Analogs to Saturday's potential for severe weather

As many people are now aware, this weekend looks to have some potent severe weather in the southern Plains.  The SPC has a day three slight risk for a wide area of Nebraska, Kansas, Oklahoma and Texas on Saturday:

 The model forecast fields are pretty good---a strong upper-level trough digging across the central Rockies with a powerful jet streak entering the base of the trough on Saturday evening.  Divergence downstream from the trough axis looks to provide broad lifting support across a very moist warm sector, in addition to enhancing strong lee-cyclogenesis in eastern Colorado.  Strong directional wind shear is forecast with southerly flow at 850 hPa to near westerly flow at 500hPa. All classic ingredients for severe weather.  Here is the NAM 60-hour forecast from this morning's run:

We're looking at surface pressure and precip in the upper-left panel, 850hPa heights and winds in the upper right, 500 hPa heights and vorticity in the lower-left and 300hPa heights and winds in the lower right.

One method that I've occasionally mentioned for predicting the eventual outcome of a particular forecast is to look for analogs in the historical record.  That is, to go back in time to when the atmosphere was in a similar state (or, better yet, when the model forecasted a similar state) and see exactly what ended up happening.  There are groups that do this sort of thing in real-time.  One is the Cooperative Institute for Precipitation Systems (CIPS) group at St. Louis University.  With every model forecast they go back through the North American Regional Reanalysis archives and try to find days from the past when the forecasted pattern was similar.  We can then look at what actually happened on those days to get a sense of what may happen now.

Take this Saturday, for instance.  If we go to the CIPS southern plains sector, we see that their number one analog for the NAM forecast for that evening is 00Z, April 26, 1984.  Here's the same four-panel plot as above, but showing what happened at that time:

You can see a lot of similarities in these patterns! Both had similar strong lee cyclogenesis at the surface in Colorado underneath a broad upper-level trough over the intermountain west. It makes sense that these are a close match.  So what ended up happening on April 25-26, 1984?  Here are the severe weather reports from that day:

Five tornado reports, mostly in Nebraska.  Lots of hail reports in Nebraska with a few scattered elsewhere.  At least we know this pattern is capable of severe weather.

Let's look at the second-best analog identified by CIPS: 00Z, April 10, 2011.  Here's the pattern:

Still similar, though the differences are more apparent.  The surface low in eastern Colorado is not as intense.  The upper-level trough is also much narrower and more positively tilted.  Still the same "overall" pattern, but there are noticeable differences.  What happened on this day?

There were still hail reports in eastern Nebraska with tornado reports in western Iowa.  There's another whole area of wind and hail (with a few isolated tornado) reports located across the southern Appalachians.  It seems that subtle shortwave in the upper-level analyses over West Virginia-Virginia may have played a bigger role than one might have thought.

Finally, the third closest analog---00Z, May 4, 1999.  That's the evening of May 3, 1999.  You know what's coming.  Here's the pattern:

Again---there are definitely differences.  The 500hPa trough for this event isn't as sharply defined as in the current NAM forecast.  There's also a jet streak across the central Midwest in the current forecast that was not there on May 3rd, 1999.  These differences are pretty important.  With stronger and sharper large-scale dynamics being forecasted for this weekend, I feel like the severe weather may organize more upscale more quickly than it did on May 3rd, 1999.  Such strong synoptic-scale dynamics can work against maintaining discrete storms for very long.  Anyhow, here's the storm reports from May 3-4, 1999:

This was the day of the first Moore F5 tornado.  Lots of severe weather throughout Oklahoma and, generally, in the same corridor that the SPC has outlined.

The CIPS site also breaks out the analogs by individual variables and how closely they "match" the forecasted pattern.  For instance, if we're interested in moisture return, the May 3-4 event has the highest correlation of these three in the surface dewpoint field.  The 1984 event, however, has the higher correlations for overall upper-level patterns and winds.  So it's a mix and match sort of deal...

How much can we trust these analogs to tell us what's going to happen?  You can see that in just the top three closest events there are already notable differences between the patterns on those days and the current NAM forecast.  Plus, as each new forecast run gets out, the closest analogs change.  We also have only a limited record (some 40-50 years) that we can go back to look for close matches.  Furthermore, in just three top analogs we've identified very different regions as areas of high impact.  What we CAN say is that this sort of pattern definitely is conducive to severe weather, and in roughly the area the SPC has outlined.  Beyond that, you still need your expert forecasting skills to help narrow down the exact areas favorable for severe weather with this particular setup.

An analog context for the cold wave

Much of the country is experiencing some of the coldest weather it has seen in many years this week with temperatures struggling to get above zero Fahrenheit and wind chills in the frigid -50 to -60 degree range in some places.  Here's this morning's surface temperature analysis:

Somewhat complicated plot, but I like it because it nicely divides the above freezing (red) and below freezing (blue) temperatres by the color of the contours, giving an idea of the extent of the cold air.  You can see the strong cold front that has been pushing through the eastern United States today as it trails behind a low pressure center that was over southern Ontario this morning.  Frigidly cold temperatures.  This air has been travelling rapidly down from the arctic over the past several days.  Here's a plot showing the air trajectories over the past three days for parcels of air in the lowest 1km of the atmosphere.  You can see that they have been travelling all the way from the Arctic Ocean down to the midwest.

One notable feature of this air transport is that it has been relatively slow to warm up as it plunged south.  The bottom panel of that plot shows the temperature of this air (in Kelvin) as it has moved along (it's read from right to left with the left side being last night).  Some warming of the air has come because the air has descended a bit.  But in terms of sensible temperature it has only maybe warmed about 10 F (difficult to estimate) over its entire journey.  Part of this is because it has traveled over an area that is now entirely covered by snow, keeping the low-levels of the atmosphere relatively cold.  Here's the latest snow depth estimate map from the National Operational Hydrologic Remote Sensing Center (NOHRSC):

You can rest assured that most of northern Canada has a solid snow pack too.  All of this snow worked like kind of a refrigerator, keeping that cold air cold as it moved south.

We can see this cold air mass nicely outlined by looking at the temperatures above the surface.  Just above the surface at 850mb we see from this morning's NAM analysis that we had -30 to -35 Celsius temperatures over the upper midwest:

When is the last time we had temperatures this cold?  One ever-developing tool in meteorology is the ability to search for analogs--that is, to find example patterns of atmospheric conditions in the past that are very similar to what we are experiencing now or expecting in the forecast.  The cold wave of January 1994 is a good analog for our current cold weather outbreak, though that cold wave was a bit longer in duration.  Here's an example of the 850mb temperatures from the middle of that event:

A similar pool of very cold air over the upper midwest.  That's -36 degrees Celsius over northern Wisconsin and Minnesota.  Wikipedia's brief page about this event has some of the following nuggets of trivia:

• Chicago got down to -21 Fahrenheit with wind chills down to -55
• Major snowfalls across the eastern half of the US
• Reagan National Airport in DC had a record low high temperature (for the 20th Century) of 8 Fahrenheit
• Pittsburgh got down to a record low -22 Fahrenheit

Do these headlines sound at all familiar?  Maybe it's because we're seeing something similar again:

• Chicago got down to -17 Fahrenheit this morning with wind chills down to -42 Fahrenheit
• The 11 inches of snow that has fallen in Chicago is the most in a single event since 2011

The bulk of the coldest air is just moving into the eastern part of the US right now.  Given how similar this event seems to be to the 1994 case, we might therefore by analog expect these kind of chilly temperatures for DC and Pittsburgh later. It doesn't look like we'll get as cold---DC is forecast to have a high of 17 tomorrow and Pittsburgh is only forecast to fall to -10---but still, this gives us an idea of what to expect.

Another side of this kind of analog forecasting is that we can use it to try and describe more abstract effects of the weather on lives and property.  Trying to describe the societal impacts of weather is something that the meteorological community still struggles to do well.  We can give you all the numbers you want for how cold it is going to get or how strong the winds will blow, but what does that mean for peoples' health and safety?  We've developed some systems to try and address this---the Saffir-Simpson Hurricane Scale, for instance, relates the wind speed of a hurricane to the type of damage we might expect from such a storm.  But we don't have these kinds of scales for everything and they are not perfect.

By looking at past analogs, we can get an idea of what we could expect based on what happened last time.  For instance, during the cold of January 1994, over 100 people died due to consequences of the cold weather.  Quoting from that Wikipedia article, United cancelled over half its flights, pipe explosions due to freezing cut off water to thousands of homes, Chicago schools closed, and hundreds of drivers in Chicago could not start their cars due to the cold.

What do we have right now?  O'Hare is backed up for four days to get flights out due to cancellations from the snow and cold, Chicago's schools are closed...we'll have to wait to see how many people suffered from exposure or lost their utilities.  My point is that by using analog forecasting we can get a hint of the kinds of consequences of the weather that our models cannot predict by turning to past experiences.  This is a powerful forecasting tool that is only beginning to be exploited...