Precipitation Type Forecasting

In most winter weather events, we see several different precipitation types ranging from snow, sleet, freezing rain, and plain rain. The temperature profile through the middle and lower levels of our atmosphere determine what precipitation type will fall.

Freezing Rain:

When freezing rain occurs it can occur from two different processes. The first one involves a snowflake aloft that enters a warm layer. This warm layer melts the entire snowflake/ice crystal into a water droplet. The water droplet then falls to the ground, where the surface is at or below freezing (32F). The temperature could be warmer than 32F, but as long as the ground temp is at or below freezing the rain will freeze. This is displayed on a Skew-T/Log-P diagram with the temperature being displayed diagonal in Celsius. The solid black line is the temperature with the hatched line being the dew point:


Freezing rain sounding from the NWS Louisville

The image taken from the National Weather Service in Louisville shows the temperature profile of a freezing rain event. The temperature could go back to below freezing after the snowflake melted in the cloud, but for ice crystallization to occur we typically need to see the droplet reside in temperatures at or below -10C. This leads us to our second way of freezing rain occurring…


Freezing rain sounding from the NWS Louisville

The sounding shows temperatures below freezing throughout the column, but the precipitation type will still be freezing rain. The lack of saturation in the cloud above 700mb only allows for temperatures of -5C in the cloud. This is not cold enough for ice crystallization to occur resulting in water droplets remaining super cooled. As a result, the shallow layer of moisture ends up producing light freezing rain that freezes on the cold surface below freezing.



Sleet has a warm layer involved in the cloud, but it is typically shallow or not as warm. Instead of the snowflake completely melting, as mentioned in the freezing rain case, the snowflake partially melts. This keeps the ice nucleus intact allowing for refreezing once the temperature drops below freezing.


Sleet sounding from the NWS Louisville,



The whole column is below freezing with ample moisture at or below -10C. This allows for enough ice crystallization for the snow to fall to the ground.



Snow sounding from the NWS Louisville,

El Nino and La Nina

Sea Surface Temperature Anomalies (SSTA) in the equatorial Pacific alternate between warm and cool periods. The cooling of the equatorial Pacific waters is known as La Nina. The warming of the equatorial pacific waters is known as El Nino.

The SSTA are measured in 4 locations 1+2, 3, 3.4, and 4:


ENSO Regions from the Climate Prediction Center.

Region 3.4 is used as the primary region of measuring for El Nino and La Nina. When the 3 month average of region 3.4 reaches -0.5C (or lower) or 0.5C (higher) for 5 consecutive monthly readings it is considered an El Nino or La Nina.

During La Nina, the easterly trade winds are stronger across the central-eastern basins of the equatorial Pacific. This creates upwelling and enhanced evaporation along the ocean’s surface cooling the waters. The warmer waters are pushed towards the Maritime Continent. The strength and orientation of the La Nina plays a factor in the pattern that follows for the United States.

The cooler waters having lower height anomalies over the equatorial Pacific create easterly momentum over the Tropics. This results in a lack of a southerly jet with the northerly jet being the big player. The ridging over the Northeast Pacific allows a dip in the Jet Stream over the Midwest and Northeast. Warmer than normal conditions is shown in the Southern half of the US extending into the Southeast.


Typical pattern associated with La Nina from the Climate Prediction Center.

Typically a more basin-wide/stronger La Nina will focus the cold further west allowing for a warmer Eastern US. Another caveat is the strength of the polar vortex, as a strong polar vortex could allow for a much flatter ridge in the Northeast Pacific with a strong pacific jet. This would bring relatively milder air to much of the CONUS.

During the El Nino winter seasons, the tradewinds are much weaker over the central and eastern equatorial pacific basins. This allows the warmer waters over the Maritime Continent to push westward. The orientation of the warmer waters in the equatorial Pacific plays a crucial role in the winter weather pattern. If the warmer waters focus in the eastern basins this would result in a much stronger low in the Gulf of Alaska. This would flood the CONUS with milder Pacific air. A central-west based El Nino opens the door to a much more favorable pattern for cold and snow across the Eastern US with the strong low centered more towards the Aleutian Islands. This acts to pump up a strong ridge in the Western US that allows cold air to dive southward.


Typical pattern associated with an El Nino winter from the Climate Prediction Center.

The higher height anomalies from the warmer water and latent heating create a strong pressure gradient at a lower latitude than La Nina. This produces a strong southerly jet stream that brings in moisture across the Southern US with below average temperatures.

Arctic Oscillation and Polar Vortex

The polar vortex is a pool of cold air over the Arctic. This polar vortex develops from the difference in temperatures between the cold arctic air and the warmer air across the southern latitudes. When the polar vortex is strong it remains in place over the arctic with the cold air being confined to the northern latitudes with a fast polar jet. When the polar vortex is weaker it is more susceptible to warming, which allows the polar vortex to be split or displaced. This displacement allows for big dips in the jet stream that bring cold air south in the Middle Latitudes.

A measure of the polar vortex strength can be figured out through the use of the Arctic Oscillation. During the positive phase of the Arctic Oscillation, the upper-level height anomalies over the Arctic are below average. This signifies a strong polar vortex with cold air being wrapped up.

During the negative phase of the Arctic Oscillation, the upper-level height anomalies are positive over the Arctic. This shows a much weaker and displaced polar vortex with cold air diving southward.

The polar vortex is a cold core anomaly with very high static stability from the stratosphere. To weaken this vortex you need to warm up it up and disrupt the strong flow around it.



Negative and Positive phase of the arctic oscillation from the NCDC

The winter of 1988-89 featured a strong polar vortex with the cold air being locked up in the northern latitudes. The 500mb anomaly of that winter is pictured below with higher heights and warming in the Middle Latitudes:


The winter of 1988-89 featured a very positive phase of the Arctic Oscillation. Reanalysis data from NCEP/UCAR.


The winter of 2009-10 featured a very negative phase of the Arctic Oscillation. Reanalysis data from NCEP/UCAR.

Higher height anomalies were seen across the northern latitudes and lower height anomalies and cold air were displaced to the south across the Middle Latitudes.

Predicting the strength of the polar vortex is very difficult in long range forecasting, but we look into clues from the ENSO forcing, solar activity, and winds in the stratosphere. The quasi-biennial oscillation (QBO) is a measurement of the winds above the tropical stratosphere. During the westerly phase, we see a strong thermal gradient from the tropical regions (very warm) to the polar regions (cold). The easterly phase reduces the thermal gradient by cooling of the tropical regions and warming over the polar region.

Other factors can further influence the prediction of the strength of the polar vortex, such as the ENSO and solar activity. During a La Nina, we tend to see momentum added to the northerly jet, which could contribute to a stronger polar vortex. An El Nino reduces the momentum in the northerly jet with the strong southerly jet stream. This increases the chance of a weaker/disturbed vortex.

During a solar minimum, this could lead to a reduced thermal gradient between the pole/equator reducing the westerlies. This is what we are currently seeing October 2016 with low solar and a westerly QBO.

North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is the difference in sea-level pressure between the Icelandic Low and the Azores High. Fluctuations in the pressure differences alter the pattern across the North Atlantic, which has impacts that affect us in the Eastern United States.

During the positive phase, lower height anomalies can be seen over Greenland with higher height anomalies to the south over much of the Atlantic. The gradient between the higher Atlantic height anomalies and the lower height anomalies over Greenland produce a very strong jet stream traveling west to east. This prevents cold air from dropping southward and allows higher height anomalies (warmer air) to occur over Eastern US during the winter.


A depiction of a positive NAO from the Climate Prediction Center.

This leads to the cold air to moving out before storm systems move towards the Northeast/Middle Atlantic.


During the negative phase of the NAO, the opposite occurs. The lower height anomalies over Greenland are replaced with higher height anomalies. The higher height anomalies in the Atlantic are replaced with lower height anomalies. This creates a weaker jet stream more susceptible to bending creating a meridional flow. This locks in cold high-pressure systems that are a key component to big snow events in the Northeast and Middle Atlantic.


Negative phase of the NAO by the University of New Hampshire

The January 2016 Blizzard occurred during a negative period of the NAO. This allowed a strong high-pressure system to build across Eastern Canada and lock cold air in place:


Analysis of the surface map from the Blizzard of 2016. Archived mesoanalysis is from Drawn in “H” and “L” to display the blocking.

Predicting the state of the NAO can be very challenging in 10-15 day forecast, but even more so in seasonal forecasting. The Sea Surface Temperature Anomalies (SSTA) make it possible for a seasonal forecast to gauge a prediction of the dominant state of the NAO.

The horseshoe SSTA pattern in the Atlantic can offer a lot of information to the probability of a negative or positive NAO:


Sea surface temperature and NAO correlation from the University of New Hampshire

During a positive phase of the NAO, the SSTA pattern shows a cold horseshoe pattern extending from the equatorial Atlantic into Greenland. The cooler waters contribute to the lower height anomalies making the Icelandic low much stronger. The higher heights stay confined to the Central Atlantic creating a strong gradient accelerating the westerlies. The stronger westerlies are less susceptible to bending resulting in a lack of blocking.

A negative phase is shown by a warm horseshoe pattern across the equatorial Atlantic into Greenland. The reduced pressure gradient over the Northern Atlantic allows for a weaker jet more likely to bend and create a block.