News DetailMontana under the gun once again!
Posted At: May 19, 2007 @ 4:53 PM
Posted By: Reed Timmer
Related Categories: Tornadoes
High-based supercells are expected to develop once again today over central Montana, with strong instability and ample deep-layer shear for rotating storms. The Storm Prediction Center has issued a severe thunderstorm watch for this region:
As seen in the composite radar image above, storms are beginning to initiate over the western area of the watch box (where wind shear and instability are marginal for supercells) and south of Billings (where supercells are more likely). The RUC analysis of the 0-1 km energy helicity index at 3 pm MDT is shown below. The EHI is a composite index that factors in both 0-1 km CAPE (low-level instability) and 0-1 km helicity (low-level shear). Very generally, 0-1 EHI values of ~ 2 or higher support tornadoes, but the cloud bases in this region will be very high, with temperatures in the 80s and mixed-out dewpoints in the low 50s. These large dewpoint spreads not only result in high cloud bases, but also maximize evaporational cooling below the storm which favors a quick transition to outflow-dominant storms. However, if supercells can develop and continue into the evening, when surface temperatures begin to cool and dewpoints rise, there is a chance of isolated tornadoes.
Monday and Tuesday still look interesting for a potential severe weather outbreak over the Central and Northern Plains. Our main concern right now is that the low-level jet will be east of where the storms develop, but this usually seems to be the case in model forecasts of Northern Plains events where a cold front is involved. Stay tuned for more updates!
As seen in the composite radar image above, storms are beginning to initiate over the western area of the watch box (where wind shear and instability are marginal for supercells) and south of Billings (where supercells are more likely). The RUC analysis of the 0-1 km energy helicity index at 3 pm MDT is shown below. The EHI is a composite index that factors in both 0-1 km CAPE (low-level instability) and 0-1 km helicity (low-level shear). Very generally, 0-1 EHI values of ~ 2 or higher support tornadoes, but the cloud bases in this region will be very high, with temperatures in the 80s and mixed-out dewpoints in the low 50s. These large dewpoint spreads not only result in high cloud bases, but also maximize evaporational cooling below the storm which favors a quick transition to outflow-dominant storms. However, if supercells can develop and continue into the evening, when surface temperatures begin to cool and dewpoints rise, there is a chance of isolated tornadoes.
Monday and Tuesday still look interesting for a potential severe weather outbreak over the Central and Northern Plains. Our main concern right now is that the low-level jet will be east of where the storms develop, but this usually seems to be the case in model forecasts of Northern Plains events where a cold front is involved. Stay tuned for more updates!
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What time are thay going to rotate. or are thay not.
once again why isn't anybody posting.
It looks like that cell north of Billings has exhibited temporary supercellular characteristics, but is definitely struggling with the cap. We'll see what happens after dark.
Nebraska may have some severe weather, the west is getting hit, and i live atleast 30-40 miles from it and we have 10mph winds, reaching 15.