Bracing for the snow

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Jason Samenow
Capital Weather Gang
Friday, December 18, 2009; 1:00 PM

The biggest Washington-area snowstorm to hit the area in years is about to become a reality, with 8-16 or more inches likely for the metro region.

The Capital Weather Gang estimates that the snow will begin between 9 p.m. and midnight from south (i.e., Prince William, Fauquier counties) to north (e.g., northern Montgomery and Howard counties) but that a little earlier or later wouldn't surprise the forecasters. It should start accumulating rather quickly because the ground is cold and it is expected that the intensity of the snow will increase steadily once it gets going.

The storm will have major impacts along the I-95 corridor this weekend, significantly impacting travel from Richmond to Boston.

Jason Samenow, chief meteorologist with the Capital Weather Gang, was online Friday, Dec. 18, at 1 p.m. ET with the latest weather information including anticipated accumulation, a snow timeline, FAQs and other information to help you with your weekend plans.

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Jason Samenow: Good afternoon. Thanks for joining me today.

All of the ingredients seem to be coming together for significant, possibly historic, snowfall for the metro region. A foot of snow is possible in many spots -- with some areas receiving a foot and half or so -- especially to the south and east.

This storm will be an "I-95 special" with heavy snow from Richmond to Boston... though our area may see some of the heaviest amounts.

Your questions are welcome...

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Northern Virginia: I'm a college professor giving a final exam tonight ending at 10:00. What are the chances the snow will start before then? Should I change the exam to a take-home for everyone's safety?

Jason Samenow: The snow will likely start between 7 p.m. and midnight. It's possible some roads may start getting slick by 9 or 10 p.m. Tough call... I'd caution your students to drive carefully.

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Alexandria, Va.: How does this storm compare to the big storm we got President's Day weekend back in 2003?

Jason Samenow: I don't expect this storm to have the duration of the 2003 storm and totals probably won't be as high. But it may come close.

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Foggy Bottom: Hi,

Thanks for taking my question. I have a 10:30 a.m. flight out of DCA tomorrow. I'm more than a little concerned about getting out of here (I have a cruise leaving Fort Lauderdale on Sunday). Do you think I should take my chances and try to fly, or would I be better off driving to Florida after work tonight?

Jason Samenow: I think area airports will be a mess tomorrow morning with long delay and cancellations. Would not surprise me if airports closed for brief periods. A drive to Florida tonight doesn't sound like fun -- you'd probably encounter snow and then heavy rain as you head down I-95.

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Alexandria, Va.: I am a Skywarn spotter. What are the latest estimated snowfall amounts for Alexandria and Eastern Fairfax County for this storm?

Jason Samenow: We're calling for 8-16" with some locally heavier amounts possible across much of the metro area. There may be a heavier strip of 10-18"+ south and east of the District.

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Alexandria, Va.: Will this storm rank in the top 3 historically for the area? How realistic is a 15+ inch snowfall event?

Jason Samenow: I'd give this 25% odds of making the top 5. Here's a link with our top 15 snow storms: http://www.erh.noaa.gov/lwx/Historic_Events/snohist.htm

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Arlington, Va.: How soon would you know whether that 10-18" area would wind up including the D.C./Arlington area? Or is 8-16" pretty much solid now?

Jason Samenow: There's still a chance we'll tweak snowfall amounts one more time. Based on the latest data I've seen, the direction would be most likely be upwards -- but modestly if we do it.

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Washington, D.C.: THUNDERSNOW? This is going to be awesome!

Jason Samenow: Big, energetic East Coast storms that originate from Gulf of Mexico can and do produce thundersnow. It's not a sure bet with this storm, but if it happens, we'd see some crazy snowfall rates... (2-3" per hour).

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Washington, D.C.: I am a graduate student trying to get home to Richmond after my final exam, scheduled for 4 p.m. Saturday. What are the chances I'll be able to drive down I-95 on Sunday?

Jason Samenow: In the past, road crews have been pretty good about prioritizing the major interstates. So Sunday travel along I-95 should be passable I would think. Getting out of your neighborhood might be a different story...

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Potomac, Md.: What's D.C.'s record December snowfall? Is it in danger?

Jason Samenow: 12" (from 1932) and yes. I'd give us at least 50/50 odds of breaking it.

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Fairfax Station, Va.: What are the variables with this storm. If it moves 20 miles east or west how will that impact the amount of snow we will receive?

RIP...

Jason Samenow: A jog to the east would probably put us closer to the low end of accumulation forecasts. A jog to the west would put us in record setting territory though it might wrap in some warmer air causing sleet to mix along and east of I-95. The snow bulls eye would shift to the north and west suburbs.

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Silver Spring, Md.: Channel 9 is now forecasting 1 - 2 feet -- is this even possible?

Jason Samenow: Yes. That's a reasonable forecast. We're not that bullish...but it's plausible.

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Washington, D.C.: Thanks for taking my question. Looking at the current satellite images there seems to be a lot of rain west and south. As a snow lover this concerns me. Is the storm behaving differently than modeled or is the air over the mid-Atlantic cold enough to keep everything as snow.

Jason Samenow: The cold air is in place. I don't think we have to worry about rain with this one. As I mentioned in an earlier response, wouldn't totally shock me to see some sleet mix in along and east of I-95 -- especially if the storm tracks a little further to the west.

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Washington, D.C.: CWG is a great service! All my friends on the East Coast are calling and e-mailing to see if the team can provide local coverage for NYC, Philly, etc. More franchises after this weekend?

Jason Samenow: Thanks so much. Really appreciate the nice feedback. We have our work cut out for us covering the complexities of D.C. weather but who knows...maybe one day we'll branch out.

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Hyattsville, Md.: Hello meteorologists,

I see that temperatures will be hovering around freezing, raising the possibility that any precipitation will partially melt and later re-freeze. What are the chances that this snowfall is going to "ice over" on roads and driveways after accumulating, making it impossible to shovel?

Jason Samenow: Because temperatures have warmed to just above freezing today, snow that initially falls may melt and re-freeze...causing very slick conditions at the onset of this event. I would be very careful traveling after 9 or 10 p.m. tonight. This will also make shoveling a little more difficult...the snow that first falls will not be powdery so you can just sweep it away.

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Silver Spring, Md.: Do you think that there will be any canceled school days this week for my kids in Montgomery County?

Jason Samenow: Our School Cast is in the 2.5 to 3 apple range for Monday...which means there's a pretty good chance of a snow day.

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Fairfax, Va.: We've seen a lot of discussion of the NAM "overperforming" and predicting too much precip. Can you explain what the NAM is designed for? (the range of conditions that it's good for, and where it falls apart)

Jason Samenow: The North American Model (or NAM) has been simulating some crazy totals. If it's correct, it would mean the high end of our snowfall predictions might be conservative. It's a pretty good short range model and has performed well in the past. But it does have a "wet" bias meaning it tends to overdo the amount of precipitation it simulates. I think it's useful as a short-range model in most circumstances, but it's a good idea to slightly discount the totals it puts out.

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I'm Dreaming Of A...: What is the likelihood that temperatures will remain low enough to keep the snow around for the 25th?

Jason Samenow: If any year shows promise for a White Christmas, it's this year. We're going to have another storm to track next week... details are very fuzzy right now though...could be a mix or even rain.

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Washington, D.C.: I've been looking at the weather radar on weather.com and the leading edge is now just outside Charlottesville/Staunton, Va., and the storm is moving at a reasonable clip. I think the snow will at least start about 5 p.m. Most predictions say somewhat later. What am I missing, if anything?

Jason Samenow: We posted earlier /voices.washingtonpost.com/capitalweathergang/2009/12/update_storm_could_effect_frid.html> about the possibility of a start time as soon as 5 p.m. -- especially in the southern suburbs. However, more likely, some of the precipitation that first arrives will dry out... so 7 - 11 p.m. appears to be the most likely start time.

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Silver Spring, Md.: I haven't lived in D.C. long enough to see a storm like this. I work late night and need to take a taxi to get home. How bad will it be at 4 a.m. tomorrow morning and how does weather like this affect how many cabs are out?

Jason Samenow: I think conditions will be pretty bad at 4 a.m. tomorrow. Be careful out there...

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Heading West: We were going to leave early early Sunday morning for Ohio, should we postpone a day? I heard blowing and drifting Sunday, and maybe the highways will be slick?

Jason Samenow: There could be some blowing snow early Sunday...but the worst will be over. If you're used to driving in snow, it might be doable.

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Wheaton, Md.: Hi Jason, We have a flight out of BWI at 10 a.m. on Sunday and a place to stay 10 minutes from there if we need to. If you were us, when would you head north? Thanks!

Jason Samenow: I would head north if it gives you peace of mind.

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Dulles to D.C.: Regardless of the NAMs propensity to spit out too much qpf in the past, why was it looked at and pretty much ignored even inside of 24 hours? It nailed the liquid amounts for the Dec 5 event and looks to be on track again for tomorrow. It was extremely persistent with a historical event beginning on Wed night and didn't let up at all.

Jason Samenow: We certainly don't ignore it.

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Alexandria, Va.: Hi Gang,

Any thoughts on what the FedCast might look like if we reach the higher end of the projected snowfall range?

Jason Samenow: FedCast (our scale for predicting Federal government operating status, http://blog.washingtonpost.com/capitalweathergang/2008/01/frequent_questions.html#what_do_the_stormcast) is low. Crews will have had 36 hours to clean from the storm by Monday morning.

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Arlington, Va.: I know what I'm going to do: my holiday baking! I visited my local supermarket last night, and stocked up on eggs, sugar, flour, nuts, butter, and vanilla extract. I'll be safe and sound in a nice warm kitchen while everyone else is unprepared.

Jason Samenow: Snowstorms are a great excuse to bake and enjoy hearty food.

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Fairfax, Va.: What is a negative tilt of the jet stream? And how does that effect the storm?

Jason Samenow: When the jet stream becomes negatively tilted, it's an indication of a intensifying storm reaching maturity. Here's a good link: http://www.theweatherprediction.com/habyhints/127/

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washingtonpost.com: Capital Weather Gang: Frequent Questions

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Stone Ridge, Va.: My family and I are planning on attending a family party in Winchester, Va., tomorrow. Will they get hit as hard at the D.C./metro area? Typically with these type of storms, how bad do the mountains get between here and Frederick County, Va.?

Jason Samenow: Winchester should still make the western edge of some of the heavier snows. 6-12" out there is a decent bet -- unless the storm jogs east (low odds).

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Columbia, Md.: How many new Twitter followers have you tracked today? Is that a record? LET IT SNOW!

Jason Samenow: Our Twitter feed is going nuts! Join us. We're @capitalweather

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washingtonpost.com: What is a "Negatively Tilted Trough"?

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Help me!: What does bullish mean regarding forecasts?

Jason Samenow: Bullish means we think accumulations may be high (in the same way when analysts are "bullish" about stocks, they think they're going up).

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Annandale, Va.: What is the chance of any air traffic moving in or out Saturday night?

Jason Samenow: We're getting a lot of questions about air travel. I think the worst time for air travel will be tomorrow morning and afternoon. The snow may start to taper some by evening which may allow some flights to take off... however, there will be backlogs because earlier flights will likely have been delayed or canceled.

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washingtonpost.com: Capitala Weather Gang on Twitter

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Arlington, Va.: I've seen predictions that snowfall will taper off Saturday evening/Sunday early morning. Any chance the storm could stall and stretch the duration of snowfall?

Jason Samenow: Not terribly likely, but some "wrap-around" snow showers behind the storm could linger into early Sunday morning. The heaviest snow should by Saturday evening.

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Washington, D.C.: I'm curious, are any of you on the "gang" over 30? You seem to be biased toward snow as a "fun" event, not taking into the account the problems it causes for the elderly and the handicapped. It's disheartening for those of us who will be house-bound because of a major snowstorm not to see our difficulties taken into account.

Jason Samenow: Thanks for your comment. While many of us enjoy a snowstorm because we're passionate about weather, we understand the challenges it poses for many people and hope the information we provide will help everyone prepare for this storm and remain safe.

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Washington, D.C.: I am a student taking a final at 4 p.m. tomorrow. How likely is it that the final will be canceled?

Jason Samenow: I think there's a decent chance your class will be canceled but be prepared just in case.

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Falls Church, Va.: My kids will want to go sledding tomorrow, but I saw one forecast that called for windy conditions. What kind of wind are we talking about? What do you think the wind chill will be tomorrow mid day?

BTW, You guys are the best.

Jason Samenow: Thanks for the nice words. Winds will probably pick up in the afternoon to about 15-20 mph (from the northeast)... with wind chills around 20.

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Alexandria, Va.: Will CWG be interested in any reader photos of the event. etc. ...?

Jason Samenow: Absolutely. Submit your photos to

Jason Samenow: http://www.washingtonpost.com/wp-dyn/content/gallery/2009/01/27/GA2009012701325.html

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Arlington, Va.: I have tickets to a kid's show at the Patriot Center, which starts at 10:30 a.m. What are the odds the roads will be safe enough to travel at that point (and to return at 12:00)?

Jason Samenow: That's going to be a pretty bad time to travel...moderate to heavy snow possible then.

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washingtonpost.com: Share snow pictures

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Downtown: What about AMTRAK? Will D.C. to NYC service likely be interrupted tomorrow morning?

Jason Samenow: I would imagine Amtrak will have some delays. I'm not sure what amount of snow would shut down their operations.

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Washington, D.C.: Any chance that this may be a good weekend to head out to W.Va., for some skiing/snowboarding? Or is the snow not extending that far west?

Jason Samenow: Those areas will get some accumulating snow but, you're right, the greatest amounts will be to the east.

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Flights: What do you think about flights landing tonight?

Jason Samenow: Flights tonight will probably be ok...but the later they come in, the higher the chance they'll be impacted (i.e. delayed).

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Anonymous: Jason, don't know how old you are but this has Feb(1979) written all over it. Yeah, I know no two storms are alike but this has thin stripe of convective thunder snow just west of the rain snow line, could set up in eastern P.G, A.A, St. Mary, Harf., Cecil counties. You would only need a 3-4 hour window of training with thunder snow to yield an extra 12" over areas just east and west of the "TRAIN"

Jason Samenow: I was only 3 in 1979... but I remember 1983, when there was thundersnow and very heavy snow rates. Agree that some favored locations will get hit really hard by this one.

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Arlington, Va.: So I have a birthday dinner reservation Saturday at 8 p.m. How clear will the roads in Arlington (Courthouse/Washington Blvd. specifically) be? Does Arlington do a good job clearing their roads?

Jason Samenow: I can't predict what road crews will be doing, but the snow will probably be decreasing in coverage and intensity around that time. Happy Birthday!

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Columbia, Md.: I really appreciate your professionalism and the enthusiasm you have for weather and forecasting. Keep up the great work!

Jason Samenow: Thanks so much. Stay safe during the storm.

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Winchester, Va.: With the temps that are forecast for Monday and Tuesday nights, is it safe to assume that whatever is not shoveled or plowed is going to become concrete?

Jason Samenow: Temps will be in the teens Monday and Tuesday night (maybe even some single digits out by you in Winchester)...so yes...any snow that melts during the day will re-freeze hard at night.

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Washington, D.C.: We moved our drive to Philadelphia from tomorrow morning to this evening. If we leave at 6, will be beat the snow?

What does the weather look like for our return trip Sunday at 1?

Jason Samenow: You should stay out ahead of the snow tonight for the most part. Leaving at 1 p.m. Sunday is a good plan if you can dig your car out. This will be a biggy for Philly too.

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Jason Samenow: Thanks so much for all of your questions. Sorry I couldn't get to all of you. Stay safe during the storm and visit the Capital Weather Gang blog for frequent updates at washingtonpost.com/capitalweathergang ... Follow us on twitter @capitalweather

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washingtonpost.com: Capital Weather Gang: Near or over a foot of snow realistic for some spots

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