As I stated in that previous post, latent heat processes from the tropics into the higher latitudes is what drives much of our weather. When these releases "ramp up" as they are doing now it can cause chaotic flow at the jet stream level and thus cause headaches in forecasting.
I'm starting to see some subtle signs that the long term weather pattern, which got established last fall, is beginning to change/fall apart. I can already see that with precipitation and thunderstorm behavior that has occurred recently.
Take, for instance, what happened late yesterday and last night across Kansas. For much of the summer, the weather pattern across the plains would have been favorable for widespread thunderstorms (and copious amounts of rainfall). What happened instead yesterday was no rain for some, and a lot of rain for others across a short spatial distance.
In the map that follows (click for a larger version), you can see the huge differences in rainfall in and around Dodge City. For instance, I only observed 0.07" while just 5 miles away there was 1.5"!
Back to the tropics. In the following hemispheric satellite image, I've highlighted the jet stream, upper lows (troughs) and 8 tropical systems (disturbances, tropical storms and hurricanes).
You might have heard of Tropical Storm Erika. I have it labeled as number 6. Even though the ocean water temperatures are very warm and very favorable for intensification, the upper level winds are such that it may never make hurricane status. We'll see. At a minimum it will bring heavy rains to Florida. Then there is Hurricane Jimena (number 3) in the eastern Pacific. This hurricane is expected to become a major one as it tracks precociously close to Hawaii, likely passing by to the north.
All eight of these systems labeled (as well as other clusters of tropical thunderstorms) are releasing latent heat that is transferring to higher latitudes. This process, in part, is responsible for the crazy configuration in the jet stream (the green wiggles). The red "L" with the arrow pointing towards it will likely become an established trough across the western U.S. and may persist until at least September 5th, if not later. What this means for the high plains is above average temperatures, moderate daytime winds and little to no chance for widespread rainfall through that period. The only hope is that a weak tropical disturbance is caught in the flow and transported northward towards the center of the country.
The Weather Prediction Center has the following outlook for rainfall through September 4th.
If there is going to be an early freeze this fall, it WON'T BE because of what has happened this summer. Rather, it will be the result of a new developing weather pattern. The only thing I have to go on right now is what some of the long range computer models are suggesting. For the past week or so the output from these computer models have suggested that an early freeze is possible. Keep in mind that each time the model is run (every six hours with new data), the resulting outlook and specifics are different.
But, just going strictly on what the contributions from tropical activity have been and is likely to continue during the next month, I'm beginning to lean towards and earlier than normal freeze.
The following map shows the average date of the first 32 degree reading of the fall:
And a map of the average date of the first 28 degree reading:
The following graph is the output from just one of the long range computer models for Dodge City. It shows nicely several strong cold intrusions (1 through 4) with the latest one being the coldest. It looks reasonable but don't take the numbers literally.
For the latitude band of Nebraska and Iowa, take about another 10 degrees off.
BTW, the same computer model has been very consistent with predicting a "wet" fall, which actually looks very realistic. More on the precipitaton outlook next week.
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