957 FXUS64 KHUN 150202 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 802 PM CST Thu Feb 14 2019 .NEAR TERM...(Tonight) Issued at 800 PM CST Thu Feb 14 2019 GOES nighttime microphysics imagery shows a scattered deck of low level stratus developing off to our west. This is occurring head of frontal boundary located over AR/MO as of 7PM. The parent low is well to our north and is moving across the Great Lakes this evening. The breezy winds from earlier today have subsided but models and VAD wind profilers via HTX/GWX indicate a 40-50 kt LLJ is still over the area. While this LLJ is forecast to weaken through the night it will continue to provide an influx of moisture into the area. So, we should see the low level clouds become more widespread. Models have all been in agreement in trying to show some light rain developing as the front nears the area and moving across the TN Valley overnight. While the better dynamics are further to the north, there does appear to be good convergence through the lower half of the atmosphere in proximity to the front. This should provide enough lift to squeeze out some light rain. Models could be overdoing the coverage of rain tonight and have reduced PoPs a tad from the previous forecast. Mainly due to the fact that hires guidance all forecasted scattered showers across western TN between 6-7pm. Latest radar imagery paints a different picture with no real returns evident in the region. Did remove the mention of thunder from the forecast as 00z soundings show a sizable cap. Temperatures will fall a few more degrees into the lower 50s to upper 40s by midnight. Then likely hold steady or possibly warm slightly by sunrise. So, the current temp forecast looks to be in good shape. .SHORT TERM...(Friday through Sunday night) Issued at 327 PM CST Thu Feb 14 2019 Latest NWP guidance still indicates that large scale ascent will increase further during the day tomorrow, as a well-defined mid- level trough (crossing the central Rockies around 12Z) shifts eastward into the Plains, and this will occur in conjunction with additional increases in atmospheric moisture as PWAT values should reach the 1.25-1.3 inch range during the afternoon. Thus, after a scattered coverage of light showers tomorrow morning, we expect both the coverage and intensity of convection to increase by late tomorrow afternoon as surface pressure falls occurring across northern TX force the cold front to either stall across the northwest portion of the forecast area or perhaps retreat northward into southern TN. Limited values of MUCAPE (~250-500 J/kg at most) in forecast soundings suggest that a few thunderstorms will be possible as well, and this has been included in the grids, in line with the latest convective outlook from SPC. Showers and a few embedded thunderstorms are still expected to be most widespread on Friday evening, as the mid-level trough advances eastward across KY and a developing frontal wave lifts east-northeastward across the CWFA. However, we expect precipitation to end rather quickly from NW-to-SE between 06-12Z Saturday, as deep-layer flow veers to the northwest and induces stronger descending vertical motions. Impacts from the first round of precipitation should be limited, as rainfall amounts will generally be in the 0.5-1 inch range. Models indicate that the height field aloft will deamplify on Saturday in the wake of the first system, and although flow at the 850-mb level may back to the southwest once again during the late afternoon, forcing for ascent appears too weak to include any chance for precipitation. There is some discrepancy among forecast sounding data regarding how quickly postfrontal stratus clouds will lift and scatter on Saturday, but regardless of this we still expect light northerly winds/CAA to keep max temps in the u40s-l50s. Flow within the surface-700 mb layer will back to the southwest and increase in earnest on Saturday evening, as the intensifying subtropical ridge to our south builds eastward into the northwestern Caribbean Sea/Florida Straits, and an initial vort max lifts northeastward out of a developing longwave trough over the western CONUS. The resultant increase in lift from deep-layer WAA will support an increase in showers in the vicinity of a retreating warm front once again early Sunday morning, with widespread showers and perhaps a few embedded thunderstorms expected on Sunday. A colder arctic airmass building into the central Plains during the second half of the weekend should force the front to push southward once again on Sunday evening, with rainfall expected to end from NW-to-SE overnight. Atmospheric moisture content will be a bit higher with the second system, and thus we expect rainfall amounts to be greater, ranging from 0.75-1 inch across the NW to perhaps as high as 1.5 inches in the SE. .LONG TERM...(Monday through Wednesday) Issued at 327 PM CST Thu Feb 14 2019 Very volatile period of heavy rainfall and potential flash flooding still looks to become established across much of the region heading into the new work week. Another frontal boundary, in what will likely be a series of frontal passages, will move southward thru the mid TN Valley going into the first half of Mon, with the axis of heavier rainfall along/ahead of the boundary translating southeast of the area. This front then looks to quickly rebound back to the north Mon evening, and perhaps stall invof of the gen area. With this front stalled along/just north of the cntrl TN Valley, multiple upper waves embedded within a well entrenched SW flow pattern, will traverse over the stalled sfc boundary heading into Tue. This looks to result in num/wide showers impacting/spreading NNE into the area late Mon night and continuing well into Tue evening. Rainfall will be aided by PWATs increasing to above 1 inch into early Tue, and continuing to climb into the 1.50-1.75 inch range late Tue into Wed. These excessive PW values will easily be in the 90th percentile or higher for this time of the yr. Deep SW flow in place below H5 also suggests the prob for enhanced rainfall rates/training along/just south of the stalled frontal boundary. Rainfall amounts generally less than a half inch thru Mon will have the potential to add another 3.0-3.5 inches Mon night thru Tue night. If these rainfall amounts materialize, this will certainly result in the prob for multiple flooding/flash flooding issues across the TN Valley, along with the potential for sig rises in area rivers/streams. Some instability/CAPE also look to spread north into parts of the region Tue night, and may result in a few embedded tstms, although the convective threat will remain fairly low. The heavy rainfall threat looks to get a secondary boost heading into Wed, as the frontal boundary to the north becomes more oriented to the WSW of the local area. Excessive moisture pooling along/ahead of this front, aided by additional upper waves passing NE invof of the front, will translate into further num/wide showers spreading into the area. Potential heavy rainfall will get additional support by an increase in low level convergence along the front, as it moves east into the area thru Wed. This looks to add another 1.5-2.0 inches of rainfall into Wed evening, with total rainfall amounts now Mon thru Wed potentially in the 5-6 inch range, with locally higher amounts. Needless to say, ongoing flooding/flash flooding issues may become more enhanced going into the middle of next week. Given the time period in question, much could change with this forecast pending further model runs, although if these rain amounts are close, this could certainly be one of the more sig flooding events to impact the cntrl TN Valley outside of a tropical system in quite a few yrs. The prob also exists for some additional embedded tstms into Wed, but again the convective threat will be secondary to the potential for heavy rainfall/flooding. Given the abundant rainfall/cloud cover xpcted be in place for much of next week, overall temps will not deviate much from day to day, with highs generally in the mid 50s/lower 60s and lows predom in the mid 40s/lower 50s. && .AVIATION...(For the 00Z TAFS through 00Z Friday evening) Issued at 521 PM CST Thu Feb 14 2019 Strong S/SW winds are advecting moisture into the TN Valley ahead of an approaching front. Cigs drop this evening from VFR (15,000ft) down to around 4,000ft by 03z. Then as the front nears the area cigs fall to 1,500ft with increasing chances for light rain. Stronger winds aloft overnight may result in a period of wind shear until 03-04z, then winds aloft begin to subside. The front stalls out over the area on Friday and should keep cigs between 1,000-1,500ft. Periods of additional lowering to 600-800ft are possible after 08z tonight and continuing through the day tomorrow. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...Stumpf SHORT TERM...70/DD LONG TERM...09 AVIATION...Stumpf For more information please visit our website at weather.gov/huntsville.