Friday, July 12, 2013

A retro upper-level low!

This isn't a "major" weather event or anything like that...but it is rather unusual.  Let's take a look at the current 500mb analysis from this morning, courtesy of the HOOT site:

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.
This upper level low is forecast to remain well south of the main jet stream, and there is no strong surface low forecast to accompany it.  As such lots of organized severe weather appears somewhat unlikely with this setup.  Though, that  separation from the main jet stream is actually helping to allow the low to retrograde westward like it's doing.  It turns out that if we had no jet stream, no zonal-mean west-to-east flow to push troughs and ridges along to the east, then all of our ridges and troughs would naturally move themselves westward.  Since this low has developed so far out of the main west-east flow (which is being deflected around it  to the north by that giant ridge over the central part of the country) we get to see the trough exhibit its natural tendency to move westward.

Definitely something fun to watch!

1 comment:

  1. Nice synopsis, Luke. It kind of looks like the upper low simply decided to join the same tropical wave responsible for Chantal, itself a somewhat anomalous July event in the Caribbean. Is the ITCZ okay? The GFS might even have that same "wave" departing Baja a week from now, at which point it will have other forecasted retrograding 500mb lows over the Pacific to interact with.