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.|
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.|
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.|
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).|
|Fig 5 -- River stage conditions as of 21Z, March 6, 2011. From the Ohio River Forecast Center (NOAA).|
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.|
|Fig 7 -- Previous 24 hours of liquid equivalent snow melt as of 5Z, March 6, 2011. From NOHRSC.|
|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.|
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.