869 FXUS64 KMRX 231041 AFDMRX AREA FORECAST DISCUSSION National Weather Service Morristown TN 641 AM EDT Fri Apr 23 2021 .UPDATE... FOR 12Z AVIATION. && .SHORT TERM...(Today through Tonight)... Key Messages: 1. Warmer on Friday despite increased cloudiness with low rain chances. 2. Rain chances increase from the west Friday night into Saturday morning. The pattern this morning is highlighted by quasizonal H5 flow across the TN Valley as troughing deepens over the Rockies into the Four Corners. At the surface, high pressure prevails over the Appalachians beneath confluent flow aloft. Currently, irsat indicates intrusion of high clouds from the west along the mean H5 flow into the OH/TN valley region, all while a few remnant low cu prevails in the valley. As for temperatures this morning and the current frost/freeze headlines, trends still look to be on track, however with some uncertainty regarding lows around daybreak given increasing sky cover. Still, with light/calm winds and much of the night being mostly clear, no changes planned on that front. Moving into/through Friday, the Rockies trough will continue to deepen as will consequent surface cyclogenesis over the Desert Southwest into West TX/OK later this morning. In response, moisture advection out ahead will setup a broad baroclinic zone across the lower MS Valley. Meanwhile downstream, a weak upper impulse will slide through the Midsouth ahead of the primary upper trough into the late morning hours. With that, guidance does hint at the possibility of light rain amidst low end pops across the plateau, which where included. Confidence is low however given lack of moisture depth and dry llv air. After the passage of this weaker initial wave, skies will look to briefly sct a bit, yet will fill back in later in the evening as the deeper system advects quickly east. High temperatures on Friday will warm a few degrees from yesterday, yet still checking in 5-7 degrees below normal. Overnight the upper system will shift across the Midsouth into the OH Valley with the surface wave traversing along the I20 corridor across AL/GA. Light to moderate rain showers will accompany this feature suggesting increasing pops overnight into Saturday morning, with sely winds picking up over the mountains as well. CDG .LONG TERM...(Saturday through Thursday night)... Key Messages: 1. Rain chances increase on Saturday with the potential for areas of heavy rainfall mainly south of I-40 and east of I-75. 2. Strong winds are possible in the Tennessee mountains and adjacent foothills Saturday morning into the afternoon. 3. Dry weather with a warming trend is expected into next week before chances for rain return by Wednesday night. Saturday... Forecast guidance is in good agreement regarding the large scale pattern but offer some differences in northern extend of the LLJ which may have some implications on how far north the heavier rain can reach. By Saturday morning, an h50 shortwave will be lifting out of the central plains with an eastward trajectory through the day. An area of robust upper lvl PVA along with upper lvl divergence will allow widespread rain showers to overspread much of the the southeast during the day. PW values in excess of 1 inch along with strong theta-e advection along the nose of a strong LLJ will allow for periods of heavy rainfall during the day. Most of this activity will be confined to the southeast portions of the forecast area such as across extreme southeast Tennessee, and southwest North Carolina. Storm total rainfall amounts will range from 1 to 2 inches in these areas with a half to one inch elsewhere across the forecast area. Extensive cloud cover and expected rainfall will keep temperatures in the upper 50s/low 60s across the higher terrain and northern valley to the mid 60s to near 70 for the central and southern valley. The other forecast challenge for Saturday is the potential for a mountain wave event and strong winds across the high terrain. Some differences in the placement of the LLJ provide some questions regarding the wind potential. The GFS is weaker regarding 850mb wind magnitude while the ECMWF is much higher. Both offer a favorable trajectory of 850mb flow that is orthogonal to the spine of the Appalchians. While some uncertainty exists, feel that there remains enough confidence to issue a high wind watch for our mountain zones of Monroe through Carter on Saturday. Will also maintain mention of this in the HWO. Saturday night through Wednesday... The upper lvl shortwave exits the area Saturday night with much drier air moving into the region. At the surface, high pressure will dive into the Midwest providing decreasing rain chances. Weak upper lvl ridging develops by the end of the weekend and continues into next week. This along with surface high pressure will continue to provide dry weather conditions with a gradual warm-up. On Monday, the surface high moves off into the mid Atlantic allowing sfc flow to veer toward the south. Aloft, amplification of the flow develops with a sharp ridge developing from the southeast into the Great Lakes. Tuesday and Wednesday will be the warmest days of the week with 850mb temperatures in the 14-16C range, translating to highs in the low 80s across many areas. Thursday and Thursday night... Large amplification in the upper lvls continue toward the end of the period. A deep disturbance moves into the Ohio Valley and Great lakes with a trailing cold front moving through the Mississippi River Valley and Tennessee Valley on Thursday. Rain chances will be on the rise with high temperatures just a bit cooler with the expected cloud cover and precipitation. Diegan && .AVIATION... 12Z TAF DISCUSSION. VFR at all sites through the first three quarters of the period, then MVFR late. Moisture will continue to increase with time as an upper system approaches from the west. Continued with lowering cigs from mid/high VFR to low/mid VFR with 5-8kt swly flow at all sites through this evening, with further lowering to MVFR closer to daybreak Saturday as shra moves in from the southwest. CDG && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 67 50 70 51 72 / 10 90 90 40 0 Knoxville McGhee Tyson Airport, TN 63 47 66 50 68 / 10 70 80 50 0 Oak Ridge, TN 63 46 64 49 69 / 20 70 80 50 0 Tri Cities Airport, TN 61 43 61 48 65 / 10 20 90 70 0 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...Frost Advisory until 9 AM EDT this morning for Cherokee-Clay. TN...Freeze Warning until 9 AM EDT this morning for Blount Smoky Mountains-Claiborne-Cocke Smoky Mountains-Hancock-Hawkins- Johnson-Northwest Carter-Northwest Greene-Sevier Smoky Mountains-Southeast Carter-Southeast Greene-Sullivan-Unicoi- Washington. High Wind Watch from Saturday morning through Saturday evening for Blount Smoky Mountains-Cocke Smoky Mountains-Sevier Smoky Mountains-Southeast Carter-Southeast Greene-Southeast Monroe-Unicoi. Frost Advisory until 9 AM EDT /8 AM CDT/ this morning for Anderson-Bledsoe-Campbell-Grainger-Hamblen-Jefferson-Knox- Loudon-McMinn-Meigs-Morgan-North Sevier-Northwest Blount- Northwest Cocke-Northwest Monroe-Rhea-Roane-Scott- Sequatchie-Southeast Monroe-Union. VA...Freeze Warning until 9 AM EDT this morning for Lee-Russell-Scott- Washington-Wise. && $$