SPC May 18, 2017 0600 UTC Day 1 Convective Outlook

By Newsroom America Feeds at 18 May 02:08

SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0100 AM CDT Thu May 18 2017 Valid 181200Z - 191200Z ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF SOUTHERN AND CENTRAL KANSAS INTO NORTHWEST OKLAHOMA... ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE HIGH RISK AREA ACROSS PARTS OF OKLAHOMA AND KANSAS... ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE MODERATE RISK AREA ACROSS PARTS OF THE SOUTHERN AND CENTRAL PLAINS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE ENHANCED RISK AREA ACROSS PARTS OF THE SOUTHERN AND CENTRAL PLAINS INTO LOWER MISSOURI VALLEY... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE LOWER GREAT LAKES REGION... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE SOUTHERN AND CENTRAL PLAINS NORTHEASTWARD INTO THE MID MISSISSIPPI VALLEY AND NORTHEASTERN STATES... ...SUMMARY... An outbreak of severe thunderstorms with tornadoes, very large hail and wind damage is expected to develop across parts of the southern and central Plains from this afternoon into this evening and overnight. Strong long-track tornadoes will be likely from parts of central and southern Kansas southward into western Oklahoma. Large hail and wind damage will also be possible outside of these areas across parts of the southern and central Plains. A severe threat is also expected across parts of the lower Great Lakes region late this afternoon and early this evening. ...DISCUSSION... ...Tornado Outbreak Likely Across Parts of the Southern and Central Plains Today and Tonight... ...Southern and Central Plains... An upper-level low will move eastward across the Four Corners region and central Rockies today as a lead shortwave rotates around the southeastern side of the system into the southern and central Plains. At the surface, a very moist airmass will advect northward across Oklahoma into southern and central Kansas with surface dewpoints reaching the mid 60s to lower 70s by afternoon. Strong instability should be in place to the east of a well-developed dryline situated from just east of Dodge City southward across far western Oklahoma by mid-afternoon. Thunderstorms are forecast to initiate rapidly during the mid to late afternoon along the dryline with numerous severe thunderstorms moving east-northeastward across the southern and central Plains during the late afternoon and early evening. A very moist airmass, strongly unstable warm sector, favorable deep-layer shear and impressive low-level shear appears likely for a tornado outbreak from central and southern Kansas southward across western Oklahoma. At this time, surface maps show a very moist airmass across the eastern third of Texas with lower to mid 70s F dewpoints at most observation sites. This airmass is advecting northward quickly and appears certain to overspread the moderate and high risk areas by this afternoon. Deterministic and hi-res solutions suggest that a strongly unstable airmass will be in place by afternoon with MLCAPE values reaching the 3000 to 4000 J/kg range. In addition, the exit region of a 45 to 60 kt mid-level jet is forecast to be located across northwest Oklahoma and south-central Kansas by late afternoon which will create strong deep-layer shear profiles over the top of a pristine airmass. NAM forecast soundings late this afternoon show 0-6 km shear in the 50 to 60 kt range to the east of the dryline with gradually veering winds with height in the low to mid-levels. This wind profile will support discrete supercell formation. Backed surface winds to the southeast, strong speed shear in the boundary layer and a strengthening 40 to 50 kt low-level jet will be very favorable for tornadoes. Low-level shear is forecast to increase during the early evening with 0-3 km storm-relative helicities reaching the 350 to 450 m2/s2 range across the moderate and high risk areas. This will be favorable for strong tornadoes with several long track tornadoes moving across the high risk and moderate risk areas. High-end tornadoes will also be possible across the high risk area. Forecast soundings also show very steep 700 to 500 mb lapse rates of 8.0 C/km. This combined with the strong deep-layer shear will make large hail likely with supercells. The more intense supercells will likely produce hailstones of greater than 2 inches in diameter. By late evening, MCS formation should take place as several severe storm clusters move eastward across northern Oklahoma and eastern Kansas. Some tornadoes along with large hail and wind damage should be associated with this MCS. Further south across west-central Texas, a dryline is forecast to be situated from near Vernon, Texas south-southwestward to just west of San Angelo, Texas. Widely spaced thunderstorms are forecast to initiate just to the east of the dryline along this corridor. Strong instability, moderate deep-layer shear and steep mid-level lapse rates evident on forecast soundings will support the formation of supercells. An isolated tornado threat along with a potential for very large hail will be possible with supercells that develop during the late afternoon and early evening. ...Lower Great Lakes Region/Mid Mississippi Valley... An upper-level trough will move eastward across the Great Lakes region today. At the surface, a cold front is forecast to advance southeastward into the lower Great Lakes region by late afternoon as moderate instability develops along to the south of the front. Thunderstorms are expected to develop just ahead of the front from northern Indiana east-northeastward into northern Ohio and western New York late this afternoon. This convection will be supported by increasing low-level convergence along the front and be aided by large-scale ascent associated with the upper-level trough. NAM forecast soundings along this corridor at 00Z/Friday show MLCAPE vales in the 1000 to 1500 J/kg range along with 0-6km shear values of 35 to 45 kt. This environment may support isolated supercell development with large hail possible. However, multicell may be the preferred storm mode due to front being oriented parallel to the low to mid-level flow. A wind-damage threat will be most likely with bowing multicell structures. ..Broyles/Mosier.. 05/18/2017 Read more

http://www.spc.noaa.gov/products/outlook/day1otlk_1200.html

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