Break through update, then what?

The following is a posting from the HMT Blog at

http://www.esrl.noaa.gov/psd/programs/2007/hmt/forecast/

We have posted some annotated figures on that site which you may find useful. This discussion focused on the American River Basin (ARB) in California for this project. I will adapt the information for this Blog.

The reasoning from our last posting on January 30th remains unchanged. Some models are continuing to advertise the break through of westerlies on the west coast with the PSD ensemble showing a distinct shift toward probability of above normal precipitation in the California region in week 2 (Feb 10-16). That notion appears reasonable given the increase of westerly flow throughout the subtropical atmospheres during the past 1-2 weeks. Tropical convection is now established over the west Pacific and continues to be active over the Indian Ocean. The tendency for convective forcing to be further west should continue to effect the North Pacific circulation with more retrogression of features. This could eventually (beyond week 2) lead to weakened westerlies in midlatitudes and a return to a trough along the US west coast, especially if a new MJO develops over the Indian Ocean (while a signal continues from the South Pacific).

Below normal temperatures are likely to remain entrenched from the Upper Mississippi Valley into the Northeast states for the next 1-2 weeks (with moderation). The rest of the country would be expected to have near to above normal temperatures. As troughs impact the USA west coast and shift inland, Arctic surges may become probable west of the Continental Divide into the Rockies and Northern Plains later week 2 into week 3 (~February 13-23). One would also expect an active southwest flow storm track across the central part of the country with warmth across the Deep South. High impact weather concerns may vary from severe winter weather from the Rockies into the northern and central Plains to heavy rainfall and severe local storms across the southeast states. Moisture sources may include an active subtropical jet (South Pacific signal) along with transport from the deep tropics through the Gulf of Mexico.

Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/3-4/2. I will try to post another discussion by the end of next week.

Ed Berry and Klaus Weickmann