If you have been paying attention to our forecasts for the past few days, you have probably heard us mention a large storm moving our way for the middle of this week. After lots of inconsistencies early on between weather models, things are slowly coming together. It is likely that we experience some form of precipitation, also likely are breezy winds Monday, Tuesday and Wednesday.
Now let’s try and answer some of those other questions: what type of precipitation will fall, how much of each will fall, and when will it fall?
Even though weather models are in better agreement, there are still some large differences. One of these differences is the placement of the low pressure as it moves northeast across the Midwest. Since winds rotate counterclockwise around low pressure, if the low stays north of us, our winds stay southerly, south of us = northerly winds. Wind direction will have a direct impact on temperatures (northerly winds = colder, southerly winds = warmer) which will impact precipitation type. Below are two images showing two different weather models with the first taking the northern route, and the second taking the southern route.
Right now I am favoring a northern route since that has been the trend through the weekend, but the most recent models have begun to shift southward. A trend we will watch closely the next few days.
When determining what type of precipitation will fall, my favorite resource to use is called a skew-t. This is a forecast of temperatures and dew points at different levels of the atmosphere. As forecasters, we look at a few key aspects of these diagrams: are temperatures below 0ºC through the entire atmosphere, if not, then what parts of the atmosphere are above 0ºC? Also of interest is the dew point compared to the temperature. The closer to dew point is to the temperature, the more saturated that layer of the atmosphere is meaning a better chance for precipitation.
The skew-t shown above is for 11 AM on Tuesday, during the start of this event. This is showing a saturated layer of air in the upper levels of the atmosphere, drier in the mid-levels, saturated around 1,000 feet up and slightly drier at the surface. In concern to temperatures, this shows us a layer of warmer air in the middle levels of the atmosphere as well as at the surface. Given this assessment, we can determine that any precipitation that does develop will be more spotty in nature (due to pockets of drier air), and a better chance of sleet (due to a pocket of warm air in the middle-levels) and rain (due to temps above 0ºC at the surface).
The skew-t below is 12 hours later at 11 PM Tuesday and is much easier to understand. This shows a saturated atmosphere from the mid-levels all the way to the surface, so precipitation is likely. Temperatures throughout the entire atmosphere are below 0ºC meaning snow will likely fall during this time.
Well now that you’ve had your science lesson for the next week, let’s move on to some simpler maps. How much precipitation will fall? The map below shows total precipitation (rain, mix, snow, etc.) accumulation for the entire storm. This shows more precip in the northern portion of the viewing area up to 0.25” and as low as 0.10” in the southern portion.
Latest model trends are for the majority of this precipitation to fall as rain with a little wintry mix. An eventual switch over to snow will be likely after dark on Tuesday, so how much snow will fall? That is still to be determined, but one model is showing less than 2” of snow with more just north of the viewing area.
Now, let’s answer our final question: when will the precipitation fall? The majority of models show precipitation (likely rain) starting near noon on Tuesday. A gradual switch over to snow will occur Tuesday evening with the majority of snow ending near noon on Wednesday with a few light snow showers possible in the afternoon. One major model shown below has precipitation starting much sooner (before sunrise on Tuesday), another aspect of the storm we will watch closely.
So, what are the impacts we can expect for this storm? There will likely be a period of slippery travel Tuesday evening as the change from rain to snow begins. Also, any snow will quickly become blowing snow with wind gusts as high as 35 mph, possibly impacting the Wednesday morning commute. Any snow that does fall will quickly melt with warmer temperatures, but rivers should be able to hold the new precipitation/melt.
Did you get all of that information? This storm is less than 48 hours away, the time when forecasts become much clearer. Expect the answer to these questions to become more clear-cut as the storm gets closer. The latest updates will be posted by following this link.
Storm Tracker 6 Meteorologist