Dr. Jeff Basara’s Guide to Forecasting Severe Weather:

Things to look for at the synoptic scale:

Are there any regions of forced rising motion?What are the synoptic surface conditions?

Things to look for at the mesoscale:

Where is the dryline, or does it even exist?Where is the low-level moisture?Are there any outflow boundaries?Is the low-level jet present?Always look at the soundings!

Things to look for at the local scale:

Where are clouds located?Where is the strongest destabilization occurring with respect to other features such as boundaries?Are any cumulus clouds forming?

Although there are many indices we can look at to forecast severe weather, there are 3 main characteristics that we focus on:

1) Instability – this is the driving mechanism for severe weather. You want to determine how unstable the atmosphere is, and where it is most unstable. We often focus on CAPE to determine this.

2) Vertical Wind Shear – how do wind speed and/or direction change from 0 to 5 km?

3) Trigger – what will initiate and sustain convection?

NSSL photo

Once convection has begun, it will frequently organize into one of 3 forms:

1) Squall lines – tend to form in regions of weak CAPE and strong vertical wind shear. The trigger is usually a strong surface front coupled with an upper level system2) Multicells – severe multicells tend to form in regions of high CAPE and weak vertical wind shear.3) Supercells – form in regions of strong vertical wind shear (both speed and directions) and high CAPE

We’ll look in more detail at supercell severe storms

NSSL photo

SUPERCELL THUNDERSTORMS

• Relatively long-lived, intense thunderstorm• Consists of one large cell, usually with exceptionally strong updrafts• Structure is such that updraft and downdraft are separated from each other• Often the entire storm can begin to rotate• Many produce large tornadoes and extremely large hail• Typically form in regions of high CAPE and strong horizontal and vertical wind shear

Supercell storms frequently result in tornadic activity

As a case study, we’ll look at the storms from 3 May 1999. These storms started out as individual supercells, but became multicellular as the evening progressed.

July 1999 3

Largest Tornado Outbreak Ever InOklahoma

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17

12

6 6

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U F0 F1 F2 F3 F4 F5

• 65+ tornadoesstatewide (57 inNorman CWFA)

• Previous state record26 tornadoes in oneday

• First F5 since 1982

• First F5 ever forOklahoma City

3 May 1999

Damage list from the 3 May 1999 tornadoes

Oklahoma and Cleveland CountiesHomes destroyed: 1,780. Homes damaged: 6,550. Apartments destroyed: 473. Apartments damaged: 568. Businesses destroyed: 85. Businesses damaged: 42. Churches destroyed: 3. Schools destroyed: 2.

Other CountiesHomes destroyed: 534. Homes damaged: 878. * Businesses destroyed: 79. Businesses damaged: 54. Churches destroyed: 2. Public buildings destroyed: 4. Public buildings damaged: 7. * Includes 53 stores at the Tanger Outlet Center in Stroud.

42 dead

795 injured

Much of the information in this presentation has been taken from

An Overview of Environmental Conditions and Forecast Implications of the 3 May 1999 Tornado Outbreak

by Richard L. Thompson and Roger Edwards, NOAA/NWS Storm Prediction Center, Norman, Oklahoma

http://www.spc.noaa.gov/publications/thompson/3may99/waf.htm

3 May 1999• late afternoon and evening outbreak• presented several difficult forecast problems

high clouds present most of the day evolution of middle and upper tropospheric flow not well forecast by models

500 mb 12 UTC 3 May 1999

Large scale trough located over western US with embedded short wave trough over Arizona. The large scale trough amplified over the Rockies by 00UTC 4 May, while the short wave trough progressed from AZ to western OK and KS

Shaded region is > 64F dew points. Dryline boundaries are drawn on warm side of each zonal moisture gradient in region of middle 50F or > dew points. Surface dew points in middle to upper 60s spread northward throughout the day.

Mesoscale view of surface conditions at 20 UTC approximately 30 minutes prior to earliest deep convection

Note 2 diffuse drylines as distinguished by the Mesonet

Eastern boundary is weaker than typically observed with a dryline

Most important – dew points in upper 60s and surface temps in middle 80s

This contributed to large CAPE

CAPE and 8 km winds 20 UTC 3 May 1999

When modified for surface conditions in vicinity of Storm A, CAPE was near 5000 and CIN less than 10

At 12 UTC the sounding revealed a moist boundary layer about 1 km deep beneath an elevated mixed layer from 825-600 mb The 00 UTC sounding indicated substantial warming from 600 mb to surface. Calculated parameters are for 00 UTC

All supercells were preceded by the development of two short-lived convective towers – that formed in a gap in the cirrus canopy between the two drylines – over northwestern Texas.

The cirrus gap shifted northeastward into southwestern Oklahoma. The convection evolved into a split storm, with the first supercell developing in the right moving storm.Storms A and B were the most prolific tornado-producers of the outbreak, including the F5 tornado that moved across the southern OKC metropolitan area

Visible satellite image at 645 PM CDT, May 3, 1999. At 651 PM CDT, the Moore/Oklahoma City tornado was located near the access junction on the H.E. Bailey Turnpike in north Newcastle, Oklahoma. Image courtesy of the NOAA Satellite Services Division (Operational Significant Event Imagery)

Supercell A

All storms displayed classic supercell features:• large updrafts, well removed from main precip core,• rotating wall clouds with associated clear slots• thin precip curtains wrapping around the west side of the low-level mesocyclones

Supercell B - Striations are indication of forced ascent of parcels through the layer of convective inhibition

Radar reflectivity image from the Twin Lakes, Oklahoma WSR-88D Radar (KTLX) at 718 PM CDT, May 3, 1999. At 717 PM CDT, the Moore/Oklahoma City tornado was located near the intersection of Santa Fe Avenue and 119th Street in southwest Oklahoma City. Image courtesy of Oklahoma Climatological Survey.

Video snapshot of the Moore/Oklahoma City tornado just southeast of Amber, Oklahoma at about 6:30 PM CDT on May 3, 1999. The view is to the north-northeast from Highway 92 just north of the Highway 92/Highway 9 intersection, about 7 miles south of Amber. The tornado was captured on video by Oklahoma Climatological Survey employees Renee McPherson, Michael Wolfinbarger, Andrew Reader and Christopher Duvall.

Aerial Photos of the Damage from the May 3, 1999Tornado Outbreak in Central Oklahoma - Photos by Doug Crowley, NWS Southern Region Headquarters, Forth Worth, Texas.

Aerial view of the Moore/Oklahoma City tornado path on May 5, 1999. The dark mud trail shows the path of the tornado in the middle foreground and center of the picture. Interstate Highway 44 is visible in the lower right through the center of the picture while 16th Street in Newcastle runs from the center foreground to the upper right of the picture. The Canadian River is in the upper third of the picture while the Moore and south Oklahoma City areas can be seen in the distance in the upper left of the photo. Note how the tornado swerved to the right to hit the intersection of I-44 and the 16th Street overpass, then moved to the left again. The photo was taken by John Jarboe (NWS Coordinator at the FAA Academy in Oklahoma City) as part of a storm damage survey .