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

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000 FXUS64 KMRX 210809 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 409 AM EDT Sun May 21 2017 .SHORT TERM (Today and Tonight)...
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Latest water vapor imagery and RAP upper air analysis shows continued eastward progression of the upper level ridge that we have enjoyed for the past few days. In it`s place, closed upper low over SW Minnesota extends trough axis SE into the Mississippi Gulf Coast. The persistent deep layer SW flow ahead of the trough axis continues to pump unseasonably moist air into the area with model generated PW values of 1.5 - 1.7 inches remaining at or above the 90th percentile through much of the period. So far in this current pcpn event, areas south of I-40 and west of I-75 have generally seen about 0.75 inches of pcpn with some localized radar estimates in excess of 2.0 inches for Saturday. Expect these types of pcpn amounts to shift NE through today ahead of the approaching front by Monday morning. With flash flood guidance of 2.0 - 4.0 inches in the SE portion of the area to 1.5 - 3.0 inches in SW Virginia, the potential for localized flash flooding, particularly in NE TN and SW VA is something to monitor. Forecast for the first half of today is a bit difficult, given the poor comparison of model pcpn to actual radar this morning. The HRRR, which had been doing fairly well for the past few days, seems to be overdoing pcpn this morning. Similarly, NAM12, GFS, and SREF are struggling with the diurnal waning of pcpn. Given this situation, have decided to back off on morning POPs slightly as it looks like guidance is overplaying it for now. However, this lack of pcpn to start the day will likely allow for more afternoon showers and tstms area wide ahead of the front and as indicated in most of the models as well. Therefore did trend the afternoon and evening POPs upward a little bit to reflect greater rain chances and in fair agreement with NAM MOS guidance for later today. Pcpn begins to wind down tonight with frontal passage just beyond the end of the period. Expect temperatures near normal today under mostly cloudy skies area wide with overnight lows tonight also dropping to near seasonal values ahead of the front on Monday. .LONG TERM (Monday through Saturday)... Monday morning cold front will be along or near the Southern Appalachians so expect western areas to be out of the precipitation and skies become mostly sunny. Eastern areas may have a few showers and a possible thunderstorm linger into the afternoon. A low pressure area is expected to form along the front late in the day over AL which may keep some clouds and precipitation along the Appalachians. As this low develops and moves northeast clouds will return quickly overnight Monday night and continue Tuesday. NAM model is most aggressive developing the low farther southwest and bringing more precipitation into the region Monday night and Tuesday with surface low moving up through the southern Appalachians. GFS farther south and slower with low and moves it east of the mountains late Monday night into Tuesday. This would be much drier for forecast area. Have gone with overall drier solution but have higher rain chances south and east Monday night and Tuesday. From Tuesday night through Thursday upper level trough digs south over the Eastern states bringing in more showers and isolated thunderstorms and much cooler temperatures as short wave energy rotates through. Highs Monday be mostly mid 70s to lower 80s near normal but fall to the mid 60s to lower 70s Wednesday and Thursday. The upper trough begins to lift out Thursday night which will end showers. A warming trend Friday and Saturday as higher pressure rebuilds.
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Chattanooga Airport, TN 80 64 79 60 / 70 40 10 30 Knoxville McGhee Tyson Airport, TN 79 63 77 57 / 80 70 20 20 Oak Ridge, TN 79 63 79 58 / 80 60 20 20 Tri Cities Airport, TN 77 61 76 55 / 80 70 30 20
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