Area Forecast Discussion
Issued by NWS Knoxville/Tri Cities, TN

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000 FXUS64 KMRX 191129 AFDMRX AREA FORECAST DISCUSSION National Weather Service Morristown TN 729 AM EDT Mon Apr 19 2021 .UPDATE... FOR 12Z AVIATION. && .SHORT TERM...(Today and tonight)... A mostly dry short term period with slightly below normal temperatures. Any lingering showers across the north associated with the exiting mid/upper level short wave will quickly end this morning. There will be more clouds north than south to start, with a gradual clearing trend from southwest to northeast during the day as drier air moves in. We will see quasi-zonal flow aloft tonight with surface high pressure over the region. The sky will be mostly clear for much of the night, although some increase in high clouds will be noted. Temperatures both today and tonight will generally run just a bit below seasonal normals. LW .LONG TERM....LONG TERM...(Tuesday through Sunday)... Key Messages: 1. Warm and dry conditions Tues give way to a cold blast Tues night and Wed with scattered showers mixing with a bit of snow in the mountains of E TN and SW VA. 2. Widespread frost/freeze is likely Wed night and Thurs night. 3. Temps 10-15 degrees below normal continue Thurs moderating to a few degrees below normal Fri through the weekend. 4. Next chance of rain is Fri night into Sun morning, but exact amounts remain low confidence. Discussion: Tuesday through Wednesday... A big change will arrive during this time as a strong mid/upper trough dives into the Midwest and western Great Lakes Tues and tilts slightly negative late in the day. This will set up a tight isothermal gradient over the upper OH Valley where a surface low will develop along a strong cold front Tues evening and deepen to about 998 mb as it lifts into NY and New England Wed. This will push the strong cold front rapidly southeastward across the TN Valley and S Appalachians Tues night. Ahead of the front, deep southwesterly flow will boost 850 mb temps into the 7-9 C range across our region Tues, and with mostly sunny skies and decent mixing anticipated in the warm sector, raised highs several degrees above the NBM yielding mid/upper 70`s. The deterministic guidance shows slight differences in the timing of the frontal passage Tues night with the GFS, ECMWF, and CMC slightly faster than the NAM which does not have the front clearing the area until after 12Z Wed. However, the 00Z runs are coming into better overall agreement, so expect a fropa in the 09- 12Z timeframe. The best upper jet support will be in the upper OH Valley and Great Lakes near the developing surface low leading to weaker frontogenetic forcing across the S Appalachians Tues night, so expect precip to be a scattered/broken band of light showers with QPF under 0.10 inches in most places Tues night through Wed morning. Strong CAA Wed morning will tank 850 mb temps to -3 to -6 C. Moisture will be exiting quickly, but a brief transition to a rain/snow mix is expected across the mountain peaks of E TN and SW VA where a trace to 0.3 inches could accumulate Wed morning. Dry air advection will allow all precip to end Wed afternoon. Highs Wed will be limited to the low/mid 50`s as the mid/upper trough axis digs into the OH and TN Valleys with continued strong CAA. This is 15-20 degrees below normal. Wednesday Night through Friday... Quieter but chilly weather for this period as the deep trough aloft gradually shifts into the NE CONUS and a large surface high drops into the OH and TN Valleys. This will maintain a winter-like NW flow pattern. Strong radiational cooling beneath the building surface high Wed night will yield lows in the low/mid 30`s with a few upper 20`s in SW VA and far NE TN leading to widespread frost/freeze headlines. Highs Thurs will only reach the mid/upper 50`s in most areas except for a few low 60`s in the far S Valley. This is still 10-15 degrees below normal. Strong radiational cooling again Thurs night will drop lows into the low/mid 30`s except a bit milder in the far S Valley, so a second night of frost/freeze headlines is likely. Heights begin to increase ahead of a shortwave trough moving through the 4 corners region Fri, and as the surface high shifts into the Carolinas, expect highs to rebound into the low/mid 60`s which is still 5-7 degrees below normal. Friday Night through Sunday... The aforementioned mid/upper shortwave trough over the 4 corners will eject into the S Plains by Sat as a separate northern stream trough progresses toward the Great Lakes. The deterministic ECMWF and CMC are still faster to phase these two features compared to the GFS which holds off on phasing until later in the weekend. This continues the trend of the ECMWF and CMC being wetter Fri night through Sat night than the GFS which keeps the greatest rainfall axis to our S. Nevertheless, confidence is increasing that this will be a fairly fast moving system which will keep rainfall amounts manageable no matter which solution verifies. Followed the GEFS/Euro/Canadian ensemble means which are very similar to the NBM giving chance PoPs Fri night expanding to likely Sat, then decreasing to chance/slight chance Sat night. A few spotty showers could linger Sun morning, but a mainly dry day is expected as a surface high builds in. Temps will continue to average a few degrees below normal next weekend. Garuckas && .AVIATION... 12Z TAF DISCUSSION. Some light showers will be around early in the period mainly TRI, so will include a brief TEMPO group there. Clouds will gradually decrease, with VFR conditions expected all sites for the period. Winds will generally be less than 10kts during the day from the north and west, then will be light overnight. && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 70 47 77 45 58 / 10 0 0 10 0 Knoxville McGhee Tyson Airport, TN 67 44 75 44 54 / 10 0 0 20 10 Oak Ridge, TN 68 43 75 43 54 / 10 0 10 30 10 Tri Cities Airport, TN 64 39 72 42 53 / 20 0 10 20 30 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$

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