April 11, 2017 – Weak weather systems will move across the area through Wednesday. Moderate storm system expected Wednesday night and Thursday.
And the “march” (although we are in April) towards the wettest water year (wrt the 8 station index) continues. With less than one inch to go to break the 1982-83 record it seems that NorCal is destined to have a water year of historical proportions. But today’s system may not be wet enough to be the record breaker.
The well defined comma cloud is splitting as it moves onshore with the well defined vort max near 40N/130W tracking a little farther north than yesterday’s runs. The trajectory of the dynamics and moisture suggests the Shasta Drainage will be the most impacted with over an inch of precipitation from the system with lesser amounts elsewhere. At press time, some light rain was moving into the Central Valley mostly along/N of I-80 with much lighter returns farther S.
Up to half of the stations that make up the 8 station index could see the necessary record breaking amounts (about .80″) while the others may fall short. Little in the way of snow impacts are anticipated as snow levels are forecast to rise to 5500-6500 ft over the Northern mountains and Sierra.
One factor we are contemplating is the potential for advisory level winds in the central and Northern Sacramento Valley later this afternoon and evening. Although the MOS guidance shows some robust wind speeds later today and again on Wed, we are skeptical of these high wind speeds. One, the NAEFS anomalies for 850 mbs show a +2 anomaly in the valley but the speeds are only 35 kts with a SWly component. Two, although the 925 mbs winds show a band of 40+ speeds in the RDD/RBL areas, the NAM BUFKIT soundings indicate insufficient depth in the momentum transfer to mix the higher wind speeds down to the surface. Most likely this is due to the WAA and the resultant relatively isothermal low profiles. Three, the pressure gradients are not that tight. Four, although there is deeper mixing with the approaching trof on Wed, the 925 mbs are not as strong, thus making advisory level winds problematic as well. Thus, we have massaged the speeds accordingly and continue to monitor the winds.
Although some light precip should continue into Wed, the system for Wed afternoon, Wed nite through Thu, still looks to be the strongest and wettest as a broad and deeper 5H trof develops over the Ern Pac on Wed and swings Ewd across NorCal on Thu. Another surge of higher TPW air associated with the deeper trof will work in behind the previous plume for Wed and Wed night. The interaction of the two the two should augment QPFs in relation to the previous system with at least 2 to 3 inch amounts over the mountains, and around an inch in the Northern Sacramento Valley tapering to around .10 inch in the Northern San Joaquin Valley. The precip amounts for the 8 station should be realized on/by Thu to surpass the previous WY record.
Mid tropospheric 5H temps in the minus mid to upper 20s Deg C range should steepen lapse rates and increase the potential for low-topped thunderstorms and primarily hail producers. This could lead to locally higher QPFs as well.
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Snow levels should start out in the 6500-7000 ft range on Wed along the 80/50 corridor before lowering to around 5000 ft on Thu. The bulk of the QPF/Snow is expected to occur during the Wed nite period. Although the Lassen NP area is expected to see about a half of a foot of snow with system #2, the potential for 2 feet or so is there for system #3, and up to 6 to 12 inches for the 80/50 corridor. This multiple wx threat for system #3 for the middle of the week looks to be the most impactful. The trof takes on a more negative tilt as it swings across NorCal and then lifts NEwd Thu nite and Fri with diminishing precip. Drier westerly flow should prevail over most of the CWA except the Shasta County mountains on by Friday afternoon.
Extended discussion (Saturday through Tuesday)
Seasonable temperatures and dry weather are expected on Saturday with a weak upper ridge over the area. More unsettled, wet weather is possible into early next week.
An upper level trough arrives during the day Sunday, bringing the potential for some light precipitation. The current timing suggest this would be spreading into the western side of the forecast area during the morning/mid day, and across the area by late afternoon. The latest GFS operational run has the system splitting, keeping much of the forecast precipitation over the northern mountains and northern Sacramento Valley. The European and Canadian models do not show the trough splitting, and have a more widespread, though relatively light precipitation event. The GFS ensemble precipitation mean is more similar to the European and Canadian, so will go with a blend of these 3 for now. This also includes the potential for the wet pattern continuing through Monday, and possibly into Tuesday as more waves move through the area. These systems are not particularly strong, but could bring an extended period of relatively cool, cloudy and wet weather.