645 FXUS64 KMRX 190848 AFDMRX AREA FORECAST DISCUSSION National Weather Service Morristown TN 348 AM EST Wed Feb 19 2020 .SHORT TERM...(Today and tonight)... A cold front was southeast of the Appalachians this morning, with the forecast area lying in the transition between lingering post- frontal moisture and incoming drier continental air driven by a +1040 mb surface high anchored over North Dakota. A few stations were still reporting some light rain as of 2am, but outside of perhaps the mountains, rain should be long over with by the time daybreak rolls around. Fog or low clouds, and perhaps some drizzle, may linger in the mountains through midday, but even there rain should be done with shortly after daybreak as well. While low clouds will slowly clear out, much of the southern zones will remain beneath a cirrus shield today and tonight, influenced by an 80-100kt subtropical jet that will be positioned over the I-20 corridor for the duration of the short term period. In the north, a fair amount of sun seems reasonable for later today. Trended resulting max temps today towards the MET guidance which favors a slightly warmer northern half of the forecast area due to likelihood of more sun. Despite the +1040mb surface high over the northern plains, it's not moving southward especially fast so highs should still be close to 50 today give or take a degree or so. For tonight, the northern plains surface high should drop to around the Omaha area or thereabouts. Cold, dry advection will continue locally. Temperatures tonight should drop into the 30s most everywhere, with areas in the northern valley, southwest Virginia, and also higher elevation locales, dropping below freezing. Dewpoints will also drop significantly, an important factor for the end of the short term and start of the long term periods. Favored the ECMWF for both lows and dewpoints, undercutting the low temps by a couple or three degrees across the board. Meanwhile, some phasing of the subtropical jet and a northern stream jet will occur over the Ohio river valley and eastward through Pennsylvania. This will leave our forecast area beneath the right entrance region, with increasing lift over an intruding cold/dry air mass driven by the advancing surface high pressure center over the plains. Much of tonight will remain dry, but a majority of guidance shows some light precipitation spreading back in from the south as we approach 7am. Despite marginal surface temperatures, substantial cooling aloft will be ongoing and with a significant low level dry layer in place the Wet Bulb Zero heights will be low enough to support perhaps some rain/snow mix all the way to the valley floor. Certainly at higher elevations snow or at worse a rain snow mix will be seen. That said, QPF amounts through the end of the short term are lacking to say the least, so even if snow is in the air there should be no impacts through 7am tomorrow morning. CD .LONG TERM...(Thursday through Tuesday)... A lot to discuss in the extended period with chances of rain and snow, some of which could be accumulating snowfall. Models are in good agreement regarding the overall synoptic pattern but differences are apparent in the overall thermal profiles with the NAM/EURO a bit colder than the GFS/CMC in the Thursday time frame. By 12Z Thursday, the h50 pattern will feature a trough extending from the Great Lakes westward into the Rockies. Nearly zonal h50 flow will traverse the Tennessee valley. At the surface an area of surface low pressure will develop across the Gulf coast with a frontal zone extending northeast and a strong high moving out of the northern plains. The eventual strength of this high will likely have a great influence on how far north the precip can move north and how quickly the cold air will arrive. Favorable upper level dynamics will overspread much of the southeast including the Tennessee valley 12Z onward which along with overrunning will lead to an expansive zone of precipitation from the TN/GA state line southward. It is expected that this precipitation will gradually make its way north through the day Thursday in response to a strengthening 850-700mb frontal zone as the surface low lifts northeast. However as mentioned above the strength of the surface high will play a role in the overall northward progression of the precipitation field. Forecast soundings, and 2-D planviews indicate the thermal profile will be sufficient for much of the precipitation to start as snow before warmer air moves north changing the snow to rain. Much uncertainty still resides in how cold the lower troposphere becomes during the day Thursday. The ECMWF and NAM12 show rather strong CAA as the surface high slides south into the Tennessee valley. This may actually offset the normal increasing diurnal temperature trend. This results in much lower confidence in snow accumulations in the southern and central valley, but believe if any accumulations would indeed occur they would be on grass and elevated surfaces. As far as model guidance is concerned the NAM/EURO are in agreement regarding overall max temperatures in the 925-700MB layer while the GFS and CMC mirror each other more. Decided to favor a model blend of the ECMWF/GFS along with the GEFS means. Ensemble plumes still show a great deal of spread regarding snow amounts but am seeing some clustering around a dusting to a half inch for locations such as CHA, and TYS. Even more spread is being observed across SW Virginia and extreme NE TN where no noticeable clustering can be seen indicating higher uncertainty across these areas. Nonetheless confidence is high enough that most locations will likely see snow fall especially from 12Z - 18Z Thursday as Omega and RH are most favorable in the DGZ during this time period. However, warming near surface temperatures and an increasing February sun angle will be working against heavier snow accumulations especially for the southern and central valley. Areas further north across SW Virginia, the Cumberland Plateau will likely see the snow last longer and could receive upwards of a dusting to a half inch of snow. The higher terrain of the Tennessee mountains will see the heaviest accumulations where 1-3 inches of snow could fall. Confidence in forecast snow amounts are still fairly low, and because of this, decided against issuing any winter weather advisories at this time. Additional changes to the forecast are most certainly going to occur and later shifts may need to issue an advisory across portions of the area. A building ridge across the west will finally kick the trough through the region as high pressure at the surface drops out of the northern plains. This will end precipitation from west to east Thursday night with much colder temperatures forecast. As the surface high moves overhead, lows will bottom out into the upper teens to mid 20s. High pressure at the surface and increasing heights aloft will bring drier conditions to the forecast area Friday through much of Sunday with moderating temperatures as highs on Saturday reach the upper 40s to mid 50s. Meanwhile further west, a closed low will move onshore the west coast on Saturday and begin to open up as it traverses the four corners region. There has been continued disagreement on when the precipitation associated with this feature will begin to move into the area. Began to ramp up PoPs area wide Sunday night and into Monday. Higher PoPs will be observed on Monday when favorable upper level dynamics begin to impinge on the area as the area will be located in a favorable region of a 120 knot h30 jet. Higher PWs will be transported northward as well which could lead to periods of heavier rain on Monday. Temperatures during this time period will likely be near to above normal. AD && .AVIATION... 06Z TAF DISCUSSION. Some light rain/drizzle is holding on across the valley, but this should move east of the terminals by 08z or so. Cold/dry air advection has bumped ceilings up from IFR/LIFR to a mix of VFR/MVFR across the region, but expect them to settle back to MVFR levels for the remainder of the night. Improvement to prevailing VFR levels is expected within a few hours after daybreak. Winds will north to northeasterly through the period, generally below 10kt. CD && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 52 39 42 26 45 / 10 60 70 10 0 Knoxville McGhee Tyson Airport, TN 52 36 41 23 41 / 10 40 70 20 0 Oak Ridge, TN 53 36 41 23 42 / 10 20 60 10 0 Tri Cities Airport, TN 50 32 39 20 40 / 10 10 50 20 0 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$