Thursday, November 24, 2011

"Turkey" of a Storm System Still Throwing Forecast Fits...

Two of the major players in the computer forecast model world, the GFS and the ECMWF, are in pretty good agreement with the handling of the Thanksgiving weekend storm and cold airmass, until you reach Sunday.

The series of images below show the GFS computer forecast model (top) and ECMWF computer forecast model (bottom).  The left half of each image shows the weather in the middle levels of the atmosphere, while the right half shows the weather at the surface.  I've drawn the cold front in on the applicable surface sides of each image.

This first set of images is valid at 6pm CST on Friday:

...and then for 6pm CST on Saturday:

Both are forecasting the arctic cold front to blast Southward to the Texas Gulf Coast by Saturday afternoon, with snow likely in the upper Mississippi Valley and upper Midwest (particularly from Minnesota into northern Iowa and adjacent portions of Wisconsin and Illinois.  At the same time, cold air will be spilling Southward rapidly through the central and southern Plains on Saturday, on strong and gusty North winds.

The "plan" diverges a bit by Sunday, as you can see on the same set of images valid at 6pm CST on Sunday:

The GFS model continues to keep the middle and upper-level storm system moving, while the ECMWF wants to hold a piece of it "back" across the Deep South.  This trend of the progressive GFS and the "holding back" ECMWF continues into the forecast images valid at 6pm CST on Monday:

If the Monday GFS forecast verifies, snow will be falling across portions of the Northern Appalachians and into New England.  If the ECMWF forecast for the same time period verifies, there will be a chance of snow Sunday Night and into Monday across portions of the Deep South and precipitation will remain in the form of rain across New England.

At this point in time, I'm inclined to lean more toward the GFS solutions from Sunday into Monday.  I'm just not seeing what would cause the energy to stop moving Eastward across the Southern states during that time period.  I don't see a "blocking" mechanism coming into play at this point in time.  You may remember that the GFS model also showed this blocking solution a few days ago, but has since moved into the progressive camp and has stuck with that solution for several runs now.  The ECMWF model is the only outlier at this time.  It will be interesting to see if the latest run holds on to that solution or begins to make the system progressive like the GFS and other major models.

With the above in mind, here is how the GFS model is forecasting snow depth across the nation at 6am CST on Sunday (most of the snow in the red circled area will have fallen Friday Night and/or Saturday, with most of the snow in the blue circled area falling on Saturday night):

Depending on exactly how this system sets up, there may be a potential for accumulating snow further South into Iowa on Saturday than the model is currently indicating.  In any case, the system will be moving fairly quickly, so I wouldn't anticipate "a foot" of snow being widespread anywhere at this time.  This looks to me like a 2-4 inch event on a widespread basis in the affected areas, with locally higher amounts up to 6 inches possible, mainly across the affected portions of Minnesota and Wisconsin.  

You can see how accumulating snows will spread Eastward into the Ohio Valley and Appalachians by Sunday and Sunday night, with the GFS snow depth forecast by 6am CST on Monday shown below (scale in inches on the right):

Folks with travel plans across the affected regions should keep abreast of the latest updates on this storm system for the weekend and early next week.

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Shamrock said...

Happy Thanksgiving, Rob....I hope you got those lights up on time. How much snow should we get here in NE Tennessee from this system? it's not unusual for us to get snow here at this fact we usually do get our first measurable snow right after Thanksgiving

Rob In Texas said...

Shamrock, Thanks, I hope you guys had a nice Thanksgiving as well!

Still a bit early, but doesn't look too heavy for your area. Current model projections are calling for a dusting to 2 inches in some spots, that would be on Tuesday for you the way it looks right now. Stay tuned for updates!