March 2006

And yes, that is broad. But higher resolution is not justifiable at this point. Northeast and North Central OK is looking even better for individual SVR cells, but I’ve paid more attention to the KS setup - what could be within driving distance.

Without the opportunity to chase tomorrow, I’ve admittedly put less into the forecast for this event than I would have in preparation for a full-on chase, but tomorrow is still looking good. This is not as clear-cut of a “cold-core” case as was previously suggested by the GFS (500mb temps have warmed, sfc moisture is progged now to be higher than previously forecast, etc…) but it will still clearly be a springtime setup - overcoming a dry boundary layer takes extra work. And it’ll be an early start.

The dryline surging east through KS will be a nice forcing mechanism, although there is still little moisture in place in this area so all necessary moisture must be advected in over the next ~18 hours over a relatively dry surface. Model forecasts for 60+ Td’s are not out of line, but it isn’t a given. Anything lower will act to lessen the differential across the focus of lift: the dryline. Additional uncertainties and difficulty in pinning a point target are introduced by the sloppy morning/midday convection that is likely in Eastern KS before the real show. It appears as though the best SVR shot is the early afternoon play: as soon as convection fires not long after noon in the areas that cleared out the earliest. There is virtually no cap in place, so whenever it gets a little kick, the show will be on, before going linear with unidirectional sheer in the afternoon/evening and marching east across the KS/MO as a nice rush-hour squall line. Attempting to forecast locations of boundaries that setup mid-day following that early convection is nearly futile this early, so the best play from my non-deployment “cyber” chase status is to remain ambiguous: As far as a relatively local target, I like the looks of Eastern KS in the early afternoon, particuarly whatever areas can clear out earliest. From the computer, I can get to any target I want, so I don’t need to be more specific than that!

Thursday (3/30) appears to be the next best shot at some severe weather for the Central Plains and already the setup is a bit intriguing, even this far out. The models are far from being in agreement as of yet, but if the current trends continue, it could make for an interesting end to the week. The GFS is firing the surface low far to the north, but the potential for another developing to the south exists. I likely will not be available to chase on this event, but will still be eager to see it play out.

Chaser Mike Perigrine of the Kansas City area recently posted a great overview of the potential for this to be another cold core severe weather setup in his posting Take a Cold Core Shower. Cold core setups and the dynamics involved are virgin material for this chaser just recently transplanted to Kansas. After reading up on some of the recommended reading by Jon Davies on 500 mb cold core lows, including a case study of a setup here in KS last fall, I am intrigued by the specific, yet often times less complex techniques that are called for when it comes to forecasting and chasing a system such as this. The first hurdle is that one must first throw out many of the traditional headlines - instability, T and Td threshholds we are used to, etc… and instead focus on a few basics. As Mike mentions:

Basically, what happens is - in an extremely narrow corridor pulling ahead of the triple point, moisture and instability pull in very close to the TP … it always seems to trigger within 200 statute miles of the triple point - so for a chaser interested in catching a cold core tornado, all you have to do is hug the TP … can’t get any more simple than that!

Further technical reading on the topic can be found in this paper:
Davies, Jonathan M., and J. L. Guyer, 2004: A Preliminary Climatology of Tornado Events with Closed Cold Core 500 mb Lows in the Central and Eastern United States. (22nd SLS Conference)

Well, in most respects*, this was one blown forecast. The much-anticipated spring snowstorm never materialized for the Kansas City region with only a dusting to two inches falling across the area. To the credit of forecasters across the region, this was a particularly difficult forecast from the start, with a strong low and models that were all over the place. The low was slated to undergo several transition periods, which it did in fact do, just not quite as forecast. Most notably, the surge of warm, dry air that intruded the middle layers of the atmosphere during the daylight hours Monday was stronger and more pronounced than any of the models had anticipated, right up until the RUC picked up on it only about 3-6 hours out. This dry slot was beautifully visible in the 24-hour water vapor loop. By that time, the seeds for a bust had been sewn: the forecast was out there, the snow had shut off, and the damage had (or hadn’t, depending on how you look at it) been done.

Water vapor image: 03/20/06 17:15Z

Secondly, the movement – the speed and the track – of the upper level low through KS did not come to fruition as anticipated. In both aspects, the models were not far off, but off by just enough to matter. In terms of speed, the upper level low stalled out in SW KS for about 6 hours midday Monday; a seemingly minor detail that was not anticipated by the GFS or NAM (or even the RUC in the short term). The GFS was the first to hint at this stall in its 12Z run, but even then it late in the game. Hindsight shows that this slowdown, while only barely indicated by the GFS, was a detail that should have served as a hint of what was to come. In analyzing model run after model run, as I did with this system, I normally pride myself in having a good grasp on the trend, the run-to-run changes. But in this case, it is clear that this trend was not given its proper recognition. The trend was nothing new either: even going back 5-7 days when the storm system first came into the sights of the models, it was progged to be a Sunday/Monday event – a full 24-36 hours earlier than what would eventually verify. Not only did this slowdown displace a good deal of the moisture westward, it also served to wrap around a nice vort max that helped zap the dynamics through the NE KS, NW MO area, thus further decreasing the significant snowfall potential for this area.

By the time the upper level low started moving again late in the afternoon Monday, it tracked slightly farther north than had been anticipated, placing the highest snowfall totals north of the KC area, in a swath from north-central KS through SC and EC Nebraska and even into Central IA. Widespread reports of 25”+ were common through central NE, forcing the closure of hundreds of miles of I-80, not to mention the closures of schools, businesses, and the rest of the usual impacts a massive snowstorm of this scale will bring.

*: One aspect of this storm that verified relatively well was the QPF: Here in KTGX, a total of 1.13” of liquid equivalent fell between Sunday morning and Tuesday morning. As early as Thursday, significant precipitation was forecast, with a 1-1.5” QPF forecast given on Saturday. While this isn’t the headline making news (in fact, this aspect has gotten virtually no media attention through Tuesday morning), it is certainly a headline with respect to the looming drought for this region. Additionally, as hinted at by the forecasts, some borderline-convective sleet showers did verify yesterday afternoon, dropping a couple tenths of an inch of sleet at a time in a series of short, pronounced bursts each lasting 4-6 minutes. Over 0.4” of liquid equivalent was received in a relatively short window Monday afternoon (~2-5pm) which included several of these bursts.

This is going to be a big one. Particularly West and North of KTGX.

The Big Picture: It appears nearly certain that some nice convective bands will set up somewhere in SC/SE Nebraska (into North Central KS?), spelling double-digit snowfall totals for much of the area, and even more importantly, some isolated 20-25″ totals. That will be the headline of this event. Widespread interstate and airport closures seem likely for tomorrow through Tuesday. I currently tend toward the GFS solution: to place the bull’s eye for snowfall totals in South Central NE and extending into North Central KS.

The 48 hr GFS:

While the 48 hr NAM seems to be favoring a snowfall maxima slightly farther East into SE NE (note: this is a snowfall product only) which would be great for KTGX:

Locally: I don’t think I could pick a tougher location to forecast for relative to this system owing to the incredibly tight gradient of snowfall totals likely to be stretched across this region. Here in Northeast KS, today’s warm air is a bit of a concern, with a current SFC temp of 44º. While some light rain showers moved through the area this AM and tried to moisten it up a bit, the warmer, drier air is winning out. While the GFS and NAM are in agreement through about 18Z MON, they begin to diverge thereafter. The GFS is more bullish, pushing the colder air in place to force a switch over from rain to snow prior to 00z Tuesday, while the NAM is a bit more conservative, lagging behind by a couple of hours. The later is the trend I tend to favor. It is currently holding a small nose of warm surface temps along the KS/MO border that have verified through this afternoon. I don’t believe the GFS is accounting for this warmth accurately. I doubt sub-freezing sfc temps will verify until the mid-afternoon hours at the earliest, putting our best snow accumulation window at ~00z - 06z Tuesday. That said, the snowfall is already piling up ahead of forecast in CO and NE; the changeover occuring earlier than the NAM or the GFS had for those areas, leading me to hope that we can exact the same fate for our area from this system. I had been wondering a bit about the dry slot affecting our area…as of this morning’s model runs, I’m less inclined to think it would be a problem.

The Bottom Line: Considering the continued slowdown of the system, I’m going adjust yesterday’s numbers a bit. I see a changeover from rain to snow occuring later; around 00z Tuesday (6pm Monday) and an increase in the forecast snowfall total to 5-7″ for KTGX.

Note: The KTGX Discussion Forums appear to be down at the moment, but posts can be made at the bottom of this post with the “Comments” button.
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Some links that will be handy over the next 48 hours or so:
NOAA Interactive Snow Maps
Snow Depth: USAF Analysis Wunderground