- Today
- Tomorrow
- Day Three
000
acus01 kwns 200546
swody1
Storm Prediction Center ac 200544
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
1244 am CDT Thu Sep 20 2018
Valid 201200z - 211200z
..there is an enhanced risk of severe thunderstorms this afternoon
and evening across the upper Midwest...
...
Severe thunderstorms are expected to develop across the upper
Midwest this afternoon and evening. In addition to potential for
damaging wind gusts, it appears that this will include the risk for
a few tornadoes, with a couple possibly strong.
...
Models suggest that much of the northern tier of the U.S. Will come
under the increasing influence of a generally zonal belt of
westerlies emanating from the mid-latitude Pacific. A couple of
significant short wave troughs are embedded within this regime,
including one forecast to accelerate east/northeast of the northern
rockies through much of the upper Mississippi Valley by late
tonight. As this occurs, the center of a prominent subtropical high
is forecast to shift across the Tennessee Valley through the
southern mid Atlantic coast region.
It appears that the short wave trough may provide support for fairly
strong cyclogenesis along a surface frontal zone now curving across
portions of the Central Plains into portions of the Ohio Valley to
the south of the Great Lakes. Models generally suggest that most
rapid deepening of the low may be across parts of the upper Midwest
through the upper Great Lakes region late this afternoon through
tonight. In its wake, a reinforcing intrusion of cool air appears
likely, with the front advancing through the remainder of the
Central Plains and upper half of the Mississippi Valley by 12z
Friday.
..plains into the Great Lakes region...
Strong thunderstorm activity with perhaps at least some severe
weather potential is possible along much of the frontal zone.
However, the most appreciable severe weather potential seems likely
to accompany the deepening surface low, particularly near peak
daytime boundary layer destabilization. This appears most probable
in a corridor east of the middle Missouri Valley through the upper
Midwest.
Warm sector destabilization could be hindered at least somewhat by
relatively warm air at mid/upper levels, with cold air aloft
associated with the short wave trough forecast to lag to the west
through the period. However, seasonably high moisture content,
including dew points in the lower 70s, may still contribute to cape
up to around 2000 j/kg with daytime heating. The moist surface air
mass will initially be confined to areas southwest of a convectively
reinforced front likely extending across eastern Iowa through
northern Illinois and Indiana at the outset of the period. During
the day, models indicate that this boundary will shift northward
with the approach of the deepening low.
Near the intersection of the developing low and this boundary, it
appears that destabilization of the boundary layer will coincide
with enlarging clockwise curved low-level hodographs. This is
expected to include wind speeds increasing to 40-50+ kt in the
850-700 mb layer. Coupled with forcing for ascent associated with
low-level warm advection, the environment may become conducive to
the development of discrete supercells capable of producing
tornadoes. A couple of these could be strong, particularly across
parts of southeastern Minnesota and adjacent northern Iowa into
parts of southwestern/west central Wisconsin.
Warm sector thunderstorm development remains otherwise somewhat
unclear. In the wake of the aforementioned warm advection activity,
forcing along the cold front in relatively close proximity to the
cyclone may be accompanied by an evolving line of storms late this
afternoon into this evening. This activity could pose a risk for
potentially damaging surface gusts, mainly across portions of
southeastern Minnesota and northern Iowa into Wisconsin, before
diminishing this evening in weaker/waning instability.
.Kerr/Wendt.. 09/20/2018
$$
Mesoscale Discussion
000
acus11 kwns 200426
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 200426
nez000-ksz000-coz000-200600-
Mesoscale discussion 1481
National Weather Service Storm Prediction Center Norman OK
1126 PM CDT Wed Sep 19 2018
Areas affected...northeast Colorado...northwest Kansas and western
Nebraska
Concerning...severe potential...watch unlikely
Valid 200426z - 200600z
Probability of watch issuance...20 percent
Summary...storms will pose a risk for isolated large hail through
about 06z over the Central High plains region. Due to the expected
limited duration of the threat, a ww issuance is not expected.
Discussion...late this evening a slow-moving cold front extends from
eastern Colorado into central NE. Forcing for ascent associated with a
strengthening low-level jet interacting with the sharpening
baroclinic zone is fostering the development of storms from
northeast Colorado through western NE. These storms are within an
environment characterized by 1500-2000 j/kg MUCAPE, 7.5 c/km 700-500
mb lapse rates and 45-50 kt effective bulk shear supportive of some
supercell structures. However, updrafts are elevated above a stable
layer north of the front which should limit primary threat to large
hail, though a few strong downdrafts could also reach the surface.
Activity will continue developing northeast this evening, but
tendency may be for storms to begin to congeal into clusters with
time, which should eventually mitigate the overall hail threat.
.Dial.. 09/20/2018
..please see www.Spc.NOAA.Gov for graphic product...
Attention...WFO...lbf...gld...bou...
Latitude...Lon 41500075 40630094 39260237 39320292 41350226 42010132
41500075
