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[1] One of the more significant atmospheric river events in recent years could be on tap for California next week.
What is an atmospheric river (AR) event, anyway? And what meteorological factors can make one unusually impactful?
![[1] One of the more significant atmospheric river events in recent years could be on tap for California next week.What is an atmospheric river (AR) event, anyway? And what meteorological factors can make one unusually impactful?](https://pbs.twimg.com/media/EsWgVqDXMAEIRR2.jpg "[1] One of the more significant atmospheric river events in recent years could be on tap for California next week.What is an atmospheric river (AR) event, anyway? And what meteorological factors can make one unusually impactful?")
What is an atmospheric river (AR) event, anyway? And what meteorological factors can make one unusually impactful?
[2] The west coast's terrain often acts like a weather machine.
A huge ocean to the west and topographical promotion of ascent, through frictional convergence and orographic lifting, means minimal input is required for precipitation: westerly flow and synoptic-scale lift.
![[2] The west coast's terrain often acts like a weather machine. A huge ocean to the west and topographical promotion of ascent, through frictional convergence and orographic lifting, means minimal input is required for precipitation: westerly flow and synoptic-scale lift.](https://pbs.twimg.com/media/EsWk513W4AELgRZ.jpg "[2] The west coast's terrain often acts like a weather machine. A huge ocean to the west and topographical promotion of ascent, through frictional convergence and orographic lifting, means minimal input is required for precipitation: westerly flow and synoptic-scale lift.")
A huge ocean to the west and topographical promotion of ascent, through frictional convergence and orographic lifting, means minimal input is required for precipitation: westerly flow and synoptic-scale lift.
[3] Evidently, a lot under the AR hood is actually based in immobile terrain.
Following this logic, the atmospheric input of an AR-based threat really only depends on the westerly flow and synoptic encouragement for lift.
To maximize an AR, these two factors must be maximized.
Following this logic, the atmospheric input of an AR-based threat really only depends on the westerly flow and synoptic encouragement for lift.
To maximize an AR, these two factors must be maximized.
[4] It follows that a midlevel set-up that allows westerly flow amidst diffluence aloft is one favorable for impactful atmospheric rivers.
Persistence of this pattern is king. Topography doesn't move, so precipitation continues for as long as a favorable atmospheric pattern can!
Persistence of this pattern is king. Topography doesn't move, so precipitation continues for as long as a favorable atmospheric pattern can!
[5] In the short term (1-3 days), persistence looks like a lot of parallels.
A low level cyclone moving W->E, parallel to the low level jet, maintains onshore flow over one location.
This can be achieved when midlevel trough motion is parallel to the height field.
![[5] In the short term (1-3 days), persistence looks like a lot of parallels. A low level cyclone moving W->E, parallel to the low level jet, maintains onshore flow over one location.This can be achieved when midlevel trough motion is parallel to the height field.](https://pbs.twimg.com/media/EsWzH4sXcAYAdyf.jpg "[5] In the short term (1-3 days), persistence looks like a lot of parallels. A low level cyclone moving W->E, parallel to the low level jet, maintains onshore flow over one location.This can be achieved when midlevel trough motion is parallel to the height field.")
A low level cyclone moving W->E, parallel to the low level jet, maintains onshore flow over one location.
This can be achieved when midlevel trough motion is parallel to the height field.
[6] In the medium term (3-10 days), persistence looks like tweet #5 happening over and over in one place.
When this occurs, it can careen from helpful rain to hazardous flooding.
This takes a special type of pattern aloft.
When this occurs, it can careen from helpful rain to hazardous flooding.
This takes a special type of pattern aloft.
[7] I've run ESRL composite analysis on two infamous 'long fused' AR series, in January 1969 and February 1986. I chose these because a USGS report called ARkStorm cites that, were they to happen consecutively, flooding could exceed the devastation of 1862.
What made them tick?
What made them tick?
[8] The culprit is clear: a powerful jet strung out along a series of midlevel lows, themselves shoved anomalously SE by Alaska ridging.
When oriented perfectly, kept in place by blocking up- and downstream, unwavering flow aloft was able to maintain tweet #6.
![[8] The culprit is clear: a powerful jet strung out along a series of midlevel lows, themselves shoved anomalously SE by Alaska ridging.When oriented perfectly, kept in place by blocking up- and downstream, unwavering flow aloft was able to maintain tweet #6.](https://pbs.twimg.com/media/EsW13KpXYAIO3tZ.png "[8] The culprit is clear: a powerful jet strung out along a series of midlevel lows, themselves shoved anomalously SE by Alaska ridging.When oriented perfectly, kept in place by blocking up- and downstream, unwavering flow aloft was able to maintain tweet #6.")
![[8] The culprit is clear: a powerful jet strung out along a series of midlevel lows, themselves shoved anomalously SE by Alaska ridging.When oriented perfectly, kept in place by blocking up- and downstream, unwavering flow aloft was able to maintain tweet #6.](https://pbs.twimg.com/media/EsW13MNW8AA_u1J.png "[8] The culprit is clear: a powerful jet strung out along a series of midlevel lows, themselves shoved anomalously SE by Alaska ridging.When oriented perfectly, kept in place by blocking up- and downstream, unwavering flow aloft was able to maintain tweet #6.")
![[8] The culprit is clear: a powerful jet strung out along a series of midlevel lows, themselves shoved anomalously SE by Alaska ridging.When oriented perfectly, kept in place by blocking up- and downstream, unwavering flow aloft was able to maintain tweet #6.](https://pbs.twimg.com/media/EsW13N1XAAAsdqO.png "[8] The culprit is clear: a powerful jet strung out along a series of midlevel lows, themselves shoved anomalously SE by Alaska ridging.When oriented perfectly, kept in place by blocking up- and downstream, unwavering flow aloft was able to maintain tweet #6.")
![[8] The culprit is clear: a powerful jet strung out along a series of midlevel lows, themselves shoved anomalously SE by Alaska ridging.When oriented perfectly, kept in place by blocking up- and downstream, unwavering flow aloft was able to maintain tweet #6.](https://pbs.twimg.com/media/EsW2FB-W8AA6wMj.png "[8] The culprit is clear: a powerful jet strung out along a series of midlevel lows, themselves shoved anomalously SE by Alaska ridging.When oriented perfectly, kept in place by blocking up- and downstream, unwavering flow aloft was able to maintain tweet #6.")
When oriented perfectly, kept in place by blocking up- and downstream, unwavering flow aloft was able to maintain tweet #6.
[9] It's important to understand how the highest-end weather events happen to try to predict 'regular' ones. With our new understanding of extreme ARs, it's apparent that next week has some pieces, and is missing others.
[10] Blocking up- and downstream will keep the pattern fairly persistent.
But the alignment isn't as favorable as our examples, preventing short term persistence. Notice misplaced Alaska ridge and a lack of bimodality.
The result? A solid, but not highest-end, AR event series.
But the alignment isn't as favorable as our examples, preventing short term persistence. Notice misplaced Alaska ridge and a lack of bimodality.
The result? A solid, but not highest-end, AR event series.
[11] I'll flesh my actual forecast thinking out in a blog soon for @WeatherdotUS; I wanted to devote this thread more to the cool historical examples that can aid my predictions.
As always, if you want to discuss/argue with anything I put in here, please reach out (:
As always, if you want to discuss/argue with anything I put in here, please reach out (:
