TL;DR
The Storm Prediction Center has issued a Marginal Risk for parts of southeast Arizona and southwest New Mexico today. Isolated severe thunderstorms may produce marginally severe hail and gusty outflow winds this afternoon and early evening, roughly between 11 AM and 7 PM local time. Activity should weaken quickly after sunset.
Where is the risk today?
The Marginal Risk—the lowest category on SPC's five-tier scale—covers portions of southeast Arizona and southwest New Mexico. This includes areas near the Arizona-New Mexico border where terrain-driven thunderstorm development is most likely.
No other severe weather risk areas exist across the continental United States today. The past 24 hours have been quiet nationwide, with zero tornado, hail, or wind reports logged.
What's driving today's setup?
A weak shortwave trough is moving across the Southwest, bringing a 35-45 knot southwesterly mid-level jet over the region. Surface observations early Monday morning showed low-level moisture slowly increasing across southeast Arizona, with dewpoints in the mid to upper 50s at lower elevations.
According to the Storm Prediction Center's Day 1 Convective Outlook, "modest large-scale ascent attendant to this feature coupled with terrain-driven influences should aid thunderstorm initiation across southeast AZ by 17-21Z as convective temperatures are reached with filtered diurnal heating."
In plain terms: the combination of lift from the upper-level disturbance and heating over mountainous terrain will trigger scattered to numerous thunderstorms by late morning through early afternoon local time.
Atmospheric ingredients
The boundary layer across southeast Arizona is not expected to become overly well mixed this afternoon, which will allow instability to build. Recent RAP model runs suggest that around 500-1000 J/kg of MLCAPE (mixed-layer convective available potential energy—the fuel for thunderstorms) will develop, with locally higher values possible.
Cool mid-level temperatures and 30-40 knots of deep-layer shear should provide enough organization for some updrafts to produce isolated severe weather. The primary hazards are:
- Marginally severe hail (quarter-size or slightly larger)
- Gusty outflow winds (50-60 mph possible with the strongest cells)
Tornado risk is not mentioned in the SPC outlook, and the atmospheric profile does not favor rotating storms today.
Timing: when will storms develop?
Thunderstorm initiation is expected across southeast Arizona between 17Z and 21Z (11 AM to 3 PM local Mountain Daylight Time). Storms will then spread east-northeastward into southwest New Mexico through the afternoon and early evening.
SPC forecasters note that "this activity is expected to quickly weaken by mid evening with the loss of daytime heating." Once the sun sets and the surface heating shuts off, storms should dissipate rapidly.
Peak severe weather window: 11 AM to 7 PM MDT across the Marginal Risk area.
What does a Marginal Risk mean?
A Marginal Risk is the first of five SPC severe weather categories:
- Marginal (today's risk)
- Slight
- Enhanced
- Moderate
- High
Marginal means isolated severe storms are possible, but coverage and intensity will be limited. Not every thunderstorm will be severe, and even the severe-warned cells may only briefly meet severe criteria. For most residents in the risk area, today will feel like a typical monsoon-season afternoon with hit-or-miss thunderstorms.
That said, any storm capable of producing hail or 60 mph winds deserves respect. If you're outdoors in southeast Arizona or southwest New Mexico this afternoon, keep an eye on the sky and have a way to receive warnings.
Looking ahead: Tuesday and Wednesday
SPC's Day 2 outlook (valid Tuesday, October 20) shows no severe weather risk areas across the country. Quiet weather is expected nationwide.
The Day 3 outlook (valid Wednesday, October 21) introduces a Marginal Risk for parts of Colorado and New Mexico as another weak disturbance moves through the Rockies. Similar to today, isolated severe hail and wind will be the primary concerns with any storms that develop over higher terrain.
No significant severe weather outbreak is on the horizon for the Lower 48 through midweek.
Bottom line
Today's Marginal Risk is a low-end event confined to a small area of the Southwest. Residents of southeast Arizona and southwest New Mexico should monitor the weather this afternoon, especially if spending time outdoors. Storms will be hit-or-miss, but a few may briefly turn severe with small hail and gusty winds before weakening after sunset.
For the rest of the country, Monday is a quiet day with no severe weather expected.
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