MIDWEST WARMTH FORGES AHEAD IN OCTOBER

The fall season is in full swing in October. Temperatures accelerate their cooling process across the entire country. Average highs cool to the 50s and 60s in the north and the 60s and 70s in the South. Some 80s remain across the Desert Southwest and closer to the Gulf Coast. Lows dip into the 30s and 40s in the north with 50s and 60s in the South. Daylight hours drop into the 10-12 hour range across most of the U.S. The peak of the hurricane season is now behind us, but the descent is slow and there are still tropical risks to be concerned with.

The pattern during the first ten days of September was driven by a ridge over western Canada paired with a trough impacting the Central/Eastern US. This allowed for multiple cold fronts to sweep across the eastern half of the US early in the month, producing below normal temperatures for much of the period interrupted by brief stretches of modest warmth. Meanwhile, the bulk of the warmth in early September was draped across the Northwest. Ridging will gradually shift towards Central Canada by the latter stages of week two in September, creating a warmup across the Central US while the cooler pattern would taper off more slowly in the East while also turning more seasonable in the West. This Central Canada ridge would remain a primary pattern driver for the second half of September, which led to above to well above normal temperatures taking hold from the Interior Northwest through the North Central US. This warmup would be less aggressive across the South and East due to troughs either overhead or nearby. Below normal rainfall was common for most of the Central/Eastern US during September. This led to areas of increasing drought to develop from the Middle/Lower Mississippi Valley towards the Great Lakes/Ohio Valley and the Northeast throughout the month. One exception was across the Northern Plains, where multiple systems brought heavy rains to the region during mid-September and led to above normal rainfall for the month. A more active monsoon pattern also led to above normal rainfall across the Desert Southwest. This even extended into Central/Southern California as moisture from the remnants of Tropical Storm Mario impacted the state over September 17-18. The Tropical Atlantic was quiet for much of September. The first named system of the month did not form until September 17 in Hurricane Gabrielle. Hurricane Gabrielle would become the second major hurricane of the season but re-curved west of Bermuda. A pair of storms would form in late September. Hurricane Humberto became the second Category 5 Hurricane of the season before re-curving west of Bermuda. Imelda formed near the Bahamas and should become a hurricane as the system turns away from the US coast as the calendar flips over to October.

October should continue to feature a ridge over Central Canada with a trough near the Gulf of Alaska to start the month. This will maintain a warm pattern across the Central US, especially in the Upper Midwest where the first 4-5 days of the month could feature record heat. The magnitude of this warmth should ease late in week one and into the second week of October, but warm anomalies should remain common through mid-October in the Central US. The Gulf of Alaska trough will extend towards the West Coast on occasion early in the month, producing near to slightly below normal temperatures. After a cooler first 2-3 days of October, a warmer pattern should emerge in the East, but a trough off the East Coast should mean that this warmth will not be as anomalous as in the Midwest. Ridging should retrograde towards Western Canada by mid-October. This should allow for a warmer pattern to emerge across the Western US. As this occurs, a trough should bring more seasonable temperatures to the East. Something to take into account at this point is the potential for a re-curving typhoon in the Pacific, as if this can occur, it could amplify this trough downstream and perhaps provide the best chance for a cool shot to occur in October across the eastern half of the US late in week two and into the third week of October. Tropical forcing passing through the Indian Ocean and the Maritime Continent during this time could also have an impact on this if it is strong enough, but this signal could be mixed out by competing tropical forcing over the Atlantic Ocean. By the final 7-10 days of October, the pattern should revert to a trough near Alaska and a ridge over Central Canada which should provide above normal temperatures to much of the US to conclude the month, especially across the Midwest.

The storm track to begin October will be most active from the Northwest through the Upper Mississippi Valley. High pressure will keep much of the East and South dry through the first week of October aside from Florida. The pattern should trend drier across the West during the second week of October as a trough is replaced by a ridge. This should increase the risk for frontal boundaries to sweep across the Midwest towards the East, but this is more likely to result in regional wetness as opposed to widespread rains, and the bulk of this activity should favor the Great Lakes through the Northeast/Mid-Atlantic by the third week of October. Barring any tropical impacts, drier than normal conditions should continue across the South, though there may be slightly better chances for rain and storms compared to the first week of October. Moisture should increase across the Four Corners during the middle third of October, with this wetter stretch expected to lead to above normal rainfall across the region for October. With the Central Canada ridge expected to return during the final third of October, the pattern should trend drier once again across the Great Lakes through the Northeast. The best chances for rain and mountain snows should return to the Northwest through the Northern Plains to end the month.

Humberto and Imelda will wind down across the North Atlantic early in October. Once this occurs, October should get off to a quiet start in the tropics due to a combination of unfavorable tropical forcing and some wind shear. Tropical forcing should become more favorable late in week two and into the third week of October. The best chances for any development initially look to be across the western Caribbean, and it is possible for multiple systems to develop in this region. A system being pulled northward from the northwest Caribbean towards the Southeast US cannot be ruled out, particularly during the third week of October. Should this occur, a track towards the eastern Gulf or just east of Florida is more likely to occur than a track into the western half of the Gulf. The potential for development in the MDR should increase during the October 14-24 time frame or so, but systems that form here are more likely to re-curve than threaten the US. Tropical forcing should become less favorable across the Atlantic over the final few days of October. This should allow for tropical risks to wind down across the MDR. There could remain some risk for close-in development, particularly in the western Caribbean, but chances should not be as high as in mid-October.