Good evening! It sure as a chilly start across central Florida with patchy frost and 30s across our northwest counties with lows down into the upper 40s all the way into the metro. It was our coldest start since mid March but, tonight, temperatures are 5-15 degrees warmer right now (9 PM) compared to this time Friday. High clouds are overhead in parts of the area and an onshore wind as kicked in; both changes compared to last night will keep our temperatures in the 40s and 50s tonight.

Thus far in this cold season, it's been tough to sustain any significant period of much below normal temperatures. That's because the upper level wind pattern, at the jet stream level, has been what we meteorologists call "progressive" -- or one that leads to systems moving quickly from west to east. This is the opposite of a "blocking" pattern -- a pattern where all of the players on the atmosphere's "field" don't move much at all. It was a blocking pattern that brought us an incredibly cold December a year ago; the chill just didn't let up!

The North Atlantic Oscillation, or NAO, is an index we meteorologists look at to determine whether the pattern is progressive or, instead, in a blocking phase. When the index is positive, the pattern is called progressive. This happens when there is a large difference in air pressure between Iceland and the Azore Islands. Whe this is the case, systems across North America move very quickly from west to east with little amplitude (you don't get deep dips in the jet stream that last for days). When there is a small difference in air pressure between Iceland and the Azore Islands, we're in a blocking phase. This means there is a large area of high pressure aloft dominating the northeast Atlantic Ocean, usually locking in a trough of low pressure (colder air) along the east coast. That's the pattern we were in last December.

The index is forecast to remain near neutral over the next several days, meaning any shots of cooler air will be fleeting across the area. This also means, though, that we'll never get too much above average temperature-wise either. In fact, for the week ahead, temperatures will climb into the low to mid 80s (about 3-5 degrees above average) before a very slow-moving cold front works southward by Thursday and Friday. The fact that the front will take so long to move through the area is more proof of the "positive" phase of the NAO: fronts are weak in this pattern and have a lot of trouble moving all the way through Florida.

Have a great night!

Brian Monahan