After a very warm start to September the temperature trend across most of the USA will begin to go slowly downward as the month comes to a close. This will be especially true over the western states during the last ten days of September when some cold air that has been building along the North Slope of Alaska begins to push south. Above 80 degrees north latitude it has been the coldest summer since 1958, so the cold air buildup up north has a bit of a head start. As the end of September turns colder there are hints that some much colder air will spread south into the northern tier of the USA by the first week of October.
In regards to tropical activity, we are near record lows in the number of hurricanes and tropical storms across the northern hemisphere. However, don’t be surprised if the last two weeks of September and the month of October get very busy with an uptick of tropical storm and hurricane activity, especially along the southeast coast along Atlantic side.
Below is the temperature outlook for October. Continued warm along the west coast and portions of the southern plains. Expect temperatures to be cooler than normal in the Desert Southwest as recent monsoon rains will keep temperatures down a bit. Also, especially early in the month, expect a wedge of colder air to invade the northern Rockies and the northern plains.
Below is the precipitation outlook for October. There will be a trend to near to above normal precipitation across the Great Lakes and the eastern portions of the northern plains. Also, some enhanced precipitation will be found in the southeast (mainly from increased tropical activity) and in the Pacific Northwest as some stronger early fall storms approach the Pacific Northwest in October.