Atmospheric high pressure will expand across northern Mexico and southwest Texas through Friday, spreading into the Rio Grande Valley for weekend before slowly receding next week. Near record heat will cover the region beginning today (Thursday) and peaking over the weekend. For today (Thursday), the combination of low level heat and humidity with light winds and just enough moisture in the low to mid levels, as well as the tail of an upper level disturbance moving across east Texas, may be sufficient for a stray thunderstorm, favoring the ranchlands from Hebbronville to Sarita. Even though the storms will be isolated, the atmosphere is very unstable and would support the potential for damaging wind and hail.
What to Watch For
A heat advisory is in effect today (Thursday) for several hours of "feels like" temperatures between 111 and 113, along/east of US 281.
An excessive heat watch is possible for Friday, for several hours of "feels like" temperatures between 115 and 118 in the same areas.
Similar conditions to Friday are expected through the weekend, favoring both areas east of US 281 as well as locations along/near the Rio Grande in Starr County. Additional watches, heat advisories, and excessive heat warnings are issued for "feels like" temperatures of 116 or higher for 2 or more hours during an afternoon.
Nighttime heat index will also be oppressive, ranging from 90 to 100 through the evening and 85 to 93 after midnight each day.
Isolated thunderstorms may sneak south/southwestward from the Brush Country late this afternoon into early this evening, feeding on the very hot temperatures and humidity.
Any cells will be capable of producing hail and potentially damaging wind gusts, as well as frequent cloud to ground lightning.
Most areas will be rain-free, however the cells could form as far south as the "mid Valley" (i.e. Harlingen to McAllen and points west)
Potential Impacts/Recommended Actions
All heat safety precautions apply through Sunday, June 9th, and likely into Monday, June 10th, before some modest relief arrives. Details at https://weather.gov/heat.
In addition to these (and those shown below), remember your pets, the elderly, and the infirm and be sure to get them to air conditioned or otherwise safe locations.
To be safe, those with outdoor activities this afternoon and evening should have a designated safe zone from lightning, potentially damaging winds, and hail. When thunder roars, move indoors to these zones
Keep your weather alerting apps and NOAA weather radio on "alert" mode in case warnings are issued.
Forecast actual temperatures will be near or just above records in Brownsville, McAllen, and Harlingen Friday through Sunday. In general, around 100 is forecast in Brownsville, 102-103 around Harlingen, and 104-105 around McAllen.