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HMT Discussion for Sunday 16 December 2007

HMT 12/16/2007 Forecast Discussion
The general forecast is similar to yesterday's discussion, but the details of the storm structures are quite different. Forecasted peak precipitation period remains the 00 UTC 12/18 through 00 UTC 12/19. A wetter system may follow sometime in the 12 UTC 12/19 to 00 UTC 12/21 period.

Best guess precipitation onset: no earlier than 02 UTC 12/17
Conservative guess precipitation onset: no earlier than 00 UTC 12/17
First heavy precipitation period: 12 UTC 12/18 (Tuesday) through 00 UTC 12/19, expect 1.25-1.75" with > 2.5" possible
Second heavy precipitation period: sometime after 12 UTC 12/19 and before 00 UTC 12/21

Current Conditions
The most critical aspects of this forecast is identifying where and how much water vapor is available and whether the short-wave trough is a coherent mass or is becomeing shredded.

The 14 UTC water vapor image showed three bands of high levels of water vapor. The eastern most plume appears to be left over from the trough passage yesterday, while the other two plumes are associated with a potent short-wave trough located near 135W at the latitude of N CA. This position is a bit south of the position valid at this time in yesterday's forecast. SSM/I at 13 UTC suggests PW values do not exceed 2 cm.

The short-wave trough over the Pacific has more structure than in yesterday's model runs. One vort max is headed into Canada with a 60-70 kt wind max at 500 mb. The vort max on a latitude parallel to N CA does not appear to have any wind observations around it. The Canadian and GFS analyses depict it as a coherent mass, but I'm skeptical with the fractured appearance in the IR baroclinic leaf and the multiple water vapor plumes.

0-72 hour & 3-5 day forecast
There is some discrepancy in the onset of precipitation between the GFS and NAM, as the GFS keeps the wave more coherent but the NAM strings it out. The NAM has a slightly later onset due to smaller values of PW and a less potent short-wave trough. All models have accumulation by 06 UTC; however, the GFS ensemble and CMC ensemble amounts suggest an onset not long after 00 UTC. The GSD ensemble uses the NAM as boundary conditions, so that the timing and intensity of synoptic waves is tied closely to the NAM evolution. Despite this, precipitation onset is sooner and accumulation greater by 06 UTC in the GSD ensemble. This may reflect a bias in the NAM onset introduced by the cloud parameterization.

Best guess of precipitation onset is around 03 UTC 12/17; A conservative guess would be 00 UTC 12/17.

Once precipitation has started, it likely won't have a complete break until Friday. This is illustrated nicely by the PSD Analogue forecast PQPF in which the probability of measureable 24-hour precipitation accumulation exceeds 70% through 00 UTC 12/22 (Saturday). The reason for this is that the wind field never turns to a northerly component, even though moisture plumes (not exactly atmospheric rivers) move through the region.

The models are in agreement that a period of heavy precipitation is expected between 12 UTC 12/18 (Tuesday) and 00 UTC 12/19 (Wednesday). Precipitation amounts range 2-4" in the NAM forecasts and a couple of GFS members but only 1-2" in the most GFS ensemble members and the GSD ensemble. NWS forecast offices call for snow levels to be fairly low, 4000-4500 ft and perhaps as low as 3500 ft, at the beginning of the precipitation period and to rise above 4500 ft during the period of heaviest precipitation.

A second period of heavy precipitation is expected sometime in the period of 12 UTC 12/19 (Wednesday) through 12 UTC 12/21 (Friday). The timing of this system is quite variable in the ensembles, though it appears that it will have a more moist moisture plume emanating from the central Pacific and that it will be a warmer event.

5-10 day forecast
Long range guidance continues to suggest a more classic atmospheric river event could happen late in the weekend or early next week, during the Christmas downtime.

10 day and beyond forecast
PSD reforecast ensemble continues to indicate ridging will develop along the West Coast beginning the middle of next week.

Chris Anderson