Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 21 2024 12:57:29 ACUS01 KWNS 211257 SWODY1 SPC AC 211256 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0756 AM CDT Sun Jul 21 2024 Valid 211300Z - 221200Z ....THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF CENTRAL AND SOUTHERN ARIZONA...AND FROM SOUTHERN COLORADO TO EASTERN NEW MEXICO... ....SUMMARY... Isolated, marginally severe thunderstorms are possible this afternoon into early evening across parts of central and southern Arizona, and from southern Colorado to eastern New Mexico. ....Synopsis... The mid/upper-level pattern will remain rather blocky over and near the CONUS through the period, with longwave ridging remaining over the West, and a persistent, positively tilted longwave trough from QC across the mid Mississippi Valley to central/south TX. A series of mostly small shortwaves will traverse the modest northerly flow aloft, in between those features, over the Plains States and central/southern Rockies. A long-lived anticyclone -- initially centered over the lower Colorado River Valley -- will continue to retrograde, drifting northwestward over parts of NV while expanding somewhat. Farther northwest, a strong shortwave trough was apparent in moisture-channel imagery over and offshore from the coastal OR/CA border. This feature should accelerate northward through the period and weaken, being absorbed into the eastern semicircle of a massive, slow-moving cyclone centered over the Gulf of AK. The trough will pass along and just off the OR/WA coastline today, and over Vancouver Island to coastal west-central BC by 00Z. At the surface, 11Z analysis showed a weak, muddled surface pattern north of an increasingly diffuse, wavy frontal zone that extended from the Hampton Roads region across the Tennessee Valley region to parts of southern OK and north TX. That front should continue to demarcate richest Gulf moisture from a lesser-modified, more continental air mass to the north, and should remain quasistationary. ....South-central Rockies and adjoining High Plains... Widely scattered to scattered thunderstorms will form from midday through the afternoon and move generally southward, with local deviance to the southeast or southwest as boundary and outflow- driven processes force. Weak low/middle-level winds will limit deep shear, with multicells the dominant storm mode. Due to the lack of greater buoyancy, and high-elevation nature of the surface limiting boundary-layer depth in many instances, much of this convection should remain relatively shallow and struggle to organize. However, the most vigorous cells, including those that move over valleys and lower Great Plains elevations with deeper subcloud layers and greater boundary-layer moisture content, will pose a threat for isolated severe gusts and marginal hail. Some cold-pool aggregation also may occur, especially over Plains areas of east-central/ northeastern NM and extreme southeastern CO. 300-800 J/kg MLCAPE should be common across this region in the preconvective air mass. The overall strong/severe thunderstorm-wind potential should diminish through the evening as the foregoing boundary layer stabilizes diabatically, and stable-outflow coverage increases. ....AZ... Scattered thunderstorms should develop today, primarily as strong insolation preferentially erodes MLCINH amidst at least marginal moisture, over the Mogollon Rim and adjoining higher elevations of eastern/central/northern AZ. With the retrogression of the mid/ upper ridge, the mid/upper flow will be largely northerly, and through a fairly deep layer (roughly 700-300 mb, sometimes higher). Activity therefore should move southward to southwestward across deeper mixed layers of the lower desert elevations, with at least isolated severe downdrafts possible. Peak MLCAPE may reach the 500-1000 J/kg range in some areas, and should be at least 250 J/kg over most of the outlook, with DCAPE commonly over 1000 J/kg. Boundary mergers, orographic effects and storm-scale outflow aggregations may focus the severe threat locally within this broader "marginal risk" area. Convection may go as far as the borderlands before weakening this evening -- hence the minor extension of the outlook area. ....Interior Northwest... Widely scattered to scattered, high-based thunderstorms are expected to develop this afternoon over much of inland OR, northeastern CA and northern NV, amid peripheral large-scale lift from the Pacific trough and strong surface diurnal heating. This activity may produce locally strong, often dry downbursts -- indeed, scattered dry thunderstorms are forecast in the SPC fire-weather outlook. Large dewpoint depressions (40-70 deg F) should result after a few hours of diurnal mixing, causing a drop in buoyancy, but with enough (200-600 J/kg MLCAPE) to support thunderstorms with subcloud/ boundary-layer depths reaching into the 500-600 mb layer. Lack of greater low/middle-level flow and buoyancy both preclude an unconditional severe outlook, but a downdrafts approaching severe limits cannot be ruled out. ...Edwards/Broyles.. 07/21/2024 $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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