Here comes another blast of Canadian air which brings the coldest air of the season Saturday morning. I have been watching the temps drop upstream where afternoon highs were in the 30s and 40s over Tennessee Thursday afternoon. The surface analysis at right shows the surface high parked over Texas and Oklahoma. Remember the wind flows clockwise around this anticyclone so Florida sits on the right side in the patch of cold air flowing out of Canada. The blue isobars are packed closely from the Great Lakes to Georgia and North Florida. Winds flow parallel to the isobars resulting in cold air advection. This horizontal movement of air is hitting us tonight and highs Friday will only reach the low 60s during the day.
The jet stream is dipping south rushing the cold our way. A freeze watch is up for most of the Jacksonville area for Friday night and Saturday morning. This will be the only night for frost during this forecast period. It is so cold that snow has been falling in the southern Appalachian Mountains. The red circles are isotachs marking streaks of faster flowing air. Those jet streaks are converging in the dip that is bending back over north Georgia. This is analogous to cars slowing up at a red light. Traffic backs up. But when the air at 30,000 feet backs up it sinks downward. This confluence is why you will see no clouds in the sky Friday and Saturday. The cold moderates quickly Sunday afternoon and no freeze is expected Sunday morning.
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