Convective Outlook


SPC 1730Z Day 2 Outlook
Day 2 Outlook Image

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1223 PM CDT Sat Aug 15 2020

Valid 161200Z - 171200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHWEST
MN INTO PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS VICINITY...AS
WELL AS ACROSS SOUTHERN AZ...

...SUMMARY...
Clusters of strong to isolated severe thunderstorms are possible
across portions of the central Plains into the southern High Plains
and northern Texas. Additional strong storms are possible across
southern Arizona. Locally damaging gusts and hail will be possible
with the strongest storms.

...Synopsis...

An amplified upper level pattern will exist over the CONUS on
Sunday. An upper anticyclone centered over southern portions of the
Great Basin and the attendant ridge extending well north into the
Canadian Rockies. Meanwhile, an upper trough will be in place from
the Hudson Bay vicinity south/southeastward through the upper Great
Lakes to the Southeast. The northern branch of the eastern upper
trough will pivot eastward across the upper Great Lakes vicinity
toward NY and the Mid-Atlantic by the end of the period with the
upper ridge persists across the West. 

At the surface, a cold front will extend from far eastern Lower MI
south/southwest into northern AR and arcing northwest toward the
OK/KS border. A secondary cold front will be shifting southeast
across the northern Plains during the morning, reaching southwest MN
and central NE by mid-afternoon. The boundaries will be the main
focus for strong to isolated severe thunderstorm development across
portions of the central/southern Plains Sunday afternoon and
evening. Further west, a seasonally moist environment will exist
beneath the upper ridge across parts of southern AZ. Strong heating
will lead to afternoon thunderstorm clusters which could produce
strong winds. 

...Southwest MN into parts of the Central Plains vicinity...

Thunderstorms are expected to develop along the surface boundary
draped from southwest MN into southwest NE Sunday afternoon. A
narrow corridor of low-to-mid 60s F dewpoints will be in place
beneath modestly steep midlevel lapse rates. This will result in a
corridor of moderate to strong instability ahead of the boundary.
Forecast soundings indicated steep low level lapse rates, with LCLs
near, to just above, 850 mb. As such, isolated convection will be
rather high-based. Low to midlevel flow will not be particularly
strong, but directional shear will provide effective shear values
from 30-40 kt, supporting high-based supercells. Forcing for ascent
will not be very strong, and only a narrow corridor of favorable
conditions will exist from southwest MN into southwest NE/northwest
KS. A few strong gusts and/or hail will be possible, but isolated
coverage and somewhat short-duration of the threat will preclude
higher probabilities. 

...Southern High Plains into North Texas...

A surface boundary will sag southward across OK/AR through the
morning hours and extend from near the Red River, arcing
northwestward through the TX Panhandle. Resulting east/southeasterly
low level flow will maintain upper 50s to mid 60s dewpoints across
the region, and moderate to strong destabilization is expected by
afternoon. Thunderstorms are expected to first develop over higher
terrain across NM and CO and shift eastward over the High Plains in
a weak-shear environment. High-based cells will generally be capable
of strong downburst winds given very steep, sub-cloud layer lapse
rates. Given steep midlevel lapse rates and effective shear values
around 25-35 kt, an occasional hail producing storm is also
possible. Additional thunderstorm development is expected along the
instability gradient and surface boundary east/southeastward from
the TX Panhandle into northern TX. Thunderstorm clusters will mainly
be capable of wet microbursts. Some guidance suggest a
westward-propagating cluster could develop via storm mergers/outflow
interactions. If this occurs, it could increase the threat for
strong/locally damaging gusts.

...Southern AZ...

High-based thunderstorms are expected to develop during the
afternoon in a weak to moderately unstable airmass. Steep low level
lapse rates, with inverted-v low-level thermodynamic profiles
evident in forecast soundings, suggest strong outflow winds may
occur with this activity. Hi-res CAMs suggest a westward-propagating
line of convection could move across the lower desert vicinity
during the late afternoon. Strong to isolated severe gusts could
occur with these storms.

..Leitman.. 08/15/2020

Read more

Updated 15 August 2020 | 12:30 pm

SPC 1630Z Day 1 Outlook
Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1129 AM CDT Sat Aug 15 2020

Valid 151630Z - 161200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS NORTHERN MN AND
THE SOUTH-CENTRAL HIGH PLAINS...

...SUMMARY...
Scattered severe storms will be possible across northern Minnesota
and a portion of the south-central High Plains during the late
afternoon and early evening.

...Northern MN...
The left-exit region of an embedded mid-upper speed max over
the Prairie Provinces will translate southeast across the Red River
Valley by early evening. While boundary-layer dew points should
largely hover around the mid 50s in the wake of convection and a
cold frontal passage yesterday, cooling mid-level temperatures and
robust surface heating should support MLCAPE of 500-1000 J/kg.
Deep-layer vertical shear will be sufficient for a few supercells
and multicell clusters during the late afternoon to mid evening,
with a primary risk of severe hail.

...South-central High Plains...
Weak low-level easterlies have yielded upper 50s to mid 60s surface
dew points across the region. Surface heating beneath steep
mid-level lapse rates will result in a plume of moderate-strong
buoyancy at peak heating (MLCAPE of 2000-3000 J/kg), within a modest
northwest flow regime aloft characterized by effective bulk shear
increasing to near 30 kts. This will conditionally support a few
transient supercells and multicell clusters.

Storm initiation north of the Palmer Divide is highly questionable
given upstream mid-level drying overspreading the region per water
vapor imagery and minimal simulated convection in morning guidance.
Greater confidence in convective development is off the Sangre de
Cristos and Raton Mesa region. This activity should develop along
the periphery of the buoyancy plume and its uncertain whether it can
spread northeast towards the richer low-level moisture centered on
southwest KS prior to nocturnal MLCIN increase. Convection may tend
to spread southeast in the weaker shear regime of eastern NM and the
western TX Panhandle. Otherwise, low-level warm advection may yield
a cluster of overnight/early morning convection across
southwest/south-central KS and northwest OK with perhaps a marginal
severe risk.

...IL to the Ozarks...
A weakly convergent cold front slowly settling southeastward should
be the focus for isolated late afternoon and early evening
thunderstorms. A confluent belt of modest 20-25 kt mid-level
northwesterlies should overlie this portion of the front, with the
greatest buoyancy expected from MO southwestward. Setup should
support isolated damaging winds region wide, with some hail also
possible over the Ozarks.

..Grams/Wendt.. 08/15/2020

Read more

Updated 15 August 2020 | 11:38 am