Updates Below (most recent 1:00pm)
It's Wednesday-- Half-way there!
As advertised, we've got our next weather-maker coming in this morning, and sticking around through the evening hours. This one figures to be a mix of rain and snow. Here's how the NAM Model is handling it:
|NAM - Wednesday Morning||NAM - Wednesday Afternoon||NAM - Wednesday Evening|
A quick review of what these maps show-- The colored blobs represent precipitation amounts over the course of 3-hours according to the legend at the bottom of the image. Also, the solid lines represent temperatures at the 850mb level (about 5000 feet in the air) that are above 0°C, and the dashed lines represent temperatures that are below 0°C.
Initially, we'll be looking at warmer air riding in at the exact same time as the moisture, especially up through Corridor-G and I-79. Both the Huntington and Charleston areas should spike into the 40s during the mid-morning hours, putting the morning commute squarely in the rain category. On the far northern fringe of the precipitation shield it is possible to get some sleet in there first, but the solution is mainly one of rain-- initially.
A Sneaky Situation (very wonky, proceed at own risk)
What gives me pause about the afternoon forecast is a small feature in the models that can generate a decent push of vertical air motion. The first thing I want to show is the GFS image for Wednesday evening:
|GFS - 700mb Chart - Wednesday Evening|
Just as we have with the surface level, we can get local areas of high and low pressure aloft too. In this map, we see a slight kink in the height-lines, indicating a localized minimum that sweeps across the tri-state area during the afternoon hours. When forecasting coastal Nor'easters, paying attention to the placement of the 700mb low is often a good approximation as to where the most likely snowfall maximums occur. The reason for this is that at this altitude level (~10,000ft), having a local area of low pressure signifies converging air motion and vertical air ascent-- key indicators for precipitation. In a situation like this, dynamic cooling also accompanies the precipitation, bringing snowfall closer and closer to staying intact right to the surface. Examining these same models in a sort of 'x-ray' view bears out a similar thought...
This model image has a bunch of 'stuff' on it, so let me describe it before we talk more about it. This map shows what's called a "Time-height cross section". The hour is listed at the very bottom, and those grey horizontal lines represent altitude levels). The purple and yellow contours indicate the 'sweet spot' in the clouds overhead where snow growth is best. The red contours indicate vertical air motion (the way the physics works is that these are negative numbers, the lower the number the stronger the vertical air motion is).
Whenever the vertical air motion spikes within the region of best snow growth, snowflakes will be created, and will attempt to make it to the surface. The next thing that should be checked is the level of relative humidity in existence in and below that snow-growth region all the way down to the surface. In addition, it's important to assess the coldest temperatures the air can be cooled to by means of evaporation (which is also known as the 'wet bulb' temperature). Here's how the same image looks with those variables enabled:
(I apologize that this gets so messy -- meteorologists often stare at 5 or 6 things at once in order to get a multidimensional picture of what's going on inside the clouds above)
This map is of the same sort as the previous one, and even includes those snow-growth contours. This time around though, two different/other variables are presented. The relative humidity is represented by the red and green blotches -- the green being humidities greater than 90%. In addition, these 'wet-bulb' temperatures are presented in contours of 5°C. The obvious one that is important for us is where the 0°C contour is and how close to the ground it is.
What we see is that the air is pretty-well saturated below the snow-growth zone, so not only is there moisture available to make flakes in the best area of the cloud for it, but these same flakes will not be lost to evaporation on its way to the ground because the air is already saturated with water. The other thing we see is that wet-bulb temperatures at the surface start dropping when all these ingredients phase, culminating with that 0°C line scraping the ground itself right around 6pm (think evening commute). This can be a dangerous situation, because if there is going to be a change-over to snow it will happen all at once and the precipitation intensity will be moderate the whole way through.
Now, I want to take the same sort of cross-sections and examine the prospects for precipitation type for both Charleston and Huntington:
|NAM - Time/Height Series - Vertical Velocity - Wet-Bulb - Precipitation Type - Charleston, WV||NAM - Time/Height Series - Vertical Velocity - Wet-Bulb - Precipitation Type - Huntington, WV|
As we go through the day, the dynamics change aloft. We'll see colder air wrapping around the back end of our surface system, and at the same time a spike in vertical velocity arrives. Notice that for both locations the 0°C wet-bulb line creeps closer to ground level, and actually gets there in Charleston. There is always wiggle-room in these models (> 90% humidity still isn't 100% you know).
So-- this is my concern, and this even includes areas all along I-64 from eastern Kentucky right through Charleston: That we'd go from moderate rain to moderate snow right during the evening commute. We'd end up with big flakes that can coat the ground (and roads) quickly. The dynamic presentation on the Charleston image is trying to post up rates greater than 0.5"/hr right at the time of change-over. A large part of our region will be on a hair-trigger where we could see a quick-flip from rain to snow right when folks don't want it.
Now, this can all be academic, and indeed it would be a good thing to keep these flakes melting above ground level and fall much more harmlessly as a cold rain. But, it is something I'm worried about for its high impact value, and if nothing else I can present another dimension of how meteorologists approach complex weather scenarios :-)
Using a different analysis, one I would call more 'ball-park' than gospel, here's what each in-close model is presenting for snowfall accumulation:
|NAM - Snowfall Projection||GFS - Snowfall Projection|
Both models are keen on the snowfall maximum in the WV mountains in response to the best moisture and the natural lift elevation provides. Winter Weather Advisories are already posted for these areas, and getting 3-6" in spots above 3000' is not out of the question. What I'm concerned about is a quick 0.5"-1.0" snowfall right along or north of I-64 during the evening commute. This is possible, though not set in stone.
Since it's midnight as this post goes live, I will be looking at updated model runs for the morning forecast on WSAZ-TV... You can catch the latest snowfall prospects there, and I will be giving my refined thoughts about this possibility there first before going to the blog with it. I certainly won't have time to work on both sumultaneously ;-)
And, of course, there's that other system looming for our weekend. At this point it's more the type of snow flurries that typically occur during deep cold spells, but it's not quite on my plate just yet. First thing's first: here's to a hopefully rainy, and therefore easier, ride home.
P.S. I realize that this post might get confusing or difficult in the middle there. I am willing and ready to address any questions about this in the discussion section if anyone has any genuine interest. Feel free to post below.
Update (1:00pm) - So far, the models have been handling the afore-described features pretty well. In fact, here's the latest analysis of the 700mb level, with the radar imagery laid underneath it:
Notice that right inside the kink in the height lines (where our 700mb Low is centered), is exactly the same place where we're seeing these quick-hitting snowfall accumulations. Rain changes to snow at temperatures around or above freezing, and as much as 1-2" has been reported. This same feature will be moving into our area this afternoon, and if there's any continuity from one area to the next we'll be getting the same exeprience, except for us it's right during the prime drive time. Be careful! You may even notice Winter Weather Advisories going up for counties along I-64. This has been something the National Weather Service branches have been doing as the change-over approached particular areas. So if that happens again for us, I've given you a little head start ;-)
|Regional Radar/Satellite with Warnings Tracking||
From the Storm Prediction Center (below): Click For a Larger Image
|Activity Overview||Storm Outlook||Watches||Potential Watches||Storm Reports|
|Temperatures||HD Doppler Radar||Estimated Rainfall||Active Warnings|
|Click For Larger||Click For Interactive Radar||Click For Larger||Click For Larger|
Have a great day everyone!