Good Saturday morning bloggers,
The snow from Friday night is long gone, but there are slick spots left over. These will melt by noon.
The weather this weekend will be rather tame with highs jumping to the 50s Sunday. A storm system will be tracking west to east across the USA Monday-Tuesday. It will get its act together east of our area Monday, but we may still see a brief period of rain as it moves by. More importantly, a cold blast will follow the storm system. See maps 1-3
MAP #1: 3PM SATURDAY
MAP #2: 3 PM SUNDAY
MAP #3: 6 AM MONDAY…HERE COMES THE COLD AIR
This cold air will settle into the Plains and Midwest Tuesday and Wednesday as a storm system moves into the southwest USA. This storm system looks rather strong as it tracks northeast into the Plains Wednesday night-Thursday. The storm will try to replace the cold air with warm/moist air. However, cold air is dense and does not like to move out fast. So, the warm air will run up and over the existing cold air. This rise of the warm/moist air creates clouds and precipitation. At cloud level, the precipitation will be in the form of snow. When it begins to fall to the ground through a layer below 32 degrees, it stays snow. If it falls through a layer above 32° it melts to rain drops. If it encounters a low level, thicker cold layer the rain drops can freeze and become sleet. If the low level cold layer is shallow, then the liquid rain drops do not freeze until they reach the ground, an ice storm. See the diagram below.
MAP #4: 3D DIAGRAM OF A WINTER STORM STORM WITH ALL PRECIPITATION TYPES
So, given this diagram where does our area lie. Below I will show to maps from the 6Z GFS at 129 hours. This is valid at 9 AM Wednesday.
MAP #5: The first thing we look at is where is the rain-snow line, also known as the 540 thickness. Cold air is dense, and when the thickness of the atmosphere from 1000 to 500 mb is 540 decameters or less, then it is below 32 through the layer. So, this means along and north of the 540 thickness you have snow, most of the time unless the surface temperature is warm. Our area is in the 546 thickness line. This means there is a layer above 32° between 1000 and 500 mb. So, any snowflakes will melt on the way down. If the temperature around 5000 feet, the 850 level, is 32° or higher then the rain drops stay liquid as they reach the ground. If the ground temperature is below 32° then the rain drop freezes on the surface, creating a glaze. If the surface temperature is above 32° then it is rain reaching the ground. Sleet will occur if the thickness is above 540, but the 5000 foot temperatures is below 32° by a few degrees. This means the snowflakes are melting on the way down, but re-freezing around 5000 feet into ice pellets. So, this is what would reach the ground.
So, again our thickness is around 546. This means the snowflakes will melt on the way down.
MAP #6: THE 850 TEMPERATURE IS AROUND 32°. So, this is likely not cold enough for the raindrops to re-freeze. They stay liquid.
MAP #7: THE SURFACE TEMPERATURE IS BELOW 32°, LIKELY MID 20S . The rain drops would freeze on contact with the surface. UH OH! AN ICE STORM. Not only does this show below 32° the surface, but in the 6 hour period from 3 AM to 9 AM Wednesday we receive .55″! This is just the 6 hour period. Total rainfall may come close to 1.00″.
Now, all of this being said it is 5 days away and can change. If the main storm tracks farther south, we are in the deeper cold air and snow is what we have. If the storm goes farther north, then the warm air gets forced in and our temperature will rise above 32° and we have rain. But, if it changes little we have freezing rain. It could start as sleet/snow if the precipitation races in before the deeper cold air exits. This kind of forecast is always complex. We will be discussing this the next several days.
Have a great weekend and enjoy the 50s on Sunday!