2019.09.27 – Chicago, Evanston in an Area of Increased Severe Weather Risk

  • A first round of storms is moving through currently and will persist for the coming hours, into the early afternoon
  • New storms will fire up in the late afternoon / evening over northern Illinois and are expected to bring heavy rainfall overnight
  • Total rain may amount to up to 2 inches or over 50 liters per square meter, posing risk of flooding & flash flooding.
  • “When thunder roars, go indoors”!
  • Drive cautiously in heavy rainfall due to risk of aquaplaning and reduced visibility.
  • The national Storm Prediction Center predicts the risk of large hail (>2.5cm / >1 in) within 25 miles to be at 10%.
  • The risk of severe storm gusts over 90km/h / 54 mph to be at 5%.
  • For up-to-date information, visit the National Storm Prediction Center
Latest radar image of Chicago & Northern Illinois

2019.09.16 – Monday’s weather for Chicago & Metro Area

Foggy weather is expected to start off the new week, however the sun will come out later.

Monday morning will start off with foggy and generally rather dreary conditions. Some light rainfall is possible during breakfast hours at around 8am. The fog is expected to largely dissolve before noon, and the remaining layer of low clouds will give way to periods of sunshine as the day progresses. Especially in the later afternoon, the sunny weather will likely dominate.

Temperatures will start off at around 19°C (66°F) in the morning, rising to around 23°C (73°F) by the afternoon.

The wind will be light, with gusts not exceeding 25 km/h (16 mph).

The general outlook for the days to come:

Tuesday: Mostly sunny, 24°C (75°F)
Wednesday: Mostly sunny, 25°C (77°F)
Thursday: Inconsistent weather, rain a thunderstorms. 27°C (81°F)
Friday: Mostly sunny, 29°C (84°F)

2019.09.14 – Tropical Update: Tropical Storm Humberto will likely miss US, Hurrican Kiko out on Pacific

Two tropical systems are currently being watched by NOAA’s National Hurricane Center – one a hurricane in the eastern Pacific, one a tropical storm in the vicinity of the Bahamas. Here are the details

Tropical Storm Humberto

Central Pressure: 1000 hPa
Maximum sustained winds: 93 km/h
Gusts: 110 km/h

Tropical Storm Humberto is currently located just to the north of the Bahamas, its precipitation and even its storm field affecting some of the nation’s islands. It is currently not predicted to make landfall in the mainland US, instead turning back out to the Atlantic and then heading towards the Northeast. The storm is, however, still expected to strengthen and is predicted to be upgraded to a category 1 hurricane by 8pm on Monday (EDT), and a category 2 by 8pm on Tuesday (EDT). On Wednesday, the storm is expected to be positioned around Bermuda. Whether the British territory is going to be struck isn’t entirely certain yet, however, precautions should be taken.

The current storm field and predicted path of Tropical Storm Humberto. Note how part of the Bahamas is currently being impacted by the storm field. Source: NOAA / NHC

Hurricane Kiko

Central pressure: 966 hPa
Maximum sustained winds: 184 km/h
Gusts: 211 km/h
Category: 3

Hurricane Kiko, though a major storm, is located well out on the Pacific and is headed Westwards, away from land. It therefore poses no real risk to North America. The storm is expected to continue on this trajectory for the days to come, eventually weakening to a category 1 storm by the end of the 5-day forecast period.

Hurricane Kiko, though a major hurricane, does not pose any threat to mainland North America at this time. Source: NOAA / NHC.

The Coast’s Curse – Why Germany’s (and Amsterdam’s and London’s) weather is so damn bad

Storms coming from the Atlantic, like this one, are the reason Europe receives so much rain. Image: NASA

There’s a reason the British came up with the phrase “raining cats and dogs”, and for why Germany appears to perpetually be in a bad mood. Western Europe receives a lot of rain. To be exact, London gets on average 630 liters per square meter (1 l/m² = 1 mm), Berlin 566 mm and Amsterdam tops both, at 810 liters annually. But why’s that?

Simply put, the reason it rains so much is because all of western Europe is close to the ocean – you are usually less than a thousand kilometers from the next coast, and on a global scale, that is almost no distance, nor is it for the humid air moving in from the Atlantic. This air eventually deposits most of its water in western Europe.

In more scientific terms, Western Europe is classified as having a “temperate climate”, meaning that there are four distinct seasons, with warm summers and cold winters, and that there is precipitation year-round. The British Isles, the BeNeLux, Germany, Denmark, the south of Sweden, south and west of Norway, all of Germany, Austria, Czechia, and much of Poland and the inland of the Balkans as well as the north of Spain fall under this classification, as do the western coasts of Iceland and the Baltic states. Further to the east, continental climate takes over, which is a lot drier than Western Europe’s rainy weather, and in the south, along the coasts of the Mediterranean, Mediterranean climate persists, with hot, dry summers and cooler, rainier winters.

Generally, the further east you go, the drier it gets, as the moisture-laden air from the Atlantic needs to cross larger areas of land, therefore typically losing its water on the way before continuing into areas such as Belarus and Russia.

Europe is surprisingly far north. At 52° North, the German capital, Berlin, is 9° further

A satellite image of the Sea Surface Temperatures of the Northern Atlantic clearly shows the warmer water being transported north by the Gulf Stream. Image: NASA

North than Toronto, and even 1° further north than Alaska’s southernmost city. Nevertheless, temperatures rarely drop far below -5°C in winter, and commonly exceed 25 and even 30°C in summer. A key contributing factor to the mild weather in Europe is the Gulf Stream – an ocean current that transports warm water from the Caribbean and the Gulf of Mexico all the way to the Northern Atlantic – both increasing Europe’s temperature, and providing the fuel for low pressure systems (bringing rain) and storms to form over the ocean, only to move eastwards and towards Europe.

There is one final piece to unlocking the puzzle of why Europe is so wet, and that goes by the imposing name of “polar front”. In meteorology, a front is the place where two air masses (areas of air with different properties, usually temperature) meet. In the case of the polar front, warm, moist air from the south clashes with cold, moist air from the north. When air masses clash, they shove each other around and lead to the formation of weather patterns. In this case, low-pressure systems are formed (which has the fancy name of cyclogenesis). And low-pressure systems typically pack rain.

The formation of a low pressure system, a.k.a. a cyclone (not all cyclones are tropical storms!). In the case of the Atlantic, the line where the arctic and subtropical air meet is called the polar front. The entire process is called cyclogenesis. Image: classconnection.s3.amazonaws.com/814/flashcards/364814/jpg/formation_of_a_wave_cyclone1322859884968.jpg

The Speed of Rain

On average, rain falls at just about 20 km/h. But what really defines rain, and are there different types?

What is rain? It is liquid water coming from the sky, so a form of hydrometeor (hydro = water, meteor = well, something that falls from the sky). But as anybody who has gotten into a torrential downpour in the summer and into misty drizzle in winter will surely have realized, not all rain is created equal.

In fact, there are distinctions between the types of rain that exist. Most notably, between “drizzle” and “rain”, two categories which are entirely distinct forms of precipitation in weather stations’ reports (using the SYNOP key). Drizzle has a diameter of less than half a millimeter, and will not exceed rainfall rates of 1.3 liters per square meter each hour (l/m² = mm of rainfall). Rain, on the other hand, has drop sizes of at least half a millimeter but is then further subdivided into light rain (under 2.5 mm per hour), medium rain (2.5 to 7.6 mm/h), strong rain (7.6 to 15 mm/h), and torrential rainfall, which exceeds 15 liters per square meter in just an hour and will often lead to flash flooding.

Rain comes in different drop sizes as well, though. The size a rain drop reaches depends on various conditions in the cloud it originates from, but is mainly influenced by the strength of the updraft, as a stronger updraft has more force and can therefore carry larger, heavier rain drops for longer before they drop (no pun intended). Once the weight of the rain drop is so great that it falls, air resistance will change its shape yet again. The larger the rain drop, the flatter it becomes, as the air resistance from the bottom forms an indentation. A drop 3 mm in diameter therefore has the shape of a jelly bean, while a drop of just 1 mm is almost perfectly spherical. The upper limit for how large a rain drop can get is typically at around 4.5 millimeters, at which point it will look almost like a tiny parachute when photographed mid-fall. Any larger, and the air resistance will tear apart the rain drop into smaller droplets.

Oh, and to provide the answer to the title of this article: an average rain drop falls at roughly 23 kilometers per hour, so would take roughly eight minutes to fall from a height of three kilometers all the way to the ground. For comparison, a snow flake will fall at about 5 km/h, taking 36 minutes, while a hail stone the size of a golf ball may reach a whopping 103 km/h and cover the same distance in less than two minutes.