The Cutoff Low - Quenching our Thirst
* Rain, Rain, Rain: Full Forecast *
As Matt mentioned in his post earlier today, thanks to the drenching today and tomorrow, our 2009 rainfall deficit may be cut in half. What's the culprit? It's a feature known as cutoff low.
By far the best description of a cutoff low I've seen comes courtesy Chick Jacobs at the Fayetteville Observer:
Think of a cutoff low as a drunk relative at the family reunion who just won't go home.
The National Weather Service's technical definition clarifies why this description is so apt:
A cut-off low is an upper-level low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current.
In other words, the cutoff low, like the drunk relative, pays little heed to its surrounding environment, and does as its chooses -- very deliberately.
Keep reading for more about the cutoff low impacting us now...
The cutoff low impacting us now, though frustratingly slow-moving, will behave relatively predictably. It's trudging into Ohio Valley and will position itself over the metro region by this time tomorrow.
As it slogs eastward, pockets of energy rotating around it will help spin up waves of low pressure at the surface. These areas of low pressure will tap Atlantic moisture and generate several bouts of rain, brewing these possible amounts:
Less than 0.5" - 15% chance
0.5 to 1" - 15%
1.0-1.5" - 20%
1.5-2.0" - 25%
2.0-2.5" - 15%
More than 2.5" - 10%
(In other words, a 70% chance of at least 1", and a 25% chance of more than 2".)
[LATE TUESDAY UPDATE: RAINFALL TOTALS MAY WELL END UP IN THE LOWER CATEGORIES -- I.E. LESS THAN 1.5"]
If we once again liken this cutoff low to the drunk relative, we curse it for dampening our spirits and annoying us, but celebrate it for, in otherwise dry times, replenishing the beer... [LATE TUESDAY UPDATE: THIS CELEBRATION MAY HAVE BEEN PREMATURE -- NOT SURE OUR RELATIVE IS GOING TO DELIVER HERE]
[Afterword: I left out a few of the geekier details about cutoff lows. But rather than re-invent the wheel (or recirculate the cyclone), I'll refer you to these fine resources:
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