Southeast Severe Storms

As a shortwave moves into the lower Mississippi Valley, a surface low will track across the Southeast today, sparking strong to severe thunderstorms. The airmass ahead of this low pressure is rather unstable with favorable environmental conditions for severe weather. All hazards are possible Tuesday afternoon & evening, which includes: tornadoes, damaging winds and large hail.

The best severe threat is in the corridor between I-20 and I-10 from Alabama east into Georgia. There is a slight risk of severe thunderstorms across southern Alabama, southern Georgia, far northern Florida and far southern South Carolina (see Fig. 1). A marginal risk surrounds the slight risk area.

Figure 1: Storm Prediction Center’s thunderstorm outlook map for Tuesday

Let’s take a look at the progression and timing of the severe thunderstorms. The severe threat will shift into the Florida Panhandle, southeastern Alabama and far southwestern Georgia around lunchtime (see Fig. 2). Heading into the early afternoon hours, the severe threat will shift farther east into far northern Florida and south-central Georgia (see Fig. 3). By late-afternoon, the severe threat will shift into southeastern Georgia and far southern South Carolina.

Figure 2: Simulated radar near lunchtime
Figure 3: Simulated radar early-afternoon
Figure 4: Simulated radar late-afternoon

Make sure you have several sources to receive weather-related information (warnings & watches). Also, make sure you have a plan in place in case a warning is issued.

Significant severe weather outbreak possible Saturday

Forecast Discussion: A strong shortwave is currently sweeping across the Four Corners region, which has induced the development of a surface low pressure system over the southeast Colorado/northeast New Mexico border. As the strong shortwave closes off into a mid-level low pressure system, the surface low will move into southwestern Kansas over the next few hours. A frontal boundary is currently stalled out across central Missouri, lower Illinois/Indiana and along the Ohio River in Ohio/Kentucky. A very moist environment exists south of the frontal boundary at and near the surface. As the surface low treks across Kansas tonight and tomorrow morning and into Missouri/Iowa late Saturday afternoon, the stalled front will begin moving northward as a warm front, replacing the dry airmass to its north with moisture-laden air. Throughout the day Saturday, surface dew points will increase across southeast Iowa and over the remainder of Illinois, Indiana, and Ohio.

surface analysis

Isentropic lift occurs when warmer air pushes up and over a colder airmass, which can generate precipitation. Isentropic lift, along with embedded weaker shortwaves, will continue to generate rainfall along and north of the soon-to-be warm front tonight into tomorrow morning across much of the lower Midwest. If you take a look at the Storm Prediction Center’s (SPC) severe weather risk area for Saturday, they have an enhanced and moderate risks extending from far northeast Missouri and southeast Iowa into northern/central Illinois, and northwestern Indiana. Please be aware that conditions in the morning (6-8am CT) may not feel like a severe weather day across the enhanced/moderate risk areas. Since the warm front will have not moved through much of the region at this point, temperatures will be somewhat chilly, especially with the rain falling. However, as the warm front surges northward, low-level moisture (humidity levels) will increase rapidly into the afternoon hours.

severe weather forecast

The surface low will move into western Iowa by mid-afternoon, strengthening further as it moves into the region. The region positioned just east of the surface low, ahead of the cold front, and south of the warm front will have the greatest risk of dangerous severe weather, as outlined in the SPC forecast. Rainy weather in the morning hours should move out quickly enough to allow for sufficient daytime heating at the surface. As the mid-level low approaches from the west, mid-level temperatures will cool, which will increase instability within the atmosphere. Surface-based instability allows air parcels (bubbles of air) near the surface and within the low levels of the atmosphere to begin rising. Surface heating and the addition of moisture makes those low-level air parcels buoyant. By cooling the mid-levels of the atmosphere, this ensures that the rising air parcels will remain warmer than the surrounding environment, allowing them to keep rising. This strong rising motion in the atmosphere on Saturday will result in deep thunderstorms developing across the risk zone, which will cause an increased risk for very large hail.

Tornado Risk: The enhanced and moderate risks also have been issued due to the tornado risk tomorrow. Within the treat zone, winds will flow from the southeast at the surface but will veer to the southwest with increasing height. We call this vertical wind shear. The position of the surface low relative to the mid-level low and shortwave will cause this turning of the winds with height. Given higher instability and vertical wind shear, tornadoes, some of which could be EF-2 or stronger, are expected. In fact, long-track tornadoes are possible. Keep in mind that the tornado risk extends southward into the Mississippi Valley ahead of the cold front as well; however, the tornado probabilities across that region will stay comparatively lower.

Chicago Tornado/Hail Risk: You might notice that the SPC only has Chicago under a slight risk; however, residents should watch the forecast closely tomorrow afternoon into the evening. The 18z NAM model is slower with moisture return across the Chicago area. By 5pm CT, the model projects dew points to only be in the mid to upper 40s. To the contrary, the HRRR model guidance has consistently projected dew points to be in the upper 50s or 60°F around the same time in downtown Chicago and well into the 60s in the western and southern metro. Timing matters a lot. If the low levels moisten sooner in Chicago, this could increase the tornado risk across the city, especially in the western and southern metro. Which model do we pick? That’s a tough question, especially when they’re trying to iron out intricate details. From what I can tell, the NAM model keeps conditions a bit cloudier across northern Illinois into the afternoon and even has storms moving through a little earlier than the HRRR model does. On the other hand, the HRRR has more daytime heating across the area, which would likely allow the warm front to advance northward more quickly. In effect, this would give developing and passing storms a much more unstable environment to work with. Honestly, I wouldn’t be surprised if the SPC bumps up the risk across areas closer to Chicago in their next update.

NAM model dew points
HRRR model dew points

Please continue to monitor the situation closely throughout the day tomorrow. These forecasts are never perfect, which means that you should expect some forecast modifications tonight and tomorrow morning.  

Active Pattern To Bring Anomalous Warmth East And Heavy Precipitation from Four Corners to New England

When a ridge of high pressure stubbornly sits over the same region, it generally brings dry weather and above average temperatures. A ridge causes sinking motion in the atmosphere, preventing deep clouds and precipitation from developing. However, along the edges of the ridge, or what I sometimes refer to as the periphery of the ridge, embedded disturbances in the flow will induce precipitation, some of which can be moderate to heavy. Notice the latest 72-hour rainfall totals (March 14-17 at 8am ET). Aside from localized convection, Florida and areas near the Gulf and Southeast coasts have remained dry through the period, due to sinking motion caused by the ridge. On the other hand, parts of the Southern Plains, Mississippi Valley, Tennessee Valley, Ohio Valley, and Mid-Atlantic have experienced wet conditions along the ridge’s periphery.   

Rainfall totals over the last 72 hours (March 14-17 at 8am ET)

Another hotspot for heavy precipitation over the last few days has been across California and other western regions. An upper-level low pressure system developed this past weekend and continues to slide southeastward along the West Coast. When a low becomes detached from the main flow, they tend to meander for a while and not move all that much. Eventually though, they get absorbed back into the main flow and make their way eastward/northeastward, which will happen by mid-week.

The evolution and track of the upper-level low will have an effect on the weather for the remainder of the week nationwide. As the feature approaches the Four Corners region on Wednesday, precipitation will develop and spread across the region. Mountainous regions across the Colorado Plateau have a high probability of picking up at least 4 inches of snow from Wednesday into Thursday, but accumulations will likely exceed at foot across the higher elevations. As the upper low moves northeastward into Colorado and then into the central Plains on Thursday, a surface low will develop just east of the Colorado Rockies. As the surface low moves across the central Plains and into the upper Midwest/Great Lakes region, a swath of accumulating snow will fall from Nebraska and the Dakotas into the far upper Midwest on Thursday going into early Friday.

The probability of snowfall accumulations exceeding 4 inches from late Wednesday into late Thursday

Also, the eastward progression of the upper low will amplify the ridge across the eastern U.S., which will drive up the temperatures substantially as the rest of the week progresses. Across the Southern Plains, Mid-South, Southeast, and Tennessee, temperatures will surge well into the 70s and into the 80s across parts of the area on Wednesday. The first round of moderate to heavy rain/storms associated with an embedded weaker disturbance will move across parts of the Southern Plains, Missouri Valley, Ohio Valley, and Kentucky/northern Tennessee on Wednesday. The rain will continue spreading northeastward into the Mid-Atlantic and lower half of New England later Wednesday into early Thursday. Some accumulating snow could fall across higher elevations regions in New England. The warm front will continue advancing northward on Thursday and Friday, expanding the warmth as far north as the Ohio Valley and Mid-Atlantic on Thursday and into the Great Lakes region/New England by late Thursday/early Friday. In fact, temperatures will probably increase overnight going into Friday across part of the Great Lakes region and western New England. Another round of rain/storms will move across similar regions in the Thursday/Friday timeframe ahead of an approaching cold front.

Green/yellow shaded regions have the high probability of having flash flooding on Wednesday or Wednesday night
Green/yellow shaded regions have the high probability of having flash flooding on Thursday or Thursday night

The cold front will sweep across most of the eastern U.S. by Friday/early Saturday, which will briefly usher in winter-like temperatures. Unfortunately, the cold front will take it sweet time fully moving through the Southeast; thus, expect Saturday to bring another day of 70s/80s across most of the Southeast outside of the Tennessee Valley and parts of the Mid-South. In fact, the cold front will likely stall out somewhere close to the Gulf Coast and northern Florida, so Florida and possibly surrounding regions just north may not experience much relief from the anomalous warmth.

Projected departure from average temperatures on Saturday morning