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Severe Weather Threat Day 1 Sat Dec 15

Synopsis...

In Mid-Upper Levels...Several Shortwaves Are Fcst To Traverse Belt
Of Cyclonic Flow Associated With Wrn Conus Mean Trough.

First Of These...Now Manifest As Closed 500-Mb Cyclone Over Nrn Mo/Srn Ia Region -- Will Move Newd To Lh By 16/12z And Weaken Considerably.

Upstream Shortwave Trough -- Currently Over Srn Portions Nv/Ca -- Is
Expected To Pivot Generally Ewd Across Nrn Az By Early Evening... Reaching Tx Panhandle By 16/12z.

Weaker And Initially Separate Perturbation -- Now Evident In Moisture Channel Imagery Over Pac Near 30n120w -- Should Move Ewd Across Central Baja Around 00z...Then Essentially Phase With More Nrn Feature....Reaching Sw Tx Around 16/12z.

Sfc Cyclone -- Analyzed At 15/12z Between Hsi-Lnk -- Is Fcst To Eject Enewd Across Ia To Upper Ms Valley...Reaching Sern Mn By 00z And Wrn Ls/Wrn Upper Mi Region Near End Of Period. Trailing Pac Cold Front -- From Ern Portions Ks/Ok Into S-Central Tx This Morning -- Should Become Quasistationary Near Or Just Se Of Present Position ...While Awaiting Arrival Of Reinforcing Caa/Baroclinicity Preceding Aforementioned Late-Period Shortwaves. Slgt Nwwd Drift Of This Boundary Also Is Possible After 06z...Particularly Where No Baroclinically Reinforcing Precip Has Developed By Then.

Portions La...Upper Tx Coastal Plain...

Ongoing Thin Band Of Convection Will Continue Shifting Ewd Across
Arklatex Region And Slowly Sewd Over Mid-Upper Tx Coastal Plain... With Gen Weakening Trend Through Remainder Of Morning.

Greatest Tstm Potential Over This Region Will Be Widely Scattered
Coverage Overnight...Especially During Last 3-6 Hours Of Period As
Subtle Large-Scale Ascent Related To Ca/Pac Shortwaves Apchs Frontal Zone. This Should Combine With Sufficient Thetae To Offset Weak Mid-Upper Level Lapse Rates...Resulting In 500-1000 J/Kg Mucape And Weakening Of Previously Stout Low-Level Capping During 09-12z Time Frame.

Although Frontal Convergence Will Remain Weak...Decreasing Cinh Should Support Increasing Convective Potential. Height Gradients Aloft Will Tighten Ahead Of Swrn Conus Shortwaves... Resulting In Strengthening Deep Shear Atop Relatively Weak Near-Sfc Winds. 50-60 Kt Effective Shear Magnitudes Are Possible.

However...Confidence Is Not Very Stg In Development Of Greater
Convective Coverage/Intensity In This Regime...Given Weak Lapse
Rates Aloft...Lack Of More Robust Flow/Convergence In Low Levels... And Erratic/Inconsistent Precip Signals In Convection-Allowing/Sseo Guidance. Any Sustained/Intense Cells That Do Develop Could Transfer Momentum To Sfc From Stg Flow Aloft...And Pose Threat Of Stg/Isolated Svr Gusts Penetrating Shallow/Near-Sfc Stable Layer.


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