The next best chance of rain for south Texas and the Rio Grande Valley will be this Friday, March 9. Rain chances will increase due to a mid-level low and a surface cold front.
Currently, mid-level ridging and surface high pressure are keeping things calm and quiet, under clear and sunny skies across south Texas. However, return flow from the Gulf of Mexico could bring a chance of some light showers Wednesday and Thursday.
Beginning Tuesday, southwest flow aloft, as well as southeast flow at the surface, will increase cloud cover through Thursday. Mid-level trough will eject out of the Pacific Northwest beginning Tuesday and will likely form a closed low over the western 4-corner states by Wednesday afternoon. There is some uncertainty what this closed low will do after that. Euro model has it continuing to dig farther south over southern New Mexico and Arizona, while the GFS model has the closed low moving into the Southern Plains by Thursday afternoon. The CMC model is similar to the Euro, except it has the low digging a little farther south.
This mid-level low will generate cyclogenesis at the surface and also determine what time the cold front will pass through the Valley. GFS has been inconsistent with the timing of the front through the RGV, ranging any time between late Thursday evening to late Friday morning.
Regardless, afternoon highs will steadily increase each day until Friday, with Thursday being the warmest day of the week. I would not be surprised if some areas reached the 90s, especially in the western Valley on Thursday.
The biggest dilemma will be QPF (quantitative precipitation forecast). With disregards to any rain that falls on Wednesday and/or Thursday (if any), anywhere from .25” to .50” of rain could fall in south Texas Friday through Saturday. However, QPF will be determined by the amount of lift ahead of the cold front AND position and timing of the mid-level low. Dew point temperatures will be well into the 60s and precipitable water values will be likely be over an inch in the Valley. Therefore, there will be plenty of moisture in the lower levels.
Temperatures behind the front will be 10 to 15 degrees cooler for the weekend.