U.S. Severe Weather Forecast (Sääkatsaus)

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 
the Dakotas. 

..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. 

.Edwards/Peters.. 05/26/2018 


Mesoscale Discussion

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. 

.Mosier.. 05/26/2018 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 35000028 35399991 35469916 34439823 33299788 32939965 
33460022 35000028