Updated Below (most recent 5:00pm Saturday)
Happy Friday everyone!
It won't be long until we're back into the swing of things (in 2013!) But before then we still have a few more opportunities to get some winter white before the year closes out.
Today will begin with some sunshine, fleeting as it may be. High pressure scrapes through the region, keeping frosty temperatures and the threat for icing once again front-and-center this morning.
|HPC - Surface Map - Friday PM|
Clouds will be overspreading in the afternoon, but the precip should hold off until after dark. Warm air will be overrunning from the south, with very little forcing from the north to keep the cold air in place initially. Normally, that doesn't make for a promising wintry appearance, but we do at least have some residual cold in place inside the dashed blue line above. That orange dashed line will end up being the battlezone. It's not the only game in town either-- there will be another system approaching right at the turn of the new year.
Storm System #1 -- Saturday/Sunday
The low pressure center situated above in the deep south will move toward the Atlantic coast, but the inverted trough (orange dashed line) will be focusing moisture right up the spine of the Appalachians. Here's how the models have it at the peak of the event...
|NAM - Saturday AM||GFS - Saturday AM||CMC - Saturday AM|
There is pretty good agreement about the max moisture moving through Saturday morning, and also the max clash between cold and warm air. One good thing going for us is that we'll be on the cold side of the 850mb 0°C line, but not quite into the -2°C and colder temperatures at that level. In a situation like the one indicated above, icing is not as much of a threat. Instead, it's a rain/snow thing. Despite temperatures being below freezing aloft, they will be marginal at the surface. This means we'll have to analyze the atmosphere in throughout the layers to see what we'll end up with. A good approximation of precipitation type can be done through looking at what are called "critical thicknesses"...
-----<tangent> "Critical Thicknesses", feel free to skip if uninterested-----
When modeling the atmosphere using physics, the math works out such that the thickness of the atmosphere through whatever column size is a good correlation to the average temperature through that column. Warmer air is less dense, so that would make a thicker column than that of a colder regime. This atmospheric thickness can be examined and even projected. Meteorologists over time have identified 'critical thickness' values for different slices of the atmosphere that more-often-than-not give us end up representing the rain/snow line for a particular model solution. The most famous one is the surface-to-500mb thickness value. For a location at the surface, that comes out to be 540dm (decameters, or 5400m). If the atmospheric column is "thinner" than this value, more-often-than-not it represents snowfall (if there isn't some crazy ice-storm element in play, etc.) For elevated areas, this critical thickness value is larger (543dm, for example, can mean snow above 1500-feet, because that lower portion of the column is technically below-ground and discounted--- believe it or not the 'atmosphere' as it relates to physics can sometimes go below the ground level. It's weird, but it works). The 540-line is not the only slice available to meteorologists. Several other levels can be examined in tandem to give the best results.
In addition to this, "ensemble" modeling can further refine the forecast. Essentially it's running the same model many times over, varying the initial conditions ever so slightly to account for any sort of 'butterfly effect' perturbation. This gives the forecaster an idea of possible divergent clusters of results or a concentration around an average. Let's take a look at the ensemble projection of "critical thicknesses" for this Saturday morning time-frame.
|Short-Range Ensembles - "Critical Thicknesses" - Saturday AM|
And voila! A complicated scenario. The indications are that we'll be above freezing in most locations, but inside "critical thickness" territory for snowfall north and west of Corridor-G and along I-79. To me, this means we'll see a rain/snow mix along that red-dashed line until we hit the mountains and their sub-freezing temperatures, snow showers in Ohio and interior WV, and rain south of the red-dashed line (except above 1500' where the critical thickness lines have a little more leeway). However, within the region where critical thicknesses indicate snow but with temperatures above 32° at the surface, we're going to have a problem of melting-- whether that means once the snow hits the ground or even in the last few hundred feet before impact. The snow may need to be a little steadier than the light stuff in order to stick on surfaces other than grass/bridges/overpasses, etc.
Here's how the Short-range Ensembles (SREF) handle precipitation type given the above imagery...
|SREF - Precipitation Type Probabilities - Saturday AM|
So you can see where the battle-line will be drawn. The usual spots near I-64 will be flirting with the rain/snow line during the maximum moisture plume, then trend colder later in the day, and in particular early Sunday. Note: Icing is not modeled to be an issue at all, as this is a different animal than the last storm, with no powerful influx of warm air over-running deep-seeded cold at the surface. It's much more straight-laced this time.
The rain/snow battle will be most apparent during the day Saturday, and eventually get to the all-snow category later that night, continuing as snow until the moisture leaves on Sunday. Here are the model-derived amounts:
|GFS - Snowfall Projection - Saturday/Sunday||NAM - Snowfall Projection - Saturday/Sunday|
I will be working on my own snowfall map for the shows this morning on WSAZ, and will update the blog post with that image as soon as I can. My first thoughts on this though are that the "donut hole" is in effect for the I-64 corridor. Whenever rain and snow mix, I don't like the idea of seeing 0.1" of rain with 0.5" of snow-- the rain will negate the snow accumulation. I'm much more positive on the snowfall possibilities in Ohio and along I-79. The modeled amounts in the southern mountains will come as the back-end upslope snow finishes it off.
Storm System #2 -- Tuesday
Ringing in the New Year with another wintry mix. Actually, this first system will be a great guide to what the second one might do, because the cold/warm air setup will be largely unchanged because these guys are back-to-back.
Using our foray into "critical thicknesses", here's what the setup is based on the models...
|GFS - Surface Map - Tuesday||CMC - Surface Map - Tuesday||ECMWF - 850mb Chart - Tuesday PM
...Another rain/snow line situation unfortunately for those snow lovers. The CMC looks more promising for snowfall, but I don't like the Canadian models. Like ever :-) ... The Euro is GREAT in these farther out than 3 days time-frame, but it too is painting a negative picture for snowfall. I put in the 540 "critical thickness" dashed line, but on the Euro you notice how despite the good thickness values (for snow) the air is drier. A zonal flow keeps the southern and northern streams of moisture from combining. The other models aren't showing this right now, but may jump on it tomorrow, or the Euro may change too. Regardless, there's still plenty of time to keep an eye on it.
Update (5:00pm) - Here's my snowfall forecast "last call" for the area given what I used during my morning shift today:
It largely reflects an accumulating snow line along and north of the Ohio River such that the Portsmouth to Parkersburg corridor can stand to see the 1-3" of snow (and points farther north as well. If you walk a line east from the Ohio River when it meets Gallipolis eastward to I-79, points north of it are also in line for the 1-3". Folks in the high points south of Pikeville and eastward into the WV mountains also stand to get accumulating snows, but not until the wrap-around moisture comes on Saturday and we all transition to snow. At this point we'll be left with the traditional upslope conditions that squeeze out the extra snow for those favored elevated areas. The gray area is the I-64 corridor and then Corridor-G. This area will start off with mainly rain, occasionally mixing with snow, but not really start accumulating until during the day Saturday when the whole region gets back to snow. At this point, most of the moisture will be gone (what else is new), but we can still eek out a 0-1" of snow-- this accounts for a variable snowfall that appears and melts and appears again, etc. Keep in mind that hilltops can always get a little extra (you know who you are). Unfortunately there'll also be a bunch of you out in Kentucky and the WV lowlands that will once again be left out, being simply too warm to permit appreciable snowfall until it's too late.
Particularly in Ohio, some folks may be surprised to find this newfound accumulations when they wake up Saturday morning. Even though overall this is a light event, don't forget all the usual safety precautions driving in that snow. It could be that nasty borderline where it's a bit too much for salt/sand, but not enough for plowing. You may be down to your own common sense on those secondary roads, if you know what I mean :-)
Update (5:00pm Saturday) - The accumulating snows are progressing to just the highlands using the typical lake-effect upslope format. Initially, Lake Michigan and Erie are in on the action, so the Kentucky mountains can get a piece too.
Post-mortem - This was a "fun" storm from a meteorologist perspective because of how dramatic the rain-snow line was. There was a little extra moisture in the system than modeled at the time of my "last call" (and more than what was presenting on the 00z models this morning too, although the NAM was onto it a little bit). The rain-snow line was fairly well forecast, with the exception of it extending much farther up I-79 than thought. The arc of accumulating snow increases steadily north of the rain/snow line such that reports of 3-6" were common on a line north of Morehead, KY to Rosemount, OH to Rio Grande, OH to Athens, OH. Several hilly spots along and north of that line were the best places to get the higher totals, while city locations within got the lesser.
Now the transition has been made to a coastal Nor'easter type (patterning the "Ohio Valley Soaker" storm type previewed weeks ago on our winter weather forecast), we will now see additional accumulations primarily reserved for the West Virginia mountains spots that are typically targeted by Lake-Effect. Sections of Nicholas County have already been reporting 4" of snow or so, so they will get that 3-6" category forecast for them on the back end.
Once again the Huntington / I-64 corridor is left in the lurch, though that should have been expected given the forecast. Maybe we'll catch a break soon here; I'll be posting a new update on the next storm as soon as I can. :-)
If you have any additional snowfall reports to give, feel free to post them in the comments section below. Thank you for your patronage and participation.
|Regional Radar/Satellite with Warnings Tracking||
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Have a great day everyone!