- هذا اليوم
- Day Three
acus01 kwns 281239
Storm Prediction Center ac 281237
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0637 am CST Tue Feb 28 2017
Valid 281300z - 011200z
..there is an enhanced risk of severe thunderstorms late this
afternoon into tonight from northern Arkansas northeastward across
parts of Missouri...Illinois...and Indiana...
..there is a slight risk of severe thunderstorms surrounding the
enhanced risk area...from the Arkansas-la-tex to the Ohio Valley...
..there is a marginal risk of severe thunderstorms immediately
surrounding the slight risk area...
Severe thunderstorms are expected to develop from portions of the
lower Mississippi Valley to the Ohio Valley through tonight. Large
hail, damaging winds, and a few strong tornadoes are possible.
A large-scale mid-upper trough over the Great Basin and lower Colorado
River valley will progress eastward to the MS valley by the end of
the period. An associated surface cyclone will develop
northeastward from the MO/Iowa border to lower Michigan overnight, as a cold
front moves eastward from the plains to the MS valley. Ahead of the
front, a relatively broad/unstable warm sector is expected from
parts of the middle MS and lower Ohio valleys southward, where
vertical shear will increase with time as the mid-upper trough
approaches from the west. Several rounds of strong/severe storms
may occur across a broad area, with all severe storm hazards
possible this afternoon through tonight, though uncertainty persists
with warm sector development through this evening.
..northern Arkansas/southern MO to the lower Ohio Valley this evening...
To the southwest of ongoing elevated storms over northern MS and
western TN, rich low-level moisture (boundary layer dewpoints of
65-70 f) is spreading northward from East Texas/la toward eastern
OK/AR. Though a subtropical cirrus plume may tend to limit surface
heating in some areas, afternoon temperatures well into the 70s
along the moist axis will drive MLCAPE values upward to 1500-2500
j/kg. Simultaneously, vertical shear will become increasingly
favorable for supercells in the warm sector, with effective bulk
shear at or above 70 kt, and effective srh of 300-400 m2/s2 expected
by this evening.
The primary uncertainty for diurnal convection in the open warm
sector will be a focus for initiation. Only a weak cap is expected
this afternoon over the warm sector given soundings modified for
upper 70s and upper 60s dewpoints, and the primary source of weak
ascent during the afternoon will be low-level warm advection.
However, uncertainty remains substantial, as represented by
inconsistency among multiple convection-allowing model forecasts.
If storms can form in this regime across central/northern Arkansas and
southern MO during the afternoon/evening, they will likely become
primarily discrete supercells and move rapidly northeastward at
50-60 kt across southeast MO toward southern Illinois. The combination of
vertical shear, buoyancy, and low-level moisture will conditionally
support a risk for strong tornadoes, while isolated very large hail
will also be possible.
..east of the surface cyclone into Illinois this afternoon/evening...
A few near-surface-based storms could form this afternoon and
persist into this evening immediately east of the surface cyclone,
along the warm front into central/northern Illinois as boundary layer
dewpoints increase to near 60 f beneath steep midlevel lapse rates.
Cloud breaks will dictate the degree of surface heating into
northeast MO, with potentially moderate buoyancy (mlcape 1000-1500
j/kg) along the north edge of the warm sector into central/northern
Illinois. If storms form within the north edge of the warm sector, strong
deep-layer and low-level shear will favor supercells with all severe
..MO/Arkansas to the Ohio Valley overnight...
As the surface cyclone develops northeastward to the Great Lakes,
the surface cold front will progress eastward to The Ark-la-miss and
lower Ohio Valley by Thursday morning. One or more bands of
convection are likely along or just ahead of the front tonight into
Wednesday morning, with an accompanying risk for damaging winds and
hail. The wind and tornado risk will be maximized with embedded
bowing segments and/or supercell structures within or ahead of the
convective band/front, in an environment with strong vertical shear
and sufficient low-level moisture/buoyancy. There will also be some
risk for more discrete storm development farther south into western
and central Arkansas late tonight.
acus11 kwns 280810
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 280810
Mesoscale discussion 0217
National Weather Service Storm Prediction Center Norman OK
0210 am CST Tue Feb 28 2017
Areas affected...parts of southeastern Arkansas...northern/central
Concerning...severe potential...watch unlikely
Valid 280810z - 281045z
Probability of watch issuance...20 percent
Summary...isolated to widely scattered strong storm development is
possible through the 09-12z time frame, accompanied by a risk for
severe hail. The need for a watch is not currently anticipated, but
trends will be monitored.
Discussion...in the presence of rising mid-level (500 mb) heights
and capping associated with warm elevated mixed layer air (based
near 700 mb), the initiation of widespread deep convective
development still seems unlikely. However, model guidance, in
general, indicates an increase in thunderstorm activity is possible
through the 09-12z time frame, as far south as the Mississippi Delta
Region. This appears mostly in response to a forecast strengthening
of southerly low-level flow (40-50+ kt at 850 mb), in a corridor
near/east of the Mississippi River. This may be accompanied by
sufficient strengthening of ascent associated with warm advection to
If/when storms form, environment profiles appear conducive to at
least some severe weather potential. Activity likely will be based
above a residual stable near surface layer, but steep mid-level
lapse rates and relatively cool mid/upper levels may be supportive
of the risk for severe hail in the strongest storms, aided by
strong, largely unidirectional shear in the cloud bearing layer.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 33289229 34629157 34848945 34538884 33248812 32728974