March 2020 started off rather windy and unstable in the Northern Plains leading to convection during a month that still sees predominately wintertime stratiform precipitation. There were several instances of showers producing lightning, graupel/snow, and convectively forced severe wind gusts in addition to synoptically driven high winds. Designating convective cells that may be more impactful than others in the cool season (late fall, winter, and early spring) can be difficult using remote sensing instruments like satellite and radar due to their shallow heights and relatively warm cloud tops. Satellite appearance of shallow convection tends to be unimpressive in the infrared using default color schemes made for taller, colder cloud topped warm season convection. Also, due to the shallow depth of the cloud layer, radar can easily overshoot this layer composed of what tends to be marginal returns. Lastly, lightning activity tends to be much lower than that in the warm season, if present at all.
But there are some ways to make subtle features of shallow convection more easily tracked by adjusting the satellite display. Let’s explore..
First let’s take a look at an event on March 4, 2020, that produced several convectively induced high wind gusts at times exceeding 75 mph. The Day Snow-Fog RGB has proven useful in monitoring wintertime precipitation, mainly because of its use of near-infrared bands to help delineate snow cover, bare ground, and cloud top microphysical composition. It is also well known that using RGB’s capable of tracking cloud features while color sorting aforementioned properties (looking at you Day Cloud Phase Distinction RGB) can be quite useful for tracking forcing mechanisms, areas of potential convective initiation, and stage in convective life cycle.
Figure 1 shows convective clouds moving through eastern Montana into the Dakotas and northern Wyoming. Convection is already ongoing in the start of the loop with a line of showers attached to a cold front extending from near Great Falls to Glasgow, Montana. The cold front can be tracked on satellite propagating south and east undercutting additional convection in the “warm sector” ahead of the front. Ahead of the front are additional clusters of growing cumulus clouds as the convective temperature was reached, aided by ample insolation over a snow free surface beneath a relatively cool column of air (focus of cloud growth probably had some influence from differential heating and convergence augmented by variations in terrain). These cumulus clouds initially exhibit a highly textured, bubble-like appearance with whiter coloring indicating non-glaciated clouds. They very quickly become bright pink while maintaining their texture signaling glaciation of clouds through ascent. Finally they turn from cumulonimbus to stratocirrus losing texture, acquire more purple coloring, and move west to east in broader upper flow as parcels detrain from their initial paths of low level ascent, thus indicating the majority of additional ascent is likely over. The ability to track these mesoscale forcing mechanisms and stages in storm growth can be important to forecasters as this may give lead time to potential initiation or advection of impactful convection, as well as convective decay.
Now let’s focus on the tristate region of the Dakotas and Montana, an area where several severe wind gusts were measured. Using Rapid City’s observed sounding as representative of the environment, this area was primed to transfer not only cloud layer winds of around 50 kt towards the surface, but any evaporation of descending precipitation cores may help accelerate these winds from aloft towards the surface (DCAPE ~700 J/kg, inverted V sounding).
We can utilize procedures meant for tracking severe convection in the warm season while making some minor adjustments to help gear towards a cool season environment. Figure 2 is an example of one such procedure. The range of the Day Cloud Phase Distinction RGB’s red component (10.3 um, Band 13, “Clean IR”) was edited from [Min: 7.5, Max: -53.5] to [Min: -10, Max -70] in an effort to desaturate red coloring by shifting this thermal sensing band towards the colder, better representing the temperatures typical in the cool season. Convective evolution is represented by a transition of colors from cyan to green to yellow/red as clouds glaciate and then cool. The inclusion of the visible spectrum within the Day Cloud Phase Distinction RGB also gave clues on storm development featuring more turbulent cloud texture near the upshear side of continually developing anvils, indicative of sustained updrafts reaching their equilibrium level. A separate, standalone look at Band 13 offered a way to track coldest cloud tops, although adjustments were also made here shortening the range from [Min: 55.0, Max: -109.0] to [Min: 0, Max -70] while making all values warmer than -25 C transparent. This allowed robust convection to be more easily tracked in an area of poor radar coverage with more red colored tops corresponding to relatively taller, stronger updrafts. One particular cluster of showers moved through Glendive, MT, with its AWOS measuring a 47 kt (54 mph) wind gust 18:56 UTC followed by Dickinson, ND, reporting 66 kt (76 mph) wind gust 19:56 UTC. This cluster exhibited relatively rapid cloud top cooling around 40 minutes prior to reaching Glendive and another prior to reaching Dickinson. Lastly, lightning activity can be analogous to updraft strength. Adjustments to the GLM FED appearance were made to shorten the default range of 1-260 to 1-20 helping draw out meager lightning rates. While overall lightning activity was low (as can be typical in cool season convection in the Northern Plains), this adjustment did help bring attention to cells that had relatively high activity compared to neighboring cells. One cell that stands out emanates from the same cluster that moved through Glendive and Dickinson but on its southern flank. Looking at MRMS isothermal reflectivity at -20C, the cell holding highest lightning activity has reflectivity exceeding 40 dBZ. All of these signals, upstream reported severe wind gusts, and our knowledge of the thermodynamic and kinematic environment can steer the forecaster into thinking this particular cell is strong and chances of producing severe downdraft wind gusts are high. This prompted NWS Bismarck’s to issue its earliest severe thunderstorm warning with an eventual 81 mph wind gust being reported at a mesonet site in Hettinger County, North Dakota.
A lightning-producing cell precipitating rain, snow, and graupel formed west of Bismarck, North Dakota, moving southeast into northeast South Dakota, during the evening of March 3, 2020. Using a similar procedure as displayed before, Figure 3 shows relatively rapid cooling of an expanding cloud top/anvil as noted in Band 13 (default color scheme shown). Values exceeding 35 dBZ in MRMS isothermal reflectivity at -20 C correlated nicely with the appearance of sensed lightning by GLM and ground based networks.
A graupel/snow depositing shower moved through Fargo, North Dakota, near sunrise March 3, 2020. This along with other showers left a trail of accumulated graupel/snow apparent on the Day Snow-Fog RGB (Figure 4). This is seen as more orange shaded streaks atop the background darker red coloring of the older snowpack. The shower moved through the Fargo-Moorhead metro right as sunlight was becoming available. Thus, it was beneficial to adjust the near infrared components of the RGB to account for little visible light to work with. This helped brighten the image revealing the metro was right on the edge of new accumulation (which lined up nicely with radar).
Later in the day, clouds peeled away revealing additional showers had deposited new frozen precip accumulation throughout eastern North Dakota into northwest Minnesota (Figure 5).
The main takeaway of this post is to encourage forecasters to become more comfortable with adjusting satellite products to help draw out convective features. Such adjustments may be especially necessary during the cool season. That’s not to say just blindly make adjustments, rather forecasters need to understand what is being adjusted and how that would affect the appearance and meaning of edited products. After determining an environment is possible for impactful convection, forecasters should think about adjusting satellite display to make subtle convective features typical in the cool season more easily tracked.
Carl Jones, NWS Grand Forks, ND