Moderate to Strong Atmospheric River Type Storm to Affect the Area Thursday Night through Saturday

Synopsis: A very moist Pacific storm will affect the area beginning Thursday night through Saturday afternoon. Light rain is expected late Thursday into Friday with heavy rain Friday night into at Saturday afternoon.

Current Conditions: The latest IR satellite image continues to show abundant moisture streaming across the eastern Pacific associated with two main areas of low pressure in a quasi-zonal flow. Relatively weak high pressure remains over the far west ahead of the developing storm systems off the west coast.   This energy associated with these developing storm systems are partially due to the remains of what was once super typhoon Jelawat that formed in the western Pacific over a week ago, now transitioning in the fast flow across the Pacific.

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The latest GFS graphic as of 18Z shows the wind pattern across the Pacific basin, where a weaker subtropical jet coming off the Asian continent becomes confluent with a more northern stream to bring a strong 140 to 170 knot jet just northwest of the Hawaiian Islands. This strong jet and associated moisture will progress east toward the west coast. Looking at the latest IVT values, an atmospheric river approaches the west coast by 12Z April 6 or early Friday morning. The IVT values indicate a narrow band of 1200-1400 kg/m/s, with a broader area from 700-1000 kg/m/s. As this high PWAT plum enters the west coast, it will weaken some, but the high precipitable water of 1.5-1.8 inches will be entering the west coast. This is on the very high end if not record end of climatology for the west coast as these types of events in April are rare.

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Looking ahead to precipitation, the GFS and ECMWF are in fair agreement about the amount of precipitation expected to fall in the Sierra and western Nevada. Since these are dynamical models, it’s difficult to pin point exactly how much precipitation will fall over the area. For the Carson City area, amounts of ¾ to locally 1 inch plus is shown in the 18Z GFS model. The ECWMF shows from 1-1 ½ inches in the local area. Some factors working for and against this storm are as follows.

Things going for significant rain and possible flooding with this storm:

  • This is the remains of super typhoon Jelawat and significant energy ejected into the subtropical jet.
  • Snow levels will start out near 10,000 feet or even higher where significant snow pack exists.
  • Rivers, lakes and streams are already running fairly high due to the last AR event and heavy snows during the month of March.

Things going against this storm:

  • The time of year for an AR event. It’s getting late in the year for these significant events. If it were Dec-Feb, it would be much more impressive.
  • There is a lack of cold air advection for these storm systems.
  • Low elevation snowpack has melted along with low elevation soils have sufficiently dried to absorb additional moisture.
  • The strong low that develops off the west coast will be pulling north into the Pacific Northwest and Canada.

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Looking at the GFS and NAM MOS forecast guidance (not shown) both indicate rather significant precipitation for the Reno area, similar to their dynamic models. The SREF ensemble plume is much less. Since there is no data for Carson City, I must factor in local weather effects such as orographics and/or shadowing and pattern recognition. Typically in these types of storms, Carson City generally receives much more rain than the models suggest. At this time I would forecast this to be another event similar to the late March storm where much of Carson City received 1+ inches east of I-580, with up to 2 inches west of US 395/I-580 especially along the foothills and Carson Range.  Expect the potential for main stem river flooding as well as small creeks and streams.  Keep flood mitigation in place as this may continue especially if the wet pattern continues.  As always, check with the National Weather Service for the latest watches and weather warnings.

Looking ahead, the overall weather pattern looks to remain active for Spring, with additional storm systems traversing the area to bring addition rains and snows to the area over the next 6-10 days at least. The amount of rain and snow remains to be seen as this is rather far out in the forecast future so forecast confidence is rather low after this strong storm winds down.

More updates to follow if forecast guidance changes.

Robert Sandberg

 

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