2006-12-06 Wx Discussion and Forecast
Forecast prepared by Chris Anderson, ESRL/GSD/FAB
2006-12-06 12 UTC Synopsis
The apex of an upper-level ridge has moved overhead of ARB. A potent and fast moving short-wave trough is about 2 days west of the CA coast. IPW values are less than 0.5" along and west of the CA coast.
Current conditions in the ARB region
Clear skies with fog in the valley. Temperatures in the valley are in the mid-30s. As one walks up the hill, temperatures ris into the low 50s. Blue Canyon is reported 53 at 16 UTC. ETL instruments at Blue Canyon measure 3" of snow. Bodega Bay reports IPW of 0.3", with winds directed offshore below 1-km. Upper-level winds are southerly at Oakland, and the 0C level is 650 mb, about 12000 ft.
0-72 hour forecast
A nearly ideal scenario has been predicted by the 00 UTC Dec 6 Canadian model forecast. You can view the forecast at http://www.meteo.psu.edu/~gadomski/CMC_0z/cmc60.html. Follow the forecast from 60-hours through 120-hours. It is, unfortunately, the only model solution produces a near ideal scenario. Much of yesterda's forecast for Friday is valid today. The first wave looks as though it will hit the coastline as a coherent, potent wave. There will be no precipitation prior to its arrival, as the IPW values are below 0.5" ahead of the storm. Most likely period of precipitation onset is 21 UTC Friday through 03 UTC Saturday. The most likely range of precipitation accumulation during the 18 UTC Friday to 18 UTC Saturday period is 1"-1.5". Even under a nearly ideal scenario, the Canadian model predicts no more the 2". Freezing levels will drop significantly to as low as 1500 m around 18 UTC Saturday.
The second wave has a much less certain evolution. As anticipated by the Sacramento forecast office, the models have shifted to an evolution in which two vorticity centers are active. The forecast question then becomes which center will become more dominant. The models are all over the map on that issue, providing a range of scenarios from bust to nearly ideal conditions. Most are somewhere in the middle. Observations are, of course, sparse. The latest water vapor imagery could be interpreted to mean the northern vort max is dominant, or it could not be. Going by the middle portion of the model guidance, t
he most likely precipitation range is 1.0"-2.0". Precipitation onset is most likely to occur between 03-12 UTC Sunday.
00 UTC Model projections
All GFS members have wave coming onshore between 12 UTC Friday and 00 UTC Saturday. No precipitation prior to landfall, during this period most GFS members produce 1" of precipitation. The control run produces 0.25" between 18 UTC Friday and 00 UTC Saturday, and accumulation between 06-12 UTC Saturday is 0.5".
NOGAPS produces 0.5" by 12 UTC Sat and another 0.5" by 00 UTC Sunday.
CMC brings first wave onshore just before 00 UTC Saturday with nearly perfect orientation of 850 mb height field. 12-hour precipitation accumulation at 00 UTC Saturday is 0.25" and 12 UTC Saturday is 1-2". It ends precipitation by 00 UTC Saturday.
Next wave comes in as a splitting wave with the northern wave progged to hit mainland between WA and BC and the southern wave hits at San Diego. First contact is between 00 and 12 UTC Sunday.
NOGAPS brings it ashore 12 UTC Sunday, with 0.5" in the previous and subsequent 120hour periods.
CMC brings it ashore earlier than 12 UTC, and keeps the northern wave much stronger. It produces 1" in the 12-hour period ending 12 UTC Sunday, during which nearly perfect upslope occurs.
12 UTC Model projection
NAM has a slightly earlier arrival of the first wave, but also pushes it northward. 12-hour accumulation ending 00 UTC Saturday, 12 UTC Saturday, and 00 UTC Sunday is 0.01, 0.3, and 0.05. for a grand total of almost 0.4".
>72 hour forecast
00 UTC 6 Dec (Wednesday) GFS ensemble continues to show a zonal pattern emerging after the shortwave on Sunday. This is good news in that it will permit higher IPW values into the area for a more extended period, enhancing the likelihood of upslope flow ahead of short wave troughs that would create heavy precipitation periods. The ensemble indicates IPW values exceeding 1.0" continuously from Mon through Friday. The timing of short wave troughs is tenuous in the model guidance at this forecast lead. It isn't unusual in this pattern to have short wave troughs affect the US West Coast every 2-3 days. Aside from the nonlinear dynamics of the polar jet stream, it does appear interaction between the westerlies and Subtropical convection is possible, as convection has been persistent over the past couple of weeks as far north as Hawaii. The first wave in the period 5-days and beyond is forecasted to affect the West Coast on Tuesday. A long-wave trough is progged to develop Thursday through Sunday. Of course, it is possible that shorter-wave troughs could be embedded within that long-wave pattern.