Major System brings heavy rains, snow to Northern California

Mark SpencerPacific Northwest, USLeave a Comment

A major system off the coast of California will be streaming in plenty of moisture, leading to several inches of rainfall and snowfall (mountainous regions) for Northern California.  One can guess this heavy precipitation will fall most along the Sierra Nevada mountain range.  A commonplace for Northern California, the Sierra Nevada mountain range provides a “block” where all the moisture will build-up on the windward side of the mountain range.

Windward v. Leeward sides

Visual of mountains in California

A brief note on the windward and lee side of mountain ranges.  This concept is certainly not foreign to those that live in either side of mountain ranges, but let’s quickly view the differences.  The Windward side is just that, the side of the mountain range the wind blows toward.  So for the Sierra Nevada range, the windward side is from the Pacific Ocean to the top of the mountain, or the west side of the mountain range.  The image at left can perhaps help visualize this better.  The leeward side of a mountain-range is the eastern side, or the side to the right of wind direction.

The primary reason moisture concentrates on the windward side of a mountain range in this situation is because all the moisture from the Pacific Ocean comes inland, travels up the mountain and rains (or snows) itself out, so there is not much – if any – moisture left as the air travels back down the other side of the mountain.  This is based on the concept that air cools as it increases with height (in this case, up a mountain), and cooler air has a lower moisture capacity (amount of moisture an air parcel can hold) – so once it reaches its capacity, it will rain out.

Now that we have covered the basics of this, let’s talk about all this rain (and snow) coming.

The setup

North American Model (NAM) guidance mid-day WednesdayWave one of moisture came inland this morning and today, bringing about plenty of low clouds and rain for those west of the mountains.  Snowfall at the tops of the mountains, for obvious reasons.  Referencing the geographic map above, the bulk of this moisture fell in the Northern Coast Ranges.  Weather model guidance suggests moisture and rain will continue to stream in off the Pacific ocean overnight and into Wednesday.  There is still slight variation in weather guidance about how widespread the rainfall will be Wednesday, but another surge of heavier rain and snow will be during the mid-day to afternoon hours.  The model at right shows us the more conservative of the two, while the model at left is more widespread and heavier.  Nonetheless, the heaviest will still be in Northern California.  Certainly no shortage of water will be found.

What can be said about this storm with regard to the rest of the nation is interesting itself.

Impacts for the rest of the United States

Model guidance further suggests this system developing into a much stronger low pressure system over the central United States by Saturday and making its way toward the Ohio Valley by Sunday and Monday.  This is a setup we saw at the end of 2016 in the Pacific Northwest with systems coming on-shore and making its way across the nation, except this time is travels up from the south instead of the northerly track through the northern plains.  You can review that system from mid-December at this link.

The image at left provides total rainfall estimates after this system is through the area.  You can almost tell exactly the mountain range outline.

Looking forward

Moving forward, it appears California is going to be hit with several systems moving in off the Pacific Ocean, so it is safe to say it will remain fairly active as we look forward.  We will likely end March next week with plenty of saturated grounds.


-Robert Carroll

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