Scientific Forecaster Discussion
000 FXUS64 KFWD 150508 AFDFWD Area Forecast Discussion National Weather Service Fort Worth TX 1108 PM CST Thu Dec 14 2017 .AVIATION... VFR conditions will prevail through the TAF period. North winds around 10 kts or less will continue overnight and through most of Friday. However, a return to light south winds will occur Friday evening, likely around 03z. Otherwise, a plume of subtropical high cirrus will continue to stream over the region within a strong upper jet streak. -Stalley && .SHORT TERM... /Issued 341 PM CST Thu Dec 14 2017/ /Through tonight/ North winds have pushed deep into South Texas, but even with the breezy conditions behind last night's front, the cold advection has been weak. The sunshine and dry air has helped daytime temperatures to effectively moderate, but even across Central Texas where cirrus has blotted out the sun, afternoon temperatures are similar to those from 24 hours earlier. While temperatures are certainly cooler than yesterday's across the bulk of North Texas, many locations reached the 60s, which is above normal for mid-December. The plume of cirrus will remain primarily across South Texas, but as an upper trough approaches from the northwest tonight, mid and high clouds will increase. The dense clouds across Central Texas will keep temperatures in the upper 30s and lower 40s tonight. But across the northern half of the area, the clouds are unlikely to inhibit radiational cooling, and Friday morning low temperatures will generally be in the lower to mid 30s. Some areas across the north and west will dip below freezing. 25 && .LONG TERM... /Issued 341 PM CST Thu Dec 14 2017/ Friday will be a pleasant day with some sunshine through the high clouds and seasonable temperatures. While humidity will once again drop below 30% area wide, wind speeds will be fairly light and limit the wildfire danger. That's not to say that fires won't start easily, but their rate of spread should be slow enough for fire fighters to contain. The upper level low near Baja today will head eastward toward the region on Saturday as another upper wave drops down the Pacific coastline. Ahead of the upper level low, southerly winds will increase and eventually bring Gulf moisture northward. Saturday will start out clear and chilly, but low and mid clouds will overspread from south to north during the morning hours with winds increasing to near 15 mph. Temperatures will stall in the mid 50s over the southern zones to near 60 over the north for highs. With winds increasing and humidity once again dropping below 30%, it raises the concern for wildfires, particularly across the NW zones. At this time it appears that the arrival of cloud cover and fairly cool temperatures will keep conditions from getting critical. We will not be issuing any fire weather watches. Rain should begin to develop over Central Texas by Saturday afternoon and spread northward into North Texas during the evening hours as strong isentropic lift organizes. While the coverage of this rain will be high, especially east of I-35, the amounts look to be on the light side with an average of less than 0.10" of an inch in the western zones ranging to 0.75" in the far southeastern zones where the better moisture/lift/instability will be located. The predominant character of the precip will be light showers, but a few isolated thunderstorms with moderate rainfall may be possible. All of the rain should quickly end early Sunday morning with clearing skies and temps warming into the 60s during the day. The second upper level system dropping through the West Coast on Saturday will move across our region in a weakened state late Monday. Both moisture and lift look much weaker with this system, but will carry a low chance of light showers over the eastern zones Monday and Monday night. Zonal upper level flow will prevail across North and Central Texas Tuesday through Thursday with high temperatures ranging from the 60s to lower 70s and low temperatures in the mid 30s to mid 40s. The big weather story is the forecast leading up to Christmas. For several days we've been seeing indications of a long-wave pattern shift across North America with virtually all of the guidance showing a flip from an East Coast trough to either a West Coast or Plains trough. The latter solutions certainly spell colder temperatures for our region. Model guidance is clustered around a strong frontal passage in about 1 week (Thursday night). Upper level ridging across the Canadian Pacific coastline will induce strong anti-cyclogenesis and tap into a pool of arctic air and send it southward into the northern Plains. Just how much of this Arctic air makes it into our region is uncertain and highly dependent upon the exact configuration of upper level pattern and how it evolves. Over the past several days we've seen wild run to run variations of the upper level pattern so it is much too early to say with confidence just how cold it will get or whether there is a threat of wintry precipitation. A stronger and slower evolving trough like what the ECMWF has been showing for several runs means a more shallow and modified cold airmass will succeed the front for the 22nd and 23rd. While a solution like the GFS which quickly digs the upper trough into the Desert SW would bring in a very shallow but arctic airmass right behind the front. Both the GFS and ECMWF forecasts raise our concern for wintry precipitation. The GFS continues to indicate a risk of freezing rain on the 23rd and the ECMWF is now indicating mixed precip on the 24th and 25th. Obviously those forecasts are 8-10 days out and we would be shocked to see a perfect forecast from either model. However the verifying forecast could be in that ballpark, or it could look like the Canadian and most of the ensembles means which are much faster and farther east with the upper level trough. This would bring a deep and very cold airmass into the region with clear skies and no chance of precip. In summary, the key points: 1) Confidence in colder weather arriving in about 1 week is high. 2) There is tremendous uncertainty regarding the post-frontal precipitation potential. 3) As always, more confidence will come with subsequent forecast data. The sooner all computer models settle on a solution the sooner we can provide more definitive forecast guidance. At this time, model spread remains MUCH too high to "sound the alarm" for a winter event. TR.92 && .PRELIMINARY POINT TEMPS/POPS... Dallas-Ft. Worth 36 57 38 59 45 / 0 0 0 10 60 Waco 37 58 34 56 43 / 0 0 0 30 70 Paris 33 54 34 57 43 / 0 0 0 10 80 Denton 30 56 31 59 43 / 0 0 0 10 50 McKinney 32 55 32 57 44 / 0 0 0 10 70 Dallas 38 57 41 60 45 / 0 0 0 10 60 Terrell 33 58 34 57 44 / 0 0 0 20 70 Corsicana 36 57 37 57 45 / 0 0 0 30 70 Temple 40 58 35 54 44 / 5 0 0 40 60 Mineral Wells 29 56 32 59 43 / 0 0 0 10 40 && .FWD WATCHES/WARNINGS/ADVISORIES... None.