By: Winn Soldani
Sweet home Chicago. The Windy City.
Welcome to the first forecast city for the CYC Weather Challenge, and a challenging one it is. Though we have many Chicagoans as part of this contest and this might seem easy to you all, forecasting for O’Hare is going to be an interesting way to start off.
While Matt and I will be pointing you to specific day-to-day forecasting resources, we were trained in our weather classes to start with climatology and bigger-picture thinking about a city we’re forecasting. So, let’s start here: what’s the weather normally like this coming week for KORD (the catchy code we use for O’Hare International Airport, based on its IATA airport code)?
Let’s look at the norms (norms are for a 30 year period. These are for 1980-2010) for Chicago in the week we’re forecasting (all the data below are courtesy of the excellent climate records page at the NWS Chicago):
Day of Month
So, looks pretty benign. But one thing we’re going to learn about May in Chicago, even late May, is that it’s still very much spring with alternating shots of cold and hot (how many Memorial Days have you worn a winter coat when you went to the Barbecue, Chicagoans?…don’t lie…). The mild norms here are arrived at by volatile weather. Sometimes, in May, it’s the cold of April, sometimes it’s the warmth of June, sometimes it’s May…but it can be any of those! Check out just how wild it can be:
Pause for a moment on this table. The highest temperature observed in the span we’re forecasting is 95 degrees. The lowest is 31. Wow.
Better yet, the high temperature has failed to touch 45 degrees on 4 out of the 5 days we’re forecasting for…and the low temperatures have been in the mid 70s…higher than the normal temps in the first table!
Precipitation…though we won’t be forecasting precipitation type, it’s SNOWED on three of the days in our forecast period! (Note that “T” stands for “trace” – less than 0.01″ but snow nonetheless!) More importantly, notice the difference between the normal precipitation (typically just over .1 inches) and the record. 1.5 to 2 inch precipitation totals happen!
What about the wind? Below is a wind rose for O’Hare. This may be new to some readers so I’ll explain. The different colors show the wind speed. The size of the colors and the direction in which they point show the frequency with which the wind came from that direction and at what speed (in knots). This wind rose for the week we’ll be forecasting for shows that over the period here (1946-2020), the wind falls largely into two buckets that reflect the battle between spring and summer we’ve described. There are a LOT of observed winds out of the NE/NNE and then a lot—though more spread out—out of the W/SW/S. SE winds and, especially, NW winds are unusual. Note that the vast majority of windspeeds are under 15 knots.
What does all this climatology tell us? Question number 1 we would ask when forecasting for O’Hare is this: is the pattern more like April or More like June? And what model(s) seem to have a handle on that pattern? Because the variability is really, really something (it is much more, than, for instance, what we’ll see in Miami in a few weeks). Get it wrong and your forecast is a sure-fire bust.
OTHER FORECAST CONSIDERATIONS
Beyond the big picture (synoptic) question of the spring/summer pattern, there are several smaller-scale (mesoscale) things that we have to think about in Chicago. Here are a few.
1) The Lake
Current nearshore Lake Michigan water temperatures are in the 50s, but with a very large area of water temperatures in the 40s in the center of the lake. This cools the overlying air and if that air advects inshore, forecasts can be heroically validated or epically bust.
With this in mind, we have to pay attention to the lake breeze. We all know this but as the land heats, it creates relatively low pressure ashore. The colder, higher-pressure lake air advects along a lake breeze front. However, at some point that front no longer has the energy to progress farther inland. Where that front stops is CRITICAL to O’Hare forecasts
- If the lake breeze makes it to O’Hare, temperatures will drop. This makes the high temperature occur at an odd time, not the typical late afternoon, but instead in the hours before the lake breeze arrives. Or, the highs may occur at a normal afternoon time but be lower than model forecasts.
- Winds will be impacted. Often there will be a brief windier period at the arrival of the front, along with a shift to a more easterly direction. When this happens, exactly, is important. If your forecast assumes the lake breeze arrives before 12 PM, your noon forecast is probably for a breeze with some kind of easterly component to it. Your max windspeed forecast may be low if the front arrives with a bit more “push” than models see.
- Storms. Though discussed more below, the lake breeze can act as a triggering mechanism for storms in certain situations. Again, if it makes it inland far enough, those storms can be over O’Hare.
As the pattern turns more summery, the main mode of rainfall changes. It becomes less stratiform rainfall (i.e. widespread showers that tend to cover an area relatively evenly) and more of a convective (thunderstorm) mode.
A key challenge for forecasters is placing the right bets on rainfall in a convective situation. The trick is that it can rain very heavily in the vicinity of O’Hare when thunderstorms are around, but where, exactly it does can make or break a forecast. Models can have skill in forecasting that an area (e.g northern Illinois) will have thunderstorms, but until the event is actually occurring, generally cannot say whether storms will impact a specific place (i.e. the weather station at O’Hare).
This can make the difference between no rain and a deluge. And it can happen either way—forecast a deluge and the storm slides by and you bust. Forecast dryness and the storm hits home and you’re also done. Often the best forecast hedges between the two rather than trying to nail the precipitation total!
I hope this overview of Chicago climatology gives you some things to think about in terms of the big picture and small picture of forecasting for O’Hare. It should be a fun and interesting week! Please stay tuned for more from us to help you prepare for the week to come!