Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
181 FXUS64 KHUN 160548 RRA AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 1246 AM CDT Mon Jul 16 2018 .UPDATE... For 06Z TAFS. && .NEAR TERM...(Rest of tonight) Issued at 1009 PM CDT Sun Jul 15 2018 Lingering pockets of light-moderate rain persist across the TN Valley this evening, as the remnants of deeper afternoon convection continue to dissipate. However, we are closely monitoring the progress of a small but decaying MCS, as it shifts east- southeastward across southwestern TN/northern MS. In spite of the recent weakening trend, it appears as if the stratiform rain shield will advance as far southeastward as northwest AL, and POPs were increased slightly across these zones to account for this. However, of more concern is the potential impact from a large outflow boundary associated with this feature, which could initiate development of new convection around the time of a subtle increase in the low-level jet closer to sunrise. Although the coverage of thunderstorms in this scenario would likely be highest beginning shortly after 12Z, we have shown a gradual increase in POPs (especially for the northwestern half of the CWFA) in the 09-12Z period. Forecast soundings depict a thermodynamic environment supportive of frequent cloud to ground lightning and locally heavy rainfall as the primary nocturnal storm threats, although a few of the stronger cells could produce brief wind gusts in the 30-40 MPH range. Other gridded forecast elements are on track at this point, with only minor adjustments made to sky cover grids to indicate a return of lower stratus clouds once again late tonight. Conditions will remain warm/muggy for the entire region, with lows ranging from the lower 70s in northeast AL to the mid 70s elsewhere. .SHORT TERM...(Monday through Tuesday Night) Issued at 306 PM CDT Sun Jul 15 2018 As an upper-level trough swings across southern Canada, it will continue to break down the ridge that was anchored across the Southeast. This will place us in a northwesterly flow pattern, allowing a series of disturbances to dive down into the region. At the sfc, a low moving out of the Great Lakes will pull a cold front into the TN Valley by Tuesday morning. With moist and unstable conditions, scattered to numerous showers and storms are possible Monday and Tuesday. Heavy rainfall, gusty winds and frequent lightning will again be the main threats with PWATS still ~2" and CAPE 1000-2000 J/kg. Steering flow will slightly improve, allowing for storms to have a lower chance of producing flooding. The increase in storm coverage and cloud cover will limit heating, and thus temps should only reach around 90 degrees. This will keep heat index values below heat advisory criteria over the next couple days. As the front pushes through late Tuesday and drier air filters in, storm chances will decrease allowing for a drier air mass to work in by Tuesday night. .LONG TERM...(Wednesday through Saturday) Issued at 306 PM CDT Sun Jul 15 2018 Large area of high pressure is xpcted to be in place across the Great Lakes/OH Valley states to start off this part of the forecast package, with a cold front located just south of the cntrl TN Valley. Some slightly drier air between H5-H85 should also be in place over the area thx to the passage of the front earlier, which should offset much in the way of rain chances except for perhaps the far SW corner of the forecast area where slightly better convergence near the front may allow for a few showers/tstms thru Wed. Temps will still be seasonably warm with highs in the upper 80s/near 90F, although the drier air will keep afternoon heat indices more in the lower/mid 90s. Rain chances begin to gradually increase Thu/Fri, as the front to the south rebounds to the north as a warm front. With the warm front moving nwd, a low pressure system out of the mid Plains states also looks to translate ewd into the Midwest/OH Valley areas, which should help with the influx of moisture into the region. The latter half of the global models are also suggesting an upper shrtwv/trough carving thru the Great Lakes, placing the region in more of a nwly flow regime. All of this should result in sct showers/tstms developing across the mid TN Valley by Fri and persisting into the weekend. Even with the increase in rainfall, highs still look to be fairly seasonal for this time of the yr, with temps again maxing out near the 90F mark for most locations, while overnight lows trend predom in the upper 60s to lower 70s. && .AVIATION...(For the 06Z TAFS through 06Z Monday night) Issued at 1246 AM CDT Sun Jul 16 2018 A weak boundary to the north of the terminals will slowly push south into northern Alabama around daybreak. At this time, included a tempo (06Z to 10Z) group for vsbys dropping to around 5SM due to fog at KHSV (due to light winds and some breaks in cloud cover expected over the next few hours). Also, included a period of IFR cigs in this tempo group, due to recent broken deck below 1000 feet showing up in current observations. This may continue periodically until around 10Z at KHSV. Further west, thicker/more persistent higher cloud cover around 5000 feet will likely keep even patchy fog from forming at KMSL. Some periodic VFR cigs around 5000 feet could affect KMSL through 08Z at times, but cigs should become more persistent after 08Z. There is some uncertainty whether a stratus deck will form at KMSL, due to the more persistent 5000 foot cigs moving in. Further east at KHSV, this persistent area of 5000 foot cigs should hold off until around 10Z, but expect at return to predominant 5000 foot cigs by then. Only included a prob30 group at KHSV between 15Z and 20Z due to lower confidence of precipitation activity. Sfc winds will remain arnd 5 kts or less, and assume a more uniform SSW component later this evening before veering to WSW by end of the TAF period. VFR and drier conditions should return after 18Z at both terminals. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...70/DD SHORT TERM...JMS LONG TERM...09 AVIATION...70/DD For more information please visit our website at weather.gov/huntsville.