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Posted at 10:15 AM ET, 03/ 7/2011

For the record: Sunday's rain and snow

By Jason Samenow

Snowfall amounts from Sunday, March 6 to Monday morning March 7. Source: National Weather Service

Heavy rains fell throughout the metro region yesterday, with amounts generally in the 1-2.5" range from east to west spanning Prince George's county and western Fairfax and Montgomery counties. West and southwest of Fairfax county, isolated locations saw around 3" or a little more. Capital Weather Gang's forecast for 1-1.5 inches in the metro area with locally heavier amounts to the west was pretty close to correct, albeit a little conservative (in the western one third of the region).

Here are some representative totals from the National Weather Service:

Brandywine (PG co.): 1"
BWI Airport: 1.05"
Reagan National Airport: 1.39" (just short of record, 1.52" from 1932)
Laurel (PG co.): 1.5"
Quantico (Prince William co.) 1.92"
Fairfax Station (Fairfax co.) 2.04"
Dulles Airport: 2.3" (new record for the date)
Gaithersburg (Montgomery co): 2.38"
Sterling Park (Loudoun co): 3.2"

Where our forecast went wrong was in the snowfall department. The rain changed over to sleet and then snow from west to east between between about 8:00 p.m. (in western Loudoun county) and midnight. No snow accumulated inside the beltway. The closest accumulating snow (mainly on grassy areas) occurred 20-40 miles to the west and northwest of D.C., with a coating to 0.5".

While we called for temperatures to drop late yesterday afternoon, we thought the precipitation would cutoff by the time it became cold enough for snow. However, a strong upper level disturbance kept the precipitation going several hours longer than anticipated which allowed for the surprise wet flakes.

We had seen this possibility of snow earlier in the week and some commenters specifically brought it up, but we never chose to include the chance in our forecast because 1) the warm ground temperatures would make it hard for any snow to stick if it snowed 2) temperatures would be at or above freezing further reducing the chance of accumulation 3) the overall pattern was the opposite of what's typically required for accumulating snow in the metro area late in the season and 4) the models went back and forth in simulating any snow at all, not lending much confidence in the scenario.

Many people pointed out the irony of the Sunday's Snow Potential Index (SPI) reading "0 (→) - Nada, naught, zip, zilch, goose egg, bupkis, need I go on?" when snow flakes were falling. I reminded readers that the SPI is a tool to assess the odds of accumulating snow, which did not occur inside the beltway (which is where the SPI most directly applies; because the area is so big, a single number can't do justice to snow odds everywhere in every situation). In hindsight, a better SPI for yesterday would've been 1 or 2.

The bottom line is that we just didn't want to hype up snow chances when the odds were so stacked against any impactful accumulation. Were we totally surprised to see a little conversational snow on the backside of a dynamic March weather system? No. Were the snowflakes consequential? No - in most areas (save western Loudoun and Frederick counties where it was enough to cause some issues). Did we miss it and should we have better factored it in to our forecast? You bet.

By Jason Samenow  | March 7, 2011; 10:15 AM ET
Categories:  Latest, Local Climate, Recaps  
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Since SPI goes from 0 to 10, it should be redefined a little so that a 1 or 2 doesn't mean 10% or 20% chance of "real' snow. Rather, 1 or 2 should mean a chance of any kind of frozen precip falling from the sky from natural causes (airliner discharges don't count). That way we don't have to argue about whether the flakes melted within 3 seconds or lasted for 10 minutes or whatever the definition of a trace is. As for yesterday in particular, the rain had ended at my house, then this weird upper level thing came through and kicked it over to snow. I don't think that was very easy to forecast.

Posted by: eric654 | March 7, 2011 10:45 AM | Report abuse

We had just about an even 2" of rain in our back deck rain gauge in N. Arlington near Marymount. Maybe a tiny bit less but between about 1.95 and 2 inches.

Posted by: Joel_M_Lane | March 7, 2011 10:49 AM | Report abuse

And to think on Saturday Ian was actually soliciting bribes from CWG posters to raise the "zero" SPI.

Posted by: FIREDRAGON47 | March 7, 2011 11:09 AM | Report abuse


When will the 70s return? I swear, I am suffering from the cold weather blues, and the deceiving sunshine isn’t helping.

Thank you.

Posted by: ThinkWarm | March 7, 2011 11:24 AM | Report abuse

12:45 AM:
As we slip slide backwards down the giant hill that is the final 2 miles of the 70 mile trek from National back to our home in Frederick County I mutter slowly to myself, "Snow Potential Index: 0 (→) - Nada, naught, zip, zilch, goose egg, bupkis."

1:40 AM:
As we waken a sister in law in Hagerstown begging a room for the night, I hum softly, "even the good get it wrong" (to tune, of course, of only the good die young).

10:00 AM:
Finally make it up hill to Frederick home. Measure 3.5" snow, take snow pics for Weather Gang, read philosophical treatises posted regarding true meaning of SPI, resolve to drive Subaru in future if there is even the slightest hint of snow.

11:00 AM:
Remind myself CWG is the best, decide to send this little post anyway, and to tell you all you have many, many devoted fans who live on the periphery of what you are defining as the Washington area, are within what is officially called the Washington-Baltimore metro area, and who often need a heads up as much as, and pay a higher price when one is not forthcoming, than do your closer in fans.

Posted by: dustygroundhog | March 7, 2011 11:25 AM | Report abuse

Near me, Campbell ES recorded 1.62" inches of rain, 0.04" after midnight this morning. Next big chance: Thursday [PM thunderstorm?].

No rain due for Mardi Gras festivities or Clarendon Ballroom dance tomorrow.

Posted by: Bombo47jea | March 7, 2011 11:42 AM | Report abuse

...oh to be at spgass' house! 5"! spgass, what did you get for commutageddon? was this your biggest snow of the season?

do you guys ever get tired of all our whining? i'm sure you thought the SPI would be a fun little thing for snow season, but it's turned into quite a source of controversy.

with all the hue and cry about the surprise accumulations 50 miles out (spgass excluded - and he's not complaining), and the oh-so-common mantra of "except north and west, where accumulations will be higher", maybe those guys need their very own SPI. though it would kill me to see their SPI 1 or 2 digits and maybe a + higher than ours inside the beltway, it might help.

maybe eric's idea about the 1 or 2 might work, or, the definition of SPI could include the idea that "north and west" is automatically 1 digit higher than the posted SPI.

and, finally, i've always thought that unless there's no precip at all coming and temps aren't getting anywhere as low as 35 degrees, the SPI shouldn't be "0".

CWG, bombo, anybody,
is there any way of accessing the weatherbug data for yesterday (w/o "signing up" or upgrading)?

"normal" high for today is 53 degrees. we're not "due" for 70s until late april:

Posted by: walter-in-fallschurch | March 7, 2011 12:03 PM | Report abuse


Thanks for the comment. We'll try to keep a closer eye on the areas you mention and regret our forecast missed the snow out there last night. But we probably won't forecast for conditions as far out as Hagerstown. Because the area is so big, we usually have to make the cut around Frederick and Leesburg on the west and northwest side.

Posted by: Jason-CapitalWeatherGang | March 7, 2011 12:15 PM | Report abuse

Thank you, Walter, but I am so hoping that a few 70-degree days will appear in March like they did in February.

Posted by: ThinkWarm | March 7, 2011 12:17 PM | Report abuse

CWG can't have it both ways. Yesterday, they were saying, "well, we only predict the rainfall from inside the beltway..." However, I seem to recall that when a much hyped snow event didn't materialize, they were all, "well, Bristow did report a few inches..."

Posted by: jojo2008 | March 7, 2011 12:19 PM | Report abuse

depends on what you mean by "no snow accumulated inside the beltway" - my back deck and stairs - inside the beltway - were covered in snow last night resulting in a slippery sheet of 1/4" of ice by this morning - nothing lasted on the grass or walkways...

Posted by: manatt | March 7, 2011 12:21 PM | Report abuse

An unexpected 4 inches of snow just east of Front Royal, with winds, and temps down into the 20's overnight (had been 54 at 11am Sunday). The snow started ~ 5pm. 2" came by 10p; the other 2" sometime after that. And all with a nice coating of ice underneath thanks to the earlier heavy rain.

It is 36 here at 1230 as I write this. Doubt it will get to 40 today, instead of the Sunday forecast of 51 by several sites.

Just wanted to share what was happening in my part of the area. Glad I am not the one trying to forecast all of this :)

Posted by: eds_kevinmjohnson | March 7, 2011 12:24 PM | Report abuse


Don't doubt there was some re-freezing of precip in spots but there were no reports of accumulating snow inside the beltway by the National Weather Service.

Posted by: Jason-CapitalWeatherGang | March 7, 2011 1:06 PM | Report abuse


We're not trying to have it both ways at all. While we probably need to state this very clearly somewhere (to point people to), we've said numerous times before our forecasting area includes Loudoun, Frederick and Fauquier counties on the west side and Calvert, St. Charles, and Mary's county on the east side. And we were very clear in what we were forecasting yesterday: "The heaviest amounts likely stay just to the west - e.g., Frederick, Hagerstown, Front Royal. But amounts in and around the District should still be near 1" or in the 1-1.5" range, with locally heavier amounts possible especially in the western suburbs." That was pretty much correct. We did NOT say we only forecast rain inside the beltway as you suggest.

Posted by: Jason-CapitalWeatherGang | March 7, 2011 1:15 PM | Report abuse

What's abundantly clear from this discussion is that we need to post a map of what CWG's forecast area is, and very clearly define what area the SPI covers (we had always based on the region inside the beltway - but didn't state the clearly enough)

Posted by: Jason-CapitalWeatherGang | March 7, 2011 1:21 PM | Report abuse

my aunt lives in muskogee oklahoma, which is south and west of dc. your forecasts are almost always totally wrong for her.

Posted by: walter-in-fallschurch | March 7, 2011 1:28 PM | Report abuse

FIREDRAGON47, it *almost* worked.

Posted by: Ian-CapitalWeatherGang | March 7, 2011 1:33 PM | Report abuse

Maybe you can avoid the controversy of the SPI if you get rid of the word "accumulating". After all it is called the Snow Potential Index (SPI), not the Accumulating Snow Potential Index (ASPI). People just want to know if it's going to snow. They want to know if they will see some snow. You shouldn't get into the nitpicking telling people they didn't really see snow because all they got was a "trace", a "dusting", a "covering", etc.

As far as how many miles out the SPI applies to, that is more complicated. You say just the DC area, then you say 60 miles out, then you say 30 miles out, then you name just the counties that count, but then you say just inside the beltway, but those counties you named have both inside and outside the beltway parts - Fairfax County for example. So not sure what the final answer should be for that.

Posted by: rwalker66 | March 7, 2011 1:56 PM | Report abuse

I agree with rwalker66: get rid of "accumulating" in the SPI. Who cares? The point is, "Will I see some snow or not?" Also, I've never understood why the beltway is used as any kind of boundary for anything weather-related. (TV mets use it all the time.) That makes no sense.

Posted by: petworthlad | March 7, 2011 2:05 PM | Report abuse

@walter-in-falls-church: "jason,
my aunt lives in muskogee oklahoma, which is south and west of dc. your forecasts are almost always totally wrong for her."

I'm just sorry that I didn't see any of the flakes - who cares if they were unexpected? That's the joy of life (at times or not re: @dustygroundhog).

Posted by: kygurl94 | March 7, 2011 2:35 PM | Report abuse

enough on the SPI...what about the 18th?

Posted by: manassasmissy | March 7, 2011 2:45 PM | Report abuse

I had a trace of snow near white flint mail (about 1 to 2 miles outside the beltway) in Montgomery county. Easy to miss, but they was a little snow on the grass, deck, and cars

Posted by: mciaram1 | March 7, 2011 2:57 PM | Report abuse

I personally liked this post on Facebook page ( "If it don't stick, we must acquit!"

Posted by: Jason-CapitalWeatherGang | March 7, 2011 3:08 PM | Report abuse

Many years ago, when Bob Ryan was asked about the difficult job of forecasting the weather for the DC area, he made a comment about how it would be too boring to be a meteorologist in Phoenix. I was thinking about the "lack of boring" as I saw the rain ebb, then increase in intensity as it starting sleeting and then snowing last night in Arlington.

Always enjoy your take on the conditions!

Posted by: busher | March 7, 2011 8:45 PM | Report abuse

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