SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0259 PM CDT Wed Jul 31 2024
Valid 312000Z - 011200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF THE MIDDLE MISSOURI VALLEY AND UPPER MIDWEST...
...SUMMARY...
Severe-weather risk continues from the northern/central Plains and
Upper Midwest to the southern Appalachians. This includes ongoing
potential for large to very large hail across parts of the
northern/central Plains, aside from a more prevalent risk of
damaging winds, some of which could be significantly severe (75+
mph).
...Discussion...
Prior outlook areas/reasoning continue to largely reflect current
thinking for the remainder of the period, with rather minor areal
adjustments made in various areas to reflect evolution of ongoing
convection.
The primary/more substantial change to the outlook is being
implemented across the Minnesota area and vicinity -- largely as
related to a convective cluster now moving east-northeastward across
western portions of the Gopher State. With a moderately unstable
airmass indicated ahead of this convection -- all the way to the
Canadian border -- it appears that severe risk will extend farther
north and east than initially anticipated (in line with recent WW
issuance across portions of northern Minnesota).
..Goss.. 07/31/2024
.PREV DISCUSSION... /ISSUED 1130 AM CDT Wed Jul 31 2024/
...Northern Plains into the Upper Midwest and Mid Mississippi
Valley...
A seasonably strong mid/upper level trough over the northern High
Plains late this morning will continue to progress eastward across
the Dakotas/NE and the Upper Midwest through tonight. At the
surface, a cold front is forecast to move southeastward across parts
of the northern/central Plains through the day, while a lee trough
advances eastward across parts of the southern High Plains by late
this afternoon. These surface features, combined with ascent
attendant to the mid/upper-level trough, should serve as forcing
mechanisms for severe thunderstorms today.
Ongoing convection across parts of ND/SD into central NE may still
be somewhat elevated. But, modifying the 12Z sounding from ABR shows
that the airmass across the eastern Dakotas ahead of the cold front
is rapidly destabilizing, and this activity should pose at least
some severe hail threat in the short term. Upwards of 3000-4000 J/kg
of MLCAPE will likely be present across eastern SD and vicinity by
early afternoon as daytime heating further erodes lingering MLCIN.
Weak low-level winds should gradually veer and modestly strengthen
to around 30-40 kt at midlevels as the shortwave trough approaches
from the west. This combination of very strong to locally extreme
instability will support intense severe thunderstorms this
afternoon, including multiple supercells posing a threat for very
large hail initially (up to 2-3 inches in diameter). Additional
robust convective development should occur southwestward near the
cold front into parts of south-central NE/north-central KS, with
large hail and severe wind gusts expected.
With time, multiple thunderstorm interactions and mergers should
lead to one or more bowing clusters consolidating from eastern SD/NE
into southwest MN and parts of western/central IA. A swath of
severe/damaging winds appears likely with any cluster that can
develop. Significant severe wind gusts of 75+ mph appear plausible
given the steep low/mid-level lapse rates forecast, and a reservoir
of very strong to extreme buoyancy that will likely be present. An
intense MCS with a continued severe wind threat could persist
tonight across southern MN into IA, and perhaps northern MO into IL.
...Central Plains into the Southern High Plains...
Large-scale ascent will become more modest with southward extent
across the central Plains into the Southern High Plains. Even so,
isolated to widely scattered thunderstorms should develop along the
cold front and lee trough by late this afternoon. With weaker
deep-layer shear compared to locations farther north, overall
thunderstorm mode may tend to be more multicellular versus
supercellular. Regardless, moderate to strong instability and
sufficient shear should support some threat for isolated hail and
severe wind gusts with any robust convection that can develop.
...Midwest/Lower Ohio Valley into the Central/Southern
Appalachians...
Similar to prior days, a belt of modestly enhanced mid-level
northwesterly flow will persist today from parts of the Midwest into
the Lower OH Valley and central/southern Appalachians. Outside of
ongoing late morning convection, much of these areas will have a
moist and rather unstable airmass present this afternoon as daytime
heating continues. Multiple organized, severe wind producing
clusters/MCSs may occur through this evening across these regions.
However, confidence in a more concentrated severe wind corridor
remains too low to include higher severe probabilities (Enhanced
Risk) with this update.
A cluster moving across western into central IN has recently shown
signs of weakening, with outflow surging well ahead of reflectivity
cores. Visible satellite imagery also shows outflow from convection
in west-central IL surging southward. A separate area of
thunderstorms in southern IA may also pose an isolated hail wind
threat as this activity spreads into a gradually destabilizing
airmass across northeast MO. It appears plausible that any of this
ongoing convection may restrengthen through the afternoon as it
continues generally east-southeastward. Additional robust
thunderstorms may develop along the various outflow boundaries. Any
cluster that can form and be sustained should pose a threat for
mainly damaging winds given the favorable thermodynamic environment
coupled with modest but sufficient deep-layer shear.
...Southern Arizona...
An upper low centered near Baja California and ongoing early day
storms across parts of northwest Mexico may influence thunderstorm
development later this afternoon, with a moist airmass located
particularly across parts of southeast/south-central AZ. With a well
mixed boundary layer and steepened low/mid-level lapse rates
expected, some of this activity could produce isolated strong to
severe wind gusts and blowing dust this afternoon into early
evening.
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