- Day Three
acus01 kwns 200554
Storm Prediction Center ac 200552
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
1252 am CDT Mon Aug 20 2018
Valid 201200z - 211200z
..there is a slight risk of severe thunderstorms this afternoon and
evening across parts of the middle Mississippi Valley and middle south
A few severe thunderstorms are possible this afternoon and evening
across parts of the middle Mississippi Valley and mid south region.
Damaging wind gusts appear the primary hazard, but there may be some
risk for an isolated tornado or two.
A modest lower/mid tropospheric cyclone, now evolving within a
southern branch of mid-latitude westerlies across the lower Missouri
Valley, appears likely to remain progressive through this period.
Models suggest that the mid-level circulation will gradually turn
eastward into the middle Mississippi Valley during the day, as a
significant speed maximum propagates around its southern through
southeastern periphery. It is then forecast to accelerate
northeastward toward the Great Lakes region, weakening and
undergoing deformation to the southeast of troughing digging within
the stronger westerlies across the central Canadian provinces and
adjacent international border area.
Downstream, a weaker southern branch impulse is forecast to progress
east of the northern mid Atlantic coast. Upstream, larger-scale
troughing now developing inland of the Pacific northwest coast
appears likely to undergo considerable deformation between ridging
to the northwest and southeast, but at least one fairly significant
elongating remnant mid-level circulation may slowly shift across the
northern intermountain region and northern rockies.
..Mississippi Valley/mid south...
The lack of colder mid-level air and generally modest to weak
low/mid-level lapse rates probably will continue to limit severe
weather potential with the remnant cyclone, particularly given its
modest strength. The lingering remnants of ongoing convection now
overspreading the middle Mississippi Valley, and associated
rain-cooled air, will also inhibit or complicate warm sector
boundary layer destabilization. However, guidance is suggestive
that seasonably high boundary layer moisture content may contribute
to moderately large cape (2000+ j/kg) with insolation beneath the
dry slot associated with the mid-level low. This appears most
probable across parts of the mid south, where a zone of stronger
differential heating may evolve and provide the focus for strongest
thunderstorm development this afternoon. It is possible that this
may include supercells, and an upscale growing convective cluster by
early evening, aided by favorable deep layer shear beneath 30-50 kt
southwesterly 700-500 mb flow. More uncertainty exists concerning
the strength of the low-level hodographs, but at least some guidance
suggests strengthening of southwesterly 850 mb flow to 30-40+ kt is
possible across the mid south into parts of the lower Ohio Valley
before boundary layer instability wanes this evening. This may
augment the risk for potentially damaging wind gusts, and contribute
to some potential for a tornado or two.
Otherwise, a warm frontal zone across northern Illinois, in closer
proximity to the mid-level circulation, could also become a focus
for convection with a risk for at least brief/weak tornadoes, given
sufficient boundary layer destabilization. The extent of this
potential, however, remains unclear at the present time.
..southern Virginia/North Carolina...
In the wake of the departing short wave impulse, models suggest that
a 30 kt west northwesterly mid-level speed maximum migrating east of
the southern Appalachians through the southern mid Atlantic coast,
could enhance convection this afternoon. Thunderstorms are expected
to form in the presence of moderately large cape, associated with
daytime heating of a seasonably moist boundary layer.
..northern intermountain region/rockies...
A relatively cool mid-level air mass and at least modest vertical
shear on the northern periphery of a 30-50 kt mid-level speed
maximum may support an isolated storm or two with some risk for
severe hail and strong wind gusts along the higher terrain of
central Idaho and southwest Montana into northwestern Wyoming.
acus11 kwns 192248
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 192247
Mesoscale discussion 1324
National Weather Service Storm Prediction Center Norman OK
0547 PM CDT sun Aug 19 2018
Areas affected...southwest Missouri into northern Arkansas
Concerning...Tornado Watch 343...
Valid 192247z - 200045z
The severe weather threat for Tornado Watch 343 continues.
Summary...the threat of damaging winds and a few brief tornadoes
continues across the watch area.
Discussion...storms continued to develop across southwest MO and
northwest AR, ahead of a weak surface low and near a diffuse warm
front. The latest surface analysis indicates 2 hr pressure falls in
excess of 2 mb, associated primarily with low-level warm air
advection. Hodograph size on the sgf VAD wind profile has gradually
been increasing, now with 0-1 srh in excess of 200 m2/s2. This trend
may persist over the next couple hours, coincident with an increase
in the 850 mb flow. As such, the north-south oriented line of storms
that moved out of Kansas into MO is becoming more of a qlcs with swaths
of damaging wind and/or brief tornadoes possible.
To the south and closer to the warm front, cells over Arkansas are more
robust overall in terms of being instability driven with hail cores
evident on radar. Low-level shear is not as strong as farther north,
but a brief tornado cannot be ruled out with this activity.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 35269249 35139403 35339419 35719416 36199421 36599432
37009432 37419435 37839434 38139256 38129247 37799186
37389149 36779125 36089124 35589154 35389189 35269249