OCTOBER 2017 30-DAY OUTLOOK
215 PM EDT Fri. Sep. 29, 2017
Atmospheric patterns, 700 MB and 500 MB height anomalies, statistical data, and various models such as the ECMWF monthly and weekly, NMME, IMME and the CFSv2, along with observed cases from past such months, data from 1895-2000 were used in this forecast.
Over the last month, equatorial sea surface temperatures were near to below normal across the central and eastern Pacific Ocean. ENSO-neutral conditions were apparent in the weekly fluctuation of Nino-3.4 sea surface temperatures values between -0.1C and -06C. While temperature anomalies were variable at the surface, they became increasingly negative in the sub-surface ocean, due to the shoaling of the thermocline across the east-central and eastern Pacific. Though remaining mostly north of the equator, convection was suppressed over the western and central Pacific Ocean and slightly enhanced near Indonesia. The low-level trade winds were stronger than average over a small region of the far western tropical Pacific Ocean, and upper-level winds were anomalously easterly over a small area of the east-central Pacific. Overall, the ocean and atmosphere system remains consistent with ENSO-neutral.
A majority of the models in the IRI and CPC suite of Nino-3.4 predictions favor ENSO-neutral through the Northern Hemisphere 2017-18 winter. However, the most recent predictions from the NCEP CFSv2 and the NMME indicate the formation of La Nina as soon as the Northern Hemisphere fall 2017.
The pattern of 200-hpa velocity potential anomalies became more coherent and Wave-1 in appearance during the past week with enhanced convection across the eastern Indian Ocean, Maritime Continent, and west Pacific. Although the CPC index based on the 200-hpa VP indicates the early stages of a strengthening MJO, the RMM index has featured a weak MJO since late August. Dynamical model forecasts indicate an increase in amplitude of the RMM index in Phase 7 during the next week. It is uncertain on how the MJO evolves through the beginning of October with the ECMWF model favoring a fast eastward propagation, more typical of an atmospheric Kelvin wave.
The ECMWF weeklies depicts above normal heights in the East. Near to above normal heights elsewhere with a zonal flow. Most of the North American Multi-Model Ensembles indicate above normal temperatures across much of the nation. However, the NCEP CFSv2 and the CMC1 CanCM3 depict below normal temperatures in the southern Plains and parts of the East.
The NAO is forecast to be generally positive through day 14. The PNA is forecast to be negative through the period.
Below normal temperatures are forecast for the Northwest, the northern and central Rockies and the Great Basin. Anomalies of 2-4F are forecast. The probability of above normal temperatures for this region is 56 percent. Above normal temperatures are forecast for the Southeast, the Ohio and Tennessee Valleys, the Mid-Atlantic and the Northeast. Anomalies of 1-5F are forecast. The probability of above normal temperatures is 57 percent. The remainder of the nation is not expected to exceed normal climatology.
Above normal precipitation is forecast for the Northwest. The probability of below normal precipitation for these regions is 55 percent. Below-normal precipitation is forecast for the middle and lower Mississippi Valley, the Midwest, the Ohio and Tennessee Valleys. The probability of above normal precipitation is 55 percent. The remainder of the nation is not expected to exceed normal climatology.
Jim Munley Jr.
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