- Day Three
acus01 kwns 291259
Storm Prediction Center ac 291258
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0758 am CDT Thu Jun 29 2017
Valid 291300z - 301200z
..there is a slight risk of severe thunderstorms from parts of the
Central High plains to the corn belt...
..there is a slight risk of severe thunderstorms from northeastern
Nebraska to northen Iowa...
..there is a marginal risk of severe thunderstorms elsewhere from
parts of Wyoming to the lower Great Lakes...
Severe thunderstorms, with damaging gusts and large hail, are
expected this afternoon into tonight within a corridor from the
Central High plains across the lower Missouri Valley region and corn
belt. Isolated severe storms are also possible over parts of the
central rockies and lower Great Lakes region.
In mid/upper levels, a zonal to gently cyclonic mean flow pattern is
expected over the northern 2/3 of the conterminous U.S. Through this
period. The best-defined at this time are (from east to west):
1. A perturbation over WI, western Upper Michigan and Lake Superior is
forecast to move eastward across eastern upper mi, northern lower
mi, and Lake Huron today, while weakening amidst gradual,
broader-scale height rises.
2. A weak, convectively reinforced perturbation -- currently over
southern NE and northern Kansas -- forecast to reach the southwestern
Iowa/southeastern NE/northwestern MO area around 00z, then northern in
and central Illinois by 12z.
3. A shortwave trough -- now evident in moisture-channel imagery
near the eastern borders of Wyoming and Montana -- should shift eastward
across the Dakotas today, reaching Minnesota overnight.
4. A strong, positively tilted shortwave trough -- presently
located over western Mt, northern Idaho and northeastern or -- will dig
southeastward across the northern rockies. By 12z, this
perturbation should extend from the Black Hills/badlands region
across Wyoming to northeastern Utah.
Other than those, numerous, minor shortwaves and vorticity lobes --
some convectively induced -- will move eastward through that belt
between the northern rockies and New England.
At the surface, an ill-defined synoptic front extended southwestward
from a low near the Keweenaw peninsula, intermingled with
convectively processed air from western WI across western IA,
east-central/south-central NE and northwest KS, then analyzed to a
surface low between spd-eha. A secondary baroclinic zone was
evident over southern SD, western NE and northeastern Colorado. The
northern low should move eastward to Eastern Lake Superior by 00z,
with the front southwestward to central Kansas and northeastern nm. The
secondary boundary will sag southward across eastern Colorado and western
Kansas. Both will be preceded by mesoscale convective outflows across
the central/eastern Kansas and central MO areas.
..Central Plains to corn belt...
A large area of outflow air is apparent in the boundary layer across
northern/eastern KS, southern NE, central/northern MO and southern
IA, and still is being reinforced by scattered to numerous ongoing
convection from north-central Kansas to parts of northern/central MO.
Synoptic models (i.E., NAM, spectral, arw-kf) and
high-res/convection-allowing guidance alike (e.G., Hrrr, until being
forced to by radar-assimilating hot starts) consistently have failed
to depict the breadth and magnitude of convection in this region
overnight. Resulting unfavorable effects on the mass fields and
thermodynamic characteristics north of the boundary likely have been
underplayed by most progs as well.
Some convective/precip reinforcement of this air mass is expected to
persist for a few more hours as the supportive low level jet -- with
associated isentropic lift to level of free convection occurring atop the outflow pool --
only gradually weakens through the remainder of the morning. This
casts more uncertainty on air-mass recovery (especially for
surface-based effective-inflow parcels), and thus severe threat,
with northward extent from the boundary. Slight northward retreat
of the boundary is possible across eastern Kansas and central MO once
the reinforcing precip shifts away and/or dissipates, and favorable
diabatic heating of the boundary layer is expected not only along
and south of the boundary, but somewhat to its north as well.
However, this is south of the stronger flow aloft, and deeper into
the eml air mass above the boundary layer, than reflected in earlier
outlooks. Given these developments and uncertainties, unconditional
severe probabilities appear reduced overall, but still may be
relatively concentrated near the boundary this afternoon and
evening. More boundary-focused re-introduction of greater
probabilities may be required as mesoscale uncertainties become more
clarified during the day.
Additional/afternoon and evening thunderstorm development near the
outflow boundary may aggregate into one or two southward- to
southeastward-propagating and outflow-driven convective complexes
tonight, potentially affecting parts of the Ozarks and/or
..Central High plains/rockies...
Afternoon convective potential will be tied largely to a combination
of upslope flow/lift and diabatic heating of elevated terrain. The
greatest juxtaposition of low-level lift, buoyancy, moisture and
vertical shear may be for convection moving eastward to
southeastward off the Palmer Divide genesis zone in east-central co,
and into a low-level convergence zone downshear. Forecast soundings
suggest MLCAPE may exceed 1000 j/kg atop a deep/well-mixed boundary
layer, yet still with 50s f surface dew points. Directional shear
(veering with height) in low levels and progged 35-40 kt 500-mb flow
will be more than sufficient for some supercell potential, with
effective-shear magnitudes around 50 kt. Activity may evolve into a
small forward-propagational complex with time and eastward extent
into KS, given the high local and potential for outflow-dominant
Farther north from north-central Colorado and the Cheyenne Ridge Onward,
moisture/Theta-E and buoyancy each will be more limited, though
steep low/middle-level lapse rates and weak mlcinh still can support
isolated severe thunderstorms developing mainly over higher terrain
and moving eastward onto the plains. This potential may linger into
the evening/overnight period as well, with the approach of the
northern rockies perturbation (number 4) and related large-scale
..NE/Iowa slight-risk area...
A separate area of widely scattered to scattered thunderstorms may
develop this afternoon near the synoptic front, ahead of
perturbation number 3, and move eastward into a pocket of relatively
high-moisture boundary-layer air not as intensely overturned by
earlier convection. Large hail and damaging winds are possible, and
a tornado cannot be ruled out. While overall Theta-E should not be
as large as south of the outflow boundaries, colder air aloft and
stronger deep shear may support some supercellular organization, as
well as the potential for a small forward-propagational cluster to
evolve. The overall threat should diminish into the evening as
activity moves over more stable low-level conditions.
acus11 kwns 290952
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 290952
Mesoscale discussion 1182
National Weather Service Storm Prediction Center Norman OK
0452 am CDT Thu Jun 29 2017
Areas affected...portions of Kansas
Concerning...Severe Thunderstorm Watch 379...
Valid 290952z - 291045z
The severe weather threat for Severe Thunderstorm Watch 379
Summary...the potential for isolated instances of large hail and
strong, gusty winds continues across watch 379, but may become more
limited over the next few hours.
Discussion...regional radar data depict a pair of ongoing clusters
of strong/severe thunderstorms across the watch early this morning.
One is a generally sub-severe southeastward-moving system over
northeast Kansas, with upshear development aided by ascent over an
outflow boundary related to the low-level jet (best observed in
upstream kddc vwp data). These storms will depart the ongoing watch
over the next hour, and further upscale growth may favor occasional
strong winds; however, their direction of movement will take them
away from the maximum in ascent related to the aforementioned
low-level jet. Therefore, downstream watch issuance/extension is not
Farther west, a small bowing segment has evolved from a
warm-advection Wing of thunderstorms advancing east/northeast across
north-central Kansas. As these storms approach the aforementioned
zone of outflow-related ascent, convective activity may increase in
spatial extent, yielding a local/brief uptick in the potential for
severe hail. However, these cells are pushing farther into an
environment characterized by earlier convective overturning. In
turn, while isolated large hail and gusty surface winds remain
possible, the severe threat may become more limited with these cells
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 38669766 39419966 39789945 40059869 40069707 39629567
39179517 38579501 38199555 38469705 38669766