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November 23, 2009

NWS: Moderate El Nino winters can be Md.'s snowiest

For those readers hoping for a snowy winter this year after a series of disappointments, there is hopeful news Monday morning out of the National Weather Service's Sterling forecast office. (Likewise, for those who loathe the ice and slush, dangers and inconvenience of wintery weather, these will be discouraging words.)

Forecaster Jared Klein has done a statistical analysis of winters since 1950 and has found 17 winters that were influenced by the El Nino phenomenon in the tropical Pacific, like this one is expected to be. The long and short of it, says Chris Strong, also at Sterling:

NOAA/NWS"With moderate strength El Nino's [like this one] we have statistically the greatest chance of above-normal snowfall."

What they're saying is that not all El Nino winters are alike for the mid-Atlantic states. Some will be snowy; some not. Here's how they tend to break down, according to Klein:

* On average, weak El Nino winters bring below-average temperatures and below-average precipitation. Not generally conducive to lots of snow.

* Strong El Ninos, on average, bring us above-normal temperatures and precipitation. The cold air tends to remain well to our north, so most of the precipitation falls as rain rather than snow.

Moderate El Ninos, on the other hand, seem to offer the greatest statistical chance that moisture and storms passing across the southern U.S. will "seed" the Atlantic coastal storms that tend to bring us our deepest snowfalls. We've already seen plenty of coastal storms this fall, including the big one last week that battered OC's dune line, and another one today.

Nothing is guaranteed, of course. There are other shorter- and longer-term climate patterns - Snowstorm 1996including the North Atlantic Oscillation - that can determine whether there will be, for example, enough cold air in place to make snow-makers out of the coastal storms.

That helps to explain why, of the 17 El Nino winters since 1950, eight produced above-normal snowfalls, while nine were below-normal. (Weak La Nina winters can produce big snow, too, as it did in January 1996, right, although that's less common.)

Still, there is plenty to look forward to this time, Klein said. "The above-average El Nino winters have been associated with some of our snowiest winters, especially during moderate El Nino episodes. With the ongoing El Nino episode expected to continue, even strengthen to moderate levels this winter, El Nino will likely play an important role with the winter climate here in the greater Baltimore and Washington, D.C. area."

Among the most memorable snowstorms in El Nino winters was the Feb. 11, 1983 storm that dropped 22.8 inches on Baltimore. Then there were three storms in 1987: Jan. 22 (12.3 inches), Jan. 25 (9.6 inches), and Feb. 22, (10.1 inches).

Here is another summary of the El Nino effect on Baltimore snowfall, also from the NWS at Sterling:

"El Niño winters in the Baltimore Region mean a milder than normal December. They also tend to be all or nothing when it comes to snowfall. Either there are no significant snow storms and season snow totals average less than 5 inches or there is a tendency toward multiple snow storms with seasonal totals above 30 inches.  These storms usually occur in January and February. November, December, and March often see little or no snow."

Here are still more statistics on Baltimore snow and ice.

Here are the snow totals for the past 10 years at BWI. The long-term average (1971-2000) is 18.2 inches:

2008-2009: 9.1 inches

2007-2008: 8.5 inches

2006-2007:  11.0 inches

2005-2006:  19.6 inches

2004-2005: 18.0 inches

2003-2004:  18.3 inches

2002-2003:  58.1 inches

2001-2002:  2.3 inches

2000-2001: 8.7 inches

1999-2000:  26.1 inches

Posted by Frank Roylance at 11:06 AM | | Comments (6)
Categories: Winter weather
        

Comments

The National Weather Service..the same N.W.S. Who tells us the past few years about all on the big Hurricans seasons were going to get...PLEASE !!

FR: Actually, the NWS did better this season. In June, they forecast 9-14 named storms, with 4-7 hurricanes, of which 1-3 would become "major" storms. They lowered the estimate in August. The final tally: 9 named storms, including 3 hurricanes, of which one reached "major" (Cat.3) status.

Woo Hoo!! Can't wait for the snow and even at 34 I love everything about snow- even digging out the car, cleaning the walkways, etc. bring it on!

Would be nice if LWX would post the data and not just the conclusion.

You're a reporter...Frank. See what you can dig up for your readers.

FR: Here's what they've posted: http://bit.ly/6ZcoV2 If you want to go deeper into the data and the math, contact Jared Klein at Sterling - 703 996-2200.

Thank you...FR!

I dug up the historical snowfall data for BWI and the CPC ENSO classifications for my own analysis. The LWX study used DCA snowfall data which is assumed to be representative of snowfall @ BWI.

I call BS on the LWX el NiƱo snowfall results. Just b/c the average is higher for categories of +ENSO doesn't necessarily mean they're statistically significant. The variance needs to be considered to determine whether the difference in the averages are truly different or an insignificant artifact resulting from a wide range of values that make up the average.

I compared the average snowfall for weak...moderate...and strong +ENSO seasons (D-J-F) using a 'z-Test: Two Sample for Means' (confidence level = 95%).

BWI AVG Snowfall and number of years:
W-: 23.9" (n= 7)
W: 30.6" (n = 5)
W+: 17.4" (n = 5)

The null hypothesis: average snowfall is the same for W and W-...W and W+...and W- and W+ were all affirmed.

Conclusion: even though the average snowfall is higher during moderate +ENSO and weak or strong events...there's no statistical difference in BWI season-total snowfall relating to +ENSO strength.

FR: I can't offer a critique on your statistical analysis. The cover letter from Strong does acknowledge that "the sampling size is small," but insists that "the signal is there in the limited statistical database." Maybe Jared will jump in and defend his work.

Dear Rebecca,

How much do you charge for shoveling? ;)

So, It'll either snow a lot or it won't. As a former boy scout I've got my snow blower and shovels all set to go anyway. The lawn mower is blocking them in the garage at the moment however...sigh. I'll fix that when the leaves stop falling.

We've got a ways to go on long range forecasting, but it's WAY better than the '60s when tomorrow was just probable, the next day was wishful thinking, and anything after that was just a guess.

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About Frank Roylance
Frank Roylance is a reporter for The Baltimore Sun. He came to Baltimore from New Bedford, Mass. in 1980 to join the old Evening Sun. He moved to the morning Sun when the papers merged in 1992, and has spent most of his time since covering science, including astronomy and the weather. One of The Baltimore Sun's first online Web logs, the Weather Blog debuted in October 2004. In June 2006 Frank also began writing comments on local weather and stargazing for The Baltimore Sun's print Weather Page.

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