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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.