Moisture products for the tornadic storm of 4 July 2020 in Saskatchewan
July 10th, 2020 by Dan BikosOn 4 July 2020 a thunderstorm developed in southern Saskatchewan that led to numerous tornadoes (video, picture, pictures).
Let’s analyze various satellite derived moisture products in the time period leading up to the tornadic storm. The following loop 4-panel loop:
Upper-left: GOES-16 visible (0.64 micron) imagery.
Upper-right: Advected Layer Precipitable Water (ALPW) product in the surface to 850 mb layer with RAP 00hr surface winds.
Lower-left: GOES-R baseline Total Precipitable Water (TPW) product with RAP 00hr surface winds.
Lower-right: Merged (GOES + POES) Advected TPW product with surface observations.
It’s important to note the status of the 3 moisture products. The GOES-R baseline TPW product has been operational in AWIPS, the ALPW product is non-operational but will become operational in AWIPS within the next 1-2 years, while the Merged Advected TPW product is very much experimental (still in development).
First looking at the visible imagery, we observe morning convection while later we see storms develop along a north-south oriented boundary. Most of the storms move off to the northeast, however one of the storms appears to develop at the intersection of this boundary with another boundary that is almost east-west oriented and this storm moves southeast. Tornadoes are associated with the storm as it’s moving southeast and exhibits inflow feeder clouds.
The RAP surface winds and surface observations provide indications of a triple point, however the signal is not strong since the winds are not strong. If we use this data along with indications of the boundary in the visible imagery to annotate with red dashed lines where the boundaries are:
The triple point shows up nicely with west winds (shortly later northwest winds) with the cold front to the west, light east/southeast winds in the warm sector and southwest winds further south with drier air (78/47 in the observation) – a classic dryline/cold front triple point although in this case much more subtle since the winds were relatively light and the boundaries and air mass differences were subtle (i.e., the weak cold front may be analyzed as a trough). The WPC official surface analysis at 18Z shows the triple point analyzed as a trough to the north and southwest, with a warm front to the east:
Note the color table and range for the bottom 2 panels of the 4 panel image above are corresponding so as to make comparison between the 2 products. One limitation of this however is that gradients at key ranges for this particular case may be smoothed out. An alternative would be a different color table that shows increased contrast across the feature of interest so that it stands out more readily such as this animation depicts:
Viewing the animation of the moisture products to focus on the dryline (the analyzed trough extending southwest of the surface low), the TPW products show subtle indications of the relatively drier versus relatively more moist airmasses. However, in comparison with the ALPW surface to 850 mb layer we see that the ALPW product really shines at highlighting the various airmass differences. This shouldn’t be surprising since low-level moisture is key for severe thunderstorms, particularly in an environment with an elevated mixed layer as seen in the RAP 2200 UTC 00hr profile from Regina, SK:
Keep in mind that the ALPW product has 4 layers, and they may be viewed for this case here:
2 key points from this loop:
1) The triple point is very much a low-level feature, it only shows up in a more subtle way in the 850-700 mb layer and does not show up above that.
2) There is rich /deep moisture in the warm sector east of the triple point in a relatively narrow corridor and the storm moves towards this region as it turns to the right (moving southeast in an environment of southwest flow aloft).
One final question worth considering, did the morning convection produce an outflow boundary that moved southwest and contributed to higher moisture / convergence for the afternoon thunderstorms? There are some indications of that in the moisture products with varying degrees of subtlety. The GOES-R baseline product seems to show some increase in TPW to the southwest of the morning convection, while the ALPW seems to show some increase in low-level moisture as well. The following animation supports this hypothesis if we make some continuity assumptions of where the weak outflow boundary would exist, but the key is that it may have reinforced the weak warm front:
Note the inflow feeder clouds towards the end of the animation, a satellite storm-scale signature that indicates the storm is likely severe.
In summary, we have a triple point pattern that led to a tornadic storm. often times we think of a triple point pattern with strong convergence along the boundaries and obvious air mass differences, making identification of the triple point pattern relatively easy. In this case, the winds are much weaker and the boundaries and air mass differences are much more subtle. It’s definitely more challenging to identify this pattern in these circumstances, however ALPW and other moisture products help with respect to identification of air mass differences and boundaries (from the PW gradients).
When using the moisture products operationally, latency should be kept in mind. Let’s first discuss latency as defined as receipt time on AWIPS. The GOES-R baseline TPW product has the least latency (~15 minutes) with ALPW latency a little over 45 minutes while the merged TPW product is about the same albeit still experimental. These products can be used for relatively short-fuse type of events, so long as it’s not so short that latency becomes an issue. Keep in mind, the products that contain POES data have a “hidden latency” in that the most recent passes that are advected to make the products are generally 2 to 6 hours old. Despite these limitations, the evolution of the triple point is captured quite well in the surface to 850 mb layer ALPW. Perhaps the GOES product may have shown this as well, however we have the limitation of cloud obscuration and it is a total precipitable water product. Some of the important changes that occur at lower-levels (therefore captured by the lowest layer ALPW) are “washed out” in a TPW product.
Posted in: Convection, Severe Weather, Tornadoes, | Comments closed
Analysis of June 17, 2020 Wyoming Snow event with JPSS products
June 23rd, 2020 by Dan BikosBy Sheldon Kusselson
ftp://ftp.cira.colostate.edu/ftp/Forsythe/LPW/Anim_GIF/2020Jun1803Advect_LPW_ALT_anim.gif
Posted in: Hydrology, Winter Weather, | Comments closed
May 2020 flooding in central Michigan
June 9th, 2020 by Dan BikosThis blog entry by Sheldon Kusselson summarizes satellite moisture imagery and products leading to the flood event over central Michigan on 18 May 2020. Comparison with previous events are included.
Animation of ALPW every 3 hours from 03 UTC 18 May to 09 UTC 19 May 2020:
ftp://ftp.cira.colostate.edu/ftp/Forsythe/LPW/Anim_GIF/2020Apr3003Advect_LPW_ALT_anim.gif
ftp://ftp.cira.colostate.edu/ftp/Forsythe/LPW/Anim_GIF/2020Mar2906Advect_LPW_ALT_anim.gif
ftp://ftp.cira.colostate.edu/ftp/Forsythe/LPW/Anim_GIF/2020Jan1209Advect_LPW_ALT_anim.gif
ftp://ftp.cira.colostate.edu/ftp/Forsythe/LPW/Anim_GIF/2019Sep2306Advect_LPW_ALT_anim.gif
Posted in: Heavy Rain and Flooding Issues, Hydrology, | Comments closed
Heavy rain event around 18 May 2020 that contributed to dam failures in Michigan
May 22nd, 2020 by Dan BikosBy Sheldon Kusselson
Posted in: Heavy Rain and Flooding Issues, Hydrology, | Comments closed
GOES/JPSS Observations of Oklahoma Severe Storms and Elevated Mixed Layer
May 7th, 2020 by Jorel TorresBy Jorel Torres, Dan Bikos and Ed Szoke
A line of severe storms moved through the southern plains on 4 May 2020, producing numerous hail and wind reports across the region (accessed via SPC). The GOES-16 Day Cloud Phase Distinction RGB is shown below, overlaid onto the GOES-16 CAPE product from 17Z, 4 May 2020 to 00Z, 5 May 2020. Notice how the atmospheric environment in Oklahoma becomes more unstable (2500-3500 J/Kg – peak observations) before convective initiation occurs. A line of agitated cumulus develops around 20Z, 4 May 2020 just north of Oklahoma City, OK. moving east. The RGB observes liquid water clouds (seen in blue) that become glaciated (green), and start to grow rapidly upscale. The rapid vertical development indicates strong updrafts within the embedded line of storm cells, where mid-to-high level ice clouds are depicted in yellows, oranges and reds within the RGB.
Another way to observe atmospheric instability is by using Gridded NUCAPS, that is a product derived from polar-orbiting satellites (i.e. in this case, NOAA-20). Gridded NUCAPS provides users temperature and moisture fields via plan-view and cross-sections. For brevity sake, a plan-view of the temperature field is observed at 1912Z, 4 May 2020, highlighting the 850mb-500mb lapse rate (i.e. temperature change with height). Although Gridded NUCAPS imagery is static (i.e. not an animation), notice how lapse rates steepen with height from central Oklahoma to southern and southwestern Oklahoma, where values range from 6.5C/km to 9C/km. The steeper lapse rates indicate a more unstable environment favorable for severe storms to develop. Conversely weaker lapse rates (less than 5.5C/km; see northeast OK and northwest AR) are a sign of a stable environment. Note stable wave clouds were observed in these respective areas, earlier in the day, due to early morning convection.
But what about the moisture component? Look no further than the Advected Layered Precipitable Water (ALPW) product that helps users inspect precipitable water values in 4 separate layers: surface-850mb, 850mb-700mb, 700-500mb, and 500-300mb. An ALPW animation (click image) is observed below from 16-23Z, 4 May 2020. Moisture is concentrated in the low levels of the atmosphere mainly between surface-850mb. Now to be fair, marginal precipitable water values are observed between 850-700mb early in the day, however dry air moves into this region at ~18Z.
Now this mid-level dry air appears to indicate the presence of an Elevated Mixed Layer (EML) which can be integral for the severe thunderstorm environment. An EML typically has a steep mid-level lapse rate, mid-level dry air, and a strong capping inversion, inhibiting convection. With an EML in place, this allows the possibility for high amounts of CAPE to exist but the question is whether the inversion can break. In this case, a lifting mechanism was present, a front, to presumably aid in upward forcing (i.e. rising motion associated with converging low level air) to break the inversion, and subsequently generate rapid convective initiation. For interested readers, more information on EMLs and how they can be identified and tracked can be accessed here: (Gitro et al 2019).
Furthermore, note the rapid drying in the moisture profiles of the 12Z and 19Z KOUN RAOB soundings, specifically from 850mb-700mb. Drying corresponds with ALPW 850mb-700mb precipitable water layer. Additionally, observe the steep mid-level and low-level lapse rates (also seen by Gridded NUCAPS above) along with the presence of a strong inversion depicted in the 19Z KOUN sounding. To zoom-in, click on individual soundings.
The EML can also be seen by GOES-16 7.3um from 16Z, 4 May 2020 to 00Z, 5 May 2020. See animation below. The low-level water vapor channel observes a narrow extent of very warm brightness temperatures oriented from southwest-to-northeast from the Texas Panhandle into Oklahoma. The southwest-to-northeast line of very warm temperatures then rapidly cool, due to the front generating upward motion, subsequently eroding the capping inversion, leading to rapid thunderstorm development.
Posted in: GOES R, POES, Satellites, Severe Weather, | Comments closed