The Most Recent Changes

Mid February 2014

I have been posting analyses on both the CONUS and Regaional Surface Analysis pages showing the surface potential temperature and its advection and a second plot of equivalent potential temeprature and its advection. The advection is depicted by vectors having magnitude proportional to the maagnitide of the advection while their direction is that of the surface wind at each grid point. These are coded in red for positive and blue for negative advection. These have proven to be excellent indicators of frontal zones and give a very good heads up for cold outbreaks. Regions of strong gradient of advection point to local frontogenesis. Over sloping terrain, negative advection usually indicates upslope motion and positive advection downslope.

For the last couple months I've had my WEB cam operational and have been posting views looking to the SSE from my house in Rapid City. At present these are being updated every five minutes during daylight hours. I hope to add a once per day timelapse of the day's scenes.

I have added to the upper-air section analyses for three low level sigma surfaces showing plots at RAOB stations (of temperature, dew point, and pressure), streamlines, shading of stronger wind areas, potential temperature and mixing ratio. The intent here is to get a better feel for the structure of low-level systems than that provided by constant pressure surfaces over sloping terrain. The latter not only go underground in places of high terrain, but even when they remain above ground, they are of varying height above the terrain, causing them to enter and exit the boundary layer. Data for these sigma levels is not readily available from the NWS NOMADS site from which I obtain the GFS data used to initialize my analyses. Hence, these analyses use only the observed data from sounding staions with a straightforward Barnes analysis of the data.

25 June 2014

A new plot is available from the 'RAOB Plots' page for 10 stations near KUNR 72662. For these ten sites (the same ones for which I do upper-air meteorologams), the plot selection now includes a 'Thetae Change Plot'. Shown on it are vertical profiles of the 12 hr (filled) and 24 hr change of thetae together with the vertical profile of 12 hr D change (departure of height from standard atmosphere for each pressure level). The change data are obtained by first interpolating to a very closely spaced set of points in sigma (p/sfcp) (about 200 points from SFC to 100 mb), finding the change for each sigma level, and returning to p space using the p vs sigma relationship for the last sounding in order to display the data. This process ensures accurate representation of changes near the surface of the earth. The data for these is obtained from the University of Wyoming RAOB data page. It is an excellent site, but suffers from the problem that when sounding stations transmit the TTAA and TTBB messages when the sounding reaches 400 mb and then retransmit later the whole SFC to 100 mb data, the latter is not picked up by U WY so the sounding is truncated. 72768, KGGW has a habit of doing this; as an upper-air observeer, I have done it myself when severe weather is possible so that SPC can see a portion of the sounding as soon as possible. But, it does lead to problems with the differences shown here. The processing of this data and the plots shown have been produced in Python as I continue to learn that language.

The latest addition to the "Model Data" section (not yet completely linked from other than model data index page) is a severe weather stability index designed to give a measure of whether convection has the potential of reaching the tropopause (overshooting tops). It uses GFS forecasts every 3 hours out to 72 hours. The index is a simple difference between the forecast potential temperature on the PV=2 level (taken to represent the tropopause) and the equvivalent potential temperature at the surface. As parcels rise in thunderstorm updrafts, approximately conserving their equivalent potential temperature, their equivalent potential temperature becomes more and more nearly the same as their potential temperature. So, when they reach near the tropopause one can compare their initial equivalent potential temperature with the potential temperature of their surroundings to test their bouyancy. Negative numbers indicate parcels warmer than their surroundings at the tropopause. As with all simple stability indices, this one is critically dependent on the initial parcel conditions. Capping is an issue, as is elevated convection. However, most of the time the severest weather, especially tornado producing thunderstorms, will be found where this index is significanly negative. These analyses are combined with the 1000 mb geopotential height to give the user a feel for low level flow.

Another relatively recent addition to the "Model Data" section is a set of GFS forecasts of: 1) surface laplacian of thetae and wind together with 1000 mb geostrophic vorticity; and 2) GFS forecasts of 300 mb streamlines and speed shading together with difluence and CAPE. Both of these should be of use in severe thunderstorm forecasting as well as other applications. Severe storms are almost always found in regions of negative laplacian of surface thetae, except when they produce a large enough convective complex to have large cold pools detectable by synoptic scale analyses and forecasts or when the convection is elevated. I am aware that difluence does not necessarily imply divergence! However, upper tropospheric difluence has long been associated with severe convection and these analyses/forecasts have already proved useful in this severe weather season. I obtain the data for these from the NWS NOMADS site and it provides PV=2 level data only for the GFS model. Hence, only the GFS is available for these plots.