Monday, January 5, 2009


UPDATE: JAN 5 - 8:50 PM. NOAA's Global Forecast System (GFS) model projection for 1:00 PM Tuesday 1-6-09 shows the 0 c line at 850 millibars (the thick blue line) nearly paralleling the Mason-Dixon line. That indicates Maryland is still a battleground state regardless of the polls or politics. If this blue line actually ends up one county farther south at the projected time, then what was going to be a nuisance ice event (less than .10") turns into a significantly disruptive one (greater than .25"). Even Norm Lewis of ABC 2 said the words: "Have your winter weather plans ready" and he said it with a cautious if not uncertain looking expression.

SUMMARY (Jan 5- 4:50 PM) The Baltimore/Washington NWS Office has modified the Winter Storm Watch into a Winter Weather Advisory from central and western Maryland to West Virginia, effective 6:00 AM Tuesday 1/6 to 9:00 AM Wednesday 1/7. This is still considered an "over-running" system which occurs when warm, moisture laden air over-rides a shallow colder surface layer. However, indications are the cold air supply will be marginal and subfreezing temperatures not as widespread. Where sleet and freezing rain do occur, total ice accumulations across the advisory areas should range between .10 and .20 of an inch by Wednesday sunrise. The areas under an advisory may face a challenging situation for school officials, parents and commuters as my projected arrival time of sleet and freezing rain in the Baltimore Metro region is between 7 and 9 am Tuesday morning. This very inconvenient onset time tomorrow suggests today should be used for developing and communicating the family contingency plan in the event schools close early. For teachers looking to incorporate this winter weather event into their lessons, it is a textbook example of how we can align "relevant weather" with essential curriculum due to several factors explained below. (Note: scroll to the bottom for the curriculum basis of this post.)

1. TIMING AND SCHOOL. I strongly suspect most districts will take a wait-and-see approach Tuesday, even if winter weather advisories remain in effect. The most likely outcome would be everyone in on time, with some western counties having a 1 or 2-hour early dismissal, followed by a region-wide 2 hour delay Wednesday. A mid-day arrival continuing into the overnight makes for great uncertainty as to whether schools will close early Tuesday, open late on Wednesday, or a variety of other possibilities. The danger of a wait-and-see approach this time around is that it *could* be a repeat of the February 2007 situation, where an early dismissal occurs into the heart of rapidly deteriorating road conditions. I'm sure all my elementary school colleagues remember that one well.

2. HISTORICAL BASIS. If we refer to recent history of weather-related closures, the outcome prior to two storms provides a glimpse of what may happen tomorow. The Monday 2/28-Tuesday 3/1 storm of 2005 is the most appropriate match. In that storm, NWS had plastered Heavy Snow Warnings all over the region. Schools across central Maryland were closed before any snow arrived. Once here, the snow fell for hours, but readers posting to this website reported little if any snow began to stick until 2 PM at the earliest, and simply made parking lots wet. Fast forward to February 13, 2007. Winter Storm Warnings were replete up and down the I-95 corridor. The enhanced radar looked like a nuclear explosion, the precip shield was descending on Maryland like a scene from Independence Day. Most schools across the region were... OPEN! Unfortunately, many of them hastily closed 2 and 3 hours early in what became the infamous "Valentine's Day Massacre" of 2007.

3. TEMP VS. PRECIP TYPE: Pertinent criteria are expected to be present by this evening for a winter weather event, but very subtle differences in temperature, humidity, cloud cover and atmospheric pressure will make a huge difference in what you see on the ground by Tuesday noon. Surface winds from the west are expected to veer more north overnight, and NWS expects this pressure gradient change will allow for a new supply of cold air to wedge down the east side of the Blue Ridge. However, we have seen this potential fail before. Despite north winds, the key is overnight temperature. NWS is forecasting some cloud cover into tonight, and those clouds act as a thermal blanket preventing excess surface heat from re-radiating. For example, if the cloud cover holds tonight's low in Reisterstown to 32 and not the currently forecast of 28, this turns into more of a sleet to rain event. Schools open on time and there is no early dismissal. Equally as possible is that the boundary layer of cold air remains in place, and the seasonally low sun angle prevents much daytime warming despite cloud cover. In that scenario, the precip is mainly freezing rain starting in the late morning, forcing school districts into the unpleasant task of closing 2-3 hours early in a deteriorating travel environment. Let's hope that does not happen, but we all would be wise to plan ahead now.

4. ENERGY TRANSFER ISSUES. Freezing rain is not only the most hazardous type of winter weather, it also presents the most interesting meteorologic process to analyze right here at the surface. When the boundary layer is at or just below freezing, the moisture falls as rain and freezes upon reaching the ground or any near ground objects that are at or below 32 degrees F. What's so weird about freezing rain is how the temperature behaves in a transfer of energy process. Let's say the morning temp in Cockeysville is 30 and rain begins around 10 AM. When the rain freezes, it causes a "latent release of heat" that allows the surrounding air to warm slowly to 32. But then, as rain continues to fall and accretion of ice increases, the surface itself remains cold while the air is briefly warming. At some point in the event, the surface is 32 and so is the air just above it. This allows for slight melting of the ice, (a phase change) and some of the newly released water molecules evaporate... a process that takes energy out of the atmosphere and produces cooling. Because there is now so much more moisture to evaporate than there was a few hours ago, the warming trend is halted by new cooling. Presto! Our storm has just created a small scale heat transfer system in your backyard.

5. HOW AND WHY OF THE FORECAST. The analysis above is why you see the NWS forecasting a high of 33 or 34 for much of the Baltimore Metro area on Tuesday. They have already anticipated that a long duration event will likely create a period of freezing rain, leading to the latent heat release, which in turn leads to evaporative cooling, and thus helps to hold the temperature right around 32 until a large scale movement of air (the passage of a warm front) ushers in warmer temperatures that scour out the cold surface air and melt the ice. You can track all the interesting give-and-take in the local NWS forecast discussion.


For a curriculum-focused analysis of our impending winter weather event Tuesday into Wednesday, I have adapted to this discussion relevant components from the MD Voluntary State Curriculum in conjunction with the Knowledge Skill and Indicators of the BCPS Articulated Instruction Module (AIM):

VSC Core Learning Goal 2. The student will demonstrate the ability to use scientific skills and processes to explain the physical behavior of the environment, Earth, and the universe.

VSC Indicator 2.3.1 The student will describe how energy and matter transfer affect Earth systems. Assessment limits: Atmospheric circulation (heat transfer systems –conduction / convection / radiation, phase change, latent heat, pressure gradients, general global circulation, Coriolis effect)

BCPS AIM Objective 12: The student will use the relationship between heat transfer and water to explain the properties of the atmosphere.
A) Use examples from the real world to describe and explain the hydrologic cycle.
B) Examine the interrelationship of humidity and temperature to explain weather patterns.
C) Use information from readings and electronic resources to explain the causes and effects of atmospheric pressure (barometric pressure).

BCPS AIM Objective 13: The student will use meteorological data to interpret a weather map and make predictions:
A) Apply weather symbols to describe and differentiate among station models.
B) Define frontal systems to differentiate among cold, warm, and stationary fronts.
C) Use weather maps to interpret the weather forecast.

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