HMT forecast discussion for Sunday 24 February 2008 (entered at 2000z)
The cooler overnight temperatures from Friday night and drying at KBLU mentioned in yesterday's telcon by WFO SAC led to cooling as the precip began yesterday around 21z that was apparently too much to overcome last night, despite a valiant effort by the warm advection, so it was mainly snow at KBLU. We discussed this in the telcon today and WFO SAC noted that between 21z/Sat-00z/Sun they were getting spotter reports of snow down to nearly 3000 ft. They noted though that this was collaborated with the Sloughhouse snow level data for this time, and they made a point today in noting how useful that data was to adjusting their forecast yesterday. Special soundings at Sloughhouse (SHS) on Saturday afternoon did show the warm advection that occurred by 21z and beyond, and there is a sharp temperature rise in the Alta time series last evening from 02-06z (from 0.9C up to 3.6C). A similar timing for the temperature rise in the BLU time series but never enough to get it above freezing overnight, with the gage snowfall showing 10-12" so far and ~1.4" of precip (tho there is a problem noted during the telcon with this sensor, stopping this morning at 1.4". As of 20z, Big Bend has received nearly 3" according to their hot plate gage. The KBLU METAR, if it is to be believed on precip type, did have a changeover to freezing rain and sleet between 07-09z, with the temp near 32F, but now (16z) it is back to 30 with heavy snow. Discussion of this in today's telcon by WFO SAC suggests that rather than UP/FZRA this is probably wetter snowflakes that fall faster, fooling the sensor, but showing that the precip during this time was as close as it got to any changeover to rain (the forecasters at SAC estimate the snow level may have risen during the 02-06z period as high as 5200 ft for a time. Peak gusts overnight from the KBLU METAR were 36 kt around 05z. The latest radar loop picks up the center of the occluded surface low approaching the far northern CA coast and heading for landfall ~21z near the CA/OR border, with a large area of precip over the ARB, including a nice band approximately north to south across KSAC area.
The 12z/NAM analysis has the upper low at 534 dm with the surface low at ~986 mb, and the rapid filling is forecast to continue, with an open wave trough passage at KBLU at 500 mb near 00z/Mon, maybe a few hours later at 700 mb, with all the energy then shifting east of the Rockies where a new storm will be born east of CO tonight. Substantial precip will continue today before ending near 06z/Mon, with the 12z/NAM forecasting up to an additional 2" over the higher terrain of the ARB. This timing is similar to our hi-res models from 12z, which show a rapid decrease in echo coverage after 06z and pretty much everything gone by 11z/Mon, and accumulations up to 2-3 inches and a little higher in some areas of the ARB (in the individual runs; ensemble forecasts from 12z are in the 1.5-2 inch range). At this point the precip should remain as snow at KBLU, but discussion of the soundings is next.
A time series of the OAK RAOBs overnight indicated warming at low levels but cooling aloft with a very unstable lower level profile by 09z, but now the 12z RAOB is more moist adiabatic with quite a bit of warming between 850-700 mb, and a freezing level rising to 5650 ft/822 mb. RNO soundings have been cooling aloft all night but in the 12z sounding very low level warming putting the freezing level to almost 6000 ft. The latest (12z) SHS sounding has the freezing level near 837 mb/1570 m, just a little higher than the 09z sounding. Time series of the winds from SHS shows a little decrease in speed but continued south to sw flow a about 40kts through 4 km MSL. A look at forecast soundings from the RUC 15z run interpolated to KBLU indicates not enough warming to change the precip over to rain today, with just a little very low level warming but the upper levels remaining about where they are. Trough passage at 500 mb is also forecast to be near 00z/Mon, similar to the NAM. The 3-km 12z/Schultz microphysics run does show the freezing level rising this afternoon near KBLU to almost 800 mb, but the precip remaining as frozen down to KBLU, so some shallow warming as also shown in the RUC forecast soundings.
As for the longer range...the strong but weakening system west of our current storm is now splitting in the NAM and GFS forecasts, but what ends up coming across the ARB midweek is a feeble-looking upper low with not much moisture left in it, even though right now this system has a great tap to a tropical plume stretching south to Hawaii. But none of the models has any precip with this when the weak southern part of the system drifts towards CA on Wed/27 Feb. The next threat is then late Friday into Sat, but the 12z GFS remains consistent in keeping this system north of the ARB and fast-moving. In fact, the deterministic run of the 12z GFS keeps it dry in the ARB through 11 March with enough ridging along the West Coast to keep the active Pacific systems north of the ARB. Some variation though in the ensemble members longer range in the GFS, as well as in the 00z Canadian ensemble system. Some members are also more favorable for the Saturday system, including the 00z/ECMWF and Canadian Global runs, so will have to watch this one as the week unfolds, though right now even if it is far enough south it looks to be very progressive.
ed szoke NOAA/CIRA/GSD