23 Jan 2012 outbreak: Synoptic evolution

The first moderate risk with tornado potential was forecast last night for portions of AR, MS, TN. A cold front aloft (CFA) associated with a short wave trough at 500 hPa came ripping across OK during the day and became negatively tilted across AR by 00 UTC. Cold advection at 700 hPa was well ahead of the low level front. QG diagnostics at 500 hPa from the 12 hr NAM forecast valid at 00 UTC show the relatively weak cross-front contribution to QG omega confined to AR while further North the along front component was dominant (courtesy of the legacy SUNYPak from UAlbany).

 Soundings were taken at LZK at 21 and 00 UTC which illustrate minor warming (+0.8C at 700 hPa) but but stronger warming (+2.60 at 677 hPa) indicative of the strengthening inversion. Soundings taken at JAN for 00 and 03 UTC showed warming around 600 hPa (+1.3C). Looping the water vapor imagery it appears as though the warming was in advance of the CFA but this effective cap was insufficient to limit convection. All of the available soundings indicate that parcel paths had zero negative area yielding uncapped near surface parcels in the warm sector. With no cap storm coverage was large and relatively uninhibited. With little in the way to focus convection multiple messy lines and clusters formed.

SRH was extreme approaching 500 m2s-2 at JAN at 03 UTC. 1st tor warning south of JAN came out around 0530 UTC. It appears that this line of storms formed along an effective dryline (mostly moisture gradient).

12-14 hr HRRR forecasts valid from 00-02 UTC showed development very similar to observations albeit just about a  or two far east and lacking the secondary more westward line of convection. Given the large 0-1km SRH from observed soundings and the lateness of model convection, it is no surprise why the HRRR failed to show any (and thus not significant) updraft helicity associated with the storms.

This point alone should highlight why it is so tough to forecast severe weather with models that may be only slight late in initiation and slow to develop. 1-2 hours late and 1-2 hours too slow to become significant (in a relative sense) means the models can be as much as 4 hours behind in convective evolution or even later if the environment is evolving and the convection doesn't follow the same evolution as observed.

As for the NAM, lets compare the 36,24,12 hour forecast of the CFA:

The 36 hr (upper left), 24 hr (upper right), 12 hour (lower left) forecast are compared to the analysis (lower right) for the frontal positions (magnitude of the potential temperature gradient) for the 700 hPa (shaded x10-5 K km-1 per 3 hrs), 500 hPa and 850 hPa (6 x10-5 K km per 3 hrs black and blue contours respectively). The difference between 36 and 24 hr is the difference in frontal position at 850 hPa into the cold front aloft. 700 hPa frontal positions were surprisingly stable, albeit with fluctuations in magnitude. So at least in theory this event had some measure of predictability associated specifically with the synoptic precursors, but the dynamical evolution of those precursors had little predictability beyond 36 (or maybe 30?) hours prior to convection initiation.

Chasing supercells

Pretty good chase day today. Any chase day is a great day, but this was good because it ended my absence on the Plains after roughly 4 years. It culminated in a splitting supercell with near tennis ball size hail in a marginal low level shear environment, but favorable bulk shear environment.

Hard to say what the issue with convection initiation was, but most of the updrafts that initiated at least marginal vertical cumulus growth seemed to tilt and then result in small turrets on the top. The clusters of clouds that did manage to grow finally did so after 2200 UTC, and by 2300 UTC we had a nice storm to chase at least from its radar presentation. The base was elevated a bit but a clear updraft bell had formed, and when we drove north on I35 to stay ahead of it, noticed pretty good size hail on the ground. Maybe 2-3 inches in diameter, low-density. I even found some that had split in half so you could see the rings. Most of the hail in the grass was unbroken, but the stuff that had hit the pavement was easily smashed.

The model scenario played out fairly well. A moisture pool was left over from the late afternoon mixing of the dryline further south. Last nights NSSL WRF simulation had 1 storm form west of Tulsa and I took this to be a positive sign that CI was probable. Of course the model had some pretty high moisture values so that was a concern. However, afternoon observations indicated that dew points would hold near 64 F. Of course the low level shear was weak as was the SRH, but the bulk shear was up around 20 m/s. Plenty good for supercells, but not so good for tornados.

The focus for initiation was along a convergence zone in Northern Oklahoma that would eventually redevelop rapidly north, at least in model land. I will have to investigate if this actually occurred.  I think its worth investigating from the modeling standpoint, especially the structure of the boundary layer that led to the mixing out event further south but kept enough moisture further north.

The storm we were on is to the east-northeast  of WDG on the image below at the time tennis ball hail was falling while we were on I-35.

Glad to be back chasing storms...

Uncertainty foci

1. The trough in question is still only half over the upper air network, thus there is uncertainty in its exact structure. It appears to be currently moving more south than east but that trend may be shifting to more east.

2. I found a website to scrutinize the 3 hourly soundings from the NAM. It turns out the cold air advection over OK around 00 UTC may actually be a reflection of a boundary layer deepening up to 550 hPa at KGAG! Thus while it is getting cooler aloft, it does not necessarily imply a forcing mechanism for ascent. Note that 3 hours later, descent is implied from the inversion yielding net warming in the same layer while the boundary layer significantly cools.  The exact role this feature plays remains uncertain.

3. Soundings from central OK show a very shallow moist layer and cap aloft which although weakens some remains strong until after 03 UTC. This is saying a lot since the previously mentioned odd double low level jet is clearly playing a big role in the development of deep moisture. Its even difficult to get moisture into KLZK under this scenario.

Severe weather potential

Interesting forecast shaping up for tomorrow evening. Somewhat strong trough will move into OK tomorrow bringing with i the chance for significant severe weather to central and eastern OK and continuing eastward overnight.

The more certain portion of the forecast is the later period late Sunday night when it appears probable that a severe MCS/squall line will tear up parts MO, AR, LA again. I think its likely as more time will allow for more moisture to advect northward in what amounts to me to be an odd linkage between a pre-existing low level jet well east of the effects of the trough and the trough. The apparent phasing of the two will occur later sunday evening. These highly dynamic environments lead to some interesting MCSs.

The early part of the threat in central OK is less certain. To me, at the moment, the most interesting part of the forecast is that the features of interest appear to slow down before entering it the Plains, then speed up once they do. This makes for a confusing scenario. The prominent features I see this morning are the dryline in western OK, the dryline warm front intersection in NW OK, and the rather strong 700 hPa cold advection over the dryline all at 00 UTC. Now, I have no idea what to expect in this type of slow then fast large scale regime. This fact alone adds uncertainty for my limited experience of significant severe weather in OK during Feb.

What is not clear is if the cold advection aloft will be enough to remove the cap, and if the instability will be large enough to be realized. We just had some rain followed by cold temperatures, and soil water fractions are high apparently. The last 10 and 30 day rainfall maps show that western OK is dry and eastern OK has seen around 2+ inches. So it is possible that strong sensible heating will take place on Sunday in areas that are drier ahead of the dryline. Will this actually matter? I will tell you on Monday.

The other major issue is how the dryline and shear align. At the moment the mean wind for sunday evening appears to be from the SW and the dryline is oriented more north-south. This angle difference will allow storms to move off the dryline. The shear vector should also orient itself across the dryline but the magnitude of that angle will be very important to storm mode.

I guess my inclination at this moment are that all the necessary ingredients can be found, it will be a matter of how they come together and when that will occur and for how long. For now, at least, it is important to get that moisture screaming northward. I could use a good storm chase.

A lot will change. But I think there is a chance for supercells. I am uncertain where convective initiation will occur, and where severe storms will get going, and what that initial mode will be. I will evaluate the Short range ensemble Forecast (SREF) when the 15z run becomes available later this afternoon.  Really this is just me getting my thoughts ramped up ... otherwise known as situational awareness.

NSSL WRF performance over OK

I grabbed some 2 meter temperature forecasts from NSSL's WRF 4 km model simulation from 00 UTC 5 February 24 hour forecast to compare with what happened on Saturday.

The model did not do terribly well but it highlighted the issue with snowpack. Lets go to the pictures:

The images depict the 2m temperature in 2 hour increments from 18 to 22 UTC; with the 22 UTC image being the warmest time of this day. What stands out is the warm air over the TX panhandle which does not expand rapidly into OK. Just east of the OK panhandle it warms rapidly, but it does not expand and penetrate eastward. Over OK, the cold patch, which aligns perfectly with the storm total snowfall and thus snow pack, does not appreciably change shape but it does warm a bit. Clearly the model had a poor representation of the snow cover, both in areal extent and depth.

From the previous post, the high temperatures even over the deep snow pack near Tulsa in the core of the model cold patch, got to near 40F ... a difference of 20 F!

The situation eases later as night arrives by 00 UTC. Below are the 24 hour forecast from the model and the initialization from the next cycle.

The differences between these 2 images is difficult to discern but they are still large, because of the eastward shift of the OK cold patch and the eastward extent of warmer air. This is an interesting case where I would expect this type of model to perform better. Diagnosing the evolution of the snow pack and the low level temperature tendencies both aloft and from within the model physics (boundary layer scheme) should shed some light on why the model performed poorly.

Temperature forecasting over the snow cover

How high will the temperature get across OK over the snow pack?

The sounding above at OUN was taken at the morning low according to the Norman mesonet site. Now it snowed another inch or two yesterday  to make the compacted snow depth anywhere from 3 to 5 inches. The snow we had was an icy consistency. I walked on it and only in the drifts was it actually loose snow.

The forecast high for today was 36 (as of this writing was upped to 38) from the NWS, and I thought that was pretty good. But it is always difficult to predict the high temperature when the ground is snow covered. The high for the day in these situations depends how far from the bare ground you are relative to the low level air mass that is moving in. The 1845 UTC visible satellite image shows where the snow is. The whiter the ground the deeper the snow:

I can't mark where Norman is but trust that we are not very far from the edge of the snowpack and points to the west of us didn't get that much snow in the first storm and barely received any in yesterdays.

However, a forecasting technique I learned at UAlbany was the 850 hPa method. This method assumes that air from above the surface will serve as the upper limit as the boundary layer grows upward.  Of course the atmosphere aloft is not usually stagnant so you need to account for that. This mornings OUN sounding had an 850 T of 1.6 C with even warmer temperatures of 6.4C at 911 hPa.  This suggests a temperature of 14.3 C from 850, or a temperature of 13.6 C from 911 hPa ... again assuming these temperatures change very little during the day. So the mid to upper 50's!

So the forecast problem is the battle between surface effects: how the incoming radiation can heat the ground and help that mixing process along versus how the mixing occurring elsewhere (in this case to the northwest and west) brings in warmer air over the snowpack. The latter is usually referred to as an internal boundary layer.

Now, the temperature is obviously controlled by the snow depth and snow cover and there should be an obvious temperature contrastor gradient across the plains:

Note, the yellow shading which tells us the potential temperature gradient pretty much aligned along the edge of the snow pack from this morning.
I will add the 18UTC and 21UTC maps when available below:

The potential temperature gradient shifted somewhat east as the snow below has melted and temperatures have warmed.

So far at 4pm local time the temperatures in western OK have risen to 62+F and Norman is currently around 45F! This is pretty much going to be the high for the day.
 I will add the mesonets max T plot when it is available:

Now where did this warm air layer aloft come from? Downslope off the mountains as the large scale trough passed through our region. Temperatures at 850 hPa warmed from -15C to 2C in 24 hours at Dodge City, and from -18C to 2C at Amarillo.

A comparison of soundings from Lamont indicated a slow warmup primarily in the boundary layer with just a minor change in the height of warm layer. However by 00 UTC it was clear that the entire column was warming significantly downstream from the mountains. LMN and OUN  were clearly situated in a warm advection regime at 00 UTC. Given the sounding structures, you could have used the 850 method with forecast soundings, not upstream soundings given how much the entire temperature profile changed.

Of course, it is very interesting that the sounding at AMA can not be used to show the large surface warming over western OK. AMA only reached 12 C which is around 50F, while western OK reached 62F. The implication is that the warm pocket over Oklahoma this morning was bigger (or may have even developed aloft) than could be revealed by the sounding network.  Either way these mesoscale details (snow cover or lack thereof, warm pockets of air aloft from downsloping) made it difficult for forecasters to utilize observations for short term forecasts of temperature. This is a much more complex scenario for what seems like a straightforward case of temperature forecasting over snowpack.


As a side note, the temperature warmed above freezing and so all the snow in the rain gauge is melting. Now a ton of it was blown around but what was secured to the gauge from the last two snowfalls should tell us something about the sleet that fell, then the snow, and then yesterdays heavier snow. The liquid total from the melt is: as of 9:40pm is 0.32".