Sunday, December 8, 2013

Rain, Sleet or Snow? Wouldn't You Just Love To Know?!

If you've lived on Long Island for at least one winter then you'll know from experience that the precipitation type associated with some storms can be quite mixed up. Take the February Blizzard of 2013, for example. Most of the Island saw rain to start which transitioned to sleet in some places before becoming snow, while other locations along the South Shore just received rain. The processes that are responsible for the precipitation type that you receive on the ground are fairly well-understood, but require really high resolution data to be able to predict it. Temperature and moisture data for the atmosphere are most important in determining what type of precipitation will fall on the ground.

Background Information
A fundamental piece of knowledge is that water comes in three forms-- solid (ice), liquid (water) and gas (water vapor). When water changes phase from one form to another it either absorbs a small amount of energy in the form of heat which cools the surrounding environment or it releases that heat energy which slightly warms the surrounding environment. For meteorology, a phase change is only really dependent on the environmental temperature, or the air temperature surrounding the precipitation particle (aka hydrometeor). Air that is below freezing or below 32F/0C will support snow and ice but air that is above freezing which is above 32F/0C only supports liquid precipitation or rain. For more information, check out some of the chemistry lessons from the Khan Academy. The air above the surface can be above freezing, but if the surface is at or below freezing then the rain can freeze on contact with the surface; this is known as freezing rain.

If you know the temperature and how it changes with height (called a vertical profile of temperature) then shouldn't you be able to predict what type of precipitation will fall on the ground? The environmental temperature changes a lot due to the motion of the winds moving air from surrounding regions around and from the precipitation itself through reactions. An important reaction within areas of precipitation are when ice and snow melt into rain that absorbs heat energy which slightly cools the environment. The reverse is true, so when rain freezes into sleet that process actually releases a small amount of heat into the environment. Therefore, the environmental temperature is quite dynamic, but tends to mostly be regulated by advection of surrounding temperature values by the wind. The National Weather Service New York Office created a phenomenal video describing how hydrometeors change depending on the environmental temperature. Take a moment to check it out!

NOAA/NWS New York created a great video and are active on Facebook to discuss mixed precip.

 Moisture is also important because if the air is not saturated then water (liquid or ice) will evaporate into the air to try to allow it to reach saturation. This is related to a temperature-dependent quantity known as relative humidity (RH). If the relative humidity is < 100% then any rain or snow that falls into that layer of low RH will partially evaporate and evaporation, like snow melting, absorbs heat from the environment which results in a slight cooling of the air. The temperature and moisture as given by the dew point temperature near the surface can provide some clues about whether to expect warm or cold precipitation. If your surface temperature is above freezing, but your dew point temperature is a lot lower (i.e. you are pretty dry at the surface with a low RH) then you'd expect your surface temperature to cool due to evaporation at the onset of precipitation when the rain or snow falls into that dry layer and works hard to saturate it. That temperature can even be readily calculated from temperature, RH and surface pressure and is known as the wet-bulb temperature.

Now that you've been briefed on how important knowing the atmospheric temperature and moisture is during times when temperatures are near freezing we can discuss the observations and forecasting of the event of 8-9 December 2013 that is currently underway!

Surface temperature values are readily available through a relatively dense network of observations that are monitored by the National Weather Service. Check out this cool interactive display (click on observations to see surface temperature, dew point and wind). What about understanding the temperature profiles with height? That's where weather balloons come in handy. The balloons provide temperature, dew point and wind data at various points extending throughout the troposphere, or the lowest layer of the atmosphere that all of our weather is confined to. The plots can be a little confusing to look at but once you get used to them, they provide a ton of useful information.

12 UTC (7 AM EST) OKX Sounding 8 Dec.
 Balloons are launched twice a day at 12 UTC (7 AM EST) and (7 PM EST) from over 100 stations across the U.S. including the NWS New York City Office located in Upton, NY (abbreviated as OKX) which is about 20 miles east of Stony Brook University. The sounding plot to the left shows two curves, the red temperature curve and a green dew point curve. They are plotted with pressure and height on the y-axis with the surface at the bottom. Highlighted on that blue bar is an isotherm, or a line of constant temperature that slope at an angle. That is the 0C isotherm or 32F which shows that the temperatures contoured to the left of the blue line are below freezing and to the right of the blue line are above freezing. At around 7 AM EST this morning, all of the air above our heads was below freezing, which would support snow as the dominant precipitation type.

While these are useful, there are only soundings available two times a day. What if the precipitation falls in between those times like it most often does? For that, forecasters must rely on model data and even model soundings, or temperature and moisture profiles that are output from the weather models. Here's where the forecasting challenge truly lies, in my opinion. We've discussed how sensitive the precipitation type is to temperature, especially whether the environment is above or below freezing. What if the favored model is off by a few degrees and those degrees mean the difference between above or below freezing and therefore rain, snow or a mix? That's where the uncertainty lies.

Forecasters monitor real-time data such as radar, satellite and surface observations to compare what is actually happening with what the models showed might happen. Updated model output is available more frequently during the day than weather balloons, but it's important to check whether the models are on the right track with reality. Forecasters take their knowledge and observations and use it to scrutinize the next model data coming in. For example if one model (e.g. ECMWF) had the temperature and precipitation all wrong for the 1 PM forecast, then at 1 PM a forecaster would take the ECMWF with a grain of salt.

A new useful tool for precipitation observations is called mPING which uses crowd-sourcing to get data. Do you want to submit your own precipitation observation? Just download the iPhone or Android app and submit something today! This tool was developed by scientists at the National Severe Storms Laboratory (NSSL), University of Oklahoma (OU), and the Cooperative Institute for Mesoscale Meteorological Studies (CIMMS) and is definitely used by forecasters and scientists so help us out and if you see something falling-- mPING it and check out other observations near you!

Event Overview and Model Forecast
As of 3 PM EST on 8 December, a moderate snow band was located across Philadelphia and stretched eastward over the Atlantic Ocean but translated eastward staying well south of Long Island. There is high pressure in place over New England that is slowly slipping eastward and as it does, low-level winds should move from out of the north to more out of the east and southeast which will advect in warmer temperatures at low-levels.

As the evening progresses, NYC and Long Island could see some light snow that may accumulate up to a couple inches before the warm air is advected into the region that may change it to sleet and then freezing rain before becoming all rain on Monday. At Stony Brook University, the column has been moistening all day from how dry it was from this morning's soundings by the precipitation aloft. The image below shows data from a vertically-pointing radar that provides fall speed at the top (how fast the hydrometeors are falling to the ground) and reflectivity which provides an idea of the size and concentration of hydrometeors. There's been snow falling but evaporating around 1.5 miles above the surface at 10 AM EST to only about a half-mile above the surface at 3:30 PM EST. According to this trend, we would expect snow to reach the ground by 5:30 - 6:00 PM EST.

Dr. Brian Colle provided this snapshot of the time (x-axis) vs. height (y-axis) data from the vertically-pointing radar on the roof at SoMAS at SBU.
 While temperatures are expected to remain below freezing for the start of the precipitation, the SBU-WRF simulated soundings of temperature with height show that after 1 AM EST, the temperatures at low-levels and near the surface should warm to well above freezing (~ 7C/44F) by 7 AM EST. In between 1-7 AM there is the chance for freezing rain, as air above the surface warms faster than the air at the surface leaving the surface temperature near or below freezing.

If you have any questions about precipitation type, don't hesitate to comment on this post!

For more information about winter weather from the NWS New York Office, please visit this site:
For SBU-WRF data, please visit this site:
If you see something falling, mPING it! For more info please visit this site:

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