Sunday, April 10, 2011

So what makes a PDS watch?

Last night was a lot more active than I expected.  Several tornadic supercells did severe damage in northern Iowa.  These storms fed off of convergence associated with the warm front that was lifting north through the state at the time.  I'll admit that in my last blog post I wasn't really looking at the warm front at all.  But, it is a front and there is convergence along it--enough to force some powerful storms.

Today the SPC has moved the moderate risk further north into Wisconsin and eastern Minnesota, mostly.  At 3:30 PM CDT this afternoon, the Storm Prediction Center has issued their first tornado watch of the day, and it's a special type of watch that many people aren't as familiar with--a PDS tornado watch, where PDS stands for Particularly Dangerous Situation.  What makes this kind of watch special?

Fig 1 -- SPC graphic showing extent and risks associated with PDS tornado watch 120.  as of 320 PM, CDT, April 10, 2011.
PDS watches are pretty rare--according to a study by Dean and Schaefer (2005), by 2005 only 216 out of 3058 tornado watches (around 7%) were "particularly dangerous situations".  I personally see these kinds of watches issued more frequently on the high plains and south central US as opposed to the upper midwest.

So what are the qualifications to be a PDS tornado watch as opposed to a normal tornado watch?  Well, as far as I can tell from reading the background literature, technically there aren't any hard numbers to describe criteria that differentiate between the two.  To list a watch as a PDS watch is a choice by the SPC forecaster to emphasize their belief that this could be an extremely destructive event.  Typically when the ingredients seem to have come together to produce violent tornadoes (EF 2 or greater) or they are expecting extremely strong winds over a large area or hail of unusually large size the forecaster will issue the watch as a PDS watch.  According to that same study by Dean and Schaefer (2005),  in PDS tornado watches on average, the number of reports of EF 2 or greater tornadoes is three times as high as in normal tornado watches.  I've also found that it's not only how potentially bad the threat could be, but also how widespread this extreme weather is expect to be that often dictates a PDS watch.  Both tornado watches and severe thunderstorm watches can be issued as PDS watches.

The explanation for this particular watch is right in the watch text:


--From the SPC PDS Tornado watch 120 text.

So what is the setup this afternoon so far?

The cold front has asserted itself and overtaken the dryline down into central Iowa.  The warm front has also moved up into northern Wisconsin by the middle of this afternoon.  Here's the SPC mesoanalysis graphic for 1900Z ( 2 PM CST) of surface temperature, winds, and moisture.  I've added my analysis of where the low pressure center is and where the frontal boundaries seem to be.
Fig 2 -- SPC 1900Z Mesoanalysis of surface pressure (Black contours) temperature (red solid and blue dashed contours) and dewpoint (colored shadings) for April 10, 2011.
We can see that northwestern Wisconsin is right in that "sweet spot" just to the southeast of the low pressure center where the most severe storms tend to form.  Also, there are parts of this mesoanalysis graphic I don't trust.  For example, notice how in north central Wisconsin there seems to be this dry bulge in the dewpoint shadings behind the warm front.  I don't trust this analysis--looking at actual reports  in that area, they show
dewpoints in the low 60s across all of northern Wisconsin--enough that they should be included in that shading.  Also notice how the mesoanalysis seems to ignore Lake Michigan--all of the contours go around it.  I don't really believe that either.  This is just more proof that you can't trust computer analyses blindly...

So we've got good moisture in that area.  But there's moisture all through that warm sector down into northern Illinois and eastern Iowa.  Why is the strong "particularly dangerous" tornadic situation being highlighted further north?

We have strong low-level wind shear throughout the area.  Here are the wind observations (from vertical wind profilers) at 1500m and 6000m at 2PM CDT:
Fig 3 -- VAD wind profiler winds at 1500m (red) and 6000m (blue) at 1900Z, April 10, 2011.
Notice that throughout Wisconsin, the winds veer with height--their direction changes from southerly at 1500m (the red barbs) to more southwesterly at 6000m (the blue barbs).  Furthermore, wind speeds also increase--from 20 or so knots at 1500m to 70-80 knots at 6000m.  So, we have a combination of both directional shear and speed shear--a good combination for rotating storms.

However, where will there be the most wind shear?  The blue wind barbs in the above image show the winds at 1500m--what if we went all the way down to the surface?  Go back to figure 2 above (the surface map) and click on it to enlarge it.  Look at the difference in the direction of the surface winds in northern Wisconsin as opposed to in northern Illinois.  The winds at the surface in northern Wisconsin have a more southeasterly direction as opposed to the winds in northern Illinois that have a more southerly direction.  This means that the winds in northern Wisconsin turn more with height in the near-surface layer than they do further south.  It doesn't seem like that much of a difference, but it can be enough to increase the potential for tornadic storms greatly.

What about instability?  The 1800Z sounding out of Minneapolis shows this:
Fig 4 -- 1800Z sounding from KMPX on April 10, 2011.
There still was a capping inversion present in the 800-850mb layer at 18Z (around 1PM CDT).  This is what has been holding back convective development so far.  Above that, though, steep lapse rates (the temperature falling off rather rapidly with height) are contributing to a CAPE value of 1824 J/kg--definitely unstable enough for storms at Minneapolis, and probably more unstable to the east.  With such a capping inversion in place, however, I'd expect the first storms to be firing off in the areas with the greatest potential for lift--the frontal boundaries.  Later on this afternoon as we get into the evening, perhaps there will have been enough heating during the day to erode the capping inversion enough for storms to fire out ahead of the fronts.  This may coincide with that dangerous time around sunset where the low-level winds really start picking up in speed.  This will increase that low-level wind shear even further and could make for some very tornadic storms.

We can see in the visible satellite image that storms are already beginning to fire--and right along the cold front, too, like we'd expect to see with a capping inversion in place:
Fig 5 -- GOES-E visible satellite imagery from 2015Z, April 10, 2011.
I've learned my lesson from yesterday--I'll also be watching that warm front draped across far northern Wisconsin and the upper peninsula of Michigan to look for more development there.  I'm also concerned with the "triple point" further south where the dryline intersects the cold front.  In the surface map above, I've analyzed that in central Iowa.  As this system moves east-northeastward, wind shear may maximize around that triple point as surface winds adjust to the complex pressure fields there.  As such, as storms fire further south along the cold front/dryline, there could be enough shear to get some stronger-rotating storms in that area too.

It's going to be an active afternoon and evening--a particularly dangerous one, according to the SPC.  Here's a summary of what I'll be looking at in the coming hours:

  1. Storms will continue to increase in coverage along the cold front and probably start firing further south along the dryline in central Iowa as well.  If these storms, though with lift provided by the cold front, can stay surface-based by drawing in warm, moist surface air from just ahead of the cold front, they could become tornadic.  Large hail will also be a concern with these storms. 
  2. Keeping an eye on the warm frontal boundary in northern Wisconsin as another focal point for convective development.
  3. As the afternoon wears on into evening, we may see some storms fire out ahead of the advancing cold front.  These storms have a higher potential for being surface-based and, if they can tap into the strong wind shear, particularly in northern Wisconsin, could produce some long-lived tornadoes.
  4. Also somewhat concerned about enhanced low-level shear near the "triple point" intersection of the dryline and cold front moving eastward through eastern Iowa.  
  5. After night falls, low-level winds will pick up even more.  This increased shear could increase the tornado threat much more broadly across the entire region.  However, if those high winds in the low levels mix down behind the cold front, it may accelerate the front and form storms into a squall line along the advancing cold front.  These storms will pack some very, very strong winds.  I'm guessing this will be the main threat late this evening for eastern Wisconsin and across Lake Michigan into Michigan.
So...if you live in those areas, be aware this evening.  Keep in touch with a good weather information source and be aware of the changing weather around you.

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