AUGUST 2015 30-DAY OUTLOOK
452 PM EDT Fri. July 31, 2015
Atmospheric patterns, 700 MB and 500 MB height anomalies, statistical data, and various models such as the ECMWF monthly, NMME, IMME and the CFSv2, along with observed cases from past such months, data from 1895-2000 were used in this forecast.
Sea surface temperatures are currently more than 1C above normal along the equator in the Pacific Ocean from the South American Coast westward to just west of the International Date Line. Sea surface anomalies are greater than +2.0C in the east Pacific from about 130W longitude to the South American Coast. Above normal sea surface temperatures extend northward from the equator to cover an extensive area of the north Pacific to the west of North America. Sub-surface ocean water temperatures to a depth of about 100 meters are considerably above normal throughout the eastern equatorial Pacific Ocean. Recent atmospheric observations over the eastern and central tropical Pacific Ocean suggest strong ocean-atmosphere coupling, with unusually active convection across the central and east-central equatorial Pacific together with large scale westerly wind anomalies at low levels over the west and central Pacific. Atmospheric and oceanic conditions suggest continued strengthening of a warm episode. Statistical and dynamical forecasts indicate El Nino conditions to persist through winter, and anomalies are likely to grow into the beginning of winter. This El Nino is likely to contribute to climate anomalies over North America this month.

During the past week, the MJO signal weakened substantially as the enhanced phase became increasingly incoherent over Africa, and no additional eastward propagation of suppressed convective anomalies over the Pacific occurred.. A strong burst of convection was observed near and just east of the Date Line, possibly due to constructive interference between a Kelvin Wave and the El Nino base state. The overall spatial pattern of velocity potential anomalies has become increasingly incoherent as the base state becomes more dominant.

Dynamical model MJO index forecasts depict a wide range of solutions, with the GFS forecasting an increase in amplitude over the Indian Ocean, several Canadian ensemble members returning convection fairly quickly to the Pacific, and other models maintaining enhanced convection over the eastern Pacific and Western Hemisphere. Persistent westerly wind anomalies aloft across Africa and the western Indian Ocean are not favorable for permitting the atmospheric ventilation necessary for significant Indian Ocean convection; however, the upper-level sub-seasonal signal is likely to continue propagating eastward. Destructive interference between the MJO and the El Nino will also increase as the former signal continues propagating eastward. Therefore, enhanced convective anomalies associated with the MJO are expected to remain weak during the next two weeks, but a remnant intraseasonal signal is still anticipated to propagate eastward across the Indian Ocean and Maritime Continent.

Dynamical model RMM forecasts offer considerable forecast spread for the future evolution of the MJO signal in the coming two weeks but most to first order forecast eastward propagation of an MJO signal of varying amplitudes into the western Pacific over the period. The solutions with the largest amplitude in Week-2, the ensemble GFS and ECMWF shift the signal to Phase 7 by the end of the two week period. Issues resolving the KW evolution versus the slower MJO envelope may in part be responsible for the varying solutions amongst the solutions.

The NAO is negative and is forecast to trend positive by day 10 and remain positive through day 10. The PNA is near neutral and is forecast to trend positive day 10 then negative bay day 14.

Most of the spatial anomalies are split with some indicating near to below normal temperatures in eth east-central U.S. and above normal temperatures in the West. The GFDL FLOR forecast near to above normal temperatures across the nation. The GFDL CM2.1 and the NASA GOES5 depicts below normal temperatures from the Rockies to eth East Coast.

Above normal temperatures are forecast for northern California, the Northwest, western Montana, and the central and eastern Gulf Coast States and portion of the Southeast. The probability of above normal temperatures for the West is 57 percent. Below normal temperatures are forecast for the Four Corners region. The probability of below normal temperatures this region is 55 percent. The remainder of the nation is not expected to exceed climatological values.

Above normal precipitation is forecast for the central Rockies eastward into the middle Mississippi Valley. The probability of above normal precipitation for this region is 56 percent. Below normal precipitation is forecast for western Gulf Coast States. The probability of below normal precipitation for this region is 56 percent. The remainder of the nation is not expected to exceed climatological values.

Jim Munley Jr.
http://www.jimmunleywx.com