Forecasting / General Post
Winter's Opening Salvo
November 15 2018 - 0200z
I have my work schedule arranged in such a way that I enjoy a few consecutive days off in the middle of the week. It gives me a chance to "catch up", physically and emotionally, as well as providing a block of time to work on writing and other such projects. There is a pizza place that I frequent on these off days; frequent a little too much, if my rough calculations about yearly spending there are anything to go by. What can I say; they make really good pizza. I left around 15z today to get lunch, and noticed that we finally seem to have turned the corner from one season to the other. There is a solid chill in the air, stemming from a cold air mass originating deep in arctic Canada, and a classic winter wind that I can still hear from time to time as I write this. Perhaps nothing signifies our concluding transition, however, as much as the start of the first significant region-wide wintry event drawing to less than a day away.
This is a true "mixed bag" event, that is, one that will, somewhere, drop every type of wintry precipitation. Many storms have unresolved uncertainties even within the H+24 to H+36 range; the classic question in so many winter nor'easters is the exact location and orientation of the "deformation band", which often yields the highest local snow totals of the storm. For this event, however, the most critical remaining uncertainties, in my opinion, pertain to local temperature profiles, rather than QPF maxima and minima. To explain, allow me to first break down how these temperature profiles relate to precipitation type ("p-type"), and how local microclimates can have major effects on said p-types. I will then provide a brief summary of my forecast thoughts for the storm, and conclude with a safety tip or two. Shall we begin?
The most effective way for meteorologists to look at atmospheric conditions as a function of altitude are through the use of atmospheric soundings. These are launched by the National Weather Service several times a day, with exact frequency depending on regional and/or special requirements (for example, special soundings may be launched prior to a severe weather event, or to gain data for model input in high-consequence situations such as an impending hurricane landfall). Soundings usually consist of a radiosonde delivered via a weather balloon; rockets are the most common alternative. NWS sounding data can be obtained on the Storm Prediction Center website, the NCAR, or from local NWS Weather Forecast Office pages. A typical sounding (in this case, the 12z or 7am sounding from Albany, NY) may look something like this:
This is a LOT of data. The bottom boxes, as well as the hodograph (wind strength and direction relative to altitude) and surrounding charts are most important for protection of severe weather, and as such, will be disregarded for this post. What I want to focus on here, out of all that, is the red line on the plot seen on the top left of the data display. The chart itself is called a Skew-T Log-P diagram; that is, the temperature lines are skewed at an angle, and the pressure lines are represented in logarithmic format. For more reading, you can access some helpful information here, here, and here. That line is a visual representation of the air temperature at different altitudes; the former measured in °C, and the latter in hPa/mb (a note of conversion: 1 hectopascal, or hPa, is equivalent to 1 millibar, or mb). Let's take a closer look, shall we?
Remember that the Skew in Skew-T Log-P denotes the "skewing" of temperature lines off to an angle. The pair of straight blue dotted lines above highlight 10°C and 0°C from left to right. The green line, for reference, measures the dewpoint. Altitude, as I mentioned above, is measured in terms of atmospheric pressure; 1000hPa roughly equates to the surface (in low-lying areas; in higher elevations, the "surface" pressure may be significantly lower). 850hPa is about 5,000 feet, and 700hPa is approximately 10,000 feet. In terms of the temperature profiles I am about to discuss, we are going to focus on the 700hPa-1000hPa range, which is the most important in determining p-types for a specific location. One should note that the exact "true" altitude of a certain hPa line changes in response to temperature; the higher the temperature, the higher the altitude, as illustrated in this graphic.
So, I promised a discussion about temperature profiles. Specifically, how different profiles affect p-types, and how microclimates factor into that affectation. Nearly all precipitation occurring during the colder months of the year starts out as snow, high above the earth where it forms. How it lands on the ground, however, is a factor of two different temperatures: air and ground. We will start with the former. Imagine a single snowflake falling through the atmosphere. As it descends, the temperature of each layer of air it passes through will determine the end p-type of the molecule. If the entire air column is below freezing, the snowflake will stay a snowflake; likewise, if the entire column is above freezing, the snowflake will melt and land as plain rain. Where things get a bit complicated are when there is a "warm nose" somewhere in the lower to mid atmosphere (commonly between 700 and 900 hPa). When this happens, the snowflake will melt and refreeze. If the warm air intrusion is relatively shallow, with a deeper layer of cold below, the molecule will fall as sleet. If the warm layer is deeper, with less cold air available near the surface, the molecule will fall as freezing rain. The chart below is a visual representation of these different temperature profiles.
Tropical Tidbits and Pivotal WX both have point-and-click forecast soundings for several models; you can also plug in an airport identifier (such as KJFK) in the viewer window to pull up a forecast sounding for that location. I encourage all of you to play around with these viewers. Compare them for your location from model to model, and draw your own conclusions.
In regards to rain versus freezing rain, ground temperature also comes into play. Rain falling onto a -5°C surface will freeze; rain falling onto a 5°C surface will not. In that regard, even if most of the layer is above freezing, a cold surface level with subfreezing ground temperatures can lead to "ice on contact". Microclimates come into play here; small, yet crucial, variations in local temperatures coinciding with elevation, local wind flow, bodies of water, etc. Different types of soil will warm and cool at different rates. The water table plays into these things. These minute variations in ground and surface air temperatures can lead to different p-types falling over very small distances. Such factors are important to consider when forecasting a storm like this, where the temperature profile is proved to be variable during the course of the storm, and outcomes may be influenced by rather small changes in temperature. Typically, each microclimate is best understood by those who live nearby. Think to yourselves; what is that one place in town that always gets an extra inch of snow?
Now, I said I'd make some predictions, and I suppose I should live up to that. Below are the forecast snow accumulation maps from the 18z 12km NAM, 3km NAM, RDPS (Canadian short range mesoscale model), and the 00z HRRR (through 8z Friday, some areas will see more after that time). These show ratios of 10:1 (that is, 10" of snow for every 1" of QPF), and exclude sleet and freezing rain.
Now, here are my thoughts (north of the black dashed line has the best chance to stay all snow):
What, you wanted more? I will leave brief word on safety to conclude this message. Drive carefully, especially in areas with microclimate variations. Rain can turn to freezing rain when you drive up a hill, and road conditions can meteorite very quickly. Plan for extra travel time. Power outages may be more of a threat than with many comparable storms due to the saturated nature of the ground. Might not take much on a tree to knock it over.
Thank you for reading. Pass it on if you wish.
#1, 2 - Storm Prediction Center
#3 - National Weather Service
#4 - Wikipedia
#5, 6, 7, 8, - Pivotal Weather
#9 - Base Map from Free World Maps / Google Image Search