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HMT forecast discussion for Sunday 27 January 2008 (entered 20z/Sunday)

Highlights: brief shot of rain last 6h or so at BLU with about a half inch so far as it looks like we may have gotten some of the southern plume. Temporary lull until the remnants of the upper low come this afternoon, with heavy precip likely. Then northern stream wave passes ~09z/Mon preceded by heavy precip that will be wet snow going to lower density snow towards morning and tapering off to snow showers that will likely continue all day on Monday at BLU. IThe 18z obs at BLU is at 34 and they went over to snow, confirmed by the radar crew that reports almost an inch on the ground (and, unfortunately, now accumulating on the radar). Heaviest precip period at BLU should be ~21z/Sun through 06z/Mon. While only limited cooling is expected this afternoon as the first wave approaches, the consensus from the telcon was that the precip at BLU will now stay as snow, heavy wet snow with the temp near or just above freezing. WFO SAC indicated 1 to 2 feet of snow would be likely by Monday morning at the radar site. Issues with the heavy wet snow on the radar may force the radar operations to shut down later this afternoon.

A review of this morning: rain started about as thought, amounts not substantial so far, but SAC did report 0.25" between 06-12z, while at BLU also 0.25" but in the form of rain, with a burst of heavier rain around 12z (with another 0.22" since 12z). Currently (1630z) at BLU +2C (35) with the wind varying from S to SSE at about 10 kts. The ESRL BLU tipping bucket gage (hotplate not available) has received just over a half inch total, and the temperature has been on a very slow downward trend with the current reading at 1C. At the Big Bend site by 18z about 0.8" with the temp now approaching 0C (elevation 1739 m/5704 ft). The burst of rain this morning was likely the western side of the southern moisture plume that we have been mentioning the last couple of days, the one that is pounding the LA area. The bulk of this plume is heading off to the east and south of the ARB, and next in line will be the remnants of the upper low. This precip is just reaching the Central California coast south of SFO as a concentrated curl of heavy rains, with the circulation still off the coast and lifting to the northeast. The 500 mb trough axis from the offshore wave will pass the ARB near 00z/Mon, while the next wave now offshore but approaching from the northwest still looks to pass around 09z/Mon. The models all show precip increasing in the ARB gradually the next few hours then really picking up mid to late afternoon with the remnants of the cutoff low coming through, then more again this evening as the northern wave approaches. The tricky part will be when it goes to snow at BLU; max mix at times or even be snow at times this afternoon as the warmest temps have likely passed at this point, but could remain mostly rain or mixed until closer to 22z or so when the remnants of the upper low get close. The really strong cold advection should come by about 06z as the northern wave approaches and passes. Ideally it will be rain this afternoon and if so could be some heavy totals; our ensemble mean has 1.5-2" from the 06z run ending at 06z/Mon at BLU, with the latest 12z ensemble 24-h forecast maxing in the 2-3" range in the ARB, a little more in the 12z NAM, and 3-4" with point values higher by 06z/Mon in individual 3 km runs from 12z and in the 06z NCEP hi-res window runs. So it should be an active afternoon, could be troublesome for the radar if it goes to snow early, and certainly will go to snow by later in the day. Heaviest precip period at BLU should be ~21z/Sun through 06z/Mon, then going to more orographic cold snow showers that may not totally end at BLU before the next system arrives.

The next wave is still on track approaching from the nw and will bring about 24h of the main period of precip to the ARB from 18z/Tue to 18z/Wed. The 12z NAM has 1-1.5" total for the ARB, the 12z GFS not quite as much, rough total from the 09z SREF ensemble mean is about an inch max. Appears to be around an inch in the Global Canadian model as well. A colder system with the snow level staying well below BLU through the event. This is the beginning of a series of fast-moving waves, with an upper level ridge well off the West Coast and CA in strong northwest flow with waves digging into the west-central CONUS. The next wave on the 12z GFS passes near 12z/Fri, then another follows with a trough passage near 06z/Sun, but that MAY be the end for awhile as the upper ridge is forecast on the GFS to then shift to near CA keeping waves farther to the north. The GFS has 0.5-1" or so max for the ARB with the Tue-Wed and Sat-Sun systems, but remains more robust with 1-2" for the Friday/1 Feb wave, and this has been consistent in the GFS forecasts for this system which is still a fast-mover but may be a little warmer than the others. The Global Canadian model is also producing more precip with the Friday storm. A look at the U of Hawaii GFS Pacific PW forecast shows that the Friday system gets a slightly better tap into the big area of tropical moisture west of Hawaii as the system crosses the Pacific. Note that when it hits the California coast on Friday this tap is no longer a direct one.

Although the 12z GFS deterministic forecast indicates the Sat-Sun/2-3 Feb wave might be the last one through the rest of the forecast period ending 12 Feb, there is considerable uncertainty on this when looking at the 12z and 00z ensembles. This is because it only takes a slight shift of the upper level ridge to the west to keep the ARB vulnerable to continuing to be in the track of fast-moving waves coming across the Pacific and then down the east side of the ridge. None of the members that brings shots of precip to the ARB ever has a prolonged system, so at this point unless something changes we would be looking at relatively fast-moving waves coming out of the wnw. However, one positive is an eastward shift in the plume of moisture across and then east of Hawaii by the 2nd week in February, which would bring it closer to the CA coast.

ed szoke NOAA/CIRA/GSD