HMT forecast discussion for Monday 28 January 2008 (entered 2030z Monday)
An interesting day and evening on Sunday in the ARB. At BLU managed to get about a half inch of rain ~08-16z with the temperature in the mid-30s with what looked like the western portion of the band of subtropical moisture that had been moving rapidly northward on Saturday. Then just enough cooling to change the precip to snow by 17z and never changed back with temps hovering near freezing into the evening. In fact at 05z KBLU METAR rose to 33F with wind 190 gusting to 40 kts (peak gust was 200 at 45 kts at 07z). Rapid cooling after 07z at KBLU with the approach and passage of the system from the northwest, and this morning (15z) the temp is down to 21F with a light sw wind and S- with showery conditions on the radar. At KBLU METAR 24-h total of 1.10", with 0.25" as of 12z/Sun, for METAR total of 1.35". This compares to the ESRL gage amount of ~1.3" after 14z/Sun (note sure of the amount before then, think it was around 0.5"; note this is the tipping bucket gage so could be snow issues). According to the automated snow gage, about 6" of snow (will be interesting to see how this compares to obs). There is a nice signal of the northern system passage with a rapid temp fall beginning at 08z. Some other amounts from the ESRL gages: at Alta just under 2" with most between 19z/Sun-08z/Mon; at Big Bend about 2.3" on the hot plate gage between 18z/Sun and 08z/Mon, then since light precip; also a nice cold front signal at 08z. It was a close call for getting more rain at the radar site but just enough cooling between the first wave of subtropical moisture and the precip with the approaching remnants of the old upper low to change things over to heavy wet snow. Dave indicated in the telcon about 6" of heavy wet snow at BLU radar site, max precip up to 3" at Big Bend with 12" of snow, with Alta about 2.25" total. So local and other models not too far off, and pretty decent forecast from the ensemble mean.
Precip is now down to light snow showers that will continue today. A series of fast-moving systems will reach the ARB this week, all at this point potential IOPs (after Tue one) but also potentially not meeting the criteria. Each wave taps a bit of the subtropical moisture in the south-central Pacific as they move eastward to varying degrees. The next system still on tap for ~18z/Tue to 18z/Wed is a fast-mover with about an inch max precip in the 00z GFS and a cold system overall with the snow level below BLU radar site. In the 12z/NAM about 1-1.25" max in the ARB for this event, timing similar to 00z GFS. The GSD 3-km runs have up to 2" for this event but this may be on the high side.
Later in the week the next in line in the series of fast-moving storms brings precip beginning ~18z/Thu/31 Jan and ending about 21z/Fri/1 Feb. Slightly warmer at first with this one with maybe some rain into BLU Thu night, but too early to tell. The 12z GFS has total precip of about 1.4" at BLU and 0.9 at SAC for this one, while the 00z ECMWF has ~1.2" with this event and with similar timing, and the 12z EC close to an inch at BLU. NOGAPS and Global Canadian model solutions look similar for this event and generally agree with the GFS and EC solutions. It appears this has a little better tap into the Pacific as it moves eastward midweek.
Another system right behind the Thu-Fri one starts precip by about 06z/Sat/2 Feb, with more precip (~1.5" max on the 12z/GFS) through midday Sunday. Right now the 0C line at 850 mb gets close to the ARB late on Saturday, so probably a snow event at BLU but will need to watch as we get closer. This event is much weaker than the GFS in the 00z ECMWF forecast, and in the latest 12z ECMWF run just in BLU only gets about 0.25" with this storm, so a big difference between this run and the GFS, even though it is actually a deeper upper low in the EC solution. The 12z Canadian Global model looks similar to the GFS for this storm and the NOGAPS somewhat more like the EC run. The 12z GFS ensembles do show some variation with this wave, with some members looking to drop the system a bit farther south as in the EC solution.
Longer range the 00z deterministic GFS has it dry after the weekend storm through 384h or 12 Feb with upper level ridge inching closer to the West Coast and systems missing to the north, then towards the end of the run more zonal flow. As noted yesterday though some variation on this ridge position so not all the ensemble runs are dry. For instance, the 00z ECMWF has a potential system on Wed/6 Feb that the GFS keeps north and east, and this is true of the 12z ECMWF also. Overall though, there seems to be a little better agreement on a drier forecast in the long range, at least in the 00z ensembles, then there was yesterday. Having noted that, the 12z GFS ensembles show a little more variation after this coming weekend, though the majority of members do keep it generally dry in the ARB.
ed szoke NOAA/CIRA/GSD