Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 20 2024 12:49:43 ACUS01 KWNS 201249 SWODY1 SPC AC 201248 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0748 AM CDT Tue Aug 20 2024 Valid 201300Z - 211200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS PREDOMINANTLY OVER PORTIONS OF CENTRAL MONTANA... ....SUMMARY... Severe thunderstorm winds and large hail are possible today, mainly over parts of Montana. ....Synopsis... In mid/upper levels, a rather blocky large-scale pattern will yield very slow evolution of major features through the period. A 500-mb low that has just closed off over southern QC should move southward over central NY through the period, with associated cyclonic flow covering the full extent of the Appalachians and most of the CONUS from the mid/upper Mississippi Valley eastward. An anticyclone will remain anchored over NM, west TX and vicinity, with attached ridging moving slowly eastward over CO and the northern Plains. This will permit strengthening southwest flow aloft to spread out of the northern Rockies and across most of MT through the period, ahead of West Coast troughing. An embedded, weak shortwave trough -- evident in moisture-channel imagery over south-central WA and central OR-- should eject northeastward over the Canadian Rockies, The 11Z surface analysis showed a cold front offshore from New England and the Mid-Atlantic, just offshore from the Carolinas, then across southeastern GA, the eastern FL Panhandle, MS/AL shelf waters, and coastal LA, becoming a warm front northwestward over east and north TX to northeastern NM. The Atlantic segment should proceed farther offshore, and sag southward into portions of northern FL and the north-central Gulf, while the western parts stalls from north-central/northwest TX to northeastern NM. Warm frontogenesis over the north-central High Plains -- just east of a lee trough -- should connect with the south-central High Plains pre-existing frontal zone by late in the period. ....Northern High Plains... Widely scattered to scattered, high-based thunderstorms are expected to form first over higher terrain of southeastern and south-central MT during early/mid afternoon, then move northeastward into a rapidly destabilizing and weakly capped (at most) boundary layer over the adjoining High Plains. Supercells with large hail and isolated severe gusts, as well as small clusters with mainly severe gusts and isolated hail, are possible. Activity should cross much of central MT before becoming weaker and more isolated as it moves into nocturnally stabilizing air this evening. While the WA/OR shortwave trough and associated DCVA/UVV field are progged to pass northwest of the area, it should tighten the height gradient in mid/upper levels. This will combine with low-level southeasterlies to yield favorable deep shear for supercells. Progged effective-shear magnitudes vary highly across models and from place to place within the same model (generally 30-55 kt) depending on depth of the CAPE layer, which mostly is a function of how vigorous and deep the mixing process is. As per its bias, RAP- rooted guidance (RAP, HRRR) appear to be mixing too much over central MT, with most other HREF members and NAM showing more realistic vertical thermodynamic profiles and greater overall buoyancy. Peak MLCAPE should reach 1000-1500 J/kg from northwestern NE to east-central MT, decreasing westward to around 500-1000 J/kg over west-central MT. Higher CAPE values are possible over a narrow, north-south corridor in the western Dakotas, but amidst weaker deep-layer forcing and stronger capping near the mid/upper ridge, inhibiting sustained and especially severe convective potential. ....South-central High Plains... Strong elevation-relative heating will support preferential removal of MLCINH, followed by and related mid/late-afternoon thunderstorm development, over the higher terrain of southern CO, on either side of the San Luis Valley. Activity should move eastward to east- southeastward over deeper, well-mixed boundary layers characteristic of the valley and the High Plains, while offering isolated severe gusts and hail for a few hours before weakening in the evening. This close to the mid/upper anticyclone center, deep-later flow will be weak, keeping shear weak and severe potential isolated and disorganized. However, steep lapse rates in a well-mixed boundary layer may support severe gusts and perhaps hail from the most vigorous, sustained cells and small clusters. ...Edwards/Mosier.. 08/20/2024 $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .