Updated: May 27, 2021
Understanding the synoptic weather ("big picture") is key to understanding the weather you might face on a flight for any given day. It helps to explain some of the details you may see in a forecast. For example, you may see a terminal forecast for freezing rain (FZRA) such as the TAF for Roanoke, Virginia beginning at 1200Z on the 13th.
KROA 121734Z 1218/1318 16008KT P6SM SKC
FM130600 VRB03KT P6SM OVC090
FM130900 VRB03KT P6SM OVC020
FM131200 VRB03KT 5SM -FZRA BR OVC009
FM131600 VRB03KT 4SM -RA BR OVC004
If you go to the prog chart (below) you'll notice the classic freezing rain setup. A strong high pressure (1041 mb) sits off the New England coast. Remember that the wind flows clockwise around an area of high pressure in the northern hemisphere. This brings in low level easterly winds off the Atlantic Ocean. Given this moist, cold, dense air tends to stay low to the ground and gets wedged up against the Appalachian Mountains. If you read the area forecast discussions, meteorologists refer to this as CAD or cold-air damming.
The next component is the area of low pressure along the Gulf coast. This is expected to move to the northeast. The warm air from the south will ride up and over the cold air wedged in at the surface especially along the foothills of the mountains in Virginia.
Notice the next prog chart valid at 1200Z (coincident with the start of the freezing rain in the TAF above for Roanoke, VA) shows a wide swath of pale orange indicating the likely potential for ice (aka freezing rain) as the warm air mass begins to make its way northeast.
Another good approach in these situations is to drill down using a Skew-T log (p) diagram. This gives you a good sense of the overall temperature profile and the depth of the freezing rain layer. Below is the Skew-T forecast for 1000Z on the 13th. Notice the temperature at the surface is a degree or two below freezing. This is the cold air dammed up at the surface. Then there is a temperature inversion present from the surface to about 5,000 feet MSL which demonstrates the warm air riding over the cold air at the surface. This is a deep saturated layer which creates the classical SLD structure. That is, snow is being produced from the deeper clouds and that snow falls into a subfreezing layer and melts into rain. Then about 1000 feet above the surface, the rain falls into a subfreezing layer and becomes freezing rain.
Eventually, the warm air begins to take over to become dominant and erode the cold air at the surface causing temperatures to rise as the event evolves over time. By 1400Z, the temperature at the surface climbs to a degree or two above freezing as shown below.
By later in the day on the 13th (0000Z 14th), most of the area that was depicting a likely risk of ice is now just showing a forecast for rain.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist