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Posted at 10:30 AM ET, 04/14/2009

The Cutoff Low - Quenching our Thirst

By Jason Samenow

* Rain, Rain, Rain: Full Forecast *

The cutoff low (about 20,000 feet up in the atmosphere or at 500mb), depicted by the big circle in the center of each map panel, will plod eastward, producing unsettled weather for the next 24-36 hours.

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".)


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:

By Jason Samenow  | April 14, 2009; 10:30 AM ET
Categories:  Local Climate  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: Rain, Rain, Rain
Next: PM Update: This Raw and Rainy Story Continues


Why don't we get these in the middle of winter? Or do we, just not recently?

Posted by: ah___ | April 14, 2009 10:48 AM | Report abuse


We do occasionally get these in winter... but they're much more common in spring and fall -- when the flow pattern is in a transitional state...

Posted by: CapitalWeatherGang | April 14, 2009 11:18 AM | Report abuse

At least this isn't May 2003 -- pretty much a month-long cutoff low that just sat over us. I remember everyone's frustration at the endless cloudiness we had. Reading WaPo articles about it having been 23 days, 24 days, 25 days since the last time we saw sunshine didn't help matters. Talk about serotonin imbalances...

Posted by: iammrben1 | April 14, 2009 11:20 AM | Report abuse

an inch of rain already in the gauge in New Balto ....

Posted by: weathergrrl | April 14, 2009 11:23 AM | Report abuse

The problem seems to be that we NEVER get cutoff lows in mid-January when we snow-lovers could use a three-day blizzard!

I'm just wondering...whether that three-foot Washington/Jefferson snowstorm back in 1772 or thereabouts was a well-positioned cutoff low. That would be interesting...Metrorail shut down by fifteen-foot drifts!

Posted by: Bombo47jea | April 14, 2009 12:17 PM | Report abuse

What three foot snowstorm? Here?

Posted by: manassasmissy | April 14, 2009 2:49 PM | Report abuse

DC had Metrorail in 1772?

Posted by: RMVA | April 14, 2009 3:28 PM | Report abuse

Washington and Jefferson Snowstorm of January 27-28, 1772:

Good afternoon, Charlottesville

Yesterday was Jefferson's birthday.

Posted by: CapitalClimate | April 14, 2009 3:46 PM | Report abuse

This system certainly hasn't under-performed in my area of Bethesda. As of 5pm we had 2.1 inches in our rain gauge (for the previous 24 hours).

Posted by: mikeBethesda | April 14, 2009 10:54 PM | Report abuse

In Ashburn, we are getting nothing: two day total = 0.01 in + 0.06 in.

Posted by: LoudounGeek | April 15, 2009 4:07 AM | Report abuse

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