This morning weak showers were noted in the forecast area as early as 13ut on Nexrad. BLU reported 41F and dropping temperatures. Auburn was 39F. The main area of precipitation remains off of the N. CA coast and of course the first forecast question is when and how intense will this system be?
There was a problem with the HMT run this morning in that the super cluster was unable to communicate data from this morning's LAPS run. As a result there is no high resolution guidance similar to what we did have with IOP7. This would have been very helpful in estimating the onset of the event and the early precipitation amounts as well as precip phase.
The NAM is again the winner with the most qpf from the system at 2.82 inches with the max precip falling between 15h and 18h 1/18/06 utc, lower than yesterday's estimate. The NAM has the event starting about 21h 1/17/06 utc and ending with light amounts in the 0.06-0.2 inch range after 00h 1/19/06 tapering to 0.0 inches by 06h 1/19/06utc. The NAM is also in better agreement with the other models today so perhaps we can derive more confidence from its output.
The GFS run begins the event slightly earlier with very light amounts in the 0.01 inch range (12h 1/17/06 utc) and really doesn't start putting down decent amounts of water (0.46 in 6hours) until 12h 1/18/06 utc. The GFS has the event tapering off after 0h 1/19/06utc and finally ending at 12h 1/19/06 utc with a storm total of 1.29 inches. The earlier GFS had a higher precip total of about 1.7 for the event so the d-prog/d-t trend is for less moisture at least in the GFS. As we saw this is also the case for the NAM.
The NAM shows favorable wind speeds and directions through the entire event with winds beginning SW at 10 at 12ut this morning. When the moisture reaches the slopes at about 09h today the winds are 30kts from the south at the mountain top levels. Winds then continue fairly strong in the 30kt SW range from 03h 1/18/06 utc until 15h 1/18/06 utc after which they fall in intensity and the RH, which up until that time is 100% on the slope, begins to lift and reaches the higher terrain at the end of the event.
Freezing levels appear to hang around 840 to 850 hPa for the entire event on the NAM with a drop in elevation only near the very end of the event. GFS freezing levels are slightly cooler but have a similar trend with the 0.0C level reaching mountaintop at 0h 1/18/06utc and dropping throughout the event to a slope surface pressure of about 860 finally falling to lower levels (900) at the end of the event. For the most part the two runs are very similar in this regard and not as dissimilar as IOP7.
The UKMET model has the precip event starting about 00ut Wednesday in line with other guidance. Unlike prior runs it appears to be progressing the wave slower and in more agreement than past UK runs. It has the best dynamics coming in aloft at about 18ut Wednesday. So in that regard it might be a tad slower.
The Canadian model has weak precip as early as 12ut today, ending the event about 12ut Thursday. This model appears to be slightly warmer than the UKMET. The Canadian model correspondingly has less dynamic support and puts most of the emphasis to the north of ARB with an abrupt breakdown in the vorticity advection late in the event.
Overall, this storm is progged to be very similar to IOP7 in temperature with less of a dramatic drop in temperatures arriving late in the event.
Max plume of moisture with good connection to the tropics is forecast to move through the forecast area today with values of 21mm PW. Overall, this event looks very similar to IOP7 with the exception that the GFS does not forecast as cold a situation late in the event but I think offers better deep layer winds and the overall event might produce more QPF.
The global trend appears to have the intense Siberian cold air breaking down over time and along with the set of short waves that have been crossing the Pacific. The 240h GFS forecast has a broad Pacific ridge in place that brings incoming systems into CA from the NW. I would consider this trend to one that would interfere with tropical moisture events that we have been seeing to date.
The GFS ensemble forecasts play out to show a minor weekend event beginning after Sunday 00ut 1/21 and maybe going through late Sunday. Probabilities for this are low at less than 0.5. Following this there appears to be a suggestion of something near then end of next week perhaps starting Friday at 00ut 1/27 and extending into the weekend; a longer duration event with the max probability (again less than 0.5) on Saturday the 28th. Then an extended dry period is forecast with a potential event out near 2/2/06 (Thursday). All of this is very speculative however; the trend appears to be on par with yesterday's ensembles for this coming weekend and very weak after that. There is a weak surge of additional moisture that follows the main thrust with origins from the northern part of the plume in the western pacific.
Moisture plumes appear to show a similar trend as the ensemble probabilities. The plume that might get into the forecast area for the weekend is now forecast to turn into a closed circulation just off the CA coast and not make it inland. It moves south of the ARB latitude as it circulates.
Dan Birkenheuer - ESRL/GSD