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Convective Outlook

SPC 1730Z Day 2 Outlook
Day 2 Outlook Image

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1230 PM CDT Thu Jun 17 2021

Valid 181200Z - 191200Z


Scattered severe storms will be probable Friday across Illinois,
Indiana, and Ohio.  Severe gusts, possibly significant, will be
capable of swaths of wind damage with the more intense storms.  A
couple of tornadoes are possible in addition to large hail.

A mid-level anticyclone will reside across the southwest into the
southern Great Plains on Friday.  To its north, an elongated belt of
strong mid-level flow will extend from the Great Lakes westward into
the northern Great Plains early Friday morning.  A mid-level
shortwave trough will amplify during the period as it moves from the
ND/Canadian border east-southeastward to the central Great Lakes
late Friday night.

At the surface, a front will become draped from the southern Great
Lakes westward into the central High Plains, with a moistening air
mass to the south over the mid MS Valley and OH Valley.  Analysis of
upper-air data on Thursday morning indicates that it is likely the
area of richer low-level moisture (15-17 g/kg lowest 100 mb mean
mixing ratio) on Thursday morning was sampled in between the raob
network centered over the Ozark Plateau when compared to surface
dewpoint observations.  It is the uncertainty regarding the spatial
extent and quality of this moisture pool and early day thunderstorm
activity that could lead to consequential effects for this forecast
across the OH Valley on Friday.

...Mid MS Valley through the OH Valley...
Scattered showers/thunderstorms will probably be ongoing Friday
morning across the southern Great Lakes where warm advection will be
maximized.  A warm front will advance east across much of OH during
the day, and possibly into western PA and NY by early evening, as
strongly veered 850 mb winds of 30-40 kt result in rapid airmass
recovery.  It is uncertain where the placement of a potential
outflow boundary over OH may reside by afternoon.  Models show a
reservoir of mid 60s to lower 70s deg F dewpoints beneath an EML. 
Models vary appreciably on destabilization but it appears MLCAPE
ranging from 2000-4000 J/kg is possible from east to west.  Current
thinking is a cluster of storms will develop during the afternoon
across IN/OH and spread southeast.  Although 850 mb flow will be
veered, the presence of the aforementioned outflow and its
enhancement in low-level shear may conditionally focus a tornado
risk.  Organized storms including supercells and bowing segments are
probable.  Weaker upper-level flow will favor a transition to HP
supercells and into QLCS/bowing segments with a wind hazard.  A
separate cluster of development seems likely to develop near the
front towards evening over the mid MS Valley and it spreading
east-southeast.  Severe gusts, potentially significant in localized
swaths, will be the primary threats with this activity.

...Central High Plains...
Model guidance shows upslope flow into northeast CO beneath 15-30 kt
westerly 500 mb flow on the southern periphery of the stronger
westerlies.  Isolated to scattered storms are forecast during the
afternoon.  Strong heating and steep lapse rates may yield a few
strong storms capable of an isolated hail/wind threat before this
activity weakens during the evening.

...Coastal LA...
A tropical depression is forecast to develop and move north across
the western Gulf of Mexico, approaching the TX/LA coastal region
early on Saturday. Coincident with the low, wind fields and shear
will also increase, along with tropical moisture aiding instability.
A substantial amount of convection is possible over LA, and a couple
tornadoes will conditionally be possible wherever mid 70s dewpoints
can reach land. Due to uncertainty in speed, there is some chance
that the better air mass remains just offshore for this forecast

..Smith.. 06/17/2021

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Updated 17 June 2021 | 12:37 pm

SPC 1630Z Day 1 Outlook
Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1130 AM CDT Thu Jun 17 2021

Valid 171630Z - 181200Z


Severe storms are expected late this afternoon into tonight across 
parts of the Upper Mississippi Valley, from central/southern
Wisconsin and southern Minnesota into northern portions of Iowa and
Illinois. A few tornadoes, damaging wind gusts exceeding 75 mph, and
hail up to the size of baseballs will be possible.

...Upper MS Valley...
Primary change from prior outlook is to expand greater probabilities
farther north/east across parts of WI and IL, with the expectation
of the wind threat persisting longer in this region tonight. 

A mid-level impulse/speed max over SD, responsible for morning ACCAS
across the eastern portion of the state, will aid in ascent along a
southeast-moving cold front. Thunderstorm initiation is expected
across west-central WI into southeast MN as MLCIN wanes near the
front during the late afternoon. Activity should eventually
backbuild southwestward into northeast/north-central IA this
evening, but uncertainty is greater with how far into central IA
convection will ultimately become sustained owing to greater
influence of the EML and persistence of a relatively hot/dry
boundary-layer across KS/southern NE into southern IA. 

While guidance will probably be somewhat overdone with the degree of
evapo-transpiration-induced moistening ahead of the front based on
12Z morning soundings and ongoing drought, large buoyancy with
MLCAPE of 2500-3500 J/kg should develop within a confined corridor
given steep mid-level lapse rates. The mid-level speed max (around
60 kts at 500 mb) will yield strong effective shear, especially with
northern extent. This suggests initial supercells will have the
potential to produce very large hail despite the relatively high
freezing levels. Enlarging low-level hodographs as a southwesterly
low-level jet strengthens this evening should maximize the supercell
tornado threat between 22-02Z. Storms are expected to predominately
coalesce and grow upscale into one or more bowing clusters with the
potential to produce significant severe wind gusts (greater than 75
mph) and brief tornadoes this evening. The damaging wind threat may
linger into the early overnight as strong low to deep-layer shear
attempts to compensate for increasingly pronounced MLCIN.

...Ozarks to Lower Red River Valley..
A remnant MCV over southwest MO will drift southward across western
AR this afternoon. Strong surface heating where dew points remain
from 70-75 F will support large MLCAPE over 3000 J/kg. Isolated to
perhaps scattered thunderstorms should develop southwest from
southern MO towards the AR/OK/TX border area. A belt of 20-30 kt
mid-level northeasterlies may provide adequate shear to support a
couple loosely organized multicell clusters. Isolated damaging
downbursts along with severe hail are possible during the late
afternoon and early evening.

..Grams/Squitieri.. 06/17/2021

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Updated 17 June 2021 | 11:38 am

SPC 1300Z Day 1 Outlook
Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0739 AM CDT Thu Jun 17 2021

Valid 171300Z - 181200Z


Severe storms are expected late this afternoon into tonight for
parts of the upper Mississippi Valley, from northern Iowa/southern
Minnesota into southern Wisconsin and northern Illinois.  Wind gusts
near or exceeding 75 mph, hail up to the size of baseballs, and a
couple of tornadoes will be possible.

...Upper MS Valley this afternoon into tonight...
A midlevel shortwave trough over the northern Plains/southern
Prairie Provinces has crested the ridge, and heights will be
suppressed east-southeastward toward the upper MS Valley and upper
Great lakes by tonight.  An associated, weak surface cold
front/trough will likewise move slowly southeastward across
NE/IA/MN/WI through tonight.  The front and any lingering outflow
boundaries from overnight convection will likely focus afternoon and
evening thunderstorm development across southeast MN and along the
IA/MN border, in phase with an embedded speed max moving eastward
from SD (as evidence by the weak midlevel convection over SD this

The thermodynamic environment will feature steep midlevel lapse
rates with a warm elevated mixed layer extending eastward from the
Rockies, atop boundary-layer dewpoints in the 65-70 F range.  Strong
surface heating in the wake of morning elevated convection will
drive large buoyancy (MLCAPE in excess of 4000 J/kg) and weakening
convective inhibition as surface temperatures warm into the 90s. 
Effective bulk shear of 40-50 kt and the large CAPE will support a
threat for supercells initially, with very large hail (up to
baseball size) and damaging winds.  Any semi-discrete storms in the
zone of stronger low-level shear/warm advection closer to the MS
River will also pose a threat for a couple of tornadoes.  Otherwise,
storms are expected to coalesce and grow upscale into one or more
bowing clusters with the potential to produce significant damaging
winds (greater than 75 mph) prior to the convection spreading east
of the larger buoyancy late tonight.

...AR this afternoon/evening...
A remnant MCV over west central MO this morning will continue
southward over AR by later this afternoon.  Strong surface heating
(temperatures warming into the lower 90s) and boundary-layer
dewpoints in the lower 70s will support MLCAPE in excess of 3000
J/kg, and isolated to widely scattered thunderstorms are expected to
form by mid-late afternoon.  Deep-layer, north-northeasterly shear
of 20-30 kt and DCAPE in excess of 1200 J/kg suggest that any
cells/small clusters will move toward the south-southwest with the
potential to produce isolated damaging downbursts and large hail for
a few hours this late afternoon/evening.

..Thompson/Leitman.. 06/17/2021

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Updated 17 June 2021 | 7:48 am