Thursday, 11:30 a.m.
It would be nice if this storm coming up tomorrow into the weekend would be made up of one disturbance - one storm, a warm front, a trailing cold front - and that's it. You know the classic Norwegian cycle model that many of us studied in science classes growing up, or that I studied back in college at Penn State. While those types of storms do occur, more often than not, it's not that cut and dried. Such is the case this coming weekend.
From everything I can look at, there are arguably three separate features that will go into this amorphous blob we'll be calling a snowstorm. In the end, there will be a swath of snow, there will be an area that gets all rain and there will be some places in between that get a little of both, even some ice.
So, what are the three pieces? Well, two of them are tied in to the same basic feature: an upper-level over southern California, seen here in the water vapor imagery from late morning:
You can see the impact of this feature already on the southern Plains, as a wide plume of subtropical moisture is carrying high-level moisture from the eastern Pacific across Mexico into New Mexico and Texas. As the surface high that currently stretches from central Texas to western Kentucky retreats from the region, the return flow on the back side of the high will quickly send more and more moisture into the low levels of the atmosphere. That, in turn, means a rapid increase in clouds this afternoon and tonight, followed by some rain breaking out late tonight and tomorrow.
By Saturday morning, the rain will spread across the Mississippi Valley into the Tennessee Valley, though as the moisture moves into the retreating cold air across the Ohio Valley, there will be some snow, too. Look at the 12z Dec. 12 NAM 850mb forecast for 1 p.m. Saturday:
Where the air remains cold enough, it'll be mainly a snow event, and that will include areas from northern Missouri and southern and eastern Iowa eastward through northern Illinois and northern Indiana into a sizable portion of Ohio.
If you look closely at that image, you will see a closed contour over Illinois and Indiana - that closed low represents the 850mb low, and there will be a surface low nearby that will work in tandem to try to pull the warm air as far north as possible. Look at the same image 12 hours after, and you'll see the three separate features I'm talking about:
The first one is beginning to fall apart as it lifts northeastward over the eastern Great Lakes, giving way to a secondary feature that is represented by a 'kink' in the height lines along the mid-Atlantic coast. To see this secondary system better, look at the accompanying surface forecast:
The secondary low, or coastal low, should steadily deepen Saturday night as it pull away from the mid-Atlantic coast and cuts by Cape Cod. The warming begun with the approach of this feature from the southwest will progress only so far before being cut off at the pass by the development of the coastal low. Just how far north that warming extends depends on which model you cast your lot with. The European model barely gets it beyond the southern New England coast. The GFS, on the other hand, takes it north of New York City and Boston before it returns from the north and northwest late Saturday night and Sunday. The latest NAM is somewhere in between.
I mentioned three different parts to this storm. The first is the primary low moving into the Ohio Valley Saturday morning. The second is the secondary storm that forms along the mid-Atlantic coast then bypasses Cape Cod early Sunday. There is a third feature, and if you look back at the two images above, you can see it farther northwest over the upper Great Lakes. Here's what that looks like at 500mb Sunday morning, based on the NAM model projections:
That may be overdone in the grand scheme of things, but the impact of the upper-level low is to help drive more arctic air across the Midwest into the Great Lakes Saturday night into Sunday, more or less guiding the primary storm out east of Nantucket. It will cause some generally light snows across parts of the Midwest into the Great Lakes Saturday and Saturday night, that will degenerate into lake-effect snow later Saturday night and Sunday.
When you put all of this together, it will be a snowstorm but not necessarily a debilitating one and certainly not destructive or crippling in any way. Here's the latest thinking from AccuWeather.com for the whole event, looking first at the western half of it, if you will:
And look at the eastern side of things:
The main concern is that parts of central and even interior southern New England could get 6 to as much as 12 inches of snow by the time the bulk of the storm is over by midday Sunday (save for Down East Maine). If there's less warming across southern New England, then those areas could wind up with more snow.
Behind the storm, it's cold again, though not quite so bitter this go round. Colder air may be waiting in the wings for midweek.
The steamy weather from the Mississippi Valley to the East Coast today will be trimmed only slightly behind a cold front tomorrow into Wednesday. It will heat up again late this week.
Summer's heat and humidity will expand into the East over the Labor Day weekend, while at the same time it turns cooler from the Northwest to the northern Plains.
A series of cold fronts will impact the weather now through the weekend, largely across the northern half of the country.
Just as I head for British Columbia to do an Ironman, the recent heat is leaving and heading onto the Plains, while it cools off in the Northwest for a spell.
A disturbance in Wyoming will be the focal point of unsettled weather into next week from the northern Plains to the Northeast, keeping heat at bay.