KMRX issues Area Forecast Discussion (AFD) valid 2019-12-10T16:30:00Z 294 FXUS64 KMRX 101630 AFDMRX AREA FORECAST DISCUSSION National Weather Service Morristown TN 1130 AM EST Tue Dec 10 2019 .DISCUSSION... A arctic cold front continues to surge through the region this morning, generally moving into/through the east TN mountains at this time. Behind the front, mostly cloudy skies and brisk nwly flow prevails with some gusting upward of 20mph observed. Likewise, cold advection is also underway favoring a rapid non- diurnal temperature drop with freezing surface temperatures already moving into the plateau, eventually spilling eastward into the valley and Southern Appalachians. As for wintry precipitation, not much has changed with the newest guidance suite as a rain/snow (with some occasional sleet) mix will march east through the afternoon/evening before a phase change to all snow occurs later tonight. Regarding accumulations, guidance does look to dry out the region slightly faster than previous runs, which would suggest a very minimal downward shift in the previous snowfall accumulation forecast, which was accepted in the update. The only fly in the ointment will be the late evening/overnight upper divergence that looks to traverse atop the area associated with the entrance region of the upper jet, which would favor a brief period of enhanced lift and thus precipitation in the deep cold air. Confidence is fairly high in this occurrence, however confidence is low regarding how much moisture will be left behind to sustain decent precipitation rates. Will take another look at additional guidance later in the day. At this time, no advisories or warnings are planned. CDG && .AVIATION... 12Z TAF DISCUSSION. Flight categories will remain low, generally MVFR or IFR, throughout the day as waves of precipitation moves through. They may bounce up briefly to VFR levels at times, but expect the prevailing category to be MVFR in nature. A transition from rain to snow will occur across the region later today behind a cold front that is currently moving through the TN valley. KTRI and possibly KTYS are the most likely to see any snow, with lesser chances at KCHA. All precip should be over with between 06z-12z depending on location, and TAFs reflect this. Confidence in how quickly MVFR ceilings will clear out once precip ends is pretty low, so kept those in through the end of the 12z TAF period for now, but expect around or shortly after 12z Wed they will clear up. CD && .PREV DISCUSSION... /ISSUED 648 AM EST Tue Dec 10 2019/ SHORT TERM (Today and tonight)... The most pressing matter in the short term period remains the chance of some wintry precipitation, and possible light accumulations of snowfall across the region. Bottom line up front here, we will cover this event with a Special Weather Statement and mention in the Hazardous Weather Outlook. Granted, there is a high degree of confidence that snowfall will actually occur. But, given some of the changes over the last 24 hours in model guidance, the fact that the duration of precipitation in sub- freezing air will be relatively short lived, and the exceptionally warm temperatures ahead of this system, the impacts from this event are likely to be relatively negligible. Light accumulations in grassy areas and just plain wet streets/roadways seem like the most likely and reasonable outcome. An advisory simply does not seem justified at this time. For more explanation, please continue on. A strong cold front was pushing through middle Tennesse early this morning, and will continue moving east into the forecast area just after daybreak. Ahead of the front, much of eastern Tennessee remained in, or actually warmed into, the upper 50s and low 60s during the overnight hours. Quite impressive for December actually. In any case, with the front arriving during the morning hours, temperatures will fall through the day today. Sub-freezing air will arrive in the plateau and areas in southwest Virginia near the Kentucky border during the late afternoon hours, then spill east to the east Tennessee mountains and finally into the Tennessee valley during the late evening and first part of the overnight time frame. This will set the stage for our winter weather event. Regional radar showed a marked uptick in rain coverage across northern Alabama and Georgia, and into southeastern Tennessee, over the last few hours. This was in response to the approaching cold front and also strengthening jet support encroaching from the northwest. This trend will continue through the afternoon and evening hours before the cold front shifts completely east of the forecast area. What remains will be more stratiform precip associated with elevated frontogenetic forcing and right entrance region jet support. This is type of precip shield is already present to our northwest over central Kentucky and into West Virginia, and will expand southeast with time as the tail end of the upper jet pivots through the forecast area. This will begin to occur around 03z (10pm) this evening, with the strongest influence and subsequent precipitation rates, between 03z-09z or thereabouts. By most guidance' accounts, by daybreak Wednesday this is all wrapping up and pretty much over with. Model soundings across the forecast area show support for a rain/snow mix over the plateau in the 18z-21z (1PM - 4PM EST) range, with all snow by 00z or so. In the valley that same process will be delayed by roughly 3-4 hours. The uncertainties lie in exactly what kind of precipitation rates and amounts we will see during the times where snow is favored as a precip type. These trends have been downward overall, and the forecast reflects this. Generally speaking, one to one and a half inches are expected in the plateau, primarily in the north, with 1-3 inches in the higher terrain of southwest Virginia and also the east Tennessee mountains. In the valley, perhaps a few one inch totals will be seen near the Virginia border, but most places will see only a few tenths of an inch. One important caveat to note here, all of this is expected to end by 4am-7am EST, and with ground temperatures being quite warm, once there is any lessening of precip rates it will likely begin to rapidly melt off roadways. For areas receiving accumulations upwards of an inch, people will probably wake up to see some snow in the grass and not much else. Elsewhere, one may be hard pressed to tell it snowed at all by the time daylight comes around. All things considered, the decision was made to cover this event with a Special Weather Statement. While literal snowfall totals may support a winter weather advisory, the impacts are likely to be negligible so this scenario doesn't seem to warrant the issuance of an advisory at this time. CD LONG TERM (Wednesday through Monday)... Dry weather greets the opening periods of the extended. Precip then returns Thursday night as a progressive mid latitude trough rolls past the MS Valley. ECMWF looks more like last night's GFS, but the latest GFS is weaker and further north too. Long story short though is that both now bring precip into the region Thursday night. But the cold air is not quite as strong either, so brief window of wintry weather in our northern areas is looking a little weaker as well. Once temps warm up diurnally during Friday, looks like most or all precip thereafter will be liquid rain with minimum temps remaining above freezing through the weekend. By Saturday a stronger H5 trough crosses the Ohio Valley, whose westerly flow will scour out the precip Saturday night. The next system in the westerlies will bring precip back into the region Monday. GC && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 57 32 47 30 52 / 100 80 10 0 10 Knoxville McGhee Tyson Airport, TN 57 30 44 27 50 / 100 80 10 0 10 Oak Ridge, TN 56 28 44 27 49 / 100 80 10 0 10 Tri Cities Airport, TN 54 29 40 23 48 / 100 80 10 0 10 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$