Archive for June, 2012
How Hail Forms!
- Published on Monday, 18 June 2012 19:41
- Jenny Brown
- 2 Comments
Now that we’ve had a few hail storms so far this year , I thought it would be a nice time time to share with y’all information on how hail forms. I promise I won’t get too scientific or technical!
Inside of all thunderstorms are warm updrafts and cool downdrafts. The updrafts carry water droplets high up into the top of the cloud above the freezing level. The water droplets then freeze and fall back into the bottom portion of the cloud partially melting along the way. Sometimes they are picked up by another updraft and carried back up into the cloud top along with additional water droplets, and the freezing/rising, falling/melting cycle repeats itself again and again. Eventually you have a hailstone with many layers of ice that can become quite large and heavy.
The mechanism that keeps hailstones lofted in the cloud is the strength, or speed, of the thunderstorm updraft. Once the hailstone’s weight exceeds what the thunderstorm updraft can hold aloft, the hailstone falls to the ground. Most garden variety thunderstorms, such as storms that just pop up during the heat of the day, only have updraft speeds in the range of about 15 to 30mph. Stronger storms such as the type that form into lines or clusters can have updraft speeds around 60mph. The larger storms known as supercell thunderstorms often have updraft speeds exceeding 80mph, and over 100mph is not undeard of.
The longer the hailstone can stay lofted by the strong updrafts, the larger it can grow. Also important is how much of the cloud is within the zone that is below freezing. In the case of the hail storm that hit Dallas back on June 13, 2012, the freezing level was measured at 14,811 feet that evening. That’s a little over 2 ½ miles up from the surface. The cloud tops for those storms were measuring roughly 53,000 feet high. That’s roughly 38,000 feet of hailstone bounce house! By my estimation that evening based on the readings for CAPE (Convective Available Potential Energy… a specific atmospheric reading that measures instability), those storms had updraft speeds approaching 100mph. Put all that together and you end up with one heck of a devastating hailstorm.
Here is a link to an animation showing the above processes. http://apollo.lsc.vsc.edu/classes/met130/notes/chapter7/52_Hail/A_52.html Once you open the link, click “Continue to Active Figure” then click “Play” and watch this basic animation of hail formation.
Hail can also come in some really unusual shapes that don’t include “round”. Hail can be spikey and/or lumpy in appearance if other smaller hail stones have adhered to a larger hail stone during its ascent. Condensation in the cloud releases latent heat which keeps the external surface of hail stones on the wet side making them more likely to stick together on their way towards the surface.
The largest recorded hail stone in the US was produced by a storm that struck Vivian, SD on July 23, 2010. The record-holding stone was measured at 8 inches in diameter, 18.625 inches in circumference and weighed in at nearly 2 pounds! The link below is to a summary article published by the National Weather Service office in Aberdeen, SD.
http://www.crh.noaa.gov/abr/?n=stormdamagetemplate
Largest hailstone recorded for the United States was on July 23, 2010 in Vivian, SD
12:30 PM Severe Weather Update
- Published on Tuesday, 12 June 2012 12:33
- David Reimer
- 0 Comments
Elevated Severe Weather Risk (Red Shading): 30 percent chance of severe weather occuring within 25 miles of your location.
Standard Severe Weather Risk (Yellow/Orange Shading): 15 percent chance of severe weather occurring within 25 miles of your location.
Low Severe Weather Risk (Brown Shading): 5 percent chance of severe weather occurring within 25 miles of your location.
Threats across Texas Hill Country and North Texas: Baseball size hail and destructive winds over 70 MPH with activity this afternoon. Another batch of storms possible later this evening with a hail/wind threat.
Threats with evening storms in West Texas: Isolated tornadoes, hail larger then baseballs, damaging winds in excess of 60 MPH
Discussion:
As mentioned in this morning’s blog update, I felt it likely that the Storm Prediction Center would extend the elevated risk of severe weather further south and east once model data agreed on an outcome. That has happened at the elevated risk of severe weather now runs all the way from eastern New Mexico, across much of West Texas into the Texas Hill Country, and then parts of Southeast Texas. Confidence is highest that several rounds of thunderstorms, some severe, will impact areas in the elevated risk areas. A standard severe weather risk surrounds the elevated risk, where severe weather is also possible.
One complex of thunderstorms is already ongoing at discussion time just east of Abilene with very large hail and damaging winds. This complex of storms will continue to move southeast and likely affect the Interstate 35 corridor within a few hours. Based on projected instability amounts, the severe weather threat will continue with that batch of storms. Additional thunderstorms will form across eastern New Mexico and West Texas later this afternoon. These storms will likely be supercellular in nature with all modes of severe weather possible. They will organize into a complex of storms and once again, move southeast, providing another round of storms for the Texas Hill Country overnight. Severe weather will also be possible with those storms.
Tuesday, June 12 Severe Weather Outlook
- Published on Tuesday, 12 June 2012 04:19
- David Reimer
- 0 Comments
6/12/2012 06Z Severe Weather Outlook
Elevated Severe Weather Risk (Red Shading): 30 percent chance of severe weather occuring within 25 miles of your location.
Standard Severe Weather Risk (Yellow/Orange Shading): 15 percent chance of severe weather occurring within 25 miles of your location.
Low Severe Weather Risk (Brown Shading): 5 percent chance of severe weather occurring within 25 miles of your location.
Threats with afternoon/early evening storms in West Texas: Isolated tornadoes, hail larger then baseballs, damaging winds around 65 MPH.
Threats with evening storms in West Texas as they congeal into a line/complex and push east: Destructive winds in excess of 80 MPH, hail up to golfball size.
Discussion:
The Storm Prediction Center has issued a large severe weather risk for Tuesday, with locations from the Texas/New Mexico border all the way east to Louisiana included. As of the early morning outlook, the highest severe weather probabilities have been placed over the southern Texas Panhandle and West Texas where supercell thunderstorms will develop by the late afternoon hours. These storms will be capable of producing isolated tornadoes and very large hail. During the evening, storms will congeal into a squall line or mesoscale convective system (MCS, a fancy term for complex of storms). Once this occurs, the threat for widespread damaging winds will increase along with a continuation of the large hail threat. This complex is expected to push southeast towards the Texas Hill Country and possibly North Texas. There is uncertainty on the exact track of the storm complex, whether it will track further south into the Texas Hill Country or further north into North Texas. With that in mind, I do expect the elevated severe weather probabilities to be extended eastward in later outlooks. The complex of storms should be weakening by the time it approaches Interstate 35 on Wednesday morning.
Severe Thunderstorm Watch will be issued Soon
- Published on Monday, 11 June 2012 15:41
- David Reimer
- 0 Comments
NWS STORM PREDICTION CENTER NORMAN OK 0332 PM CDT MON JUN 11 2012 AREAS AFFECTED...W-CNTRL AND NW TX CONCERNING...SEVERE POTENTIAL...WATCH LIKELY VALID 112032Z - 112130Z PROBABILITY OF WATCH ISSUANCE...80 PERCENT SUMMARY...THE THREAT FOR LARGE HAIL AND DMGG WIND GUSTS IS INCREASING OVER PARTS OF W-CNTRL AND NW TX. A WW WILL LIKELY BE NEEDED WITHIN THE NEXT HR. DISCUSSION...SFC ANALYSIS FROM 20Z PLACES A DEEPENING SFC LOW NEAR SPS...WITH A SLOW-MOVING COLD FRONT ORIENTED SW-NE ACROSS N TX. RECENT VISIBLE SATELLITE IMAGES SHOW INDICATIONS OF A DEVELOPING SFC-BASED CU FIELD 40 NW MWL OCCURRING ALONG WIND SHIFT...AND ADDITIONAL TOWERING CU DEVELOPING APPROXIMATELY 30 N SJT. VERY STRONG BOUNDARY LAYER HEATING...WITH TEMPERATURES REACHING NEAR 100 F...HAS BEEN SUFFICIENT TO OVERCOME A WARM LAYER NEAR 850 MB OBSERVED IN 12Z SOUNDINGS. FORCING FOR ASCENT IS EXPECTED TO BE WEAK...BUT GIVEN THE STRONG HEATING AND CONTINUED WEAK CONVERGENCE ALONG/NEAR WIND SHIFT AND ATTENDANT DRYLINE...INITIATION OF AT LEAST ISOLATED STORMS APPEARS INCREASINGLY LIKELY. WITH MLCAPE VALUES NOW EXCEEDING 4000 J/KG IN SOME AREAS...STORMS WILL LIKELY BE CAPABLE OF PRODUCING VERY LARGE HAIL AND ISOLATED DMGG WIND GUSTS...BUT STORM ORGANIZATION SHOULD BE SOMEWHAT LIMITED BY WEAK EFFECTIVE SHEAR /NEAR 20 KTS/. ..ROGERS/MEAD.. 06/11/2012
3 PM Severe Weather Update
- Published on Monday, 11 June 2012 15:28
- David Reimer
- 0 Comments
6/11/2012 20Z: Severe Weather Outlook
Here is the latest severe weather outlook from the Storm Prediction Center. This outlook remains nearly unchanged compared to the one we posted in our morning discussion. The orange shaded regions are in the standard risk of severe weather with a 15 percent chance of severe weather occurring within 25 miles of any given point, such as your house. Locations outside the orange zone but in the brown zone have a low risk of severe weather, or a 5 percent chance of severe weather within 25 miles of any given location, such as your house. The Dallas/Fort-Worth Metroplex is included in the standard risk of severe weather.
Visible Satellite Imagery at 3:16 PM CDT
This satellite image shows that we have the beginning development of a cumulus field from Abilene northeast to Jacksboro. This cumulus field will continue to expand over the next few hours as the cap begins to weaken. Latest short term model guidance suggests thunderstorms will be ongoing south of Abilene to around Brownwood by 6 PM. It looks like thunderstorms make take a little longer to develop northwest of Fort Worth, but all model guidance is consistent in showing thunderstorms in progress by 8 PM from Bowie to Decatur. As storms northwest of Fort Worth begin to congeal into a squall line or Mesoscale Convective System (MCS, a fancy word for a complex of storms), the risk for damaging winds would increase.
Severe Weather Risks with Strongest Storms:
Baseball or larger size hail
Damaging winds to 70 MPH (If storms congeal into line/complex, wind threat will increase and some gusts could exceed 75 MPH)
Isolated, Brief Tornadoes
Later in the evening when thunderstorms congeal into a line/complex of storms, the risk for damaging winds would increase along with a continued risk of large hail. Obviously, these storms would be prolific lightning producers.
I’m in Little Elm having lunch right now and plan to stay put for a few more hours until storms begin developing a county or two to my northwest. When that happens, I’ll move to intercept them and bring the live stream online.
National Radar Composite
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