U.S. Severe Weather Forecast (Convective Outlook)

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. 

..northern rockies... 

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. 

.Leitman/Dean.. 08/17/2018 


Mesoscale Discussion

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. 

.Kerr/Thompson.. 08/17/2018 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 39019044 39468966 40128899 40878834 40338769 38948821 
37888888 37659004 38199054 39019044