Will Sunday's storm produce any snow?
Some are probably wondering why I haven't updated Monday's discussion of the storm slated for late Saturday night into Sunday. The primary low looks increasingly like it will track towards the Great Lakes region which should bring enough warm air into the region and upper levels to keep the bulk of the precipitation as rain.
However, considerable uncertainty continues about the precipitation type at onset and at the end of the storm. In essence, there is still a chance that our western suburbs could start as a wintry mix, probably sleet or freezing rain late Saturday night or early Sunday morning and then there is also a possibility that the precipitation could end as a period of light snow. I'll try to explain some of the reasons for the forecast uncertainty and then will make a stab at what I think might happen.
The discussion will start with what we do know. A storm will track towards the Great Lakes and will be located near northern Illinois or Northern Indiana by 7 p.m. Saturday (see figure below, left panels). By that time the temperatures at around 5000 feet are expected to be above freezing in much of the Washington area. The low level flow ahead of the Great Lakes low is fairly weak at that time which will keep the low level warm advection and mixing to a minimum which could allow surface temperatures in the colder suburbs to remain near or below freezing at the precipitation's onset.
Much of the uncertainty later in the forecast stems from questions concerning where and how fast the secondary low will form and how much energy will be available with the upper level system lagging behind it. The two models I'm analyzing, the American (GFS) and European (ECMWF), diverge significantly by 7 p.m. Sunday.
The American model forms the secondary low quicker and farther east than the European model. In essence, the American model favors less low level warming ahead of the system with a greater chance of having some snow at the back end of the system as temperatures at the surface stay in the 30s during Sunday providing there is enough upper level energy to provide lifting. By contrast, the inland track of the European model and slower movement would allow for warmer temperatures making it harder to get snow at the end of the storm.
The ensemble mean and bulk of the individual GFS model runs (or ensemble members) suggest that the upper level trough will be unusually strong (see below and note the blue and purple area) as it crosses the region raising the possibility of snow. The ensemble members are pretty similar in terms of the strength of the trough as can be seen by how closely clustered the blue lines on the right hand side of the chart below. All are indicating the trough is a strong one. The difficulty with the precipitation call during the latter half of the storm is that there will be two opposing forces acting on the weather. Low level cold advection (a precipitation inhibitor) will be battling the dynamics associated with the upper trough (a precipitation producer). Unfortunately, the numerical models often don't correctly foretell the winner of these atmospheric conflicts.
The GFS (American) model does forecast snow at the back end but during much of its snow, the temperatures in the lowest 500 ft of the atmosphere are forecast to be a little below freezing during the period when the bulk of the precipitation is expected to fall. On the chart below a vertical temperature profile is shown. The period when the precipitation is forecast is the period between the two blue lines. at the end of that period temperatures start collapsing. Delay that cooling and the event is all rain, speed it up a little and any period of snow could be prolonged a little.
Will we see a few flakes, maybe, especially if you are like me and look in the mirror. We could see a brief period of snow if we are lucky (or unlucky). This idea is also supported by the NAM model that just cam in at around 10 AM this morning. The GFS model that just came out redevelops the low to our north and it implies less snow on the back end than last night's runs.
Storms that reform to our northeast rarely produce much if any accumulations unless there is a closed 500 low to the southwest. My guess is if we see any flakes, they will not accumulate.
The temperatures early next week should mirror or be more severe than those from this past week. More strong northwesterly winds will bring frigid weather into area. The same northwesterly wind will bring another bout of heavy snow squalls into the mountains of western Maryland and West Virginia in the same areas hit hard earlier this week.
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