Just a quick update this evening. The GFS and other models are beginning to come to an agreement that a major pattern shift will occur over the next five days, most likely in response to the rapidly-eastward propagating MJO in the Pacific. Here’s the synoptic setup and the possible conequences:
A weak cutoff low will drift over NorCal on Monday and Tuesday. Although models do not generate any precip with this low, I do believe that there is a chance of isolated showers/thunder as this low moves overhead. Not the extensive mid-level cloudiness currently asociated with the low and the moisture tap it may be developing on its eastern side.
Beginning on Wednesday, a strong pacific jet will begin to carve out a deep trough off teh coast of CA. The jet will be well-positioned to deepen and nearly stall the trough, which will feature a strongly curved cyclonic jet stream near its base. There will be a West Pacific moisture connection with PWs near 2 inches. Also, the latest models continue to indicate that the trough will be able to develop a solid subtropical moisture tap, which will possibly generate extensive shower and even thundestorm activity over the entire state on Wednesday or Thursday. After that, several cold fronts will begin to move in, enhanced by the powerful je and deep moisture tap. There will also be a potentially deep surface low not far off the CA coast, and this could further increase the dynamic strength of the system, in addition to bringing strong winds. Some iterations of the storm on the GFS have looked extremely impressive; others have looked merely decent. There does seem to be a lot of variability amongst the models as to whether the system will be able to bring significant rainfall to SoCal. Some runs of the GFS brought up to 5 inches or so to the Central Coast, so the potential is certainly there for some major rainfall. I do expect the model solutions to waffle around some more as we approach the pattern change, as the MJo is quickly evolving and could change model outcomes quite a bit from run to run. In any case, I do think we’re going to have a rather wet Halloween across at least the northern 2/3 of the state.