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I've made a point in my posts this week to mention how the current weather pattern supports significant temperature fluctuations and gusty winds. When storm systems charge across the country as they currently are, pressure quickly rises and falls in response to frontal passages (so do temperatures). That creates tight pressure gradients that generate wind.

This graphic below is the 3K NAM surface depiction showing our next strong cold front arriving late Friday evening. The front is in Illinois and behind it the black lines that are tightly spaced across Iowa are isobars (lines of equal pressure). The close proximity/packing of the isobars means pressures are rapidly rising in response to the cold dense air surging across Iowa. Ahead of it, Friday will be a windy but mild day with highs upper 40s north to mid 50s south.

Friday night, the cold front hits and at the evening, gusts of 35-40 mph from the NW are showing up in Iowa. While they are not indicated, a 50kt low level jet is streaking into the area that combined with strong cold air advection, has the potential to mix down gusts of 45-50 mph for a time Friday night. Again, ahead of the front southerly winds will be strong during the day with potential gusts up to 40 mph as well. If you have outdoor Christmas ornaments not securely attached the next 24 hours, they could be gone in the wind if you know what I mean. Wind advisories are quite likely.

Here's what's driving the wind. Temperatures in NW Iowa are at 15 degrees while in SE Iowa it's 40. Differential heating is what prompts pressure gradients and wind within them. If temperatures were the same at all locations, there would be no wind. It's nature's way of attempting to stabilize the thermal imbalance that's far stronger in winter than it is in summer.

Fast fronts also mean moisture return is negligible restricting precipitation potential. The dynamics are impressive but in cases such as this, most of the precipitation is restricted to the forcing in the deformation zone NW of any surface low development. While we may get a brief shower along the front and some post frontal snow showers in the cold advection they won't amount to much. As you can see , one needs to be much further north to get any significant precipitation. Here's the EURO's projected totals through Saturday. Notice amounts do go up out east where moisture eventually intercepts the system.

Essentially, this tug of war between warm and cold periods will go on into early next week meaning more of this type of weather. However, if my theory is correct we will trend toward colder weather by the middle of next week. It's based on the AO (Arctic oscillation) and NAO (North Atlantic Oscillation) going negative. Both the GFS and EURO show it. Here's what the EURO indicates for both of those teleconnections through December 16th.

The AO


What negative phases imply is blocking at polar regions is growing, especially in Greenland. That indicates high pressure is dominant in those areas. Higher air pressure will force cold air under the block. The shear density of the cold air will allow it to push and ooze southward into the Midwest. This graphic shows the typical results of a negative AO /NAO couplet.

The only thing keeping this from really getting out of hand in terms of bitter cold is the fact the MJO is currently in warm phases. I think in this case, the strength of the negative AO, NAO, and EPO (eastern Pacific Oscillation) will offset the MJO . However, unless the MJO swings into colder phases later in December (which would not surprise me), we may be able to keep the worst of the nasty cold up north. Obviously, there are some contradictions in modeling and teleconnections at this time and I'm hoping the noise in the pattern goes down in the next 7 days.

As it stands now, the EURO weeklies show temperature departures like this leading up to Christmas.

Week 2 December 7-14th

Week 3 December 14-21st

Week 4 December 21-28th

For good measure, the weekly means have this for total snowfall the next 32 days.

Who knows, maybe we'll get that White Christmas after all. Whether we do or not, I always dream of it. That's all for now, happy Friday and roll weather...TS


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