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

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735 FXUS64 KMRX 211922 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 322 PM EDT Fri Jul 21 2017 .SHORT TERM (Tonight and Saturday)...
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The southern sections of our area should have the best chances of showers and storms through the evening, with most activity being diurnally driven, ending before midnight. However, as the mid/upper level ridge to our west drifts slightly to the south tonight, convection across IN may track in a more SE direction and into WV/SW VA late tonight. The NAM may be overdone with sustaining convection overnight, but the RAP supports it and to a lesser extent, the HRRR as well. So a slight chance PoP will be kept for SW VA overnight. On Saturday we will have very little forcing and a weak cap in place, so most convection will likely be terrain-based and diurnally-driven. Temperatures tomorrow look pretty similar to today, but with slightly higher dewpoints, heat index values near or a bit above 100 are expected in the central and southern TN Valley. .LONG TERM (Saturday night through Friday)... A weak upper level high pressure ridge will move south through the Mississippi Valley through the beginning of the work week. At the same time...a weak low level boundary will move south through the Ohio Valley and just to the east of the upper level high pressure. Models have moved this boundary through the area a little faster...bringing the rain chances beginning early Sunday afternoon and lasting through monday afternoon and evening. Ample instability is present Sunday with greater than 3000 J/kg expected...along with PW values around the 90th percentile for this time of year. With low level flow from the SW...mid level flow from the west...and upper level flow from the NW...decent shear is present. However winds will be far too weak to influence storm structure or motion. For Sunday afternoon and evening...can expect scattered strong to isolated severe showers and thunderstorms for downburst winds and large hail...with the greatest threat in areas north of I40. With the large PW values for this time of year...and the mid level flow parallel to the boundary...could see the possibilty of localized flooding in many areas for storms training over the same locations. Monday...models disagree on timing of the frontal boundary moving SE of the southern Appalachians. Right now...have left rain chances in through the day. Instabilities and winds will be much weaker on expecting scattered showers and thunderstorms...with a chance for a few strong storms but nothing severe expected at this time. There does remain a threat for localized flooding with storms that move across the same area as PW values remain high. Coverage of showers and storms is expected to weaken overnight...with only a few showers remaining along the higher elevations. Tuesday...scattered showers can be expected in the higher elevations of E TN and areas south of I40 where forcing is still present. By Wednesday and Thursday...models disagree on timing of a frontal boundary swinging through the area. Have included slight chance precip chances for both days for scattered showers and thunderstorms.
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Chattanooga Airport, TN 76 95 75 90 / 20 20 20 50 Knoxville McGhee Tyson Airport, TN 73 94 74 89 / 20 20 20 50 Oak Ridge, TN 74 94 75 89 / 10 20 20 50 Tri Cities Airport, TN 69 92 71 88 / 20 30 20 60
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&& .MRX WATCHES/WARNINGS/ADVISORIES... NC...None. TN...None. VA...None. && $$ DGS/JW is the U.S. government's official web portal to all federal, state and local government web resources and services.