Snow Monday, Rain Tuesday

Happy Valentine’s Day to all the ladies out there!
And to the guys, Happy Sunday After The Super Bowl.

It appears our “Squeeze winter into Four Weeks” bonanza is almost over, with one final hurrah. Models in very good agreement the potential for a major winter storm has evaporated, all this despite the fact I’m doing US Drought Monitor (a virtual lock for major storms, earthquakes, or furloughs). Nevertheless, 2-part storm coming, with snow on Monday and rain on Tuesday.

Monday’s snow will be courtesy of an approaching warm front. With plenty of cold air in place, this will start as snow everywhere during the morning commute and continue for much of the day. Most guidance shows 2-4″ before snow changes to freezing rain and rain during the afternoon and evening from south to north as a brisk south wind pulls in warmer air. The GFS is faster with the changeover and is more like 1-2″, while the NAM is a bit slower and heavier with the snow, and has some getting 4-6″, with even more out to the west of Frederick. By evening, it should all be rain or freezing rain.

Monday night, the warm front lifts north into PA and the pcp shifts to our north along with it. Any lingering freezing rain will change to plain cold rain east of the mountains, while freezing rain could linger out in the colder mountain valleys.

Tuesday, a low will ride up the mountains, putting us on the warm eastern side of the storm. With no high to our north to lock in the cold air and a storm track to our west that puts us on the warm eastern side of the low, this is an all-rain event… and potentially heavy rain at that. Tuesday’s temperatures are expected to climb into the 40s east of the mountains, and 50s near the bay. Even west of the mountains we should see temps in the upper 30s and lower 40s.

After that, I do believe we’re done for a while. Looks like we squeezed a winter’s worth of snow into 4 weeks… all this after the warmest December on record.

More Fun; Blame Me!

We’ve got a few small events coming, along with some bitter cold over the weekend, and then a potential for a higher-impact winter storm (and potential change over to rain) for next week… the last of which you can blame me.

Rehashing the Tues Snow, one last time:
First, amazing to see the difference with the last storm between the “high-elevation” Piedmont snowfall and areas south and east, which got next to nothing… bummed it wasn’t more widespread. I have noticed over recent years the Piedmont “influence” has become more pronounced as we get more and more of these marginal snow events, with temperatures often hovering in the lower 30s. I did some looking a while back, and I discovered what I think is the main answer for the difference. Yeah, the Piedmont is farther inland, more north and west, and it’s higher up. But being 500-1000′ up doesn’t make that big a deal with respect to the temperature… you lose ~ 3.5°F for every 1000′ you go up in a saturated atmosphere (one producing rain and snow)… so really, the Piedmont elevation will shave roughly 2-3°F. I think it has more to do with what I read from the folks up out of SUNY/Albany: falling snow typically will melt after descending ~1,000′ through above-freezing temperatures (assuming it’s not 60°F, of course). Basically, the Piedmont elevation allows these areas to “meet” the snow before it finishes melting in marginal events. At least that’s my take. The slightly colder air and more north/west locale add to the chances for maintaining snow, but for this last event, it really seemed almost entirely contingent on elevation once you got north of VA/DC.

Next up: Friday. Cold front will blast through, producing a period of light snow or snow showers as it approached Friday eve. Not a big deal, but if you’re out on Friday be aware we could see snow coat the roads.

Saturday: Some models show a band of lake-effect snow showers setting up over nrn MD as the wind trajectory coming into nrn potions of the region comes off Lake Huron, Lake Superior, and Lake Erie. We’ve seen this before… if you can line up these 3 lakes in our upwind trajectory, we can see a band of accumulating lake-effect snow. Given (1) how cold the air rushing in is and (2) the lakes aren’t frozen, this is worth mentioning. Could be enough to even coat the ground (though with the wind and increasingly cold, dry air, it won’t last)

Monday/Tuesday: Blame me. I have the U.S. Drought Monitor duty over the next 2 weeks. In the past, I have had a litany of things interrupt my shift, including snow storms (more than once), a tropical storm, a furlough, an earthquake, and a Papal visit. That leaves pyroclastic flow and earth’s magnetic field swap on the list… or, just tap into my greatest hits. Naturally, models show a storm approaching from the southwest Monday, then moving overhead (or nearby) Monday night into Tuesday. This would get us a period of moderate snow Monday into Monday eve, then changing to rain Tuesday. At least that’s how this looks now. The models are targeting the heavy snow more to the west as the low is expected to track more inland, but some snow/ice appear likely before the changeover, and it could be a moderate to heavy before the change over.

Again, this last one’s all on me. Sorry!

Snowfall Prediction and Snow-to-Liquid Ratios

I was asked a while back how we predict snowfall… and I totally forgot to answer it. I’ll try and address it here. The answer is simple yet complex.

We get the liquid total, then apply “ratios” of expected snow-to-liquid to get a final amount. In our area, that ratio is usually around 10::1, so you just take the liquid pcp and x10… to get snowfall in inches. If it’s colder, you use a higher ratio… like 12::1 to 14::1. Some of our fluffiest snows we’ve gotten near 30::1, but these super-fluffy events are by nature usually small and not laden with moisture. To get a lot of moisture, you need to tap the Gulf and/or Atlantic, and those air masses are warmer and will consequently have lower ratios (a more normal 10::1, maybe 12::1) but more pcp. We rarely will get a foot of powdery snow. It’s happened, but not often. Sometimes, it’s the opposite… where our ratio drops to like 8::1 or 6::1, if it’s relatively mild yet still cold enough for snow.

Interesting side note: I looked up our liquid pcp estimate up here for Feb 9th, and it’s right around 0.40″… and I measured 4.0″ of snow, so the climo 10::1 held. This was likely more a function of the cold air aloft, cos we were in the upper 20s and lowers 30s for much of it.

Speaking of aloft…. It gets more complicated around here, where warm air aloft will lower your ratio by altering the snow structure where it’s created. It can also flat out kill the snow entirely by causing it to fall as sleet, freezing rain, or rain. I’ve seen cases where it was bitter cold at the surface but warmer aloft, and we had a dry but dense snow. In 2003, we had temperatures in the teens at the surface… but readings aloft were in the upper 20s and lower 30s, so despite the bitter cold at the ground, we ended up with a dry snow that was quite dense… and I believe our ratio was right around 10::1 despite the arctic air mass at the surface.

And, from my co-worker Brad Rippey…

The good folks at St. Louis University have studied thousands of frozen-precipitation events over the past several decades and have put together some cool graphics. I’ll only bore you with one of them. J

The mean snow-to-liquid ratio in the D.C. area is 10.5 to 1, but as Eric said we might occasionally see 30:1 or higher when the snow is very powdery.

A lot of our low-ratio events tend to be mixed snow and sleet – we call it a “slurpee.” If I recall, a pure sleet event tends to average around 3:1.

Snow-to-Liquid Ratios for DC/MD via LSU

For future reference, here’s the list of all available WFO snow-liquid graphics via St. Louis University (and CIPS):
Snow to Liquid Ratios

It’s really interesting to compare some of the coastal and inland locations.

Where the snow fell: Piedmont-centric

As far as where this snow fell, it was a Piedmont-centric event, with most folks in the Piedmont getting a solid 3 to 6″ of snow. The rub was outside of the Piedmont, where there was little if any. I got some notes from folks that already had 4″ early in the day, and I’ll bet those folks got 6 to 7″, especially in the highest Piedmont locales well north and west of town. Here in Jacksonville I measured a solid 4″ on a table top, probably a hair more. I also noticed a measurement submitted down the street of 2.5″… and I’m not sure how they got that number. If you used anything on the ground, your total was lower… as I noticed clearing the driveway. But this was an easy-to-measure table-top snow event. No wind, and a very pretty snow to boot. And if anything, it was hair over 4. For anyone interested, here’s the map of reports from the NWS:
Snow Map from the NWS for Feb 9

Also, there’s another batch of light, perhaps even moderate snow on the PA border dropping southeast. This could dust the roads again tonight, though I really hope not. I need schools to open on time tomorrow. You can see the snow as of Tues eve dropping slowly southeast on the MD-PA line.
Local NWS Radar Loop (via AccuWeather)

In closing, while not what I would call a “hit”, not a total miss either. For some, the original idea was the accurate one. But for others, not so much… as the folks that got little to no snow covered a much larger area than I thought.

Wow! GFS vs NAM

A little side note for you. Two of the main models we use for forecasting (there are more, but these are two of the big hitters) are the GFS and NAM, both run by the U.S. NWS. We can get point-specific forecast for airports (which we then extrapolate or apply to the surrounding area), and the latest from these two for BWI show what we’re up against.

The models give a precipitation forecast for the airport, but in liquid equivalent (we’ll assume a 10::1 ratio for snow, so you take the liquid as snow and multiply by 10 to get snowfall)… here are the details of the two for BWI.

Total Precip: 0.267″
Pcp as Rain: 0.120″
Pcp as Snow: 0.145″ (1-2″ of snow)

Total Precip: 0.648″
Pcp as Rain: 0.0″
Pcp as Snow: 0.648″ (6-7″ of snow)
* Some — but not much — of the GFS snow is with a light snow it has forecast for the end of next week, which is less than 0.1″ (1.0″ of snow)

Wow. Talk about a difference. The NAM was what rolled in first last night, and what made me pull the plug. If the GFS is right, that was a bad call on my part. Huh… curious to see where this goes. Obviously, the NWS didn’t like what they were seeing either cos they pulled the plug too. How ironic would it be if the GFS verified…

GFS pcp fcst for BWI

Where it stands – Tues, Feb 9th

Good morning,

The storm is not panning out to be what I/we thought, but there may still be a small area that gets more than 3″ of snow. For most looking for a decent snow event, this will be disappointing… with 1-3″ primarily on grassy surfaces, and the bulk of it north of DC. However, some of the late-night models went back to the idea of locally heavy snow, but they have it on an almost microscopic scale when looking at this globally (which we have to do). At this point, just keep an eye on radar…

You can see the snow beginning to “fill in” as the coastal low takes over, and the heaviest right now is north of DC… and west of Baltimore, though the snow in the Baltimore metro area is beginning to fill in as well. Again, I think unless something crazy happens at the last minute, 1-3″ north of DC, but there may still be some folks who get locally more if the coastal low gets going.

Needless to say, this has been a frustrating forecast for you and me both, but at least we’re getting something.

Hope you all have a good day.

Northern Snow, Sketchy Farther South

Got a few minutes, and no major changes. Before I continue, I will try my best to address most — if not all — on this list, but there is going to be a wide variety of snowfall totals from this storm. Looks like “3 zones” of snowfall… heaviest in the north, a transition zone, then a southern area that gets a fair amount of rain.

Super Quick Take:

Northern areas (from Howard County/BWI and points north… generally speaking, the I-70 and 695 corridor north into srn PA): Looks like mostly snow, with accumulations in the 4 to 8″ range. Could be higher totals if the coastal low sets up in the right spot. Pcp starting as rain this eve, changing over to wet snow by midnight. Snow could be heavy Tuesday.

Central areas (east and north of the DC Beltway to I-70): This area will likely see mostly snow as well (also rain at the onset this eve), but the band of heaviest snow is progged by the models to set up a bit farther north. This could change… but for now, I’ll put this area in a 2-5″ forecast, with the higher totals farther north.

Southern areas (DC South): Rain will cut into snowfall totals, likely limiting these locales to an inch or two of slushy wet snow. But I’ll add this “southern area” may get more snow, esp around DC and immediate burbs, if some of the models are correct.

How it’s gonna play out:

Enjoy the bright blue skies, cos today may likely be the last of the sun we’ll see for the rest of the workweek. We’re in between systems, with a coastal low off of New England while a broad “swirl” of unsettled weather dives southeast from the Great Lakes. This swirl is courtesy of a deepening trof (a southward dip in the jet stream), and what makes this forecast even messier is the number of smaller disturbances (look like eddies) embedded within the swirl.

Non-met/non-technical folks, skip this italicized part…
Geeky sidebar: If you want to see what this swirl looks like on the satellite, check out the water vapor loop; this image/loop shows the movement of the air aloft via water vapor (moisture content) ~ 15,000′ up:

If you’re not really into weather and just want a forecast, don’t bother.

Back to the guts.
This eve (Monday eve)… rain changing to wet snow, but not a big deal. First round of pcp will arrive from the west and southwest, starting as rain in most areas before changing to wet snow in northern MD ~ 10 pm, north of DC ~ midnight, and changing to snow in southern areas in the wee hours of the morning. This snow is not the main event, and when we all get up tomorrow morning, it may not be all that bad out. Dare I say, we may very well wake up to very drivable conditions… wet snow, but treated roads will likely be fine.

Tuesday, models agree a new coastal low will spin up, causing snow to intensify from mid-day Tuesday into Tuesday evening. It’s during this time that the conditions could deteriorate rapidly. Models vary on where the rain/snow line sets up Tuesday; the NAM changes snow back to rain during the day in DC and burbs, while the Canadian has moderate to heavy wet snow falling all the way down into DC during the day. Models also show potential for thundersnow… a classic side effect of a rapidly developing coastal low. Again, I suspect the very drivable conditions Tues morning will become a nightmare in the afternoon if the models are correct. Snow, heavy at times, north of DC. If the Canadian is right, this will include all of DC. If the NAM is right, it’ll be north of DC. Both models did well during the blizzard, so not 100% sure.

Tuesday night: Models have the coastal low departing, but there’s enough energy still left over (those embedded spinners within the overall swirl I mentioned) that snow lingers, especially in the north, until Wednesday morning.

I see no point in second guessing the NWS forecast for this storm, as I think they’ve got a very good handle on it… in fact, when I saw their forecast, I was like “That’s exactly what I was thinking!” … 4-8″ in the north, 2-5″ in the central, 1-2″ in the south.
I’ll share the NWS snowfall forecast links here.

Expected snowfall:
Max snowfall (if things break just right):

I’m sure this will change. Always does… but for now, that’ll do it.


Tuesday Target

Good eve, Happy Super Bowl Sunday… good game so far.
And as the first half ends, THAT’s the Michael Oher I remember… though I have nothing against the guy. Wish him the best.

Doing this now cos Mondays are usually a busy day at the office… and this’ll give you and me a springboard… a starting point. Models in agreement we will see snow Monday night into Tuesday, but are targeting central and northern MD, with areas from DC south more marginal due to warmer temperatures and storm track. Models also seem split… with a group in the 1-3″ range, and another in a heavier 4-10″ range. The threat for heaviest snow is primarily across northern and northeastern MD. Again, areas near and south of DC may end up on the low side, with maybe nothing. Snow — possibly starting as rain — will arrive Monday night, and continue into Tuesday. Temperatures in nrn MD will be in the upper 20s and lower 30s, while data shows areas south of Baltimore will be mostly near or above freezing.

Details: Wish there was an easy way to describe what’s coming. It involves vort maxes, divergence aloft, steep lapse rates, deformation zones, and coastal redevelopment. The kind of stuff I’m sure you all are just dying to hear me wax poetic about. But I’ll spare you. If I tried to describe the setup, this email would easily approach Thesis length… and I don’t have a Masters or PhD, so let’s not go there. In plain English, a strong upper-air low will swing over and to our south on Tuesday, putting us in a good spot for pcp. Temperatures support mostly snow, though areas south of Baltimore may wrestle with rain entering the equation. Models also show the upper level energy creating a coastal low on Tuesday; if, where, and when this happens will prove vital in our final storm tally. The other item of note is that this will not be the uniform region-wide snow storm we saw 2+ weeks ago. Rather, there will likely be a focus of snow in northern areas, while areas farther south may not see much.

As far as snowfall… I was chatting with a fellow weather buff (Mike C) and the number/range that came popped to mind was 3-6″, 4-8″ for MD Piedmont… my first “gut” on this, but certainly not the high-confidence forecast of our blizzard. For now, I think much of central and nrn MD… areas along and north of I-70/I-695/I-95 will be in the 3 to 8″ range, with the highest totals in the north and northeastern reaches of this northern region. In central MD south toward DC, we may be looking at 1-2″, perhaps 3″, but this area will be tough as a slight tweak in storm track will make a world of difference. I’ll try and fine tune this for you Monday. I’m thinking 1-3″ as a general rule for areas west of I-95 and north of DC to the Baltimore, then – as mentioned previously – amounts climb as you head north from there. Not a big storm, but could prove entertaining if we end up getting the heavier snowfall. On the flip side, if the coastal low disappoints, then this will be 1-2″, maybe 3.

Nothing concrete for you yet, but then again, it wouldn’t be me if I did otherwise!

Addendum – Models, for true gluttons:

NAM (the winner in the Blizzard of ’16): 2-6″ from I-70/695 north, 6″+ in ne MD, 1-2″ in DC metro.
GFS (also did well on the blizzard): 5-10″ across central and nrn MD, 10″+ in ne MD, 2-6″ in DC Burbs.
Canadian: 1-4″ across the entire region, including DC
European: 1-3″ across the entire region, including DC
UKMET: 1-2″, with the heavier snow up in PA

Other outlier models:
Japan’s GSM: 2-6″ across the entire region, including DC
Germany’s ICON model: 2-6″ across the entire region, including DC, with a max of 5-10″ over west-central and north-central MD
France’s ARPEGE Model: 2-5″ in Baltimore metro, 1-2″ near DC

Today’s Close Call, Early-Week Issues

A bit of a special Sunday update. I mentioned the close call for Sunday… and it’s close, but is going to pass to our east. Check out the satellite look from Sunday showing how close this coastal low is:

The spinner off the Southeast Coast is our close call, but it’s being kicked out to sea by the spinner over the upper Midwest. Some rain and wet snow could clip the Eastern Shore as this thing swing by tonight, but it looks to be nothing more than a cloud maker for most of us.

The spinner over the upper Midwest is expected to drop southeast and once it gets at or south of the same latitude of the coastal low, it may actually start to pull moisture back to the west on Monday. But as the Midwest feature gets closer to us, a bunch of factors will lead to increasing chances for snow Monday night thru Tuesday night. I’ll get into that later, but it appears we’ve got accumulating snow coming, starting Monday night and lasting perhaps into Wednesday morning.

Models vary on who gets what… with the greatest likelihood of a decent snow from DC north into srn PA and srn New England, including much of central and northern MD. I won’t get into detail on model snow amounts just yet… but they’ve got everything from light snow (1-3″) to quite heavy (8-14″) I’d say lets get past this coastal low, and then I’ll get back to you this eve.