SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0748 AM CDT Tue Aug 20 2024
Valid 201300Z - 211200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS PREDOMINANTLY OVER
PORTIONS OF CENTRAL MONTANA...
...SUMMARY...
Severe thunderstorm winds and large hail are possible today, mainly
over parts of Montana.
...Synopsis...
In mid/upper levels, a rather blocky large-scale pattern will yield
very slow evolution of major features through the period. A 500-mb
low that has just closed off over southern QC should move southward
over central NY through the period, with associated cyclonic flow
covering the full extent of the Appalachians and most of the CONUS
from the mid/upper Mississippi Valley eastward. An anticyclone will
remain anchored over NM, west TX and vicinity, with attached ridging
moving slowly eastward over CO and the northern Plains. This will
permit strengthening southwest flow aloft to spread out of the
northern Rockies and across most of MT through the period, ahead of
West Coast troughing. An embedded, weak shortwave trough -- evident
in moisture-channel imagery over south-central WA and central OR--
should eject northeastward over the Canadian Rockies,
The 11Z surface analysis showed a cold front offshore from New
England and the Mid-Atlantic, just offshore from the Carolinas, then
across southeastern GA, the eastern FL Panhandle, MS/AL shelf
waters, and coastal LA, becoming a warm front northwestward over
east and north TX to northeastern NM. The Atlantic segment should
proceed farther offshore, and sag southward into portions of
northern FL and the north-central Gulf, while the western parts
stalls from north-central/northwest TX to northeastern NM. Warm
frontogenesis over the north-central High Plains -- just east of a
lee trough -- should connect with the south-central High Plains
pre-existing frontal zone by late in the period.
...Northern High Plains...
Widely scattered to scattered, high-based thunderstorms are expected
to form first over higher terrain of southeastern and south-central
MT during early/mid afternoon, then move northeastward into a
rapidly destabilizing and weakly capped (at most) boundary layer
over the adjoining High Plains. Supercells with large hail and
isolated severe gusts, as well as small clusters with mainly severe
gusts and isolated hail, are possible. Activity should cross much of
central MT before becoming weaker and more isolated as it moves into
nocturnally stabilizing air this evening.
While the WA/OR shortwave trough and associated DCVA/UVV field are
progged to pass northwest of the area, it should tighten the height
gradient in mid/upper levels. This will combine with low-level
southeasterlies to yield favorable deep shear for supercells.
Progged effective-shear magnitudes vary highly across models and
from place to place within the same model (generally 30-55 kt)
depending on depth of the CAPE layer, which mostly is a function of
how vigorous and deep the mixing process is. As per its bias, RAP-
rooted guidance (RAP, HRRR) appear to be mixing too much over
central MT, with most other HREF members and NAM showing more
realistic vertical thermodynamic profiles and greater overall
buoyancy. Peak MLCAPE should reach 1000-1500 J/kg from northwestern
NE to east-central MT, decreasing westward to around 500-1000 J/kg
over west-central MT. Higher CAPE values are possible over a
narrow, north-south corridor in the western Dakotas, but amidst
weaker deep-layer forcing and stronger capping near the mid/upper
ridge, inhibiting sustained and especially severe convective
potential.
...South-central High Plains...
Strong elevation-relative heating will support preferential removal
of MLCINH, followed by and related mid/late-afternoon thunderstorm
development, over the higher terrain of southern CO, on either side
of the San Luis Valley. Activity should move eastward to east-
southeastward over deeper, well-mixed boundary layers characteristic
of the valley and the High Plains, while offering isolated severe
gusts and hail for a few hours before weakening in the evening.
This close to the mid/upper anticyclone center, deep-later flow will
be weak, keeping shear weak and severe potential isolated and
disorganized. However, steep lapse rates in a well-mixed boundary
layer may support severe gusts and perhaps hail from the most
vigorous, sustained cells and small clusters.
..Edwards/Mosier.. 08/20/2024
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