Monday, October 13, 2014

Monday morning update on today's severe weather threat

UPDATE -- 9:00am Monday

As suspected last evening, the Mid-South has been upgraded to a Moderate Risk for severe storms today. Probability of damaging wind within 25 miles of your location is now at 45% with a 10% chance of wind over 75 mph. The tornado risk is also at 10%.

Current SPC severe weather outlook for Monday - a Moderate Risk is in effect for the metro.

Current SPC probabilities for damaging wind. Pink is 45% and the black hatching indicates a 10% or greater risk of wind over 75 mph.

Current SPC probabilities for tornadoes. Yellow is 10% and any black hatching indicates a 10% or greater risk of strong (EF-2+) tornadoes.

As far as timing, supercells capable of high wind and a few tornadoes will be possible by late this morning, while the squall line itself is on track for an afternoon (1-4pm) arrival. Everyone in the metro should prepare for the possibility of severe storms from late morning through early evening.

The remainder of the blog, posted last night, is still applicable, particularly the severe weather preparations discussed near the bottom.

--Erik Proseus,, MWN Meteorologist

ORIGINAL POST -- 8:25pm Sunday

As we blogged about yesterday, severe weather looks likely on Monday for a large portion of the middle and lower Mississippi Valley. In fact, the Storm Prediction Center has indicated as many as 42 million people from the Midwest to the Gulf Coast are under a risk of severe weather.

The greatest likelihood, where there is currently a 30% risk of severe storms within 25 miles, exists from St. Louis across the Mid-South to northern Louisiana. This same area also is under a threat of "significant severe" weather, which includes EF-2 or stronger tornadoes, 75+ mph wind, or 2"+ hail. The Memphis area is in both the 30% area, as well as 10% risk of significant severe storms. SPC has indicated that there is a decent chance parts of the 30% area could be upgraded to a Moderate Risk tomorrow, the first for our area since June 5 when a derecho caused widespread damage.

The Mid-South is currently forecast for at least a 30% chance of severe weather within 25 miles, as well as a 10% risk of "significant severe" weather. These probabilities could go up tomorrow morning.  
The area is currently sitting under a warm and moisture-laden southerly flow of air from the Gulf of Mexico, evident by dewpoints in the upper 60s this evening and south wind. Overnight, wind will begin increasing from the south and southwest aloft, further priming the pump for severe weather as moisture and wind shear increase. This will bring about a chance of a few thunderstorms tonight, though no severe weather is expected.

Forecast dewpoints as of 7pm Monday evening are still near 70 degrees. This amount of low level moisture is more than sufficient for strong to severe storms and is very muggy for October when dewpoints typically are in the 40s and 50s.
By tomorrow, low-level wind will really start to crank up, and in fact, wind at the surface ahead of the potent cold front will increase to 25-35 mph during late morning and afternoon, prompting the issuance of a Wind Advisory for the area from 10am - 7pm. If you need to, tie down any loose objects outdoors tonight or before leaving for work or school in the morning. Wind in the lowest 5000' of the atmosphere will also pick up to more than 50 mph during the afternoon, setting up a low-level wind shear scenario that is favorable for rotating storms and possibly tornadoes.

Forecast wind at about 3,000' early Monday afternoon. Values are in knots, which can be converted to mph by adding 15%. Eastern AR has values approaching 60 mph, while they are near 50 mph over Memphis. Strong storms can push this wind to the surface, creating a damaging wind scenario. The wind, coupled with changes in direction or speed above and below this layer, can create a wind shear scenario, which is necessary for severe storms.

Forecast wind shear values as of 4pm Monday afternoon. Values near 50 knots (present over the metro) are more than sufficient to allow storms to rotate, which is a precondition for supercells.
With an abundance of warm air and a period of dry weather expected during the day tomorrow, temperatures will warm to at least 80, more likely the lower 80s, by mid-afternoon. This heating will add fuel to the fire in the form of instability, another necessary ingredient for strong to severe storms. However, even if temperatures don't quite reach 80, there are plenty of other factors in play that could allow for storms to become severe.

Forecast CAPE values at 1pm Monday are near 2000 J/kg, which is sufficient for strong storms, given other factors which are also forecast to be present.
Given all of the above parameters - instability, moisture, shear - the final ingredient, lifting of the air into the primed airmass, will be provided by the cold front, or perhaps more likely, a trough out ahead of the front. Storms will move into western AR early Monday and then travel across the state during the day ahead of the front. A squall line of storms (or using meteorological jargon, a QLCS) will sweep across the metro most likely during the late afternoon to early evening hours. As of now, we're expecting this line between 3pm-8pm, but are favoring a rush hour impact in the metro. In fact, one of the high-resolution models (below) agrees.

Simulated radar (not a forecast), shows a squall line pushing into the city at 5pm. This type of forecast product is used for TRENDS. Radar is NOT expected to look exactly like this at that time, but gives us a good idea what to expect.

The main threat will be damaging straight-line thunderstorm wind of up to 60-75 mph with the squall line. However, as we saw with the last big squall line, "loops" or notches in the line are favored spots for rotation and a few tornadoes can NOT be ruled out.

The better chance for any rotating severe storms capable of tornadoes, however, will be in any renegade cells that form ahead of the line during the afternoon in a sheared environment. If these storms form, and can tap into the shear and become supercells, tornadoes would be possible, as well as some large hail. These threats are CONDITIONAL though, meaning we're not positive that this scenario plays out. If it does, the threat becomes more likely.

Once the line moves through, expect a quick drop in temperatures and a few hours of rain, perhaps heavy. The heavy rain along and behind the squall line could be capable of producing some flash flooding, so be aware of that threat during rush hour and into the evening as well.

Note a quick drop in temps behind the line, as shown by the high-res NAM model valid at 5pm. Temps in the 80s fall quickly into the 50s/60s behind the line.

Bottom line: prepare for the likelihood of severe storms tomorrow afternoon and evening, possibly first as supercells, then as a squall line around rush hour. Damaging wind is the main threat and appears to be a good possibility. Tornadoes and large hail are a secondary risk. A Tornado Watch is expected during the afternoon tomorrow. Know your plan for wherever you will be during the afternoon and evening should a warning be issued. Avoid windows even as a line of storms moves through due to the threat of high wind and lofted objects.

We'll bring you the latest all day tomorrow, including nowcasting of the events as they unfold, on our social feeds below. We highly recommend having a severe weather app to alert you to severe weather conditions. We prefer ours (links to info and download below), but honestly don't care as long as you have a reliable way to get warning information.

Also, remember that outdoor tornado warning sirens are for OUTDOOR use and the policies vary by municipality and county. If you hear one and aren't sure of the policy, take cover. In general (in Shelby County), Bartlett, Collierville, and Germantown only sound their sirens when the respective municipalities are under a threat. Memphis and Shelby County sound their sirens for any threat in Shelby County.

Click here for the latest forecast, or get it on our apps.  Stay safe!

Erik Proseus,
MWN Meteorologist

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