Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
000 FXUS64 KHUN 192347 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 547 PM CST Tue Feb 19 2019 .UPDATE... For 00Z TAFS. && .NEAR TERM...(Tonight) Issued at 332 PM CST Tue Feb 19 2019 An unusual event is unfolding across the TN Valley today. We started the day with a mix of rain and sleet as a result of dry/cold sub cloud layer. As rain moved in it was able to saturate this layer closer to noon converting everything over to all rain. Soundings via UAH as part of VORTEX SE operations at 18z showed strong WAA occurring aloft. Strong lift in this region coupled with cold/steep lapse rates was resulting in numerous thunderstorms. Due to the colder nature of the atmospheric profile we have had reports of pea to dime size hail with some of the stronger storms. What makes this so unusual is the fact that temps have remained below 40 degrees so far. The main surface based warm front is still anchored along the Gulf Coast and much of these storms are developing along the elevated frontal boundary moving through the area. Strong W/SW winds aloft oriented almost parallel to this boundary are resulting in some training of storms across MS and into NW AL. MRMS rainfall estimates with the band of storms indicate totals closer to 2 inches. Saturated soils were resulting in almost all of this rain being converted over to all runoff. This is something we will have to watch closely as some of the storms moving out of MS move into the area. Any training of storms over a particular area could warrant Flash Flood Warnings, especially in many urban locations. Now in terms of what will occur for the rest of the evening, an upper level system was shifting out of the desert SW and into the Central Plains. This was causing pressure falls over the TX Gulf Coast with a surface low expected to develop and track N/NE along the MS River basin tonight. A strong 50-60kt LLJ tied to this surface low is forecast to move N/NE towards the area overnight. This coupled with a stronger jet aloft will be able to transport near record moisture into the TN Valley. As it stands now, the latest guidance continues to show that best lift/forcing associated with system moving into the Plains will stay to our west with the heaviest rainfall axis being centered along the MS River tonight. However, current radar/satellite imagery indicate a narrow band of showers/storms moving north out of New Orleans area and into central MS. Moving that forward in time would bring it up over the area later this evening, which models are not quite accounting for. The current forecast will show a slight decreasing trend overnight as the warm frontal boundary aloft moves north into TN. But as mentioned above we'll have to watch the narrow band of precip in MS. Rainfall totals are likely too low and will be higher where we see any training of precip. We will continue to monitor and make adjustments to the forecast this afternoon/evening. The Flood Watch was updated this morning and went into effect at Noon and will run through Friday. The one other concern that we might be underforecasting the winds tonight. Model soundings indicate a sizable inversion over the area and the stronger winds may not quite be able to mix down to the surface. Some of the higher winds might be possible in the terrain in NE AL and southern middle TN and have tried to show some of those gusts there. .SHORT TERM...(Wednesday through Thursday night) Issued at 332 PM CST Tue Feb 19 2019 The surface low will be shifting into western TN Wednesday morning and the heavier rainfall axis will be centered near the low. As the low moves into western TN and the pressure gradient tightens over the area, it looks like a CAD situation might develop in the Lee of the Appalachians. Some of that is going to spill into the TN Valley and likely could help to reinforce our low level inversion through the morning. The other impact this might have is that it could really enhance the convergence over the area and increase forcing. As the surface low shifts into the OH Valley it will push a cold front into the area later in the day. Latest guidance is probably a bit too quick with the frontal passage tomorrow and move it east of the area by Wednesday evening. Think that even though the CAD is projected to weaken through the day this may end up slowing the progression of the front and it might stall over the area for a brief period of time tomorrow. When analyzing forecast soundings, there is upwards of 500 J/kg of MUCAPE expected along and ahead of the front. With near record PWATs, strong convergence, and a rather unstable environment for late February could result in intense rainfall rates of 1 to 2 inches per hour. Winds from 850mb to through 250mb would be paralleling the frontal boundaries and we could see training of storms. When taking all of that into consideration was a bit reserved to use the lower QPF amounts guidance was providing. So, have tried to show higher amounts during the afternoon hours as the front moves into the area and where we might see training of storms. High pressure shifting E/NE into the OH Valley will help to push the lingering frontal boundary south of the area Thursday morning. While the core of the heavier rain will be south we should still see some light rain continue. River flooding should be ongoing at this point and the concern that we have is the models trending lower with rainfall amounts has brought forecasted river levels below where they might actually end up after all this rainfall. Taking into consideration where QPF might be, flooding could be comparable to the Christmas 2015 flood at the low end. While this front is moving across the area a trough will be digging into the desert SW and should result in a continuation of the strong S/SW flow over the area. Shortwaves ejecting out of this trough could bring the front back into the area Thursday evening. Some guidance is showing a few bands of heavier rain moving across the area as these waves cross the area. Rainfall amounts won't be nearly as high due some slightly drier air aloft. But any rain will continue to impact river flooding. Thursday night into Friday could end up similar to what we had today, a warm front lifts back to the north with strong convergence focused along the boundary. Light to moderate rainfall moves back into the area as we head into Friday. .LONG TERM...(Friday through Monday) Issued at 332 PM CST Tue Feb 19 2019 The extended period begins Friday morning with a warm front nearly overhead/slightly displaced just N of the CWFA, with light to moderate rain likely ongoing by this time. Friday overall will be a wet day, thanks to the stalled frontal boundary and several disturbances riding NE along it through the day. This round of rainfall will exacerbate any ongoing/lingering flooding concerns from heavy rainfall received earlier in the week. Rainfall totals for the Friday/Friday night timeframe look upwards of 1.5-2", with isolated higher amounts from any thunderstorms that occur during this time. It goes without saying that this will add to flooding issues ongoing across Northern AL/Srn Middle TN. The bigger story in the extended revolves around the forecast during the day/evening timeframe Saturday. A potent upper level system takes shape across the Intermountain West later this week, spawning cyclogenesis Friday night into Saturday over the TX/OK Panhandles region, and ejecting NE into the Ern KS/MO through the day Saturday/Saturday evening. This part of the forecast will definitely require some fine-tuning in the next couple of days. The pattern is favorable for a QLCS for our area, with perhaps QLCS/supercells to the N/W over portions of Wrn TN. Any shift of the upper level pattern would increase/decrease our threat for severe weather during this time, and this will need to be watched closely. For now, the ingredients are there for at least strong to possibly severe wind gusts along the line of storms that march across the area Saturday evening/night. Daytime high temperatures Saturday will top out in the lower/middle 70s, which is unseasonably warm for this time of year. Dewpoints will creep up into the lower 60s as well with stout Srly flow ahead of the front. An 850mb jet of 50-60kts, 300-400 m2/s2 SRH, and 0-1km shear of 40kts (with 0-3km shear of 50kts) will lead to the potential for 50-60mph wind gusts along the line, with a lesser chance for a brief spin-up tornado. Add to that PWat values around 1.6", which are record values for both BMX/OHX SPC PWat Climo values for this time of year, and you're looking at a mixed bag of weather on tap for Saturday. In addition to the severe threat, moderate/heavy rainfall will accompany this fropa through early Sunday, adding another 1-2" (with isolated higher amounts possible). Again, it sounds like a broken record, but this will further exacerbate the ongoing flooding conditions across the area. For this reason, the Flood Watch may need to be extended further in the weekend (as it currently runs through 00Z Saturday. This is in the 'fine tuning' that will happen over the next couple of days, so stay tuned. The fairly active pattern isn't quite done yet. There is an end in sight, however brief, that will occur in the wake of this fropa this weekend early Sunday. DRY WEATHER WILL BE EXPECTED FOR ALMOST A FULL 36 HOURS, so get out there and enjoy it! Dry conditions are expected mid-morning Sunday and all day Monday, before more rainfall develops and overspreads the region on Tuesday. Depending on your model of choice that far out in the extended, another 1" (at least) is on the horizon (you guessed it: further adding to the lingering flooding concerns across the area). A lot can change between now and then, though, so stay tuned for further forecast updates between now and then. && .AVIATION...(For the 00Z TAFS through 00Z Wednesday evening) Issued at 547 PM CST Tue Feb 19 2019 Inclement flying weather will continue across the area for the TAF period. Reduced CIG/VIS values into the MVFR/IFR category will continue, with possible reductions to LIFR in the heavier showers and/or tstorms. ESE winds in the 10-15kt range early, will increase into the 20-30kt range with some higher gusts later this evening, continuing to around daybreak Wed. Forecast models indicated stronger winds developing aloft, enough to include a Low-Level Wind Shear for the mid evening through after midnight, with ESE winds at FL 2kft to 60kt. Those winds should subside somewhat in the overnight, but widespread showers and embedded tstorms will continue into Wed afternoon. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...Flood Watch through Friday afternoon for ALZ001>010-016. TN...Flood Watch through Friday afternoon for TNZ076-096-097. && $$ NEAR TERM...Stumpf SHORT TERM...Stumpf LONG TERM...12 AVIATION...RSB For more information please visit our website at weather.gov/huntsville.