Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
000 FXUS64 KHUN 181508 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 908 AM CST Mon Feb 18 2019 .NEAR TERM...(Rest of Today) Issued at 907 AM CST Mon Feb 18 2019 A mixture of low and high clouds continues across the TN Valley this morning. Visible satellite showed some of the lower clouds beginning to break up underneath the cirrus. The strong jet stream just to the north will allow for a constant stream of cirrus over the area but I think it may thin enough to allow for some sun later this afternoon. The cold front that moved through last night is now sitting along the Gulf Coast. High pressure over the OH Valley will keep our area under a CAA regime for much of the day. Later today if we can get a bit more sun we should be able to warm into the mid to upper 40s. Nudged temps up by a degree or so to account for current trends. The rest of the forecast is in good shape. .SHORT TERM...(Tonight through Wednesday) Issued at 243 AM CST Mon Feb 18 2019 Tonight, strong isentropic lift develops on I290-I300, saturating the layers above the shallow cold layer. This cold/dry layer will continue to be enhanced by a rather distinct cold air damming situation spilling drier air southwest through SC/GA into north AL. This may delay/limit heavier precipitation until late Tuesday afternoon and evening. Also, the strongest core of the LLJ will be closer to the MS river valley, so the heavier rain band may be larger in extent to our west. However, one cannot ignore the strength of the 850 mb warm front lifting north Tue afternoon, with SIs dropping slight below 0, so will continue to include a few thunderstorms through Tuesday night with a narrower band of moderate rain. The concern is that with the 850mb warm front lifting north/northwest of our forecast area Tue evening, the heaviest rain core will shift with this driving force. Thus, QPF is lowered considerably through Wed morning. As of now, for Tue-Wed, QPF is on the order of 2-3 inches (heaviest NW) which is near what the GEFS QPF plumes are centering around. This is on top of the ~0.8-1.3in from Monday which has pushed smaller streams upward quickly this morning. On Wed, QG lift weakens, but the cold frontal band will be slower to shift east. So, duration may be more of a factor for QPF on Wed. After WFO coordination, we are going to hold off another shift to hoist a Flood Watch. We also have a full dry day and night ahead preceding the rain threat, so would rather issue closer to the event. Smaller streams are already responding quickly to the 1-1.25 inches we had on Sunday, so we will likely see rivers go into flood quickly despite the lower QPF. .LONG TERM...(Wednesday night through Sunday) Issued at 243 AM CST Mon Feb 18 2019 Volatile wet period for the cntrl TN Valley will continue past mid week, as a frontal boundary stalled invof the gen area provides a continued focus much for numerous/widespread showers, along with some embedded tstms. Both of the latter portions of the 00Z ECMWF/GFS indicated a frontal boundary stalled just south of the local area from cntrl AL/GA WSW into the cntrl Gulf regions starting Wed night. Multiple upper waves embedded within an active/stagnant SW flow regime over most of the region will continue to traverse NE over the sfc boundary and into parts of the mid TN Valley. Given the SW-NE orientation of the flow pattern coupled with the location of the stalled front, much of the showers may actually look to impact more of the ern half of the area from late Wed night into Thu, and may actually provide for a few breaks in the excessive rainfall during the day Thu, mainly for locations west of the I-65 corridor. This is highly dependent though on the exact location of the stalled front being tempo displaced to the south thru Thu. Any nwd bias in the location of this front could result in more excessive rainfall/showers for much of the area. As alluded to above, some weak instability may also be in place Wed night into early Thu to result in the prob for a few embedded tstms. In fact, an H85 SW jet near 50kt in place Wed night may allow for a few strong tstms capable of brief gusty winds mixing to the sfc. Whatever tempo break in the rainfall/showers develops on Thu will quickly begin to fill back in Thu night into Fri, as the stalled boundary to the south meanders back to the north. This should then allow for more upper disturbances to traverse NE across much of the cntrl TN Valley heading into the end of the work week. Deep layer SW flow in close proximity to the stalled front oriented in a gen SW-NE direction would suggest the prob for banding/training of mod/heavy showers thru Fri night. The whole axis of rainfall over much of the region will then get some boost/enhancement going into the weekend period, as another sfc low pressure system out of the mid/srn Plains states lifts NE into the Midwest region. The attendant cold front associated with the sfc low looks to sweep thru the Mid South/Lower MS Valley areas Sat into Sun, with rainfall likewise tapering off from the west across the area into the day Sun. Total rainfall amounts from Wed night into Sun look to be around 3.5-4.25 inches, with perhaps some locally higher amounts. Total rainfall thru the whole forecast package, including around an inch or so of rain that has already fallen looks to be in the 6.5-7.5 inch range. While this may not be quite as high compared to some earlier totals, again this will be highly dependent on the location of the stalled front across the gen area after mid week. In any case, these rainfall amounts will still result in many hydrologic/flash flooding issues, along with sig rises on many of the area rivers/streams. Given all of the cloud cover and rainfall xpcted to impact the region thru the end of the work week/into the weekend period, overall temps are xpcted to be fairly stagnant, with highs predom in the 60s and lows mainly in the 50s. && .AVIATION...(For the 12Z TAFS through 12Z Tuesday morning) Issued at 442 AM CST Mon Feb 18 2019 Scattered to broken layer at 020-030agl will exit KMSL and KHSV by 15Z. VFR ceilings at or above 200agl will scatter or clear for a brief period this afternoon (21-01Z). Ceilings at or above 150agl will fill in thereafter. At KMSL, the ceilings may lower to at or above 040agl by 11Z. The ceilings will also lower just after this forecast period (~12Z) at KHSV. North winds will gust at 13-18kt today, then veer to the northeast this evening at 6-10kt. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...Stumpf SHORT TERM...17 LONG TERM...09 AVIATION...17 For more information please visit our website at weather.gov/huntsville.