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HMT discussion for Tue/20 Mar

All indications this morning is that our decider guy made a good decision to not have an IOP as the system continues to split and as a result most of the precipitation thusfar has been to the northwest of the HMT area (over an inch in far nw California), and now the greatest amounts from here forward look to be farther south. Blue Canyon (BLU) has yet to report any precipitation through 18z and the winds have been pretty light. SAC radar indicates scattered precipitation to the north of the HMT area with a more general area from SFO southward. The 12z models are all down considerably in the amount of precipitation forecast with this event, the GFS with only 0.23 inches total at BLU and the NAM about the same. Both have more to the south now. For the NAM this is in contrast to last night's run that had a solid area of .75-1.0 inches, while the GFS did better in keeping amounts much lower, generally in the .25-.5 inch category. Our 3 km models continued to be on the high side last night, with max amounts in the HMT area of around an inch and 2 to even 3 inches in that spot 70 miles to the nw. This mornings 12z local model runs are down on the total amounts but still high versus other models, with generally over half an inch forecast in the HMT higher terrain for storm total. A loop of the radar imagery suggests the large area of echoes just to the south will just miss the HMT area today, hence the lower amounts expected. Had this been just a little farther to the north then this might have been an event approaching an inch in places.

Now looking ahead, the other good news is that the system for early next week continues to look good, with a much more significant event possible. Looking at the big picture, the jet that has moved off of Asia is very impressive, with the 12z GFS analysis having 215 kts at 250 mb in the core near 35N/155E. This jet progresses across the Pacific this week and by 00z/Sat/24 Mar stretches from the Pacific Northwest back to the Dateline, where a trough will be deepening in the entrance region of the jet. The lead part of the jet will bring a dump of moisture this weekend but this should stay to the north in the Pacific Northwest, although it could get close to the area late Sunday. Our system will be the trough in the entrance region that gradually strengthens as it approaches the West Coast. The 00z models were all in good agreement with a moist system in the Monday to Tue/26-27 March timeframe except that the GFS was 12 to 18 h faster. The 00z GFS ensembles showed quite good agreement with a system, as did the Canadian ensembles, which often have quite a bit more spread. The ensemble mean of NCEP and Canadian members has over an inch of precipitation for the system, which seems respectable for a mean amount. The 12z GFS just coming in is slower, in agreement with the 00z model consensus, although it is not quite as deep as the earlier run, and so has a 2.75 inch total precipitation maximum more to the nw of the HMT area. The 12z GFS ensembles also support a Mon-Tue timeframe and there is good agreement among the members for a good system. The bottom line right now is this looks to be a much stronger system that could slow down somewhat (and in fact the ECMWF model does slow it down as it closes off the system just west of SFO on 12z/Tue in the very latest 12z run just coming in). There is a tropical tap back to the western Pacific initially with this system that gets stretched out as the jet moves eastward so when it does hit the West Coast there is not any real atmospheric river present, but the system is strong enough and will apparently carry enough moisture to have a 1 to 3 inch type potential precipitation event.

Beyond this system things remain active with the models consistent in showing another trailing cutoff low but there is a lot of spread in when or if this low would ever make it onto the coast, and that would be no sooner than the end of March and possibly not until early April.

ed szoke NOAA/GSD


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