Revisiting the Isochrones

One of the innovations introduced last year was the drawing of isochrones, or lines of equal time. These isochrones indicate the start time of the four-hour period where 95% of the reports at a point were expected to occur – each point has one four-hour period. This year, to aid in the drawing of the isochrones, participants now also draw hourly report coverage areas each hour from 18Z to 03Z. The final product looks something like this forecast from 25 May 2017:

Dave Isochrones for 25 May 2017

The above image says that the area to the east of the 19Z line will see its most severe weather from 1900 UTC – 2300 UTC, and the areas east of the 23Z line will see the peak severe weather from 2300 UTC – 0300 UTC the following day. Ideally, these lines will be displayed on the 15% threat area (the “slight” risk equivalent) to determine the eastern bound of the final line – currently participants and the NSSL desk lead have this forecast as a background when drawing their isochrones.

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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.

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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?

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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.

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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.

All indications from the CAMs were that ample convection would occur over a large area, raising concerns over potential outflow boundaries and convective mode. There were also concerns about elevated vs. surface-based storms, which could produce different hazards.

On all days in the experiment, we look at guidance from CAMs and CAM ensembles alike, and yesterday the paintball plots of reflectivity greater than 40 dBZ from the ensembles produced Jackson Pollock-esque maps by 00 UTC. The Storm-Scale Ensemble of Opportunity (SSEO) had many members showing a linear structure stretching from Arkansas to Texas and individual cells stretching along the warm front near the Arkansas-Missouri border. However, the members were in more disagreement over what was occurring over north-central Oklahoma, showing both linear systems and discrete cells.

The NCAR ensemble showed a relatively similar area of convection, albeit with more coverage by the convective cores. The increased coverage may be partly a function of having more two members in the NCAR ensemble than in the SSEO. The linear structures stretching down the cold front at 0000 UTC were slightly more segmented than some of the SSEO members, and there appeared to be many more linear structures along the warm front than in the SSEO. Some members gave the indication of a double line structure, with linear convection along the cold front behind the main squall line.

Participants also considered the HRRR ensemble, which had linear structures further to the north, convection all across Arkansas and perhaps less focused on the warm front, and more segmented structures from Arkansas to Texas. However, it was also showing somewhat of a double-line structure along the cold front and behind the initial squall line, as was seen in the NCAR ensemble. The HRRR ensemble at this point also showed less convection in north-central Oklahoma than the NCAR ensemble and the SSEO.

These are just snapshots of the reflectivity greater than 40 dBZ at one point in time. Participants also considered the updraft helicity tracks and probabilities, the overall reflectivity fields, and the entire period from 1600 UTC to 1200 UTC the following day. Besides the ensembles, extra focus was given yesterday to the operational and experimental versions of the HRRR, which at 0000 UTC showed very different structures underneath the center of the low.

The operational HRRR (top) has heavier convection with more coverage over north-central Oklahoma, and more stratiform precipitation behind the linear structures in Arkansas. There were also more hints in the operational HRRR of cellular structures along the warm front, but the experimental HRRR (bottom) depicted more intense cells across northeastern Oklahoma.

While the CAM ensembles suggested mixed-mode signals for this hour, the upscale growth of convection was forecast to be mostly complete by this hour. How did the CAMs do?

The radar images tell the tale – there was a small double line structure, with a large squall line stretching into Texas and a second line across northwestern Oklahoma. Cells along the warm front in Missouri and Arkansas did not congeal into a line, and a few cells were present in central Oklahoma.

Prior to 0000 UTC, significant hail and wind reports across much of north-central Oklahoma (including a 4.25″ hail report!), associated with supercells close to the core of the low pressure system. This case study will be an interesting one for CAM developers, encompassing mixed modes, early elevated convection, significant severe weather, and two active fronts. The atmosphere was more than happy to provide us with any type of midlatitude convective weather we could ask for yesterday, offering researchers and forecasters alike new questions and challenges.

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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.

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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:

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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….

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