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Sunday, September 26, 2021

SPC Sep 26, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0752 AM CDT Sun Sep 26 2021 Valid 261300Z - 271200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHERN PARTS OF ARIZONA AND NEW MEXICO... ...SUMMARY... Isolated severe hail and/or gusts are possible today across southern parts of Arizona and New Mexico. ...Synopsis... A progressive synoptic-scale pattern will be noted through the period, as the eastern trough deamplifies and shifts east, and so will the ridging now over the Intermountain West and Rockies. Upstream, a strong cyclone was evident in moisture-channel imagery over the northeastern Pacific, centered near 49N142W, with troughing extending approximately 1,000 nm southward. This system will shift eastward through the period and remain offshore, while amplifying southward, with the parent cyclone west of the BC coastline by 12Z tomorrow. Still, a broad area of associated cyclonic flow and height falls aloft will spread inland tonight ahead of the trough, and across northern CA and the Pacific Northwest. As that occurs, a longstanding cut-off now -- now over northern Baja -- will eject northeastward over southwestern through east-central AZ. At the surface, 11Z analysis showed a cold front offshore most of the Atlantic Coast except central FL, where it became quasistationary and extended westward over the Gulf. A field of Pacific moisture was manifest at the surface over southern AZ, with dew points commonly in the mid 50s to low 60s F (with slight decrease due to mixing expected today). PW commonly around 1.25-1.5 inches was noted at 12Z over southern AZ, based on the TUS sounding, limited GPS PW sampling, and objectively analyzed RAP fields. ...AZ/NM... Convective coverage is expected to increase from midday through the afternoon over the region as the boundary layer destabilizes and MLCINH becomes negligible. By midafternoon, widely scattered to scattered thunderstorms are expected across much of southern AZ and far southwestern NM, offering isolated gusts near severe limits and marginally severe hail. Convection subsequently should spread in a fan-shaped pattern (strongly resembling the difluence of mean-wind and mid/upper flow vectors) northward and northeastward toward both the high country north of the Mogollon Rim, and toward south-central NM. The threat for isolated severe may be maintained as far as near the southern NM I-25/Rio Grande corridor before evening weakening takes place, and the outlook has been somewhat enlarged eastward accordingly. Intensity and coverage should diminish this evening and overnight as activity encounters a more-stable boundary layer, both from outflows and nocturnal diabatic cooling. Forecast soundings suggest that the preconvective environment will be characterized by: 1. Somewhat greater low-level moisture content and steeper midlevel lapse rates westward contributing to 1000-1500 J/kg peak MLCAPE, but weaker low/middle-level flow/shear, and 2. Steeper low-level lapse rates with larger DCAPE and stronger mid/upper winds eastward. These factors only overlap diffusely, but will contribute to potential for at least brief/localized organization with attendant marginally severe potential. ..Edwards/Jewell.. 09/26/2021 Read more LIVE: spc.noaa.gov