Tuesday, January 24, 2012

A note on "forcing"

It is easy to take a hard look at the maps and determine easily what constitutes strong "forcing". Usually we see a highly dynamical setup (e.g. deepening surface low, intensifying short wave trough) and immediately point to strong "forcing" as a reason for an outbreak. So what did yesterdays outbreak look like in terms of forcing, where we can be specific and look at two metrics of forcing: 700 hPa Q-vector divergence (shaded) and thermal absolute vorticity advection (contour). The Q vector divergence is an approximation for the QG omega equation forcing function. Thermal vorticity advection is the Trenberth approximation for QG omega; though as noted by Sanders (1990) the divergence of Q vector method may be more reliable in frontogenetical forcing. So here is the map of these two forcing functions on the NAM 12 km grid:
The same convection as yesterdays post is used comparing the 36 (upper left),24 (upper right),12 (lower left) hr forecast to the model analysis (lower right). Although this plot is for 700 hPa the one at 500 hPa was similar. Clearly the front (e.g. cold front aloft) was present the forcing is not that strong according to the model analysis, though the forecasts suggest a much greater forcing than diagnosed through the data assimilation system. One could reach the same conclusion from yesterdays plot of the derived QG omega (through the harmonic method used in SUNYPak) at 500 hPa (shown again below). At most both of these plots suggest that forcing in the region of the outbreak in AR and later in MS and AL was more weak to moderate than strong. The forcing for ascent shown here is localized to the front aloft. Where there was strong forcing indicated by the direct retrieval of 500 hPa QG omega was located around Kansas City ahead of the upper low relative to its translation as a negatively tilted trough. Certainly we can say this was a strong upper low but by no means was the outbreak area under strong dynamic forcing.

Monday, January 23, 2012

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.

Friday, January 13, 2012

Tornado days

Revisiting the tornado data set (1950-2010), I summed up the tornado's per day to take a look at the dependency of reports per day, daily path length, daily fatalities and daily injuries. The figure below shows these variables with respect to the daily maximum tornado magnitude. Given the magnitude of this years April, the April climatology is highlighted in Red.

 10 of the 43 E-F5 days (23%) occur in April contrast that with 71 of the 317 E-F4 days (22%). The April E-F5 fatalities have a median of 22 while injuries have a median of 290. This all occurs with a median of 26 tornadoes and a minimum of 11. The 3 April 1974 outbreak is the largest outlier in the E-F5 category with 148 tornadoes, 2553 miles path length, 368 fatalities, and and 6149 injuries.This years April had roughly 200 tornadoes with an estimated path length of 1950 miles. Final official numbers probably won't be available until March. I will update the graphics then.

 March has 7%, May has 35% and June has 20% of the E-F5 tornado days to make up the monthly distribution. The E-F4 tornado days are distributed as such: March has 10%, May has 23%, and June has 16%. These 4 months comprise the most deadly and numerous tornado days.

Always know your data: Note the outlier in the E-F0 category for Path Length. That is an error in the database associated with one tornado on 14 AUG 2006 in New Mexico. Apparently these types of errors appear now and again and are hard to officially remove.

Sunday, January 8, 2012

Tornado reports year in review

I queried the storm reports page from SPC, collecting the tornado reports for the last 7 years (2005-2011). I wanted to see what kind of year this was. I started by looking at (convective) days over the last 7 years (2557 days) where tornado reports were received (1186, or 46.4%).

When 30 or more tornado reports were received daily (sample size of 68), the yearly distribution was:
2011: 13
2010: 11
2009: 7
2008: 19
2007:  5
2006:  4
2005: 9

The top 6 report days over this period were:
27 APR 2011: 292
15 APR 2011: 146
12 MAR 2006: 140
16 APR 2011: 139
  5 FEB 2008: 131
25 MAY 2011: 127

So 2011 stands out both in terms of the maximum tornado report day on 27 APR (twice the reports of the next closest day), and 4 out of the top 6 report days. Quite the year for regional outbreaks, but does not beat 2008.