Tuesday, February 1, 2011

The Dry Slot--Blizzard Killer?

Well, it's around midnight central time now--and the snow is still falling for many people.  But it ended rather soon for many as well--too soon, given some of the forecasts.  However, earlier Tuesday it was a very, very impressive looking storm--
Fig 1 -- Radar mosaic from 2008Z, February 1st, 2011.
This storm has a very well-defined structure.  The center of the low pressure at this point (20Z) was in southeastern Missouri.  This is slightly faster and further north than our forecast yesterday led us to believe--here was Monday's 36-hour forecast of the surface pressure field at 00Z Wednesday--four hours after the above image.
Fig 2 -- GFS 36 hour forecast of surface temperature (shaded) and mean sea level pressure (contoured) for 00Z, February  2nd, 2011.
Note how it had the low pressure center in nearly the right place (maybe a bit too far southeast) but...it had it there four hours later than it actually got there.  So this storm was moving faster and slightly further north than models anticipated.

Is it really that big of a deal?  The models generally got it right after all.  However, this small timing and track difference has huge implications for what areas expect to see the heaviest snowfall, what areas will see ice instead of snow, and what areas will see the snow end sooner than expected.

A lot of it has to do with the position of something commonly called the "dry slot".  This is a standard feature found in most well-defined mid-latitude cyclones.  So what is it?

Mid-latitude cyclones are often described as having "belts" of air moving around them.  Think of this like "jets" of air being moved around the low pressure center.  As we know, air wants to move from high to low pressure and air spins counter-clockwise around areas of lower pressure in the northern hemisphere.  We can track air from different regions as it is drawn toward and around the cyclone.  I'm going to focus on two different kinds of air--warm, moist air from the south (think Gulf of Mexico) and cooler, drier air from the northwest (think the high plains or central Canada).  How will these different kinds of air be drawn into the low?
Fig 3 -- Schematic of different "conveyor belts" of air moving into a classic mid-latitude cyclone.
The above schematic shows what typically happens.  Warm, moist air to the south is advected northward to the east of the low.  In fact, the advance of this warm air helps define and drive the warm front to the east of the low.  However, the air already in place gets colder the further north we go.  So as this warm air moves northward, it also begins rising over the colder air already in place.  The closer to the low pressure center the air gets, the more it gets "caught up" (there are more technical details behind this...) in the circulation and gets "wrapped around" to the backside of the low. Because it rises up off the ground more, this air tends to move much more freely and can be somewhat easily brought around to the back side (western side) of the low.

In contrast, the cool, drier air to the northwest isn't necessarily warmer than the surrounding air.  In fact, it's being brought south toward much warmer air.  Because it's cooler than its surroundings as it heads south, this air doesn't rise much and tends to stay near the surface where there's lots of friction (among other things).  As such, this air doesn't move as freely and, while it still somewhat gets caught up in the low's circulation, it can't be as freely advected around the low.  So it usually kind of just gets drawn into the center of the low.  We can see that in the tan arrow of the diagram above.

Of course, to get any kind of precipitation, be it snow, rain, freezing rain, ice, whatever--you need moisture.  The air in the "warm conveyor belt" has lots of moisture.  The colder, drier air moving in from the northwest does not.  As such, we only really see precipitation associated with the warm conveyor belt.
Fig 4 -- Schematic of precipitation areas in association with "conveyor belts".
The area where there is much drier air being advected around and into the low is relatively devoid of precipitation because there is no moisture there.  This area is what is called the "dry slot."

The above diagram should make a whole lot of sense--we see this all the time in our cyclones.  Look how this diagram matches up to the radar image above:
Fig 5 -- Overlay of "conveyor belt" diagram with the above radar image.
This explains that comma-shaped pattern to the precipitation--the warm conveyor belt feeds the areas of precipitation.  Dry air being wrapped into the low from the northwest/southwest, in contrast, forms that "slot" where there is no precipitation--the dry slot.

Now we can explain why subtle changes in the track forecast for this storm are so important.  Take the area around Saint Louis.  Previous forecasts had called for 12-15 inches of snow in the Saint Louis metro area on Tuesday.  They were based on the more easterly track of the low pressure center models had been hinting at.  This would bring that heavy "wrap-around" precipitation on the back side of the low right through Saint Louis and give them an extended period of snow well into tonight.

But...the low was slightly further north and faster than expected--so instead, much of Saint Louis and points south got into the dry slot, which they were not necessarily expecting.  I've heard spotter reports of only 4-5 inches that have fallen instead of the forecast 15.  In contrast, areas north of Saint Louis like around Quincy Illinois, did receive some 10-15 inches of snow--they stayed in that wrap-around precipitation while the dry slot stayed to the south and east.  This is why small changes in forecast position can mean the difference between a good forecast and a bust.  Of course, Saint Louis is still seeing blizzard-level winds, so the blowing and drifting of that snow is creating huge problems.  The blizzard warning is by all means warranted and it is still a very dangerous situation out there...

One additional note about dry slots.  As the leading edge of this low-level drier air advects northward along with the surface low, enhanced convergence can often form near the surface.  Think of this kind of like a sort of dryline like we see during severe weather episodes in the southern plains.  Convergence along the edge of this dry air mass mixing along can be enough to fire off some convection.  Here's a radar image from late Tuesday evening as the leading edge of the dry slot began pushing into southern Michigan:
Fig 6 -- KIWX 0.5 degree base reflectivity from 0706Z, Feb. 2, 2011.
Note how the precipitation in the area I circled seems more cellular--less like the widespread swaths you see up north?  That is convection forming along the leading edge of the dry slot.  Often these are the best places to find that awesome weather phenomenon that is thundersnow.  In fact, as the dry slot approached Chicago earlier Tuesday evening, thundersnow was indeed reported there.  Pretty amazing stuff...

Anyhow, this storm continues to move east--tomorrow is going to be a hectic day for much of the northeast and quite the headache in the central US as recovery from the storm begins.

No comments:

Post a Comment