Preview for May 18, 2020 – KORD

By Matt Gallagher

Good morning everyone! Our first forecast is due tonight, May 17, at 00 UTC (technically May 18), or 7 pm Sunday Chicago time.  Here’s a preview of the setup for tomorrow.

Day 2 image not available

The WPC shows Sunday’s storms largely having cleared to the east Monday morning though the threat of precipitation is still very real in the Chicago area on Monday.  You’ll learn that forecasting precipitation is one of the hardest things to do in the Weather Challenge, as precip can be very localized – areas just a mile or two apart can see very different amounts of precipitation (Forecasters will often use the term QPF, or ‘quantitative precipitation forecast’ as a shorthand for precipitation).  More on this for a future post.  Precipitation is an area where models, more than MOS, can give you the best information on which to base your forecast, but resist the temptation to look at just a single model. You need to gather as much information as you can from all the models, as well as MOS, to best hedge your QPF forecast.

The low that’s currently centered over Illinois will be moving off to the east slowly Sunday and Monday, with precipitation likely Monday and Tuesday. Also watch those winds – as the low moves east, the flow will clock from the SE all the way around to the NE, across the relatively cool waters of Lake Michigan. 

A great place to start – after examining the big picture forecast from sites such as the WPC and considering the threat of severe weather via the SPC  –  is the Area Forecast Discussion. These are issued several times a day by every NWS local office in the country (sometimes hard to find on the individual websites – look for ‘text products’.). These are a narrative of not only the forecast, but also what the forecaster was thinking when they came up with the forecast that they did.  While the quality of these varies widely, one thing does not – the forecaster signs their forecast with their name. Unlike most government bureaucrats, there is accountability not cloaked in anonymity. If you read these daily you start to get a sense of the forecasters’ personalities, but better yet you get the advice of an experienced forecaster with local knowledge. 

So the forecast challenges for tomorrow, I think, really focus around that QPF. Good luck! Feel free to post questions here, or on the Facebook group.

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