PARDON THE INTERRUPTION

Before we get to the small bump in the road early Friday, temperatures Thursday morning were cold enough to support frost in some locations but not others. My climate guru Steve Gotschalk in Lowden, tells me he got down to 34 and had lots of frost on the ground. 25 miles to the southeast, the NWS in NW Davenport was 7 degrees warmer at 41. Elkader had a frosty 31, while 40 miles away in Dubuque it was 12 degrees warmer at 43. Both had clear skies and light winds from the same direction. What the heck is going on?

The simple answer to the riddle is elevation and a process called cold air drainage. In the case of Elkader, the city sits in a valley with an elevation of 732 ft. while the airport, in Dubuque, is on top of a ridge with an elevation of 1076 ft. That 344 feet of height was the primary reason for the 11 degree spread between the cities, even though the latitude is not that much different. Essentially, cold air drainage is the downhill flow of cold, dense air along slopes into valleys, a process driven by gravity, similar to water. This phenomenon occurs on clear, calm nights as the ground radiates heat, making the air closest to the surface denser and causing it to sink. Cold air drainage leads to colder temperatures and the potential formation of frost in valley bottoms.

Another oddity Thursday morning was a lot of places saw frost even though lows were above freezing in the 34-37 degree range. How is that possible? Well, the height of the thermometer on most weather stations is situated about (6 feet) above ground level (AGL). Because cold air sinks relative to warm air, and because the ground radiates heat very efficiently during calm, clear nights, the temperature at or near ground level can often be several degrees cooler than the temperature at the 6 ft. thermometer height. To illustrate this example, the NWS in Aberdeen conducted a simple experiment back on October 2nd, 2008 - during a night with clear calm conditions. One thermometer recorded temperatures at the standard 6 ft. level, while a second thermometer was placed near ground level (directly below). The temperature trace of both thermometers is shown during the period 3pm on October 1st, through 8am on October 2nd. At 7:00am, a 2 degree difference was noted between the ground thermometer and the one at the 6 ft level. Fun with weather!

BACK TO THE BUMP IN THE ROAD

Getting back to that bump in the road previously mentioned, it's tied to some clouds and possible showers early Friday morning. An elongated trough ahead of a cold front is trying valiantly to tap into a narrow tongue of moisture. About the time it arrives, the forcing along the trough is on its way out. We should be able to reach the saturation necessary for some rain, but it seems banded and light, generally 1/10th of an inch or less. My counties west of the Mississippi have a better chance of seeing the showers, yet even there coverage looks spotty, with any rain short in duration. There is a chance a couple of thunderstorms could fire, which might lead to some isolated higher totals. Those should be few and far between. What does occur, should move out of the area from northwest to southeast during the morning. Clearing will follow in the afternoon, allowing highs to bounce back close to 70. Here's what models are suggesting for rain totals through Friday morning.

The EURO

THE 3K NAM

The HRRR

The GFS

The National Blend of Models

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A FINE WEEKEND, THEN THINGS GET MORE INTERESTING

The upcoming weekend appears to be another beauty with dry weather back in place along with above normal temperatures. Highs should start near 70 Saturday and climb into the low to mid 70s Sunday through at least Monday of next week. Towards the middle of next week, we are still seeing signs of an atmospheric realignment that results in the mean storm track emerging from the southwest. Several chunks of energy will emanate from the base of the trough, which keeps reloading in that area. How this all evolves is giving models fits, particularly regarding phasing. The general idea is that several systems should impact the Midwest, each with a chance of precipitation. Due to the uncertainty associated with the amount of phasing, it's tough to determine the timing and impacts of individual waves, not just precipitation but temperatures as well. Deterministic models show a lot of variance after 7 days. I've resorted to the ensembles of the EURO and GFS, which consist of many members to try and get a balanced solution. The takeaway is that with each passing disturbance, temperatures will gradually trend cooler, something we've been advertising on here for a week. Both models end with highs in the 50s out around the 24th or 25th of the month. Take a look.

The EURO

The GFS

Week 2 rain chances are looking better with improved access to moisture and the storm track. Both the EURO and GFS indicate near to perhaps even above normal precipitation in the week 2 period. We could certainly use the rain, so hopefully the pattern materializes as it looks now.

If you are curious, for the first time today I saw some positive signs that winter may be closing in come November. This is the EURO control 30 day temperature departure outlook, October 25 to November 24th.

The 7-day departures for the week leading up to Thanksgiving were really cold.

That same week ending November 24th, the EURO control showed this for snowfall in the central Midwest. Up north, the snow started to accumulate much earlier in the month and is shown to be quite substantial there.

Let me say this about what I've shown above. Models such as the weeklies, while showing precise numbers, in reality are far less accurate in their depiction. It's what we call eye candy in the weather world. So what good is a model that goes out 6 weeks? The simple answer is the value that comes from the trends it depicts. This run was cold and snowy, mid-November on. Now the question becomes does this trend hold up in future runs over the next couple of weeks. If, and only then, do we see the beginnings of a solid trend. If that happens, I will be excited. Until then, take it with a grain of salt. For you snow and winter lovers, that was some tasty candy.

Happy Friday, and thanks for visiting the page. If you enjoyed the content, please hit the like button. Roll weather...TS