Monday, November 13, 2017

MikJournal Monday 11/13/2017...Rollercoaster Weather

Good Monday to ya! A little dreary outside my place this morning with temperatures in the low 40's and a fairly high barometric pressure reading of 30.45". For you weather geeks like me, convert that to millibars (what you typically find on a weather map) by multiplying the inches of mercury by 33.8638815...and you have about 1031 millibars (mb), a fairly stout high pressure reading. By the way, the record highest pressure reading at Louisville is 30.98", or 1049 mb tied on February 12, 1981 and initially set on January 6, 1924. Okay, while I am on the subject, the lowest reading for Louisville is 28.93" (~980 mb) set also in the month of February on the 28th in the year 1902. And the only other time a barometric pressure reading dipped below 29" of mercury was February 10, 1960 at 28.98" (or 981 mb), good enough for a strong Category 1 hurricane. Thank you NWS Louisville for providing us with the numbers.

Typically, in the late Fall and Winter, high barometric pressure readings originate in the Arctic regions that slide southward and obviously carry much colder air. Again you weather geeks know that cold air sinks. In addition, as the air sinks, it warms (relatively) and expands, exerting a higher pressure at the surface and dries out the atmosphere just above the surface. So, often you have fair and cold weather associated with such areas of high pressure. For example, the February 12, 1981 temperature at Louisville (the day we recorded the highest pressure reading of 30.98") was 21 degrees for a high temperature and 0 degrees for an overnight low.

I provide this brief, amateur definition of high pressure because you will probably be exposed to pressure readings this week. The relatively high pressure today will be replaced by a monster low pressure system later this week. I have read and heard that the pressure readings associated with this low pressure may reach 990 millibars, or about 29.25" of mercury. That's a pretty strong storm system and will likely carry a lot of wind ( a result of difference between high and low pressure). The warmer air ahead of it will be pumped into our region via the Gulf of Mexico. Then, post-frontal winds will crank from the northwest and usher in much colder air, coming from the Arctic region. This setup provides a classic battle between warmth and cold and has potential to produce some high octane storms for our region later this week. Stay tuned.

As of today, we are still above normal in the temperature department. The Climate Prediction Center gave a fairly accurate outlook for our region through the 12th that temperatures would average out above normal for the first 12 days of the month. Well, how about the next 2 weeks?

The outlook for November 18-26 (most of this week not included) calls for a higher percentage of being below normal in the temperature department and near normal to below normal in the precipitation department for Kentucky.

A preliminary outlook for this week from the Weather Prediction Center through next Monday morning is for rainfall amounts to be in a range of 0.25" to about 1.00" statewide. Of course, these values are subject to change as upper air readings will be sampled throughout the week to determine how this volatile week will eventually unfold in our region.

No really significant extremes in temperatures yesterday. The highest reading was 89 degrees at Tucson AZ and lowest reading was 0 degrees at Bottineau ND.

One feature I like to look at is the temperatures in Alaska. The coldest temperature was -9 at Gulkana yesterday. That's not bad for them. Fairbanks has not recorded a below normal day yet this month. They are running over 12 degrees above normal so far. The outlook from the CPC has northern Alaska at above normal through the 26th while below normal for the central and southern parts of the state.

That should allow for chunks of cold air to reach our area. However, the air is still not truly 'Arctic' yet. Even Siberia is forecast to be above normal. That does not mean swimsuit weather. However, a weak polar vortex can still send down very cold air, at least according to our standards, even modified Arctic air.

I am currently working on a brief regional outlook for the winter as well as a December outlook. Please note I will not be posting seasonal snow totals again for this upcoming winter, since there is no sound basis for making such forecasts. However, I am posting a percentage factor similar to how the CPC provides in their outlooks for temperature and precipitation. Such outlooks will provide odds of above normal, near normal, and below normal categories.

A little preview for you. I do have a higher than average chance for ice (that is regular rain falling on ground at or below 32 degrees, not sleet) for the state. Also, a higher than average chance for above normal snowfall at localized areas of the state. Stay tuned.

Also, another little morsel. I like to follow teleconnection signals such as the Arctic Oscillation and Pacific North American. Well, I follow about 4 of these signals. They all have to be in agreement to support an increased chance for wintry weather. Admittedly, it may still be too early in the season to be incorporating such signals in a forecast. But, if this was late December/January, I would be getting really excited. Nearly all 4 signals are supporting a chance for wintry weather. There is one that is still oscillating, no pun intended. But, one of the signals is a bullish call for wintry weather later this month. Keep an eye on the trend. That's what I want to do.

I will be posting a mid-week report about the second half of the month in detail. Until then, signing off.

MS







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