having the chance of severe weather somewhere in the center of the country. Yesterday, we had three separate areas of potential severe weather to consider:

The first area was concentrated across northern Iowa and far southwestern Wisconsin, the second area stretched from central Nebraska south through western Oklahoma, and the third area was in western South Dakota. Since SFE forecasts cover a subset of the contiguous United States, choosing which areas to forecast for is an important part of the forecast process. In this case, the worst severe convection was anticipated within the eastern two areas, and the forecast domain was chosen to encompass as much of those areas as possible.

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:

Wednesday, May 03, 2017

Snow Forecasting Experiment??

Strange considerations can crop up in the SFE. In previous years we have forecasted in areas of low radar coverage such as the mountain west, determined which side of the U.S.-Mexico border a storm would form on, and dealt with the severity of convection coming onshore from the Gulf. However, remnants of last weekend's storm threw a highly unusual wrinkle in the forecast....