Scientific Forecaster Discussion
000 FXUS64 KFWD 171701 AFDFWD Area Forecast Discussion National Weather Service Fort Worth TX 1101 AM CST Sat Feb 17 2018 .AVIATION... Light surface winds in the wake of this morning's shortwave should eventually veer around to the north this afternoon, and then northeast this evening. This will be short-lived, however, as return flow cranks back up overnight in response to a deepening West Coast trough and its associated lee-side surface trough. A northward surge of low level moisture should send a deck of stratus northward Sunday morning, and MVFR cigs have been added to all TAF sites near or after 12Z. An initial shortwave will begin to spread enough lift to generate showers by Sunday afternoon, so VCSH has been introduced to the extended DFW TAF. 30 && .UPDATE... /Issued 952 AM CST Sat Feb 17 2018/ The disturbance responsible for this morning's rainfall is moving quickly to the east and taking the precipitation with it. The main change with this update is to end POPs from west to east and remove the mention of thunder for this afternoon. Models are indicating some lingering light precipitation this evening over Central Texas, so will maintain the low POPs across the southern- most row of counties tonight. No other updates are planned at this time. 30 && .SHORT TERM... /Issued 400 AM CST Sat Feb 17 2018/ /Today and Tonight/ A shortwave currently over eastern New Mexico will move east across the Southern Plains today. Expect widespread showers and isolated thunderstorms this morning northeast of a Graham to Hamilton to Palestine line, with a chance of showers and isolated thunderstorms farther to the south. Skies will start to clear across the northwestern zones by late morning. Rain chances should shift to the southeast of a Goldthwaite to Granbury to Sulphur Springs line this afternoon. After morning lows in the upper 30s northwest to lower 50s southeast, temperature will rise into the mid 50s southeast, where the cloud cover will be the highest, to the mid and upper 60s across the northwest where there will be the most sunshine. West to northwest winds of 5 to 10 mph will become north to northeast this afternoon. Some showers are expected to linger into tonight southeast of a Lampasas to Athens line. Some patchy fog and drizzle may develop across Central Texas toward daybreak Sunday. As high pressure centered to our north moves east, northeast winds of 5 to 10 mph will veer around to east at around 5 mph. Overnight lows will be mostly in the 40s. 58 && .LONG TERM... /Issued 400 AM CST Sat Feb 17 2018/ /Sunday through Friday/ An active pattern will take shape across the Southern Plains for the upcoming week. This will mean some good opportunities for widespread rain across a good chunk of North and Central Texas. The latest model guidance continues to suggest a highly conditional risk for strong to severe storms on Monday and again on Tuesday. Temperatures will generally remain near or above normal with a brief cool down on Wednesday and Thursday. A moderating temperature trend is possible as we head into the last weekend of the month. For Sunday---Rain chances on Sunday look lower compared to 24 hours ago. The 850mb baroclinic zone that has helped to trigger widespread rainfall on Saturday will slowly lift northward. The surface front will lag behind this feature, but it'll also lift northward. With increasing low level moisture, there could be some morning fog/drizzle--- especially down across Central TX. Farther north, wind speeds may keep the PBL overturned enough such that this potential is largely mitigated. While the magnitude of WAA is decent, it (the WAA) will more than likely be the only lifting mechanism. Given that ascent along the warm front is typically tame (in the absence of strong instability aloft) owing to the gradual slope of the frontal surface, I've lowered PoPs into the 20-30 percent range. This is supported by TTU WRF and NAM NEST hi- resolution guidance. There may be an isolated storm or two as well, but with elevated instability being quite meager (only a few hundred J/kg), I'll leave this out of the forecast. Despite widespread cloud cover, there's a decent 1000-500mb thickness ridge across the area. This should equate to above normal temperatures with afternoon readings in the 70s. Some mid 70s are possible out across the Big Country along the spine of the thermal axis. Across the I-30 corridor, temperatures may remain 10 to 15 degrees lower than the rest of the area due to limited WAA. By late Sunday afternoon into Sunday night, a conveyor of rich low level theta-e will stream in from the southwest and spread across the northern Big Country, Red River and eventually into the Ark-La-Tex. This likely indicates an increase in instability and should be sufficient at generating a bit more in the way of more widespread and vigorous, but non-severe convection. The best chance for showers and a few thunderstorms will be northwest of a Comanche to D/FW to Sulphur Springs line late Sunday afternoon into Sunday night. Coverage of convection isn't expected to be spectacular, however. For Monday---There will continue to be a chance for isolated to widely scattered showers and a few thunderstorms roughly across the northwestern third of the area during the pre-dawn hours on Monday. Continued strong low level flow will help to keep the PBL mixed overnight and this should lead to a very mild and breezy Monday morning. Into Monday afternoon, models are in good agreement that a dryline will sharpen some across the Big Country. Strong southwesterly flow aloft will advect a layer of warm air atop the moist boundary layer, likely keeping most areas free of rain during the mid- morning and into the afternoon hours. I'll maintain a 30 PoP, however, for northwestern zones where theta-e advection is expected to persist. The dryline will slowly move eastward through the day and there should be some convergence along the boundary. This convergence, however, will be the only meaningful lift and so this lowers confidence that this will be enough to breach the cap. What is a little concerning is forecast temperatures vs. the convective temperature. Minus the NAM, even the coolest guidance in the remainder of the model suite advertises mid to upper 70s and low 80s across the area. Convective temperatures are forecast to be in the mid 70s. If current forecast temperatures are to verify, it's possible that surface based convection *could* develop. Instability is modest with values of up to 1,000 J/kg, however, and the instability profile is very slender. Updrafts developing into an environment characterized by 30 to 40 knots of deep layer shear and this instability profile may not develop favorably to mature into strong updrafts. IF the updrafts can survive, they *could* become strong to severe. Unfortunately, this far out, it's a little difficult to accurately depict storm scale processes and as a result, we will continue to highlight only a conditional risk for severe weather on Monday afternoon. Late Monday into Tuesday---Convection will more than likely blossom late Monday into Tuesday. While the lack of diabatic heating tends to dampen the overall instability and thus the severe weather potential, it's important to not let your guard down when dealing with strong low level flow. Low level winds will crank upwards of 40 to almost 50 knots across the northwest and this should transport a healthy batch of low level theta-e. If convection is ongoing or develops, there is a possibility that it could grow upscale across northwest zones and move eastward. I'm not ready to buy this scenario completely given the lack of higher resolution guidance to support this, but it's not out of the realm of possibility. I'll keep 50 to 60 percent PoPs to the northwest where this is most likely to happen and for forecast consistency. Again, a conditional risk for severe storms will be present with this convection. Tuesday and Wednesday---Convection will likely be ongoing into Tuesday morning and I'll continue to show widespread rain chances (60-80%). The best rain chances will be along the Red River. The ECMWF has remained consistent with a slower frontal progression than most of the other guidance. The last few frames of the 00 UTC NAM and a majority of the 00 UTC GFS ensemble members have started to shift towards the ECMWF solution as well. In fact, the deterministic 00 UTC GFS has also slowed a bit (but still remains the fastest solution). With that in mind, I've pushed the forecast to be more in-line with the ECMWF/NAM/GFS Ensemble guidance. The bottom line here is that widespread rain chances will be possible a good portion of Tuesday and into at least the first half of Wednesday. The slow moving nature of the front, coupled with ascent courtesy of jet entrance forcing actually resembles a Maddox Type I (one) heavy rain pattern. Given that we've been quite dry for sometime, I'm not ready to jump on the flood bandwagon, but depending on the amount of rainfall received Late Monday into Tuesday morning, it may not be out of the realm of possibility. Highest QPF will generally be along and east of I-35 during the midweek time frame. While that is a broad geographic descriptor, we will likely refine this as we near the the potential heavy rain event. It'll turn cooler on Wednesday as cooler air overspread the region from the north with brisk northerly winds in the wake of the front. Thursday and Friday---Model guidance continues to diverge during this portion of the forecast and as a result, I've leaned heavily on the blended guidance. The operational GFS, which seems to be much of an outlier, dries much of the area out fairly quickly on Thursday and most of Friday. The ECMWF on the other hand develops quick return flow. Both models have a pretty large trough to the west and I'd be inclined to believe the ECMWF output which suggests lee-side cyclogenesis and resultant low level moisture return. 24-Bain && .PRELIMINARY POINT TEMPS/POPS... Dallas-Ft. Worth 62 46 70 60 80 / 100 0 30 30 30 Waco 58 46 73 62 80 / 30 10 30 20 20 Paris 56 42 59 55 75 / 100 0 20 40 30 Denton 62 41 66 59 78 / 100 0 40 30 50 McKinney 58 42 64 58 75 / 100 0 30 40 40 Dallas 60 47 70 60 79 / 100 0 20 30 30 Terrell 57 45 68 61 78 / 60 0 20 30 30 Corsicana 55 47 69 62 79 / 40 5 20 20 20 Temple 59 47 75 62 76 / 30 20 30 10 20 Mineral Wells 65 43 71 61 81 / 100 0 40 30 30 && .FWD WATCHES/WARNINGS/ADVISORIES... None.