Alert archive / Virginia / Mesoscale Discussions in 2021
Mesoscale Discussion 1550
The Storm Prediction Center issued this mesoscale discussion at Aug 18, 2021, 8:46 AM EDT for Frederick County, Washington County, Maryland; 6 counties in Virginia; Berkeley County, Jefferson County, West Virginia. It was in force for 119 min.
The area
The outline as issued.
The facts
- Issued
- Aug 18, 2021, 8:46 AM EDT (12:46 UTC)
- Expired
- Aug 18, 2021, 10:45 AM EDT
- In force
- 119 min
- Issued by
- the Storm Prediction Center
- Concerning
- Severe potential...watch possible
- Chance of a watch
- 40%
Counties Frederick County, MD, Washington County, MD, City of Manassas, VA, City of Manassas Park, VA, Clarke County, VA, Fauquier County, VA, Loudoun County, VA, Prince William County, VA, Berkeley County, WV, Jefferson County, WV
The radar
The Sterling radar (KLWX), 39 km from the middle of the area, began a volume at 12:49 UTC, the scan nearest the moment this went out.
Open the radar archive at this moment
The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.
The watches that followed
Watches SPC issued over any of the same counties within four hours of this discussion.
The nearest sounding before it
Washington/Dulle, VA (KIAD), launched 12:00Z, 0.8 hours before the alert went out and 47 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.
- Mixed-layer CAPE
- 1011 J/kg
- Mixed-layer CIN
- -1 J/kg
- Mixed-layer LCL
- 592 m above ground
- 0-6 km bulk shear
- 30 kt
- 0-1 km storm-relative helicity
- 151 m²/s²
- 0-3 km storm-relative helicity
- 175 m²/s²
- Significant tornado parameter
- 0.6
- Supercell composite
- 4.6
The text
As transmitted.
830 ACUS11 KWNS 181247 SWOMCD SPC MCD 181246 MDZ000-PAZ000-VAZ000-WVZ000-181445- Mesoscale Discussion 1550 NWS Storm Prediction Center Norman OK 0746 AM CDT Wed Aug 18 2021 Areas affected...Parts of the eastern West Virginia Panhandle...northern Virginia...central Maryland and adjacent south central Pennsylvania Concerning...Severe potential...Watch possible Valid 181246Z - 181445Z Probability of Watch Issuance...40 percent SUMMARY...The development of occasional isolated supercell structures with the potential to produce tornadoes (generally short-lived) may increase through 11 am-1 pm EDT, mainly west through northwest of the District of Columbia into the higher terrain around Martinsburg, WV and Hagerstown, MD. It is not yet certain a watch will be needed, but trends are being monitored for this possibility. DISCUSSION...The remnant surface low of Fred has migrated northward into areas south/southeast of Parkersburg, WV during the past few hours. As this has occurred, very moist boundary layer has advected northwestward across the Mid Atlantic region. This includes dew points as high as the mid 70s F into the higher terrain centered around the Martinsburg, WV vicinity, where low-level convergence is becoming focused, and an area of enhanced mid/upper forcing for ascent is forecast to overspread through 15-17Z. Beneath a narrow belt of 30+ kt southerly flow around 850 mb, this also appears likely to coincide with where clockwise-curved low-level hodographs will become maximized, with the terrain contributing to the maintenance of backed/southeasterly near surface flow. As breaks in the overcast allow for insolation to contribute to boundary-layer destabilization, it appears that the environment may become increasingly conducive to supercell structures with the potential to produce relatively short-lived tornadoes. ..Kerr/Edwards.. 08/18/2021 ...Please see www.spc.noaa.gov for graphic product... ATTN...WFO...CTP...LWX... LAT...LON 39777813 39787740 39007724 38347764 38807819 39777813
SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Aug 18, 10:58 AM EDT | Mesoscale Discussion | SPC | Anne Arundel, Baltimore, Calvert, Carroll, Charles, Frederick and 9 more | 92 min |
| Aug 18, 11:37 AM EDT | Tornado Watch | SPC | Allegany, Anne Arundel, Baltimore, Calvert, Carroll, Cecil and 12 more | 11 hr |
| Aug 18, 3:29 PM EDT | Mesoscale Discussion | SPC | Anne Arundel, Baltimore, Carroll, Cecil, Frederick, Harford and 5 more | 91 min |
| Aug 18, 4:57 PM EDT | Mesoscale Discussion | SPC | Anne Arundel, Baltimore, Carroll, Cecil, Frederick, Harford and 6 more | 93 min |
| Aug 17, 10:04 PM EDT | Mesoscale Discussion | SPC | Bedford, Bland, Botetourt, Carroll, Craig, Floyd and 17 more | 116 min |
| Aug 18, 2:38 PM EDT | Mesoscale Discussion | SPC | Charles City, Gloucester, Isle of Wight, James City, King and Queen, Lancaster and 20 more | 97 min |
Nearby
Mesoscale Discussions in Virginia, 2021Mesoscale Discussions in Maryland, 2021Mesoscale Discussions in West Virginia, 2021Every mesoscale discussion in 2021
Sources
The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.