Monday, January 26, 2009


A SLAM DUNK OF UNCERTAINTY

School call 1-26-09

REVISED UPDATE: MON JAN 26 - 11:15 PM. It looks likely that the central Maryland/southern PA schools may have to contend with going in on time followed by early dismissals. Great, just the "daymare" we were hoping to avoid. If that occurs, it will be monitored closely here with updates each hour. If you've prepared for pandemonium as I warned yesterday, then you know what's coming. As for dismissals, I estimate that the decision on a 3-hour must be by 10:00 AM, 2-hour by 11:00 AM and so on. So if you're reading this in the classroom, and snow is comin' on down, it's only a matter of time before they pull the plug. I realize many of you might be confused about the Winter Weather Advisory vs. Winter Storm Watch deal, but it is quite simple. The first round will not be as significant as the second, and NWS is waiting to see how Tuesday plays out before deciding how to handle Wednesday.
EARLIER UPDATE: MON JAN 26 - 9:15 PM. As with every storm, there is always uncertainty and the irrational fear that while you were in the bathroom, it all started falling apart! Just let physics be your friend, and it will work out fine. The North Atlantic Oscillation trended to near-zero neutral just as I expected. This sight southward push of upper level pressure differences in far northern Canada then allows the surface Highs and cold air to hold longer across the Midwest and Northeast. Despite whatever the computer models are saying, solid real time data about what the atmosphere is doing RIGHT NOW will rule the day (and night). Some of the few key data sets I've examined are:

1. What is the current atmospheric saturation, and at what levels?
ANSWER: Plenty moist. At 850 millibars (5000 feet), I can see little black dots all over my map, and that tells me "the air is wet up there." Temp/Dewpoint differences are not much, 5-8 degrees maybe. Heck in Roanoke, VA the difference is just 2 degrees, so you can see why they're already under a Winter Storm Warning. The moisture is just waiting for that short-wave to set it off. According to the water vapor loop, there's tons more on the way.

2. Are there any short-waves embedded in the flow that will touch of an early AM round of snow? ANSWER: YES! There's one moving across West Virginia that'll do the trick just fine. A short-wave is a ripple in the atmospheric pressure flow at certain levels, in this case we're measuring 850 millibars or 5000 feet. Once this little ripple in the atmosphere crosses Maryland, the air is plenty moist that it will provide just a tad of instability, but enough to initiate development of snow crystals. Voila! Light snow begins falling between 3-4 AM (update: that's for western MD. The I-95 corridor may have to wait until, gasp... after daybreak). I know it is a bold and gutsy call, but that's why you come here, right? Note: click below for a larger map.

Short Wave Lesson

3. How might Tuesday's results influence Wednesday's outcome? (now there's a cliffhanger!). ANSWER: The deeper, more extensive short-wave ridge-trough combo located over Arkansas will travel through the flow, and reach the Mid-Atlantic tomorrow night. This is the one of the culprits behind a change to freezing rain, directing inland warmer air on a southeast fetch. More details posted shortly, but first I have to see what tale Stormin' Norman is going to spin up.

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