SYNOPSIS: Still anticipating 3-5" of pcpn at Blue Canyon (KBLU) between now and midday Sunday 11 Feb, when the welcome wet spell that began this morning is anticipated to be about ended.
Precipitation that began in the ARB this morning continues, although intensity is not heavy at this time. This is ahead of the cold front associated with the first system anticipated yesterday to give pcpn to the ARB today. The surface low-pressure center (~992 mb) well off the OR coast is a bit deeper and northwest of where it was anticipated to be located this time yda. The cold front appears to be within 100km of Cape Mendocino at the moment, and oriented NNE-SSW at the latitude of the ARB. This front is expected to traverse the coast ranges late this afternoon and early this evening and clear the ARB by 06-07Z Th, that is by 11pm PST. John McGinley reports that the WRF-LAPS ensembles from 12Z this mrng are indicating a 60% chance of > 1" pcpn and 20% chc greater than 2" (I think this was for KBLU). I am inclined to be a little more pessimistic on QPF, settling for .75 to 1" as the most likely amount for KBLU by 12Z Th 8 Feb. Snow levels may rise a little this afternoon as the air mass moistens up, but then will drop to 4,500ft or so after tonight's frontal passage.
The next wave is estimated near 34N/147W currently, based on 6.7 micron water-vapor imagery, and progressing E-ENE. It has a large cold cloud shield ahead of it, strongly suggestive of the warm advection property the progs have been indicating it would have when it reaches the coast. There are also 2 ship reports with SE winds of 35 and 40kts at 18Z, supporting the notion that this will be a stronger feature than the one now affecting the ARB. The surface low with this is forecast to be a little deeper and a bit farther NW than the GFS high-res "deterministic" run of 12Z yesterday, with the warm advection a bit weaker over ARB. Nevertheless, I think this still has promise of being a significant event, with 1-2" at KBLU, beginning late Th aftn or early evening, and probably letting up Friday morning after 12Z. Snow levels will rise to perhaps 6,500ft by Friday morning PST, but don't appear likely to rise as much as was anticipated yesterday.
Details after this point are less certain. The GFS and some of its ensembles are suggesting that this second pcpn event will end with some cold advection, laying out a frontal zone south of the ARB. With the main upper trough still offshore on Friday, the GFS is suggesting the focus of the pcpn will shift south with the frontal zone, into the southern Sierra. According to HPC, the SREF WRF runs are keeping the pcpn focused more on the ARB vicinity. GFS ensembles are also a bit scattered by 48h into the forecast, strongly hinting at considerable uncertainty in details of timing and intensity of pcpn over ARB after this point. I think most likely is for there to be a break in the pcpn after 12Z Friday, and that the daylight hours of Saturday (10th) will be wetter than daylight hours of Friday, mainly because Saturday the ARB will likely be in synoptically related ascent as the upper trough axis approaches shore. There are hints in some of the GFS ensemble members of wave development offshore along the front by Saturday morning.
The trough axis is anticipated to cross the West Coast Saturday night or early Sunday. This indicates that the pcpn will become mainly showers by daylight hours of Sunday, although the possibility that the progression of the upper trough could be enough slower than forecast to allow continued steady pcpn into the daylight hours of Sunday can't be ruled out at this point.
Looking farther ahead, the upper ridge is outlooked to slowly amplify over the Eastern Pacific early in the week. Nevertheless, there will be at least one additional significant upper short-wave trough that will approach the west coast early in the week. This is seen both on the GFS and the ECMWF from this morning. Both these models leave open the possibility, however remote, that this wave will be IOP material.
Its been fun...John B.