Sunday, March 6, 2011

Here comes the flood...

Flooding concerns are going to be big this spring after all of the huge snowfalls we've had in the northern part of the country.

For instance, here's today's watches, warnings and advisories map from the National Weather Service.
Fig 1 --  NWS watches, warnings and advisories for 2010Z, March 6, 2011.
We're in between two storm systems in the country at the moment.  Up in the northeast we can see advisories associated with the heavy rain and snow from the same trough that brought tornadoes to the southern US over the past few days.  Another trough is moving across the intermountain west and when the divergence aloft associated with this feature moves over the eastern slopes of the Rockies, cyclogenesis at the surface is forecast to occur.  Cold temperature in the area will bring fairly decent snowfall to the central and northern plains in the next few days.  Hence, we're already seeing all the purple winter weather advisories pop up in areas around Wyoming, Nebraska and western Kansas.

But what I wanted to focus on today is the threat of flooding, something that we've been worried about for a few months now given the big snow events we've had in the northern US.  There's still a lot of snow on the ground in the northern part of the country, as shown in this National Operational Hydrologic Remote Sensing Center (NOHRSC) analysis from today:
Fig 2 -- Snow depth as of 0600Z, March 6, 2011.  From NOHRSC.
The northeast is gaining snowpack even today with wintry conditions as we saw in the warnings above.  But all of the northern plains and upper midwest still have snow on the ground too--over a foot, probably, in much of the northeast and even parts of the upper midwest (the color scale on these images isn't the most high resolution...).  If you think about all of the major rivers in the US whose source tributaries lie in the areas that still have snow--the Missouri, the Red River of the North, the Mississippi, even the Ohio River--you can see that there's still a lot of water locked in snow that will be entering the river systems in the coming months.

We can look at snowpack in another way--the ice-water equivalent.  If you remember from my discussion on snow ratios, one inch of snow doesn't translate to one inch of liquid water on the ground.  Often, one inch of snow is more like a tenth of an inch of liquid water on the ground.  Furthermore, as snowpacks age they tend to compress and go through cycles of partial melting and refreezing, all of which complicates the snow-to-liquid conversions even more.  There are actually many scientists who study how snowpacks on the ground evolve in time.  Such information becomes very important not only for hydrological modeling, but also when we are trying to predict areas where avalanches are likely, figuring out how glaciers are going to evolve, etc.  I find it fascinating how much there actually is to say about how snow on the ground changes and evolves.

Anyhow, the NOHRSC has an algorithm to attempt to compute the liquid water equivalent of the snow on the ground, and they produce a product that shows this:
Fig 3 -- Snow water equivalent as of 0600Z, March 6, 2011.  From NOHRSC.
Any areas where you start seeing the brighter pinks mean 6-10 inches of liquid water equivalent contained within the snowpack on the ground.  That is, if you melted it all down, there would be 6-10 inches of standing water everywhere.  Everywhere.  Now all of that is going to be running off into rivers and streams as it melts this spring--yikes.

We're already seeing the effects in some places.  Notice that even though Iowa, central Illinois, central Indiana and Ohio have seen a lot of snow this winter, the weather has already warmed up enough so that much of it has melted.  There's little to no snow cover in these areas on the maps above.  So all the water contained in that area is filling up the rivers.  Here's an image showing the current river stages at areas across the central midwest:
Fig 4 -- River stage conditions as of 2050Z, March 6, 2011.  From the North Central River Forecast Center (NOAA).
The image above shows the current flood stage at various river observing stations in the region.  Anything orange, red or purple indicates a river that is at or above minor flood stage (in various degrees of severity).  Notice that up north where the snowpack is still on the ground and relatively frozen,  all the rivers are still running below flood stage.  But move into central and southern Illinois and you see that several rivers, particularly the Illinois River, are experiencing areas of moderate to major flooding from all that snow that has since melted away.  It's a similar story if we go east into Indiana and Ohio.
Fig 5 -- River stage conditions as of 21Z, March 6, 2011.  From the Ohio River Forecast Center (NOAA).
Much of the lower Ohio River is beginning to see some flooding.  Other rivers like the White and Wabash Rivers in Indiana are also beginning to see moderate flooding.  As areas further upstream keep melting snow, the number of locations seeing flooding will start heading up...

So where will we see flooding next?  Every cyclone that moves through brings another wave of melting.  A sudden warm spell with rain and thunderstorms on top of snow cover can initiate very fast melting.  Any time liquid water mixes with snow, lots of melting begins.  So in a radar image like this on the east coast right now:
Fig 6 -- Base reflectivity radar mosaic over the northeastern US at 2118Z, March 6, 2011.
...we're seeing a lot of rain falling on areas with snow cover still on the ground.  Granted, much of the precipitation in far western Pennsylvania, upstate New York and northern New England is falling as snow, so that's just adding to the snowpack.  But down in eastern New York, eastern Pennsylvania and the middle Atlantic states, we're just seeing heavy liquid downpours.  Recalling in figure 2 above that some of those areas had some decent snow still on the ground, melting is going to occur--and rivers will rise.  Here's the NOHRSC's analysis of how much liquid-equivalent of snow has melted in the last 24 hours across the US.
Fig 7 -- Previous 24 hours of liquid equivalent snow melt as of 5Z, March 6, 2011. From NOHRSC.
Note there's a wide swath of 1-2 inches of liquid water that as been released due to melting snow in a swath stretching from southern Michigan through southern Ontario and into New York and Pennsylvania.  That's a lot of water being released to drain into the river system--particularly since a lot of the tributaries of the Ohio River start up in the western Appalachians of Pennsylvania and New York.  So we might expect flooding to translate down the Ohio River Valley.  Here's the Hydrometeorological Prediction Center's latest significant river flood outlook between now and Thursday:
Fig 8 -- HPC Significant River Flood Outlook valid through Thursday, March 10, 2011 and issued on Saturday, March 5, 2011 at 2034Z.  From the HPC.
They expect significant flooding to continue in the lower Illinois River valley and parts of the Wabash, White and lower Ohio Rivers through Thursday.  However, note that they also state that significant flooding is likely or possible in much of southern New York and northern Pennsylvania--right where we're seeing all that rain melting the snow pack today.

Now, just imagine what would happen if we had a storm with a very northerly track that brought a significant warm sector, downpours and thunderstorms to Minnesota and northern Wisconsin where there's a lot of liquid equivalent snowpack still on the ground.  All this flooding "potential energy" is just waiting to be released up there.  We will have to wait and see how the Mississippi and Red Rivers (of the north) react as snows continue to melt throughout the spring.  In the meantime, though, flooding is already happening, so if you live in an area prone to flooding, watch your local forecasts and be prepared.

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