Highlight: Explaining the Great Plains unexpected wet regime.
Fig. 1: The 7-day northern hemisphere upper air pattern reveals an upper ridge over a warm SSTA zone in the northeast Pacific and expansive upper trough over a cool zone south-southeast of Greenland.
Fig. 2-3: The Gulf of Mexico is warm (+0.41C) but has cooled significantly (by 0.55C) the past 2 weeks.
Fig. 4-6: ECM upper air forecasts for Saturday, next Tuesday and May 29 continue to show the general correlation between the “warm blob” of SSTA in the Northeast Pacific upper ridge and the cool upper trough in the North Atlantic related to the North Atlantic warm hole SSTA pattern.
U.S. discussion: An excessive rainfall event continues over the northwestern Gulf of Mexico States. Widespread flash flooding has affected Texas and Louisiana. Rainfall amount will exceed 10 inches in some areas and the wet fetch off the Gulf of Mexico continues for another week (or more). Some of this moisture works northward into the northern Great Plains.
The wet pattern is somewhat unexpected as drought concerns in the northern Great Plains and north/east Ohio Valley plus southwest Great Plains to southern Texas were previous concerns of expansion.
However, a semi-permanent upper air pattern present since 2013 emerged in May and although shifting has a general persistent presence into early June.
The semi-permanent upper ridge over a vast area of warm SSTA north and northeast of Hawaii and known as the “warm blob” present since 2013 has the usual attendant upper-level high-pressure ridge in-place (Fig. 1). The compensating cool pool of SSTA south-southeast of Greenland has an attendant semi-permanent upper trough widening influence in May. Downstream from the Northeast Pacific ridge a subtropical low in the upper atmosphere persists over the Southwest U.S. inducing a moist inflow of moisture off the Gulf of Mexico to cause the Texas rains (Fig. 2). The Gulf was unusually warm heading into this pattern but cooled significantly as strong southeast flow ejecting moisture into Texas has cooled the northern Gulf in this pattern (Fig. 3).
The upper air forecasts by ECM over the next 10 days reveal this pattern more-or-less continues and explains in-part why unexpected beneficial rains are welcomed by the Great Plains region. On Saturday ECM depicts another trough into the Southwest U.S. downstream from the Northeast Pacific “warm blob” ridge (Fig. 4). The North Atlantic Warm Hole (NAWH) upper trough splits with one center east of the Carolinas (Fig. 5). The result is an in-between (the two troughs) warm upper ridge for the East/Southeast U.S. By late month the susceptibility for a wet fetch out of the Gulf of Mexico causing beneficial rains for the Great Plains continues ahead of another Interior West upper trough (Fig. 6).
Traders/analysts are reminded that the influence of warm/cool SSTA “blobs” in the North Pacific and North Atlantic change their influence on the atmosphere as summertime arrives. In June, the NCEP CFS V2 upper air forecast indicates a dry/warm ridge over the Central U.S. (Fig. 7).
Fig. 7: The unexpected wet pattern in the Great Plains during May can easily reverse warmer and drier in June as indicated by the upper air forecast from NOAA.