Sunday, February 27, 2011

Update on severe weather expectations for tonight

Latest severe weather risk outlook for tonight from the NWS/Storm Prediction Center
It's amazing how similar tonight and last Thursday night's severe weather episodes seem to be, outside of the difference in timing of the event.

Thursday, I was expecting scattered severe storms to fire up in the mid to late afternoon and a squall line to race through in the late evening. The scattered storms never got going much, outside of one near Jackson, TN, while the squall line came through a couple of hours earlier than expected. Today, we are once again watching for scattered severe supercells to possibly form through the evening hours, followed by a potential squall line early Monday morning.  Both events saw the Mid-South in a moderate risk a day ahead of the event, though for tonight the Memphis metro is on the southern edge of the moderate risk versus in the middle of it thanks to a slightly more northerly track to the culprit low pressure system. It will be interesting to see exactly how this event plays out.

We'll be watching StormView Radar closely for isolated to scattered storms to fire up anytime through the evening and early overnight hours.  A little Weather 101... If these storms can "break the cap" and "become surface-based," we have a threat of tornadoes on our hands. If not, the threat is lower. This is what is called a "conditional" chance of severe weather - the severity of the storms is based on the conditions named above. The "cap" is like a lid on the atmosphere that keeps storms at bay. Without going into detail, if the lid, which is holding all of the energy under it, is released (or broken), then storms can literally explode upwards. The storms also need to become surface-based, which in essence means they get their energy from the heating at the surface of the earth. If a storm can't tap into the energy of the lowest levels of the atmosphere, it is what is referred to as an "elevated" storm. Typically, a storm must become surface-based to be tornadic.

So, the details on tonight's forecast... or at least my best educated guess. If storms can form and meet the above parameters, we could be looking at scattered supercellular thunderstorms beginning anytime and lasting through the early overnight hours. I put the chance of storms such as these at about 40%. If they form, you need to have your severe weather action plan in place. Later in the night, probably between 3-6am, a line or lines of storms are expected to move into the region from Arkansas and sweep across the metro area.  This line could be very similar to Thursday evening's line, posing a damaging wind threat, along with isolated tornadoes.  Any tornadoes that form tonight could be strong, as wind at the lower and mid levels of the atmosphere is strong enough to support them.

By morning rush hour, most activity will be moving east and scattered showers will remain as the cold front itself passes through. Lows tonight will be in the mid 60s, but expect wind to shift to the northwest and temperatures to fall into the 50s during the day. Calmer weather should persist for much of the rest of the work week.

When planning for severe weather, please visit the MWN Storm Center for the latest radar, severe weather watches and warnings, and severe weather safety tips. It's also an excellent idea to check your NOAA Weather Radios for fresh batteries and have them programmed to alarm in case of severe storms. Overnight severe weather is a great concern as many people could be unaware of what's going on outside until it's too late. MWN will also provide coverage of the severe weather threat as best as possible throughout the overnight hours via Facebook and Twitter and MWN's web products.

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