Month: May 2018

FV3 Configurations

Posted on by Burkely Twiest.

Though we do lots of forecasting in the Spring Forecasting Experiment, we also evaluate cutting-edge numerical weather prediction. This year, we’re looking at many iterations of the Finite Volume Cubed model, or FV3, which is slated to eventually replace the GFS as the next generation global prediction system. This year, we have versions of FV3 provided by the Geophysical Fluid Dynamics Laboratory (GFDL; this is where FV3 was developed), the National Severe Storms Laboratory (NSSL), and the Center for the Analysis and Prediction of Storms (CAPS). GFDL and NSSL are each providing one member, while CAPS is providing an ensemble of eleven members that use different microphysics schemes and different planetary boundary layer (PBL) schemes. These different configurations can help illuminate the behavior of this new model core with multiple sets of physical parameterizations, grid spacings and convolutions, and FV3 versions. For more details, see the Operations Plan.

Last Wednesday, May  23rd, provided one of our most interesting case studies for the different versions of CAPS FV3, with differences in the storm structure, location of precipitation, and thermodynamic environment between the different microphysics and the different PBL schemes. The below panels show the reflectivity fields for CAPS FV3 members with four different PBL schemes, all of which use the Thompson microphysics.

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Domain Decisions

Posted on by Burkely Twiest.

When severe weather is imminent across the country, forecasters at the SPC must consider all of the possible areas. However, here at the SFE where we are considering multiple experimental NWP ensembles, we have to select our domain of interest. When the severe weather is spread across an area larger than a single domain, we must choose which region we want to focus on. To do this, the facilitators consult not only our experimental numerical weather prediction models, but also the upper-air data collected by radiosondes. Typically, the domains have quite a bit of overlap, but on days like last Thursday (17 May), a decision must be made between two very different areas.

After evaluating the previous day’s forecasts, the first forecasting activity for the current day that participants do is the hand-analysis of upper air maps at six different levels: 250 mb, 500 mb, 700 mb, 850 mb, 925 mb, and the surface. This activity ensures that participants get their hands onto the observed data, and develop a thorough understanding where a convectively favorable environments will occur. A map discussion follows (pictures of which can be found on the NSSL Flickr account) where participants share what they’ve learned through their contours. On Thursday, the conversation was far-reaching, as we had to decide between two domains that had very little overlap:

The two potential areas of interest for 17 May 2018, with the eventual selected domain highlighted in green.

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Consider the Messaging

Posted on by Burkely Twiest.

One of the advantages to having forecasters from the Storm Prediction Center (SPC) working on both the Innovation Desk and the Severe Hazards Desk is that they have experience with issuing outlooks, and how the public reacts to those outlooks. Thus, we can explore the nuance of forecast issuance better than if we were operating without the SPC forecasters. An example of this dynamic occurred on Tuesday this week, with the well-forecast line of storms that moved through the northeastern United States, bringing many reports of wind damage, injuries, and even fatalities. The experimental forecasts issued by both the Severe Hazards Desk and the Innovation Desks were the equivalent of a categorical Moderate Risk from the SPC, which can be triggered by either a 45% forecast of wind with significant wind gusts or a 60% forecast of wind without the significant wind gusts:

On the Innovation Desk side, there was a lot of debate about which type of moderate risk to issue. A categorical high risk was also considered, by debating whether or not to issue a 60% coverage contour with significant severe. Although we thought that there would be pretty high coverage of severe wind reports, the low-level shear and the strong low-level flow that would typically warrant a high-risk scenario were missing on Tuesday. High risk issuance for wind from the SPC is rare (as are high risks in general) and typically reserved for derecho-type events. The last wind-driven high risk was issued by the SPC on 3 June 2014 across Nebraska. Thus, an aggressive moderate was the message the Innovation Desk tried to pursue, with a 45% area and an area of significant severe (wind gusts in this case, although the Innovation Desk considers all types of severe convective weather in issuing its probabilities):

The Severe Hazards Desk issued a moderate that emphasized the high coverage potential of the event, going with a 60% contour with no significant wind highlighted:

Both of these outlooks indicated that the meteorological conditions were unusually favorable for a wind event, with strong mid-level flow and excellent lapse rates for the northeast. However, maintaining the equivalent of a moderate categorical risk communicated that this system, while dangerous and powerful, would not rise to the level of some of the most infamous derechos of years past. In meteorology, as in many things, communication is key.

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Forecasting During Quieter Days

Posted on by Burkely Twiest.

Although May is the heart of severe convective season across most of the United States, some days still have relatively little severe convective weather. This has been the case for most of this week, with yesterday having the most severe convective reports of the week (48) according to the Storm Prediction Center’s (SPC’s) storm report page. However, just because there are fewer storms occurring doesn’t mean that operations halt during the Spring Forecasting Experiment – quite the opposite! While we may have fewer probabilistic contours to draw if we don’t have as high of probability on a given day, the placement and magnitude of these forecast contours are still a significant challenge.

These types of days challenge the models in a different way, showing how they perform on days that are less strongly-forced on the synoptic scale or may not have as much available moisture than the days with more storm coverage. Considering that only about twelve days per year reach the level of a moderate risk according to the SPC, lower end days are far more common and thus require thorough testing as well.

This week has so far exemplified a number of different lower risk days, mainly associated with a slow-moving trough progressing across the contiguous United States. On Monday, a smattering of wind and hail reports affected western South Dakota and eastern Wyoming. Absent of strong upper-level flow, terrain was a large consideration in our forecasts. Tuesday saw a handful of hail reports and a tornado report in eastern South Dakota and across Iowa. Ongoing elevated convection in the morning complicated this forecast, as the main area of convective concern was not starting with a clean environment. Yesterday a morning mesoscale convective vortex (which were originally known as “Neddy Eddies” after retired SPC forecaster Ned Johnston) led to questions about when convection would initiate and how well the models captured the relatively small-scale vortex.

Even when our forecasts are challenging and don’t perform as well as we’d like, we still perform subjective verification the following day. Below are the full-period forecasts for Monday for total severe from the Innovation Desk (left) and for wind from the Severe Hazards Desk (right):

Tuesday for total severe from the Innovation Desk (left) and for hail from the Severe Hazards Desk (right):

and Wednesday for total severe from the Innovation Desk (left) and again for wind from the Severe Hazards Desk (right): 

While these forecasts were far from perfect, we don’t update these full-period forecasts later during the day and issue them by ~10AM CDT. Therefore, these represent our initial impression of the weather for that day after only an hour or two of consideration – and we are getting additional observations and numerical guidance as the day goes on. Particularly on Wednesday, this updated guidance caused us to shift our short time period forecasts (which we do update) as the afternoon wore on. Clearly, more marginal forecasts still present challenges, just different ones than the high-end days. We always have something to consider here during the SFE!

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Short-Term Forecasting Methods

Posted on by Burkely Twiest.

This year, the Spring Forecasting Experiment is focusing on the Day 1 time period more than ever before, eschewing the long lead-time forecasts that we have made in previous years in favor of honing in on timing information and allowing participants to delve into the data. Since more data than ever before is available within the drawing tool where participants draw their forecasts, we’re excited to see how participants probe the new data within the various ensemble subsets.

One short-term experimental forecast product being generated on the Innovation Desk this year are Potential Severe Timing (PST) areas, which indicate which 4-hr period severe weather will occur in over the general area of 15% probability of severe. By identifying the timing of the severe event and displaying all of the timing contours on one graphic, the end product is hoped to be valuable for emergency managers and broadcasters for their advance planning. Small groups of participants generate these forecasts around subsets of the CLUE and HREFv2 ensembles, meaning that on any given day we’ll ideally have 5 separate sets of PSTs. After the participants separate into their small groups and issue their forecasts, we ask them to come back together and brief one another on what their particular ensemble subset was doing. This way, each group of participants can delve into the data from their subset more deeply than if the activity were to take place as one large group. This briefing period also allows the participants to be exposed to different lines of reasoning in issuing their forecasts, and has thus far sparked several good discussions.

Here are the PSTs from 3 May 2017, or Thursday of last week:The different ensemble subset groups compose the top row and the left and middle section of the bottom row, while the bottom right hand panel shows the forecast from the expert forecaster facilitator on the Innovation Desk. Several different strategies are evident within the panels, including some groups that chose not to indicate timing areas for all of the 15% area of our full-period outlook (shown below).

 The reasoning from the groups for their different areas gave insight into the model performance as well as the different forecasting strategies employed by the different groups of people. The group using the HREFv2 decided not to use the NMMB member when generating their forecasts, because the depiction of morning convection was so poor. The HRRRE group had very large areas, which they attribute to the large spread within the HRRRE. The NCAR group decided to discount the guidance in the north of the domain, because of erroneous convection in the northern domain. Instead, they felt more confident in the southern areas where the ensemble was producing supercells. Their group thought that the thermodynamics of the northern area was less conducive to supercellular convection. The group using the mixed physics ensemble from CAPS placed their first area based on where they thought convective initiation would occur, indicating that they thought convection would quickly become severe. Their southern PST was very late to cover any severe threat overnight, but they considered that it might be more of a flood threat (which we do not forecast for in the Spring Forecasting Experiment). The stochastic physics group (another ensemble run by CAPS), on the other hand, had an ensemble which showed almost no signal in the southern area of interest. It also showed a later signal than the other ensembles, contributing to the spread in the time of the first PST.

All of these details came out during the discussion of the PSTs, after participants dove into the data from their subensemble. How did the PSTs do? Here’s a snapshot of the PSTs with reports from 18-22 UTC overlaid:Ideally, all of the reports would fall into the 18-22 UTC contours, which mostly occurred for the expert forecaster and did occur for the HRRRE and Mixed Physics group, although both groups had large areas of false alarm. Here’s a similar image, but showing reports from 22-02 UTC:At this point in time, all groups missed the activity in Kansas, although some groups captured most of the reports within a 22-02 UTC window.

The day after the forecasts, participants are able to go through and give ratings based on the reports that have come in, and choose the group’s forecast that they thought performed the best. Who performed the best for this case? 3 votes for HREFv2, 2 votes each for the HRRRE and the CAPS Stochastic Physics ensemble, and one vote each for the CAPS Mixed Physics and the NCAR ensemble group. Clearly, the complexity of this case provided plenty of nuances to evaluate, and I would bet that more complex cases such as this are on the way….after all, we’ve only just begun Week 2 of the 2018 SFE!