Broad trends for increased westerlies into the US W coast still on track, but latest trends in progs suggest for the broader scale (above the scale of individual short waves and jet streaks) more of a tendency for a high over low block to develop in Ern Pacific by next weekend. Details of timing continue uncertain in my view, but it looks as if Th 8 Feb would be the very earliest for an IOP-quality event in the American River Basin.
There are 2 main routes of middle and upper tropospheric tropical moisture into the midlatitudes at the moment. The most persistent has been a weak, semipermanent low-latitude trough off Baja that has been bringing tropical moisture across central Mexico into Texas and the Gulf of Mexico for several weeks now. The other is as pointed out on Friday by Klaus and Ed in the western Pacific where there is a feed from the vicinity of the Phillipines E and ENE toward the general vicinity of Hawaii. This one has been largely lacking through much of the winter so far. Temporarily there is a third, being produced by an upper trough near 30N/140W that is becoming detached from the main polar jet.
This upper trough is forecast by the models to shear apart, with the upper portion propagating eastward to reinforce the low-latitude mean trough near 115-120W (and the northward feed of moisture to its east), and the middle tropospheric portion tracking north to north-northeast with a weak manifestation at the surface moving toward the Oregon coast. The main effect of this latter portion is to flatten out the ridge and set up a baroclinic zone in the eastern Pacific that is oriented SW-NE near the coast. In the progs this then acts as a kind of wave guide for future waves coming out of the central Pacific, undercutting the high latitude portion of the major ridge near the West Coast that has been with us for many days. As well, and perhaps more importantly, it provides a conduit whereby the western Pacific water vapor feed may reach the mid latitudes. The first system of this new regime to affect the HMT domain is due to arrive about Thursday. This almost certainly will be followed by at least one other of more significance.
D(prog)/Dt of GFS ensembles suggests that the Thursday system is not to be written off as an IOP candidate at this point. However, the most promising event still looks to be Friday-Saturday for HMT. Beyond this, details are fuzzy, but there is the distinct possibility of more IOP-candidate material following.
In summary, Thursday (8 Feb) is the earliest possibility for an IOP, and prospects continue good for an IOP in the Friday-Saturday-Sunday range.