We are now approaching the total solar eclipse on April 8 so the skills of our models are now in high demand.
And many of these increasingly accurate forecasts represent a real threat to good visibility in Texas and much of the Midwest, where many plan to go.
A double threat exists: clouds involve two characteristics: subtropical jet afound an area of low pressure moving from the southwestern United States
This blog will describe these important features and current uncertainties in the forecast.
Let me start with the cloud cover forecast near the time of Monday’s eclipse (11 a.m. PDT, 2 p.m. EDT, April 8) by the Eurocentric model this morning. The center of the total path is shown by the blue line. If this forecast is accurate then everyone in Texas will be fine, but the cloud fields will definitely change a bit.
There are two main cloud areas of concern. The first is a long line of clouds (red arrow) associated with the subtropical jet stream (or jet stream). The other is a cloud field associated with a low pressure center over New Mexico and Colorado. A change in either feature is, pretty much guaranteed, going to be a real problem.
The US model-predicted cloud field (GFS) is also bad for eclipse enthusiasts (see below), with subtropical jet stream clouds moving directly into Texas and most The path of the total solar eclipse is covered by clouds.
So what is the subtropical jet?
Many of you have heard about it polar or mid-latitude jet stream, a region that typically affects the Pacific Northwest. But there is another jet stream to the south, the subtropical jet stream (see figure below), located near the northern boundary of the tropics.
The subtropical jet stream typically draws moisture (and clouds) from the tropics into the mid-latitudes. In fact, there was a nice subtropical front this morning that brought a lot of moisture to the southwestern United States.
The other key element of this forecast is an upper low/trough in the southwest, with a second low over South Dakota (see chart below at the same time as the images above). This chart shows the situation at about 18,000 ft (500 hPa). There is upward motion and clouds to the east of these features.
There is technology to determine this forecast uncertainty by looking at a set of multiple forecasts, each slightly different. The following “stream chart” shows the amount of water vapor above Dallas, TX for each forecast. Critical moments are represented by red arrows. Many forecasts call for a lot of water vapor, which means lots of clouds. Not good.
The new US forecast (GFS) has just been released (below). Subtropical jet stream moisture blankets Texas and clouds from low and associated northern fronts are disturbing much of the path across the Midwest.
At this point, I’ll probably be aiming for northern New England. There’s still some uncertainty in the forecast, but we’ll know pretty much the final story by Friday.
