- Day Three
acus01 kwns 170545
Storm Prediction Center ac 170543
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
1243 am CDT Fri Aug 17 2018
Valid 171200z - 181200z
..there is a slight risk of severe thunderstorms across portions of
the northeast this afternoon and evening...
Isolated severe thunderstorms with mainly a strong to damaging wind
threat will be possible this afternoon and evening across parts of
the northeast. Additional storms may produce sporadic strong wind
gusts from the Central High plains to the Ohio and Tennessee Valley
vicinity, and across parts of the northern rockies.
A larger-scale upper trough will shift eastward across the Midwest
and northeast today. Several embedded shortwave impulses will
provide focus of strong to severe thunderstorms today from the
Southern Plains into the Ohio/Tennessee Valley vicinity and portions of the
northeast. Additional strong storms are possible across the
central/southern High Plains in moist upslope flow regime as a weak
perturbation drops south/southeast on the eastern periphery of The
Rockies upper ridge. Isolated strong wind gusts also are possible
across parts of the northern rockies as a weak shortwave impulse
provides support for isolated high-based thunderstorms.
A warm front will quickly lift northward across northeast New York and
Vermont/New Hampshire this morning. South of the front, surface dewpoints in the
upper 60s to low 70s are expected. Modest heating, with highs
generally from the upper 70s into the 80s, will be sufficient for
the development of 1000-2000 j/kg MLCAPE. Deep-layer
south/southwesterly flow will increase with the approach of the
upper trough and thunderstorms are expected by early afternoon in
the vicinity of a prefrontal trough across eastern PA/NY, as well as
along the warm front across Vermont/New Hampshire into southern New England. 25-35
kt effective shear will be sufficient for organized clusters and
marginal supercell structures capable of strong, locally damaging
wind gusts and marginal hail. While low-level shear is not
particularly impressive, backed low-level winds in the vicinity of
the surface trough and warm front, coupled with forecast mean mixing
ratios around 16 g/kg, a brief tornado or two can not be ruled out.
Additional thunderstorms may develop later in the evening as a
surface cold front shifts southward from Quebec, but severe
potential will be limited as boundary-layer stabilization ensues
with loss of daytime heating.
..AR into the Ohio/Tennessee Valley vicinity...
Ongoing convection at the beginning of the period will impact where
stronger storm develop/re-intensify later this afternoon. Current
best guess is that convection moving into the lower Ohio Valley
vicinity this morning may persist and re-strengthen across Kentucky/Tennessee
where a moist and unstable airmass is in place. Modest deep-layer
shear will result in multicell clusters and downburst winds will be
the main concern with any surging/bowing line segments that develop.
Additional convection is possible along residual outflow across Arkansas
into western TN/MS. Shear will become even weaker with southward
extent, but strong instability combined with precipitable water values near 2 inches
will continue to Foster downburst wind potential with any stronger
..central/southern High Plains...
Thunderstorms will develop in weak upslope flow regime as a weak
impulse migrates across the central rockies. Strong heating and
modest boundary layer moisture will result in a corridor of moderate
instability across eastern Colorado into parts of the OK/Texas panhandles.
High-based convection will mainly pose a strong wind threat as cells
struggle to maintain intensity due to lack of stronger deep-layer
shear. Additionally, some marginally severe hail is possible with
any stronger cell that can develop given steep midlevel lapse rates.
Strong heating and modest boundary-layer moisture beneath steep
midlevel lapse rates will promote isolated thunderstorm develop
during the afternoon and evening. High-based thunderstorms will
mainly pose a dry microburst threat given inverted-v thermodynamic
profile, and a few strong wind gusts are expected.
acus11 kwns 170341
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 170340
Mesoscale discussion 1305
National Weather Service Storm Prediction Center Norman OK
1040 PM CDT Thu Aug 16 2018
Areas affected...parts of east central Missouri and southern/central
Concerning...severe potential...watch unlikely
Valid 170340z - 170545z
Probability of watch issuance...20 percent
Summary...isolated to widely scattered strong to severe storm
development is possible through midnight to 2-3 am CDT. While the
severe weather threat seems limited enough that a watch is not
currently anticipated, trends will continue to be monitored.
Discussion...a mid-level closed low now over central Iowa is
forecast to gradually turn eastward into the middle Mississippi
Valley through 06-09z. As it does, it appears that associated
forcing for ascent will continue to support scattered vigorous
thunderstorm activity, which is already ongoing in an arcing band
across northern through west central Illinois. Ahead of this band,
the boundary layer remains relatively warm with surface temperatures
in the upper 70s to around 80f in a narrow corridor, from around the
St. Louis area northeastward through central Illinois. Coupled with
dew points in the lower/mid 70s, this appears to be supporting
moderate to large cape on the order of 2000-3000 j/kg.
Although shear is still somewhat modest, the eastward progression of
a belt of 30-40 kt flow in the 700-500 mb layer may contribute to at
least some strengthening before the mid-level forcing and associated
convection progresses through the lingering instability axis
overnight. This could be accompanied by at least a short-lived
period with some increase in potential for storms capable of
producing severe hail and perhaps locally strong surface gusts.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 39019044 39468966 40128899 40878834 40338769 38948821
37888888 37659004 38199054 39019044