Alert archive / Montana / Mesoscale Discussions in 2021
Mesoscale Discussion 789
The Storm Prediction Center issued this mesoscale discussion at May 28, 2021, 3:55 PM MDT for 10 counties in Montana; Sheridan County, Wyoming. It was in force for 2 hr.
The area
The outline as issued.
The facts
- Issued
- May 28, 2021, 3:55 PM MDT (21:55 UTC)
- Expired
- May 28, 2021, 6:00 PM MDT
- In force
- 2 hr
- Issued by
- the Storm Prediction Center
- Concerning
- Severe potential...watch unlikely
- Chance of a watch
- 20%
Counties Big Horn County, MT, Carbon County, MT, Carter County, MT, Custer County, MT, Fallon County, MT, Powder River County, MT, Rosebud County, MT, Stillwater County, MT, Treasure County, MT, Yellowstone County, MT, Sheridan County, WY
The radar
The Billings radar (KBLX), 163 km from the middle of the area, began a volume at 21:55 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 nearest sounding before it
Glasgow Intl Airport, MT (KGGW), launched 12:00Z, 9.9 hours before the alert went out and 294 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
- 0 J/kg
- Mixed-layer CIN
- No buoyant parcel
- Mixed-layer LCL
- 1536 m above ground
- 0-6 km bulk shear
- 40 kt
- 0-1 km storm-relative helicity
- 346 m²/s²
- 0-3 km storm-relative helicity
- 399 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.0
The text
As transmitted.
240
ACUS11 KWNS 282155
SWOMCD
SPC MCD 282155
MTZ000-WYZ000-290000-
Mesoscale Discussion 0789
NWS Storm Prediction Center Norman OK
0455 PM CDT Fri May 28 2021
Areas affected...portions of southeast Montana into extreme northern
Wyoming
Concerning...Severe potential...Watch unlikely
Valid 282155Z - 290000Z
Probability of Watch Issuance...20 percent
SUMMARY...Large hail and gusty winds may accompany the stronger
storms into the early evening hours. A WW issuance is unlikely given
the expected sparse coverage of severe wind/hail.
DISCUSSION...Multicellular thunderstorm clusters have been gradually
increasing in coverage and intensity across Yellowstone Country over
the past couple of hours given increased deep-layer ascent by a
passing mid-level trough. These storms are developing atop a dry
boundary layer, with the latest METAR observations showing up to 30F
temperature/dewpoint spreads, and RAP forecast soundings indicating
a dry sub-cloud layer extending up to 500 mb. Despite the dry
boundary layer, 8.5-9.5 C/km low and mid-level lapse rates are
promoting up to 1000 J/kg SBCAPE, with some convective
organization/sustenance possible given 35+ kts of effective bulk
shear.
Much of the CAPE is contained above 700 mb, and is characterized by
low CAPE density. As such, large hail may accompany the more robust,
longer lasting multicellular clusters. Damaging gusts are also
possible given efficient evaporative cooling and downward momentum
transport within the steep lapse rate/dry low-level environment.
Nonetheless, overall convective coverage and associated brief severe
threat is expected to be relatively isolated overall, and a WW
issuance is not expected.
..Squitieri/Edwards.. 05/28/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...UNR...BYZ...RIW...
LAT...LON 45950950 46340762 46490643 46500498 46460453 46000409
45620414 45090428 44740467 44590564 44640684 44820766
45000887 45390944 45950950Nearby
Mesoscale Discussions in Montana, 2021Mesoscale Discussions in Wyoming, 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.