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

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827 FXUS64 KMRX 210829 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 329 AM EST Sat Jan 21 2017 .SHORT TERM /Today through Sunday Night/...
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A threat of severe thunderstorms, locally heavy rainfall and flooding are possible later this afternoon/evening and again Sunday and Sunday night. A lead shortwave trough will quickly shift northeast into the Southern Appalachians late this morning, bringing a broad area of scattered showers and thunderstorms across the region from the late morning through the mid-afternoon, before exiting the far northeastern zones late in the day. Attention will then shift to the SW as a rapidly NE advancing warm front will move into the southern Tennesse Valley. Depending on how much clearing can occur behind the first wave of precipitation, the atmosphere across the southern Plateau and Valley (and perhaps as far north as the I-40 corridor) will be able to destabilize somewhat ahead of the next round of storms that will roll in during the 06-12z time period early Sunday morning. Progged CAPE values off of the model KCHA soundings are as high as 1500 j/kg, with strong 0-3 km speed/directional shear. This will allow for some strong to severe thunderstorms within this warm sector. Damaging winds and marginally severe hail are the main threats with the strongest activity, but a very isolated tornado cannot be ruled out (mainly along the far Southern Plateau/Valley) due to the uncertainty of how far north the warm front will make it. For Sunday, the aforementioned wave of showers and storms will continue it`s march through the area during the morning hours.Coinciding with this is a strengthening LLJ along the mountains which will lead to some breezy/gusty southerly winds during the day on Sunday. These downslope winds will help to keep high temperatures on the mild side despite the precipitation and cloud cover. To the west, a deepening closed upper-low will quickly race toward the Southern Appalachians, bringing a third and much longer duration shower and thunderstorm event late Sunday afternoon into Sunday night. Given the strong shear, along with some marginal instabilty in place, the potential for strong to perhaps severe thunderstorms will exist once again. This will particularly be true if the track of the upper-low shifts a little more to the north. However, the primary concern with this deepening upper-level low pressure system is heavy rainfall and the potential for localized flash flooding (particularly along the Great Smoky Mountains). There are several concerns regarding this system with respect to the heavy rainfall potential. First, a deep, moist southerly flow off the Gulf of Mexico parallel to a slow-moving northeastward tracking cold front. This could result in training of clusters of showers and thunderstorms Sunday night into Monday. Also, A deep warm cloud layer, with some appreciable instabilty to support organized updrafts. Widespread storm total QPF amounts from tonight through Monday show widespread 2 to 3 inches of rainfall across much of the CWA, with locally higher amounts possible along the East Tennessee Mountains. Though it is a little early for a watch, we have opted to issue a Hydrologic Outlook and have mentioned these threats in the HWO as well. .LONG TERM /Monday through Friday/... The deep upper low will slide further northeast into the Mid Atlantic on Monday. Rain showers will continue across much of the area as moisture continues to be slung around the top of the low and back into our region. As the low lifts further north Monday night into Tuesday, rain showers will gradually taper off from southwest to northeast. Winds will become quite breezy on the backside of the low, especially along the Smokys where winds may approach advisory levels by Monday night. Will mention gusty winds in the HWO. As cooler air funnels in Monday night, a brief period of light snow is possible across the highest elevations of the Smoky Mountains. Weak ridging will build in for the second half of Tuesday, bringing a brief period of dry weather. Southwest flow on Wednesday will warm highs back into the 60s for one more day, before a cold front sweeps through the area Wednesday night into Thursday and temperatures turn colder. From here, model solutions diverge, with the GFS showing a much wetter frontal passage than the drier ECMWF. For now, have broadbrushed slight chance to chance PoPs Wednesday and Thursday in association with this feature. A few snowflakes may mix in Thursday morning across higher elevations as colder air once again works into the area. It will turn much cooler for the end of the week as a deepening upper trough builds over much of the country. A weak shortwave will ripple through this trough Thursday night into Friday. There`s not a lot of moisture associated with it, but with predominately northwest flow there should be enough for some orographically driven snowfall across the higher elevations. Some snowflakes may mix in even at lower elevations with temperatures dipping to near freezing Friday morning, but no accumulation is expected in the valleys at this time.
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Chattanooga Airport, TN 66 55 63 47 / 60 80 90 90 Knoxville McGhee Tyson Airport, TN 65 53 65 49 / 50 70 80 90 Oak Ridge, TN 65 53 64 49 / 50 70 80 90 Tri Cities Airport, TN 64 50 65 48 / 50 50 80 100
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