Reality Update

Tropical convective forcing remains stationary ~0/140E with development of a new region ~160W within 15 degrees of the equator. The latter is associated with downstream twin subtropical cyclones that were part of the response with the recent Madden-Julian Oscillation (MJO). Most diagnostic tools have shown a rapid weakening of the MJO signal during the last week. The remnants of the MJO combined with the pattern of tropical SST anomalies, including the warm ENSO signal, suggest organized tropical forcing during the next couple of weeks is most probable around 10S/160E while extending from the South Pacific back into Indonesia. Some shift slightly to the west may occur afterwards.

Zonal mean westerly flow aloft has been increasing throughout the tropical and subtropical atmospheres (within 15 degrees of the equator) during the past week. In fact, some of this westerly flow has already worked its way around through southern Asia and is enhancing the East Asian jet. This supports a GSDM Stage 1 weather-climate pattern more typical of La Nina conditions than El Nino. The current negative phase of the Pacific-North American teleconnection (PNA) is a charcateristic of this stage. Westerly flow should continue to increase throughout the subtropics. As shown by most models, the current trough-ridge-trough wave train across the PNA region should progress eastward leading to a ridge position just off the North American west coast for week-two (GSDM Stage 2). A strong subtropical jet is expected to “undercut” the ridge. During weeks 3-4 it is probable this pattern will shift westward and allow the subtropical jet to impact southern California. This scenario differs from an El Nino composite where a combined, extended jet across the North Pacific would be expected (GSDM Stage 3). However, this possibility cannot yet be ruled out for this winter/spring.

After the major winter storm of the next few days, much of the USA is in for a cold and dry regime, particularly the northern and central states during week 1. Locations in the Deep South will have freezing temperatures while Santa Ana conditions may develop across California. As the subtropical jet begins to undercut the ridge, another cold/wet storm system may impact locations from the Desert Southwest/Deep South into the Mississippi and Ohio Valleys and on to the northeast during week 2 (starting roughly next weekend). Afterwards, this storm track may shift northwest and locations such as the California coast may finally get some much needed precipitation going into February.

Please see http://www.esrl.noaa.gov/psd/programs/2007/hmt/forecast/
for images.

Ed Berry and Klaus Weickmann