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SPC Jul 15, 2025 1300 UTC Day 1 Convective Outlook

1 month 3 weeks ago

SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0734 AM CDT Tue Jul 15 2025 Valid 151300Z - 161200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF NEBRASKA... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF WYOMING... ...SUMMARY... Scattered severe gusts (some 75+ mph) and large hail are probable across parts of Nebraska late this afternoon and evening. A mix of scattered severe/damaging winds and large hail should also occur from the Upper Great Lakes to Wyoming this afternoon to mid-evening. ...Northern/Central Plains to the Upper Great Lakes... A belt of stronger mid-level westerly flow will remain confined generally along/north of the international border today. Even so, multiple weak mid-level perturbations, some of which are related to prior/ongoing convection, will aid in additional thunderstorm development later this afternoon and evening across portions of the northern/central Plains to upper Great Lakes. This convection will also be aided by a slowly southward sagging cold front across SD into NE, as a weak low develops northeastward across the Upper Midwest into Canada along this boundary. As diurnal heating occurs today, a moist and moderately to strongly unstable airmass will exist by this afternoon along/south of the front from NE into parts of SD, MN, and the U.P. of MI. While stronger flow aloft will tend to remain to the north of the front, there should still be sufficient deep-layer shear across the warm sector for some updraft organization. Current expectations are for robust thunderstorms to develop along the front in MN by early afternoon, and quickly spread eastward across northern WI, the U.P. of MI, and Lake Superior through the rest of the afternoon into the evening. Sufficient instability and effective bulk shear should be present along/ahead of the front to support organized convection, including some supercells. Large hail will be possible with the initially semi-discrete thunderstorms, before a rapid transition to a cluster/linear mode increases the threat for severe/damaging winds. This activity should quickly weaken by late evening as it encounters a less unstable airmass with eastward extent across the upper Great Lakes. A somewhat separate area of thunderstorms should initiate along/south of the front across parts of southern SD into NE by late afternoon/early evening. Greater instability will likely present across this region compared to locations farther north, but deep-layer shear should be more modest (around 25-30 kt). Still, some large hail may occur initially given the presence of steep mid-level lapse rates and ample instability. The threat for scattered to numerous severe/damaging winds is expected to increase through the evening across central NE as an intense bowing cluster develops and spreads southeastward. Isolated significant gusts of 75+ mph appear possible given the rather favorable thermodynamic environment. ...Great Basin into Wyoming... The upper trough digging southeastward over the northern Rockies today will influence convective development this afternoon across parts of the northern Great Basin and WY vicinity. High-based convection that occurs across parts of eastern NV into UT will occur in a relatively dry profile, with limited total precipitable water. Still, a deeply mixed boundary layer coupled with weak instability should support some risk for severe gusts with this activity as it moves generally east-northeastward this afternoon/evening across the northern Great Basin vicinity. Greater low-level moisture, with surface dewpoints generally in the low to mid 50s, is already present this morning over parts northern WY. Convection that initially develops near Yellowstone in northwest WY may grow upscale fairly quickly into an organized cluster given 25-35 kt of deep-layer shear, while posing a threat for mainly severe/damaging winds given the presence of steepened low/mid-level lapse rates. Isolated large hail may also occur. The Slight Risk across WY has been expanded northward/eastward some based on latest guidance trends. This cluster should eventually weaken late this evening/tonight as it approaches the WY/NE/SD border vicinity. ...Arizona... Modest mid-level east-northeasterlies will be present today across parts of AZ on the southern periphery of a mid-level anticyclone over the Southwest. Scattered thunderstorms should once again develop this afternoon over the higher terrain of southeast AZ and southwest NM. With very steep low/mid-level lapse rates expected with ample daytime heating, this activity may pose an isolated threat for severe winds as it spreads into the lower elevations of southern AZ through the evening. ..Gleason/Bentley.. 07/15/2025 Read more