Severe thunderstorms developed along a cold front across the Upper Midwest and Canada during the afternoon of 16 Sep 2021. During the early afternoon, NWS/MPX (Twin Cities/Chanhassen, MN) forecasters were monitoring various GOES-East products to assess convective potential. From the MPX AFD: “Regional radar and enhanced satellite imagery is showing convection developing along the international border in northwest Minnesota. RGB enhanced satellite does show more glaciated clouds across northern Minnesota with only water clouds across the central part of the state.” Given the analysis, while convection was developing further north, there was still quite some time before it would get started in central Minnesota, but clear skies would allow for instability to increase ahead of the front. Figure 1 depicts the Day Cloud Phase Distinction RGB being discussed up to the point of AFD publication.
The discussion continues: “In addition to the RGB images, the satellite derived stability indices does show an enhanced area of both higher K-Indices, and CAPE values over west central Minnesota this afternoon. … once the cold front
arrives later this afternoon/early evening along this enhanced area of greater instability and moisture, convection should develop.” The discussion is captured in Figure 2 and 3. CAPE values are increasing and arrive at levels sufficient for severe convection, while K-index values allude to scattered-to-numerous thunderstorm coverage. The analysis is a great example of utilizing various satellite tools to assess the current convective environment and produce a short-term forecast based on current trends.
Shortly thereafter (at 2047 UTC), a NWS/SPC mesoscale discussion was issued for much of Minnesota ahead of the CF, with a 80% chance of watch issuance. The MD mentions: “Visible satellite imagery further indicates inhibition has rapidly eroded as CU field across the area has become more agitated with vertical development over the past hour or so. While CAMs guidance suggest convective initiation may not occur until closer to 00z, observational trends suggest storms could develop sooner.” The visible imagery discussed is shown in Figure 4 ending just after the issuance of the MD, with corresponding Day Cloud Phase Distinction RGB imagery in Figure 5. Both capture the vertical cu development in north-central MPX and southwest DLH, with the RGB also revealing the glaciation of cumulus clouds (cyan changing to bright green/yellow). Convection would initiate over the next hour (first GLM flash at 2153 UTC), and mature over the following hour (first warning, TOR, at 2245 UTC). This is a great example of a forecaster relying on observational trends in decision-making and as a check on model guidance. In this case, storms did indeed develop sooner than was suggested in CAM guidance, per the forecaster’s discussion.
Convention would continue to grow upscale thereafter, with numerous severe thunderstorm and tornado warnings issued across MPX’s area (Fig 6). These storms produced large hail (to 2.5″ in diameter) and strong/damaging winds. The 1-min visible imagery near sunset reveals cloud texture well with increased shadowing, allowing one to easily diagnose features such as OTs and AACPs associated with individual updrafts.
A longer VIS animation with GOES CAPE and surface obs overlay shows convective evolution along/ahead of the cold front and within the CAPE max (Fig 7).
Severe convection also developed north of the border in southwest Ontario. These storms were moving quick, with Env Canada warnings listing storm motion at around 60 mph! The GOES-East 1-min mesoscale sector included this area, with visible imagery and AWIPS tools confirming the fast storm movement (Fig 8).
A storm relative animation of the 1-min VIS+IR sandwich better characterizes the large overshooting top and downstream warm anomaly with this storm. The animation also reveals the influence of the storm on the surrounding environment, particularly the appearance of low stratus bending toward the storm updraft in the inflow region of the system.
Bill Line, NESDIS and CIRA