Rare Excessive Wet And Very Warm Climate AUG/SEP/OCT 2021 for U.S.

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11/05/2021, 3:52 pm EDT
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Super-warm (and wet) late summer/autumn (so far)

Fig. 1: The very warm regions of the U.S. rankings (circles) for AUG/SEP/OCT 2021 with the precipitation rankings added (squares). A rare very wet warm and wet national combination.

Discussion: Propelled by super-warm sea surface temperature anomalies (SSTA) off the U.S. East Coast stretching into the northern Atlantic the attendant high-pressure ridge aloft lead to the warmest AUG/SEP/OCT on record for the Northeast U.S. and top-5 warmth for the Upper Midwest/Midwest regions. The entire U.S. was warmer-than-normal for the 3-month period and ranked 2nd warmest in the 127-year climatology. Most of the time anomalous heat is associated with drought. Wet climate generally suppresses heating especially during the day. However, the U.S. ranked 8th wettest for AUG/SEP/OCT 2021 including 7th wettest in the Northeast States to accompany the record warmth. The entire U.S. was wetter than normal except normally wet in the 4-Corners regions and Texas/southern Great Plains. The primary contributor to the wet (and warm) U.S. pattern was the super-warm North Atlantic basin coupled with an elongated upper trough in the Gulf of Alaska toward the Northwest U.S. Coast. The moisture was entrained into the upper flow pattern south and east of that trough causing an elongated area of wet weather while the upper ridge was sufficiently powerful to drive the anomalous heat. The warm temperatures combined with a wet climate caused exceptionally humid conditions.

Fig. 2: The North Atlantic basin SSTA analysis for Sep. 1, 2021 identifies the widespread anomalous warmth across the region.

Fig. 3: NOAA statewide temperature rankings for October 2021 given the 127-year climatology.

Fig. 4: NOAA statewide precipitation rankings for October 2021 given the 127-year climatology.