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

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000 FXUS64 KMRX 201922 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 322 PM EDT Sat May 20 2017 .SHORT TERM (This Afternoon through Sunday)...
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The upper level ridge has held strong across the forecast area today with only a couple of isolated showers/storms so far. Expect this will continue to be the story throughout the remainder of the afternoon with the main surge of moisture expected after sunset tonight. Over the last hour or so, storms have pulsed up to severe limits at times across NE TN and SW VA and expect this will be possible over the next couple of hours. Current water vapor imagery and RAP upper air analysis indicates a closed low spinning across the Central Plains with a ridge stretching northward from the Carolina coast into the Great Lakes. Another shortwave trough can be noted moving northward across Mississippi. The surge of moisture will occur with this wave with PW values increasing into the 1.6-1.8 inch range later tonight into early Sunday morning. These values are near the maximum value observed for mid-May and have increased the PoPs and QPF values during the overnight timeframe. Due to highly saturated nature of the atmosphere, mid-level and upper level lapse rates are not overly impressive and are near moist adiabatic. Therefore, expect the chance for any strong to marginally severe storms overnight is very low at this time but there could still be a stronger storm or two with gusty winds being the main threat. The high PW values and persistent moisture advection ahead of the front will lead to QPF values in excess of 2 inches in a few locations and expect localized flooding will be possible. Currently 3-hr FFG is over 3 inches for most locations and will forgo issuing a Flash Flood Watch but will continue to monitor things throughout the overnight hours and into the day on Sunday. The cold front will remain to the west of the area throughout the day on Sunday with showers and storms continuing. PW values remain elevated in the 1.5-1.7 inch range. Therefore, have continued with 70-90 PoPs. Once again, SBCAPE values should remain below 1000 J/Kg during the day on Sunday with 0-6 km Bulk Shear values around 20-30 Kts. Overall, this would suggest maybe a stronger storm or two again but no severe weather is anticipated. Widespread cloud cover and precipitation will lead to cooler temperatures on Sunday and have reduced highs by a couple of degrees with highs generally in the mid to upper 70s for most valley locations. .LONG TERM (Sunday night through Thursday)... Cold front will be sweeping through the area overnight Sunday night into Monday morning and as it moves through precipitation will come to an end Monday from the west to the east. We will get a quick peak of sunshine on Monday as drier air comes in behind the front, but this will not last for too long before out next system. Unfortunately the long term models diverge on how to handle this next system. They all forecast a surface low to slide northward along the stalled frontal boundary, but they disagree on the strength/timing of the low. This leads to discrepancies with how much precipitation might move into our area. Best chances look to be in the Southern Appalachian Mountains and to their southeast, but we can`t rule out widespread precipitation moving into the Tennessee Valley. If this system does bring widespread rain to our area we could get flooding depending on how much precipitation falls this weekend. At this time went with lower end precipitation chances since models haven`t really been consistent with this system, and will adjust as we get closer. A secondary front will move through Wednesday/Thursday and bring another round of rain to the area. This system looks to move through quickly and dosen`t look to bring much in the way of widespread thunderstorms. After continuously cooler temperatures with these few systems we will see our "coldest" day on Thursday with temperatures expected to be about 10 degrees below normal. A quick warmup will begin at the end of the work week and into the weekend getting us back above normal.
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Chattanooga Airport, TN 69 77 63 80 / 80 90 70 20 Knoxville McGhee Tyson Airport, TN 68 77 60 77 / 80 90 70 20 Oak Ridge, TN 69 76 61 79 / 80 90 70 20 Tri Cities Airport, TN 65 74 60 76 / 50 80 70 30
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