I hope you enjoyed the sun today because aside from today and most of tomorrow, it looks like that will be all we see of the sun until as late as Friday. The lack of sun will be counter-balanced by rain, and, what looks like, lots of it.
Chart showing 500mb level vorticity (lift) and heights…
Of note above is the closed (circular) lines over western Mexico…
What we have is what weather guys call a “cut-off” upper level low. What that means is that an upper low circulation has separated from the jet stream. These types of systems typically move slower and usually lead to quite a bit of instability in the affected regions to the right of the upper system. Heavy rains, thunderstorms, and sometimes severe weather can accompany these systems.
With this particular system, south Texas and Southeast Texas could see all the above weather types with a larger focus on heavy rainfall for Southeast Texas.
HPC Hazard Map for Tuesday
The event will begin Monday night as a surface low forms over west-central Texas…that will help bring a warm front and lots of moisture northward from the northern Gulf of Mexico into SE Texas. Rain and storms (some severe) with begin in south Texas, but expect some quick-moving showers Monday night in southeast TX. Rain and storms will pick-pickup in southeast Texas on Tuesday and Tuesday night. Heavy rains will begin training inland from the Gulf of Mexico as the upper low system and accompanying surface low rotates over central and north Texas. It is the training of thunderstorms and heavy rainfall that will increase the chances of localized flooding across southeast Texas.
Forecast for early Wednesday showing warm front along SE Texas Coast
The Hydrometerological Prediction Center has a 5 inch bulls-eye for SE Texas over the next 3 days, most falling on days two and three (Tuesday and Wednesday). A front will push, slowly, through on Thursday. Some light showers seem possible on Thursday with the passage…then the rain should come to an end. Friday and Saturday look dry before the next system approaches late next weekend.
HPC 3-day Rainfall Totals
Map showing frontal passage on Thursday
HPC has southeast Texas in a slight risk region for rainfall exceeding flash flood guidance for any particular county during this event. I also fully expect the National weather service to issue a Flood Watch for the region by tomorrow…if not sooner.
Map showing HPC Flash Flood Exceedance probability
In order to exceed flash flood guidance and prompt Flash Flood Warnings, SE Texas, in general, would have to receive 2.5+ inches of rain in an hour, or 3.5+ inches over 6 hours (see graphics below). Some locations could definitely see Flash Floor Warnings Tuesday night and Wednesday.
How much rainfall the area receives will be determined by how fast the system moves, where training rainfall occurs, and how quickly the atmosphere destabilizes between episodes.
Optimally, the system will move a bit faster and the 3-day rainfall will average about three inches area-wide, and flooding will not be a big concern. I do believe that areas SW of Houston will receive higher totals, and localized flooding will occur. Which areas receive the most rainfall and flooding will not be known until the even begins to wind down on Thursday.
Keep your umbrellas handy from Monday evening through Thursday morning. Pay attention to the National Weather Service and media for possible Flash Flood Warnings that may be issued for certain areas Tuesday night and Wednesday for the Houston/Galveston area.
You can check the forecast page for temperature information, and the sidebar for tweets from the National Weather Service, Storm Prediction Center, and Weather Spectrum (@WxSpec).
Have a safe week and try to stay dry!