Scientific Forecaster Discussion
000 FXUS64 KFWD 190433 AFDFWD Area Forecast Discussion National Weather Service Fort Worth TX 1133 PM CDT Thu Oct 18 2018 .AVIATION... /06Z TAFs/ Concerns and challenges will be similar to the previous 00z discussion, as low VFR cigs at DFW and current high MVFR cigs at Waco begin to slowly lower by 12z Friday and after. An approaching system and increasing, widespread rain/showers arrive by 18z and continue through 00z-03z across the region. A shallow dry layer under an relatively shallow frontal inversion will take some time to saturate, but as large-scale ascent gets going on Friday, the widespread rain/showers will result in saturated and a quick lowering through MVFR with IFR vsbys/cigs likely between 19z-01z or possibly longer at all airports. A slow improvement back into low MVFR is possible by 03z Saturday, as the system begins shifting east of the area and rains diminish in both coverage and intensity. Otherwise, light N/NE winds around 5 knots will become E or even ESE by 06z Saturday. 05/ && .SHORT TERM... /Issued 350 PM CDT Thu Oct 18 2018/ /Tonight/ The high pressure circulation across the Florida Peninsula will continue to advect moist, Gulf air into Texas through the night. Given the cool, dense air is still settled across the state, warm air advection will once again lead to another wave of precipitation, mainly west of Interstate 35. There will likely be the presence of weak instability across Central Texas overnight, leading to the threat of isolated storms. Rain rates outside of any storms will likely not be significant, generally less than half an inch before 7am. If thunderstorms occur, there will be a threat for slightly higher rainfall amounts around sunrise. For that reason, a Flash Flood Watch will be in place beginning 6 AM and last through the day tomorrow. Lows tonight are expected to be a few degrees warmer compared to last night. The warmest temperatures will be across the Brazos Valley, where upper 50s are expected. Winds will remain out of the north/northeast, between 5-10 mph. Hernandez && .LONG TERM... /Issued 350 PM CDT Thu Oct 18 2018/ /Friday Through Thursday/ The ejection of the upper level low over the southwestern US into the Plains will result in increasing isentropic lift and warm advection above a shallow frontal layer. Winds just above the frontal inversion will become southerly at speeds of 20-25 knots resulting in good moisture flux and precipitable water values surging toward 2 inches across much of North Central Texas. Theta-E values will also increase and models are in better agreement in bringing MUCAPE values of a couple hundred J/KG in across the southern CWA, and saturated EPV progs suggests CSI/CI will also be present well into the heart of north Texas. While the threat for thunderstorms will be limited to primarily the southern and southeastern zones, the increased instability combined with the surging moisture content suggests that either bands of heavier rainfall or cellular convective cells will be possible Friday. The actual QPF guidance from the models is not as clustered as I'd like to see and it is possible that some of the drier forecasts of less than an inch everywhere will be the ones that pan out. However, there is a signal in several of the models highlighting all of the ingredients we look for to anticipate heavy rainfall across the southwestern into the central zones of the CWA that can't be ignored. This coincidentally is the region that has seen the highest amount of rainfall in the last week and soils just aren't capable of holding any more rain. We have issued a flash flood watch in this region to highlight this potential of heavy rainfall. These heavy rain ingredients suggest some locations may see more than 3 inches if and where any banding sets up, but in general the average amounts still look only like 1-2 inches over the watch area. We do expect rivers, creeks, and streams to be very sensitive to any additional rainfall which would cause flooding. While widespread rain is expected Friday in areas outside of the watch, locations farther to the north have not seen as much rainfall and this round is not expected to be as heavy there as moisture content and instability diminish which should keep rainfall rates low. Temperatures will be cooler Friday due to the rainfall and weak northeasterly surface winds continuing. Most locations will spend the day in the 50s with low 60s in the east and southeast. The bulk of the rain will come to and end from west to east Friday evening as the shortwave moves off to the east and the isentropic lift ends. This will also send a cold front into the area with much drier air filtering into the region Saturday. Still a few showers are possible along the front as it moves through the region Saturday morning, but by late Saturday the entire area will be rain free with some sunshine filtering in through the lingering high clouds. Dry and cool weather looks to prevail Sunday and Monday and these look like pretty nice days compared to what we've been through in the last week. Highs will generally be near 70 Saturday and in the 60s on Sunday and Monday. Lows will drop into the 40s. Unfortunately the rain will return by the middle of next week as another trough moves toward the region. The weekend front will stall across south Texas and therefore that rich moisture will be quickly drawn back northward. We will bring the chances of rain back Monday night, and show them increasing Tuesday and Tuesday night. While confidence is high in the chance of rainfall Wednesday, what is still in question are the amounts. That's because rainfall amounts are contingent on whether a tropical system will develop in the eastern Pacific Ocean and gets drawn northward by the upper trough into Mexico. Should this occur it would provide strong upper level dynamics for lift and result in a swath of widespread heavy rainfall across Texas. We've seen both the operational GFS and ECMWF flirt with this scenario on occasional runs and we know from climatology that this is a pattern setup that has produced many instances of 10+ inches of rain. Again, we will not start advertising or anticipating this until we get better consistency with the possible tropical system (that hasn't organized yet), but we're watching this closely as it'd be a worse case scenario after all of the rain we've already seen. Temperatures in the midweek period should stay cool with the frontal boundary remaining to our south with ongoing rain. The good news is that a more progressive upper level pattern will organize later in the week into the following weekend. This will get us back to normal - with a period of drier weather in between successive systems. TR.92 && .PRELIMINARY POINT TEMPS/POPS... Dallas-Ft. Worth 56 58 55 70 50 / 50 100 70 20 0 Waco 55 60 58 68 52 / 60 100 80 40 0 Paris 52 56 54 69 47 / 30 90 70 10 0 Denton 54 58 55 71 49 / 60 100 60 10 0 McKinney 55 58 55 70 49 / 40 100 70 10 0 Dallas 56 60 56 71 51 / 50 100 70 20 0 Terrell 56 59 56 69 51 / 30 90 80 20 0 Corsicana 56 60 55 67 50 / 40 90 80 40 0 Temple 56 61 58 65 54 / 60 100 70 40 0 Mineral Wells 54 59 55 70 48 / 80 90 50 20 0 && .FWD WATCHES/WARNINGS/ADVISORIES... Flash Flood Watch from 6 AM CDT Friday through Friday evening for TXZ115>119-129>134-141>146-156>161-174.