Your comprehensive race day forecasting
Posted: Oct. 3, 2018 Quick Summary Mostly cloudy, with a small chance of drizzle or light rain. Temperatures starting in the low to mid 40s (F) at race start, topping out in the upper 40s to low 50s by race end. Winds switching from the northeast to more out of the east at about 5-10 mph. Details It has been too long since my last update on TCM, and since then, the forecast has become a lot less uncertain. Last time I plotted a spaghetti plot of the 500-hPa heights from all of the GFS ensemble members showing that there was a trough to the west of the Twin Cities and a ridge to the east, but the exact positions of the cold front and jet stream were really hard to get in the vicinity of the Twin Cities. Since then, the spaghetti plot shows a lot more agreement between ensemble members. For this reason, we have more confidence looking at a single GFS forecast to get more details about the likely weather for Sunday. Below is a plot of the surface temperature and winds at 7am, just one hour before the starting gun, on Sunday from the GFS. Minneapolis is on the cold side of a stalling cold front (which later becomes a warm front ahead of the next storm system). This means for the Twin Cities metro area, temperatures will be in the low to mid 40s, maybe even some pockets of upper 30s Sunday morning at sunrise, with winds out of the northeast at 5-10 mph. By noon, things have warmed up a tad to the mid to upper 40s, with winds become more easterly at about 10mph. Right now, the models suggest both a low-level cloud deck and potentially high clouds as well, so that's keeping the sun and warmer weather at bay. This is an area where the forecast might be a little less certain and we'll keep an eye on that detail. If the clouds are being overforecast, then it could get slightly warmer toward the end of the race. But clouds seem like a good bet, with even a relatively small chance of drizzle or light rain. After the race, the stalling front becomes a warm front and moves back towards the Twin Cities, with increasing chances of rain later in the day. At the moment, it does not look like this front and rain will get there early enough to impact the race. Potential race challenges: While I thrive in temperatures that are as cool as forecasted on Sunday (must by my Siberian blood), it may be on the cool side for some, especially with a slight breeze and with some nonzero chances of drizzle or light rain. Therefore, some racers may choose to wear a warm layer or wind breaker, along with light gloves for those with extremities that are sensitive to cold. Another challenge is that miles 8-15 and 21-26.2 will be going against the wind. This may be refreshing but add a small layer of challenge. But many racers are from the area and are some of the heartiest individuals in the nation, putting up with far worse weather conditions year round. May the cool weather carry you to a fast finish!
-Chris
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Most of the forecast models are shaping up within the more predictable 10-day range (although there can of course be variation to these forecasts as they get closer to game day). Right now, depending on which model you look at, there may be light rain/drizzle or mostly sunny skies. The difference will be important. Even in the 50s, full sun can impact your surface temperature and the temperature of surrounding pavement/black top/buildings. Like all cities, Chicago can turn into a heat island. I'll get into this more closer to race day, but Chicago tends to be hotter (and windier) near the end of the course. There are more open spaces, darker road surfaces exposed to the sun, and it's later in the morning when you arrive. To make things worse, crowd support is low to nill in the last 6, when things inevitably start to go wrong. I've always wished that they would reverse the course so that runners could get the crowds and the lower temps/low wind near the end.
Ideal temperatures for a 'perfect' marathon should be in the upper 30s to low 40s, but Chicago looks like it may be close. I'm projecting mid 50s to low 60's at the end, with North winds around 10mph. For those finishing in under 3 hours, you'll still be in the upper 50s. After that, it could feel a bit warm. With a north wind, temperatures will be cooler, but you'll hit a slight headwind in the first 6 miles, a cross from 13 to 18, and a strong headwind in the last 3 or so. Plan accordingly! Race morning (Oct. 7th) is 11 days away. This is pretty far out for a trustworthy forecast of temperature and weather, but it's close enough in time we can start to get an idea of the overall weather pattern. There are a number of ways we can try to peek into the future that far out. Today, we'll look at a forecast tool known as a forecast ensemble to judge how well a numerical weather prediction model is handling its weather patterns. As we'll see, race day is shaping up to be a difficult forecast but we at least are starting to get an idea of the weather pattern that's setting up on race weekend. So please read on to learn more about how to use an ensemble to get useful weather information at longer range. And if you skip far enough ahead, we'll look at how race day is shaping up. I am happy to answer questions in the comments below. Before getting into the forecast, I will try to explain here how an ensemble works. A forecast ensemble is just a forecast model that is run a few times at each forecast time. For example, this morning NOAA ran its GFS ensemble with 20 forecast members out to 384 hours, as it does four times a day. That is, four times a day, 20 predictions are made with the GFS model. Each of these forecast members are made unique by adding small, unique perturbations to the initial conditions of each of the ensemble forecast members. An ensemble is intended to show the amount of uncertainty in a forecast since the small initial perturbations grow larger in time and space as a model is run (typically on a supercomputer) forward in time. So, for an 11-day ensemble forecast, if we see that all of the ensemble members generate a huge range of weather possibilities, we know making a specific forecast isn't very useful. If the ensemble members, on the other hand, sort of resemble each other, or at least show some kind of narrowed range of weather pattern possibilities, then we can get useful information. As a caveat, it is worth mentioning ensembles suffer from what is called "underdispersion", where the range of predicted possibilities is too small (i.e., too confident) and not consistent with the actual model errors. I won't get into this issue too deeply, but it's a way that ensembles can fool a forecaster. For now, we'll just use an ensemble to see if there's some kind of pattern emerging. To keep this post from getting too long, I'll pick on the American ensemble system today. But I will include other ensemble systems into future blog posts since incorporating variety leads to a more accurate forecast. Below we have a "spagetti plot" of the so-called 500-hPa geopotential height fields contoured at three values (5520/blue, 5760/green, and 5880/red gpm) for all of the ensemble members of the GFS forecast model (GEFS) on Sunday morning (Plot credit: COD Weather at http://weather.cod.edu/forecast/ ). 500-hPa is a pressure level (pressure decreases with height) representing a slice of atmosphere at around 18,000 feet above sea level. It's a good level to discern large-scale weather patterns. To bring this plot down to earth, the green curves are rough approximations of where the jet stream is positioned in each ensemble forecast member, and the black curve is the non-ensemble version of the GFS. Typically, it is cooler north of the jet, whereas it is warmer south of the jet. Low pressure systems often move along and with the jet. At first glance, this plot does resemble spagetti and seems like a giant mess. But actually, if you cross your eyes, you can see the colored lines sort of cluster together. The jet (green lines) seems to dip south over the Rockies and head north over the Great Lakes at this time. This suggests the potential for warmer than normal conditions over the eastern third of the country, and cooler air moving in over the Dakotas. This puts Minneapolis right in the battle zone. And just looking at this chart alone, it seems plausible a low pressure system might be positioned somewhere over the Great Plains during this time. So, while the pattern looks fairly well parsed out, being in the region of the jet makes this a challenging forecast for Minneapolis, since any one of those green lines can mean profoundly different conditions on race day. This means there could be a cold front to the west of Minneapolis with brisk, warm winds out of the south or that a cold front might have already passed by the starting gun with refreshing, cool winds out of the northwest. Alternatively, a low pressure system may sit to the southwest, with a warm front in the area, accompanied by the potential for some rain and thunder. Some of these possibilities sound concerning, but given the wide range of possibilities at this point, it's hard to bank on any given scenario. We only know that a pattern is emerging with a storm system and accompanying cold front setting up somewhere over the central US. There exists pitfalls and silver linings in any of the scenarios for the marathoner to consider.
One other thing to consider with these ensembles is weather extremes. I'd put those red lines in the category of extreme. If those lines were close to Minneapolis, that would mean very warm weather might be possible. The good news for now is those red lines are quite a bit south, so even if the cold front hasn't passed on Sunday morning, a reasonable estimate of the upper bound on temperatures is in the 60s to maybe 70s. And again, it would likely be refreshingly breezy in that case since the pattern suggests a storm system happening somewhere in the central US. Likewise, January weather looks unlikely, although cool weather cannot be ruled out. I will not get into precipitation/thunder specifics until later forecasts since that's even harder to predict than the large-scale weather pattern. Finally, to repeat the earlier caveat about underdispersion, we can get too confident that the pattern will occur as predicted in the above GFS ensemble, but it's quite possible the forecast members are latching onto a certain pattern whereas other patterns are more likely than shown above. This is a frustrating aspect of ensembles, but at least they start to show the range of uncertainty you can't get from just looking at a single forecast. And hopefully help you plan for race day scenarios that may occur. I will provide updates in the coming days as the forecast picture becomes clearer, but for now, I'll leave you with the Climate Prediction Center's probabilistic outlook for temperatures in the coming 8-14 days. It's no coincidence it resembles the GFS ensemble above. It shows Minneapolis on the edge of likely cooler than normal weather, consistent with a cold front sweeping southeastward out of Canada in the 8-14 day timeframe. So let's hope it is ambitious and gets there in time for race day! -Chris It's a bit early at this point to post a reliable forecast, but in past years, Chicago has been warm, even hot. In fact, in 2007 it was so hot that they shut the course down early. Temperatures in the 80s were recorded in 2008, 2010 and 2011. Right now, models are showing that there is an above-average chance of high temperatures on race day, which should come as no surprise. The earth, after all, is warming substantially, and runners of early fall marathons are almost certainly going to be victims of that trend. That being said, there are things you can do prior to the race to prepare. Overdress for your training runs, and run during the hottest time of the day to acclimate to the heat. There are probably advantages to doing this regardless, but on race day you will be more prepared than others who were hoping for that sweet spot in the upper 30s or lower 40s. We will continue to post updates on the forecast as race-day gets closer!
- Jesse |
Chris Rozoff and Jesse Sweeney, content contributors.Chris is the expert meteorologist and MVP. Jesse is someone who watched the weather channel a lot as a child and breaks it down for the masses. Chris runs ultras. Jesse runs road and ultras. Archives
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