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Severe Weather Threat Day 1 Wed Jun 12

Upper Ms Valley Srn Great Lakes And Oh Valley Region...

Only Minor Changes Have Been Made To Ongoing Forecast. Threat For A Significant Widespread Damaging Wind Event Still Appears On Track From Srn Portions Of The Great Lakes Into The Oh Valley Region High And Moderate Risk Areas.

Storms Are In Process Of Developing From Nern Ia Into Srn Wi Within
Zone Of Dpva And Ascent Accompanying A Progressive Shortwave Trough Generally Ne Of Sfc Low And North Of E-W Quasi-Stationary Front. The Initial Sfc Based Storms Developing East Of Sfc Low May Organize As Discrete Supercells With An Initial Tornado Threat. Storms Should Eventually Consolidate And Transition To A Forward Propagating Mcs With An Increasing Threat For Significant Wind Damage As It Moves Esewd Through The Srn Great Lakes And Oh Valley During The Evening And Overnight.

Prev Discussion...

A Significant Severe Weather Outbreak Appears Increasingly Likely
Across Portions Of The Upper Midwest/Lower Great Lakes From This
Afternoon Through This Evening. Intense Storm Development In A Very Unstable Airmass...Coincident With A Compact And Intensifying
Surface Low Should Result In Numerous Damaging Wind Events As Well As Scattered...Possibly Strong Tornadoes.

Ern Ia/Nrn Il/Nrn Ind/Nwrn Oh And Adjacent Areas Of Extreme Srn
Wi And Swrn Lower Mi...

Significant Severe Weather Ingredients Have Congealed Ahead Of A
Compact And Progressive Mid/Upper Trough And Associated 50-60kt Mid Level Wly Flow Currently Tracking East From The Nrn/Cntrl Plains. An Expansive Warm/Moist Sector Exists Ahead Of The Upper Forcing And Developing Surface Cyclone With 2m Dewpoints Well Into The 60s And Lower 70s F And Pw Values In Excess Of 1.25 Inches Across Much Of The Midwest From Ia East Across Il/Ind/Oh. Morning Soundings Across This Same Region Indicate A Pronounced Eml Advecting East From The Srn High Plains With 700-500mb Lapse Rates Approaching 9c/Km.

Despite Relatively Strong Capping Across The Warm Sector...Degree Of Heating And Large Scale Ascent Aiding Surface Cyclone
Intensification Across Ern Ia By Late Afternoon Will Prove Adequate
For Surface-Based Storm Initiation. Additional Near-Surface-Based
Deep Convection With Both Hail And Wind Potential May Precede Truly Surface-Based Storms To The North Of The Surface Low And
Quasi-Stationary Front Situated From Nern Ia Across Srn Wi To Lower
Mi.

Given Degree Of Capping And Magnitude Of Instability...Expect
Potentially Explosive Storm Development To Occur In The Presence Of Modest To Locally Strong Storm-Relatively Helicity /Srh/.

Any Convection Initiating Near The Surface Low And Front Will Quickly
Acquire Supercell Characteristics Given Degree Of Effective Vertical
Shear Of 35-55kt. Potentially Strong Tornado Threat May Be Maximized
During This Early Development Phase /21z-00z/ Near The Low And Front While Storms Remain Discrete Amidst High Instability And Effective Srh In Excess Of 200 M2/S2.

Organizing Influence Of The Deepening Surface Cyclone Rippling East Along The Warm/Quasi-Stationary Front...In Concert With 50-60kt
Mid-Level Jet Streak Directed Preferentially Into/Across The
Developing Mass Of Convection Suggests Upscale Progressive Mcs
/Possible Derecho/ Evolution Through The Evening Hours.

Given Antecedent Airmass Characteristics...Strength Of Large Scale
Forcing...And Depiction Of Current Conditions...Relatively High
Confidence Exists In A Number Of Storm-Scale Model Simulations
Showing Mcs/Derecho Evolution Within The High Risk Area Through The Evening Hours.

Wrf-Arw Simulation From 00z Takes The Apex Of The Progressive Mcs From Chicago To Detroit In Under 6 Hours With A Forward Speed In Excess Of 40kt. Shear And Instability Within And Ahead Of This Convection Will Remain Supportive Of Both High Winds Possibly Well In Excess Of 60kt...As Well A Few Tornadoes Possibly
Associated With Supercell Structures Embedded Within The Squall
Line/Qlcs.

Oh East To East Coast/Southeast...

Warm Moist Airmass Also Exists Well East Of The Stronger Forcing
Across From Oh East Across The Appalachians To East Coast. Some
Model Simulations Depict The Possible Evolution Of Small Convective
Complexes Developing From Initially Multicellular Storms Across The
Higher Terrain This Afternoon. Large Scale Forcing And Shear Remain Subtle/Weak Across These Areas Until Late In The Period.
However...Degree Of Anticipated Instability Could Result In A Few
Hail/Wind Events Through The Evening And Possibly Into Late
Tonight/Early Friday Across These Parts Of The Slgt Risk Area.

Mt/Nern Wy...

Strong Impulses Rotating Through The Base Of Larger Trough Across The Pacific Northwest Will Aid Ascent Across The Nrn Rockies Through The Afternoon. Steepening Low Through Mid-Level Lapse Rates And Strengthening Shear Should Contribute To A Number Of Supercell Storms Posing Both Hail And Wind Threats. An Isolated Tornado May Also Evolve From Initial Discrete Cells Given Relatively High Boundary Layer Moisture And Sufficient Directional Shear In The Storm Updraft Layer.


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