Tornado Forecasting

An overview of how I do forecasting before a storm chase.

A Source of Lift

Usually, thunderstorms won't initiate without some type of lift. Lift is usually produced by jet stream dynamics, surface boundaries, and sometimes topography (orographic lift). When the flow aloft is diffluent, it tends to produce lift underneath it. Frontal boundaries, drylines and outflow boundaries are sources of lift as airmasses with different densities converge. Lift can also be produced by upslope flow.

Convective Available Potential Energy (CAPE)

A measure of instability. When air near the Earth's surface is lifted, it expands and cools. Eventually condensation can occur, which releases latent heat and further increases buoyancy. This is what drives thunderstorm development. CAPE measures how much buoyancy is available. Usually I'm looking for CAPE of least 1,000 J/KG.

Convective Inhibition (CINH)

Convective Inhibition measures how much warm air aloft is inhibiting thunderstorm initiation (negative buoyancy). Usually this needs to be less than 50 J/KG to allow thunderstorms to initiate, but it varies depending on the strength of the lifting.

Deep Layer Shear

In order to sustain thunderstorm updrafts, precipitation needs to moved away from the updraft by sufficient vertical wind shear (change in wind speed or direction with height). Usually I look at the effective bulk wind difference (EBWD). It should be 25 knots at a minimum, and preferably higher than 40 knots.

Low Level Shear

Shear in the lowest 1 KM is important for tornado formation. 0-1 KM storm relative helicity (SRH) should be at least 100 M2/S2, and preferably higher then 200 M2/S2.

Veering With Height

Discrete supercells are the most favorable storm mode for tornadoes. Supercells are more favored when the wind veers with height (turns clockwise). Ideally, winds at the surface will be from the east, and as you go up from the south, then from the west.

Lifted Condensation Level (LCL)

The lifted condensation level determines the height of the cloud base. When rain falls below the cloud base, evaporative cooling occurs. If the cloud base is too high, storms tend to produce too much cold outflow for tornadoes to form. Generally, I want LCL heights to be below 1.5 KM.

Storm Motion Vectors

Discrete supercells are most likely when the storm motion takes it away from the boundary that initiated the convection. If storms are moving parallel to a boundary, they tend to merge into a line. If storm motion is perpendicular to the boundary, supercells will be favored. Storms should also be moving faster than the boundary or they may be forced into a line or even be undercut by the boundary. For storm chasing purposes, a storm motion to the east if preferred because it is more aligned with the road network. Slower moving storms are also preferred. It can be difficult to keep up with a storm moving to the northeast at 40+ mph if the roads are all east-west and north-south.

Huricane Dorian likely to hit Florida

The HMON, HWRF, GFS, GDPS, ECMWF, GEFS, CMCE and EPS all indicate Hurricane Dorian will landfall somewhere in central or southern Florida. Landfall is most likely Monday night of Tuesday morning (Sept 2nd or 3rd). Dorian may slow as it approaches Florida, which could prolong the impact of wind and storm surge, and increase the risk of flooding. Though it's still early, the models are now generally putting Dorian's track near West Palm Beach or Port St. Lucie.

Dorain is currently a category 2 hurricane with wind estimated at 105 mph. Dorian will be entering an area with light shear and water temperatures in the mid 80s. This should allow Dorian to strengthen to category 4.

April Snowstorm

A very late season snowstorm produced up to 33" of snow in central Wisconsin from April 13-15.

Some snow totals:

14.3" Sioux Falls, SD
15.8" Minneapolis - St Paul
24.2" Green Bay, WI
21.2" Appleton, WI
28.0" Stevens Point, WI
31.6" Carlsville, WI (Door County)
33.0" Amherst, WI (Portage County)

At Green Bay, the 24.2" that they received was the largest April snowstorm on record. The previous record was only 11.0" on April 4 - 5 in 1977. Incredibly, it was the 2nd largest snowstorm on record. The largest had been 29.0" on March 1 -2 1888. There have been numerous roof collapses in the Green Bay Area. Green Bay has had 36.7" of snow in just April alone. The previous snowiest April was just 15.1" in 1907.

The first half of April has been up to 16F below normal for the Midwest:

NWS Des Moines reports that temperatures for the April 1 - 15 period have been 11.5 - 16.0F below normal, and the coldest since 1881:

Estherville, Mason City, Waterloo, Ottumwa and Lamoni currently 
rank as the coldest start to April on record /April 1-15/. Des 
Moines is currently 2nd coldest, behind 1881, as it is the only of
the sites with records dating back that far.

First Week of January

The first week of January was more than 20F below normal from Illinois to West Virginia.

Full Unedited Video of Greensburg Tornado

Tomorrow is the ten year anniversary of the F5 Greensburg tornado. On the night of May 4, 2007 a tornado grew to a width of 1.7 miles and destroyed most of Greensburg, KS, where 11 people were killed. It was the first F5 in 8 years.
 

Texas Snow and Tornadoes

On April 29th, six tornadoes hit eastern Texas, including an EF4, an EF3, an EF2, and three EF0s. Four fatalities were confirmed.

The April 29th 12Z (7 AM CDT) sounding for Fort Worth showed that there was already 2,500 CAPE:

As dew points rose into the mid 70s in the afternoon east of Dallas, CAPE values exceeded 5,000 by 21Z (4PM CDT) according to NAM:

There was strong low level wind - greater than 40 knots at the 1 km level - and a clockwise turning wind field with height, producing sufficient shear and helicity for tornadoes.

On April 29 and 30, heavy snow fell across the High Plains as far south as the Texas Panhandle. Amarillo received 3" and Dalhart had 9". There were over 50,000 power outages in the Texas Panhandle caused by high winds. At Amarillo, winds gusted to 54 mph and visibility dropped to 1/2 mile.

Garden City, KS had blizzard conditions on the 30th and 11" of snow, causing three structures to collapse. 14" fell in the southwest corner of Kansas at Elkhart. One farmer near Sublette estimated that he had lost 80% of his crop. At Wallace in northwest Kansas, 28" of snow fell, with drifts of 8-10 feet. 33" fell at Lander, WY and 39" fell at San Isabel, CO.

Flooding has killed 7 in Arkansas and 3 in Missouri. In southern Missouri, the town of Houston received 11.15" of rain.