Severe Weather Threat Day 2 Thu Dec 13

Synopsis...

The Spread Among The Various Model Ensemble Data Remains Large For This Forecast Period Concerning Short Wave Developments Within A Strong Mid-Latitude Jet Propagating Eastward Across The Pacific.

This Includes One Impulse Forecast To Emerge From The Jet And
Progress Through Amplified Upper Ridging Across The Northeast
Pacific...Before Digging Toward The British Columbia And Pacific
Northwest Coast. In The Process...This Feature Appears Likely To
Split...With One Portion Migrating Eastward In A Zonal Branch Of
Flow Extending Eastward Into Canada...While Another Portion
Continues To Dig In A Branch To The South...Curving Into A
Significant Upper Trough Across The California Coast.

Variability Exists Among The Models Concerning The Magnitude/ Evolution Of The Perturbations Evolving Out Of The Split...And This Begins To Increase The Spread Concerning The Eastward Acceleration And Evolution Of The California Trough And Embedded Closed Low.

This Latter System Does Still Appear Likely To Progress Into The Great Basin And Southern U.S./Northwestern Mexican Plateau... While Interacting With Another Belt Of Westerlies Emerging From The Subtropical Eastern Pacific. As It Does...Strong Upper Jet Intensification Is Forecast Across The Northern Mexican Plateau Into
The Crest Of Amplifying Short Wave Ridging Across The Southern
Plains.

In Response To Upper Developments And Associated Surface Pressure Falls Across The Central/Southern High Plains...A Return Flow Of Low-Level Moisture May Commence Off The Western Gulf Of Mexico...While A Plume Of High Level Moisture Advects East Northeastward Along The Southeastern Fringe Of The Upper Jet.

Substantive Boundary Layer Moistening Is Not Expected Into Deep South Texas Until Late In The Period...But May Occur Above A Residual Cool Stable Surface-Based Layer As Far North As Parts Of Central And Northern Texas. However...Lower/Mid Tropospheric Warming Above The Layer May Cap Potential Destabilization.

As A Result...Convective Potential Across The Southern Plains Is Expected To Remain Negligible Thursday And Thursday Night. With The Possible Exception Of A Lingering Risk For Thunderstorms Near Southeast Florida Coastal Areas...This Will Also Be The Case Elsewhere East Of The Plains.

Great Basin/Southern Plateau/Rockies...

Significant Moistening Is Forecast Across The Region On Southerly
Low-Level Flow Ahead Of The Upper Trough. In Conjunction With
Boundary Layer Warming And Mid-Level Cooling With The Approach Of The Trough...At Least Weak Destabilization Is Expected. Coupled
With Large-Scale Forcing For Upward Vertical Motion...This Should Be
Sufficient For The Development Of Scattered Thunderstorms Late
Thursday Into Thursday Night.

Highest Probabilities Appear To Exist Along And South Of The Mogollon Rim...Across The Lower Deserts Around Phoenix.

However...The Risk For Thunderstorms May Not Be Negligible Across Parts Of Eastern Nevada Into Western Utah...And Northeast Of The Rim...Into The Four Corners Region And The Southwestern Two-Thirds Of New Mexico Late Thursday Night.


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