We'll focus on the black contours for now--those are height contours, telling us the height of the 500mb pressure level above sea level. The green shadings here indicate where there is a lot of moisture, but that's not as important for what I'm looking at. The basic flow pattern shows a shallow trough over the Pacific Northwest (which is making is more cloudy than sunny in Seattle right now), a big ridge over the central US and a cut-off low over the upper Ohio River valley. Not entirely unusual...we're seeing some rain along the east coast in association with that low, but otherwise nothing too odd.
Let's see what our models are doing with this weather pattern over the next few days. Here's the GFS 24-hour 500mb height forecast for tomorrow morning. Now we've switched to where the color shadings are vorticity, but the black contours are still the height of the 500mb surface.
Mostly the same pattern, but the west coast trough has dug in a little more and the cut-off low over the Ohio River valley has drifted a little to the southwest even more. Let's fast-forward another 24-hours to Sunday morning's forecast:
Now is where things really are getting unusual. That cut-off low is moving rather steadily to the west! Normally our troughs in the mid-latitudes move from west to east--not from east to west. But here we have a relatively strong cut-off low moving westward. Another 24 hours, now the forecast for Monday morning:
The low continues to move west. It's also interesting to observe how this low is forecast to undercut that giant ridge in the central part of the country. On the larger scale, that ridge isn't really forecast to go anywhere...the low just kind of slides in underneath it. Tuesday morning's forecast now:
The cut-off low has migrated back all the way to the desert southwest. Broad ridging is forecast for the eastern 2/3 of the country with that low still holding on down in the southwestern corner. By next Wednesday the GFS has the low weakening to a remnant trough with is probably being forced by the ongoing "monsoon" season in Arizona. Enough latent heat is being released with the ongoing convection there to lower the pressure and heights and maintain a "monsoonal trough".
So what does this westward-moving low mean for our weather? The unusual orientation of the low will mean some atypical wind patterns. As the low moves westward across the plains, easterly winds will help advect moisture straight toward the Rocky Mountains in Colorado and New Mexico, setting the stage for some heavy rain there late this weekend as the GFS is hinting:
Likewise, extended periods of southerly winds on the eastern side of the low will help to bring up very moist air from the Gulf of Mexico through the southeast, Ohio River valley and into the upper Midwest by the end of the weekend. This "precipitable water" forecast shows the the forecast total amount of water vapor in the atmosphere above any location. Lots of moisture in a plume to the east of the upper-level low.
Definitely something fun to watch!