- Day Three
acus01 kwns 261259
Storm Prediction Center ac 261257
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0757 am CDT Sat may 26 2018
Valid 261300z - 271200z
..there is a slight risk of severe thunderstorms from central
Montana to portions of western North Dakota...
The primary risk for severe storms, with damaging gusts and large
hail, is for this afternoon and evening over parts of central and
eastern Montana to western North Dakota.
In mid/upper levels, a highly amplified yet progressive pattern will
persist through the period. The main two troughs will be:
1. A western synoptic trough -- now located from the interior
Pacific northwest southward across coastal Southern California. An embedded
cyclone -- centered initially over the Sierra near Lake Tahoe --
will move eastward over the Great Basin today and tonight, its
center reaching western Utah by 12z.
2. A broad, weak area of troughing from the upper Great Lakes
southward across the lower Ohio Valley to the central Gulf. With
time, this troughing is expected to incorporate what now is
classified as subtropical storm alberto; please refer to latest NHC
forecast advisories for all track/intensity/wind-radii guidance on
this system, as well as all related tropical watches/warnings.
Farther north, across the northern fringes of the synoptic-scale
height weakness, a negatively tilted shortwave trough -- now evident
in moisture-channel imagery over mb and northwestern on -- is
forecast to move eastward across northern on (including the Lake
Superior vicinity) through the period.
At the surface, 11z analysis showed a wavy, mostly quasistationary
frontal zone from Maine across extreme southern qc to Northern Lake
Huron, Southern Lake Superior, to a weak low near inl, and westward
over northeastern Montana. This feature should move southward over New
England while remaining quasistationary elsewhere and generally
becoming more diffuse. Cold frontogenesis over western Montana should
shift eastward through the period, with the resultant front
positioned at 12z from a low over western South Dakota southwestward across
western Wyoming. A dryline and Lee trough will be noted over the central
and southern High Plains.
..northern rockies to northern plains...
Widely scattered to scattered thunderstorms in clusters are expected
to develop this afternoon across a broad area from central/eastern
Montana back across the mountains into portions of central Idaho and across
Nevada. Isolated severe gusts are possible from northeastern Nevada and
south-central/southwestern Idaho over the bitterroots, as adequate
low-level moisture, seasonally cool midlevel temps, diabatic surface
heating, well-mixed boundary layers, resultant steep low/middle-
level lapse rates contribute to sufficient buoyancy (around 500-1000
j/kg mlcape) and downdraft-acceleration potential. Weak
low/middle-level flow will limit vertical shear and the potential
for sustained convective organization, however.
A somewhat greater and better-organized severe gust/hail potential
should develop this afternoon over portions of central/eastern Montana
and persist into this evening, while shifting eastward. Widely
scattered to scattered, high-based convection should form, some of
which may exhibit at least transient supercellular character, with
some aggregation into clusters also possible. Although this area
will lie under, or very close to, the synoptic ridge residing in
between the troughs documented above, a series of low-amplitude
perturbations/vorticity maxima should penetrate that ridge after
moving through the strongly difluent mid/upper flow regime northeast
of the Great Basin cyclone. These will act in tandem with surface
diabatic heating to steepen low/middle-level lapse rates, amidst 50s
f surface dew points, resulting in MLCAPE 1500-2000 j/kg (locally
higher) either side of an axis extending northwestward from western
South Dakota across east-central Mt, with deeply mixed subcloud layers.
Low-level shear initially will be modest, but with favorable
cloud-layer shear. With time and eastward extent from late
afternoon into this evening, low-level absolute and storm-relative
flow will strengthen, resulting in enlarging hodographs and stronger
storm-scale lift for maintenance of some strong-severe convection
into western ND overnight. Isolated large hail also may occur from
nocturnal convection in the elevated zone of low-level warm
advection and isentropic lift extending eastward over portions of
..upper Great Lakes...
Widely scattered thunderstorms are expected to develop this
afternoon over two primary areas -- portions of the Michigan Upper
Peninsula/northeastern WI, and north-central/northeastern lower mi,
with a relative gap in coverage in between where weaker lift is
expected. Activity should form primarily south of the frontal zone
and beneath the strongly difluent upper-flow regime south/southeast
of the northern-stream shortwave trough. Initiating foci should
include lake-breeze, differential-heating and outflow boundaries.
Modified 12z apx/DTX/grb raobs and forecast soundings suggest
well-mixed boundary layers suitable for maintaining isolated
marginally severe hail to surface, as well as downdraft
accelerations leading to locally damaging gusts. Weak low/middle-
level flow and shear should temper organized severe potential with
the predominant storm Mode being multicellular.
..FL Keys to southwest Florida...
Although the forecast track of Alberto keeps the center well
southwest/west of here, the relatively expansive low-level wind
fields characteristic of a subtropical system will enlarge
hodographs across the outlook area as the cyclone translates
northward over the eastern Gulf, per NHC guidance. Coverage/number
of relatively discrete cells in this environment is uncertain, but
the moist/low-LCL thermodynamic profiles, juxtaposed on
strengthening low-level shear over the region with time, suggest at
least a marginal/conditional supercellular tornado risk remains
warranted -- primarily for the latter half of the period.
acus11 kwns 260218
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 260217
Mesoscale discussion 0526
National Weather Service Storm Prediction Center Norman OK
0917 PM CDT Fri may 25 2018
Areas affected...western OK...western North Texas
Concerning...Severe Thunderstorm Watch 121...
Valid 260217z - 260345z
The severe weather threat for Severe Thunderstorm Watch 121
Summary...isolated strong wind gusts possible for the next hour or
so across western OK and adjacent portion of western North Texas.
Discussion...thunderstorm moving across western OK has become
outflow dominant, evidenced by the linear structure and fast
southward progression. Fdr VAD is currently reporting southeasterly
winds ahead of this system and the resulting moist inflow will
likely result in storm persistence for the next hour or so. Isolated
strong wind gusts are primary severe threat. Additional development
is not currently anticipated across the remainder of the watch area.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 35000028 35399991 35469916 34439823 33299788 32939965