Mesoscale Discussion 1799

Issued

Oct 5, 2021, 11:18 AM MST (18:18 UTC)

Expired

Oct 5, 2021, 1:15 PM MST

In force

117 min

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch likely

Chance of a watch

95%

Counties Coconino County, AZ, La Paz County, AZ, Maricopa County, AZ, Mohave County, AZ, Yavapai County, AZ

The Flagstaff radar (KFSX), 145 km from the middle of the area, began a volume at 18:21 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.

• Severe Thunderstorm Watch 512Oct 5, 11:45 AM MST

866 
ACUS11 KWNS 051819
SWOMCD
SPC MCD 051818 
AZZ000-NVZ000-052015-

Mesoscale Discussion 1799
NWS Storm Prediction Center Norman OK
0118 PM CDT Tue Oct 05 2021

Areas affected...western/central Arizona

Concerning...Severe potential...Watch likely 

Valid 051818Z - 052015Z

Probability of Watch Issuance...95 percent

SUMMARY...Ahead of the upper low, widespread thunderstorms will
likely organize this afternoon/evening. Damaging winds and large
hail appear likely though a tornado may also be possible with any
supercells. While uncertainty on the best temporal window for severe
remains, a weather watch is likely within the hour.

DISCUSSION...As of 1800 UTC, regional radar showed scattered to
widespread thunderstorms ongoing across much of central and western
AZ into southern NV. Ongoing in broad ascent ahead of an upper low
ejecting out of southern California, additional storm development
appears likely over the next couple hours. Visible imagery shows
breaks in the clouds are supporting modest surface
heating/destabilization with 500-1000 J/kg of MLCAPE on SPC
mesoanalysis. Deep southerly flow aloft around 40 kts will be
sufficient to support organized storms, including a few supercells,
with bulk shear of 40-50 kts. Low-level shear remains weaker, but
low-level backing of the flow ahead of the upper low may support a
brief tornado or two along with the relatively greater threat for
strong wind gusts and large hail. The steep low and mid-level lapse
rates may also support a few significant gusts or hail greater than
2+ in as well.

Convective trends and hi-res guidance point to the increase severe
potential across western and central AZ in the next couple of hours
as the ongoing cluster of storms from southern NV, eastern CA and
western AZ move northeast. Some uncertainty exists on the convective
evolution of the northern cluster as it approaches an area of lesser
buoyancy under more persistent cloud cover across north-central AZ.
Additional thunderstorms developing within cloud breaks farther to
the south and west will likely have the greatest severe potential as
they track into central AZ over the next few hours. Given the
increasing threat for damaging wind gusts hail and perhaps a
tornado, a weather watch is likely.

..Lyons/Guyer.. 10/05/2021

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...TWC...FGZ...PSR...VEF...

LAT...LON   32761218 32661318 32721359 32921385 33251401 33541413
            34131407 34531421 35451475 36011458 36431414 36591339
            36411256 36101177 35671140 34861113 34261092 33691081
            33311076 33041090 32871161 32761218

Mesoscale Discussions in Arizona, 2021Every mesoscale discussion in 2021

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.