DFW FORECAST DISCUSSION
521 FXUS64 KFWD 131242 AAA AFDFWD Area Forecast Discussion...UPDATED National Weather Service Fort Worth TX 642 AM CST Thu Dec 13 2018 ...Winter Weather Advisory West...Strong Gusty Winds to Impact Much of the Region... .AVIATION... /12Z TAFs/ A powerful storm system will bring low ceilings, thunderstorm chances, and strong gusty winds. Patchy showers this morning are helping to moisten the low-levels, and transient MVFR ceilings will become more persistent as the day progresses. MVFR visibility in the soupier air across Central Texas will improve as the boundary layer warms and wind speeds increase. As an intense cyclone approaches from the west, strong forcing for ascent will clutter the radar scope with convection. There will be a window for thunder late in the day, but the stronger updrafts will be east of the Metroplex TAF sites into the evening hours. Steadily veering winds will become westerly and increase in speed toward nightfall. Speeds will continue to increase tonight, and the wind speed criterion for an airport weather warning may be met during the overnight hours. Despite the intensity of the wind, the direction should pose minimal crosswind issues. Snow is expected on the higher terrain of western North Texas, but as the 850mb low reaches the I-35 corridor this evening, the distance between the snow production zone aloft and the surface will increase. As a result, little if any snow is expected at our TAF sites. Any ambitious wet flakes that do reach the surface would quickly melt, and no impacts from wintry precipitation are expected. The snow potential will end during the daylight hours of Friday morning. Although the rain will taper off, the gusty winds will continue throughout the day. 25 && .SHORT TERM... /Issued 457 AM CST Thu Dec 13 2018/ /Today Through Friday/ The strongest storm system in recent memory will impact North and Central Texas today and tonight. Severe thunderstorms will be possible in the warm air ahead of a cold front that will usher in extraordinarily gusty winds. Although the postfrontal air will not be particularly cold, intense dynamic cooling within the core of the cyclone will produce some snowfall, particularly across the higher terrain of western North Texas where a winter weather advisory will be in effect. While this storm system is still intensifying in New Mexico, an unrelated shearing impulse is tracking through the eastern half of the state. Although it has been gradually dissolving into the subtropical jet downstream of the more potent low, it has maintained enough fortitude to generate some elevated thunderstorms within North Texas. However, its potency has been most effectively realized with the richer moisture across Southeast Texas, the ongoing complex within which will track into East Texas this morning. Outside of the isolated storms, the echoes within North and Central Texas have yielded little rainfall at the surface overnight, but this has served to moisten the column in advance of the stronger system still upstream. The remainder of the morning will be the proverbial calm before the storm. But as strong forcing for ascent begins spreading across the region this afternoon, showers and thunderstorms will proliferate across North and Central Texas. The most favorable area for strong/severe thunderstorms will be across our eastern and northeastern zones, but the boundary layer instability will likely be tempered by the morning precipitation and lingering cloud cover. While the best shear will lag well behind the best instability, the increasing vorticity as the synoptic-scale cyclone approaches will enhance updraft strength and hail growth potential. However, the quality of boundary layer parcels will be diminished both by morning rain cooling the near-surface air and the initial system`s disruption to the low-level pressure fields. Based on the track of the surface low, our northeast zones will see the greatest backing of the surface winds, but the tornadic potential will be dampened by the quality of the boundary layer parcels and the rapid storm motion that will tend to limit the lifespan of individual updrafts. An attendant wind threat may result from the downward transport of stronger momentum aloft. Within the northwest quadrant of the approaching cyclone, winds at the 925mb level will exceed 50kts. Some of this postfrontal momentum will reach the surface in our western zones this afternoon, then spread across the region from west to east through tonight. Although the track of the surface low, which will be right through the heart of our CWA, will initially limit the wind speeds, the strong winds will eventually envelop the bulk of the region. The wind advisory area will remain intact; high wind warning criteria (sustained 40 mph, gusts to 58 mph) are unlikely to be met. While these winds will be of lesser strength than those that accompany a severe thunderstorm, the duration of the winds today will increase the likelihood of downed tree limbs, power outages, and other wind damage. Secure any outdoor furniture and holiday decorations. Most winter weather events in North Texas involve an arctic air mass, but this event will be entirely driven by dynamic cooling within the core of this powerful cyclone. Despite the intensity of the system, the snowfall forecast is by no means a slam dunk; it may be more akin to a blindfolded slam dunk that could go terribly wrong. Guidance is in remarkably good agreement with the large scale features, including the track of the 850mb low. But with a potentially unprecedented scenario within the working history of numerical guidance, this consensus can`t preclude a deviation in the eventual path. Regardless, confidence continues to improve in accumulating snow in our western zones late this afternoon and into tonight. It`s not often we talk about the elevation of the snow line in a winter weather forecast, but it will be a critical piece of the puzzle today. The favored areas for snow at the surface will initially be the higher terrain of the Big Country to our west, then gradually spread east. Even if snow is mentioned in the forecast for your area, rain may still be the dominant precipitation type, particularly early in the event and along the fringes of the wintry weather potential. Heavier bursts of snow may accompany the core of the low, but surface temperatures above freezing and low snow/water ratios should keep totals under our warning criteria (4 inches in 12 hours), hence the advisory rather than a warning. East of the advisory area, snow production aloft will continue within the core of the cyclone, but warmer temperatures within a mile of the surface will enhance melting, and little if any snow should reach the surface along and east of the I-35 corridor. For areas that see significant snow, visibility will be limited with strong wind scattering the falling snow, but once on the ground, the heavy wet snow will be less willing to blow around. Lightning has already accompanied some of the snow in West Texas, and thundersnow will be possible within North Texas as well. These heavier snow showers will be more likely to accumulate and may even do so on roadways. Roads that accumulate snow, particularly those more heavily travelled, should remain slushy. However, the snow cover may allow temperatures to dip below freezing Friday morning, and the morning commute could feature some icy bridges and overpasses in the advisory area. 25 && .LONG TERM... /Issued 457 AM CST Thu Dec 13 2018/ /Friday Night Through Wednesday/ Our vigorous low pressure system will continue to move readily east into the Lower Mississippi Valley Friday night, then across the Southern Appalachians and southeast states on Saturday. The residual and deep cyclonic north-northwest flow in wake of this system will easily be able to translate momentum down to the surface and maintain gusty northwest winds 15-25 mph over most of the CWA Friday evening, before decoupling of the much drier airmass and a slow-relaxing pressure gradient allows winds speeds to diminish by Saturday morning. The northwest winds will remain brisk east of I-35 on Saturday, as similar processes of mixing keep speeds up to at least 10-15 mph. Otherwise, ridging aloft and associated strong subsidence will allow for plentiful sunshine, though colder conditions this weekend with lows in mainly in the 30s with highs struggling into the 50s Saturday, but modifying into the lower-mid 60s on Sunday. One quandary will be if we have any lingering effects from snowfall over our current Winter Weather Advisory area and impacts on both low and high temperatures. For now, I did go slightly cooler, but not much cooler without really knowing how this all plays out. Moving into the early part of next week, a strong northern stream shortwave disturbance will move southeast from the Central/Northern Plains into the Mississippi and Tennessee Valleys. This system will dampen the shortwave ridge overhead and allow for a reinforcing shot of cooler, but not extremely cold air southward. Meanwhile, mid level energy will amplify southward across the Eastern Pacific Ocean early next week and push a broad and weak lead shortwave within the southern branch of the westerlies toward West Texas. The reinforcing cooler airmass from the north and increasing mid-high cloudiness arriving from the west will likely mean slightly cooler days, especially across the northeast half of the CWA Monday-Tuesday. Modified low level moisture return and the return of southerly breezes will help low temperatures remain more mild between 35-45 degrees. Otherwise, I did leave some low chances for showers in the mid- late week period, as divergent southwest flow aloft occurs overhead in advance of an organizing system over the Rockies and Desert Southwest. Medium range models and their ensemble members continue to struggle with the energy within the main trough to our west. In addition, uncertainty on our airmass modification and resulting instability has me leaving any mention of thunderstorms out of the forecast(for now). We`ll focus more on this system this weekend and beyond once we have our current system long gone. It does appear it will be much milder mid-late next week with no concern for any wintry precipitation at this time. South-southwest low level winds, with plentiful cloud cover associated with the developing southwest flow aloft will generally hold low temperatures in the 40s, while high temperatures only modify into the upper 50s to mid 60s. 05/ && .PRELIMINARY POINT TEMPS/POPS... Dallas-Ft. Worth 62 40 49 38 59 / 80 90 70 5 0 Waco 63 39 49 38 59 / 50 80 50 5 0 Paris 63 44 49 38 56 / 90 90 70 30 0 Denton 60 39 49 36 59 / 80 90 70 5 0 McKinney 63 40 48 37 56 / 90 90 70 10 0 Dallas 64 40 49 38 59 / 80 90 60 10 0 Terrell 65 42 50 38 57 / 80 80 60 10 0 Corsicana 65 40 50 38 56 / 50 80 50 5 0 Temple 63 37 49 38 59 / 30 80 40 0 0 Mineral Wells 55 35 44 33 56 / 80 80 60 5 0 && .FWD WATCHES/WARNINGS/ADVISORIES... Wind Advisory from 3 PM this afternoon to noon CST Friday for TXZ092>095-103>107-118>121-133-134-145-146-158>161-174-175. Winter Weather Advisory from 3 PM this afternoon to 9 AM CST Friday for TXZ100-101-115-116-129-130-141>143-156. Wind Advisory from noon today to noon CST Friday for TXZ091- 100>102-115>117-129>132-141>144-156-157. && $$ 25/05