Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
000 FXUS64 KHUN 210851 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 251 AM CST Thu Feb 21 2019 .NEAR TERM...(Today) Issued at 251 AM CST Thu Feb 21 2019 Latest water vapor imagery shows an amplified upper trough over the west coast, with weak ridging over the east. The TN Valley remains within an area of southwest flow aloft, which is keeping a steady stream of moisture over the area. At the surface, a cold front remains draped from its parent low over the eastern Great Lakes, southward through the Ohio and TN Valleys. Behind this front, temperatures have fallen into the upper 30s to lower 40s. Radar imagery shows a band of moderate to heavy rainfall south of the I-20 corridor, with a few isolated showers developing north of this main band. This activity is being generated by a boundary at 850 mb, which is expected to slowly drift northward through the day. At least through daybreak, all but the southeastern portion of the area will remain dry except for some patchy drizzle. The wet pattern will continue through the day and into the short term period, as the weak ridge remains to our east and the aforementioned upper trough slowly moves eastward. The cold front will move just to our south by mid morning and then stall. Where it stalls is still in question, which will play a role in where the heavier bands of rain set up. Meanwhile, the 850 mb warm front will drift northward through the day, with showers and thunderstorms becoming numerous by this afternoon. As the instability will remain elevated today, thunderstorms will generally be isolated. However, within any storm that does develop, locally heavy rainfall will occur. Overall, rainfall totals through sunset will be on the lower side, compared to previous days, with generally less than half an inch expected. But again, locally higher amounts are likely and actual amounts may end up higher if the banding occurs over our area. Even with half an inch of rain, localized flooding is possible, especially west of I-65. Temperatures today will be mild, with highs in the lower to mid 50s. .SHORT TERM...(Tonight through Saturday night) Issued at 251 AM CST Thu Feb 21 2019 The upper trough over the Intermountain West will continue to progress eastward overnight, with the weak ridge holding over the southeastern CONUS. It's almost a rinse and repeat pattern, as more rounds of rainfall are expected to move through the TN Valley. The most intense rainfall looks to occur late tonight, as an 850 mb warm front begins to lift northward and stalls just to our north. Meanwhile, the nocturnal jet will intensify and help aid in strong lift to generate bands of heavy rainfall. The axis of the heaviest rain looks to set up right over our northwestern counties, with as much as 1.5 inches of additional rainfall possible between midnight and sunrise. This is a bit on the pessimistic side and confidence is not overly high that this much rain will be realized. However, given the potential impacts, would like to err on the side of caution. So, with that said, significant flooding is possible across areas north of the TN river and west of I-65, where the heaviest rain has already occurred. To make matters worse, this will occur at night, when it's much harder to see flooded roadways. So extreme caution should be used late tonight if travel is necessary. Again, confidence is on the lower side but guidance is fairly consistent on bringing that heavy band over our area. Numerous showers will remain possible through the morning hours on Friday as the 850 mb front remains stalled. So the moderate to heavy rain will continue through 18Z, with an additional half an inch expected. So all in all, we could see upwards of 2 to 3 inches of rain in the 12 hour period from midnight to noon on Friday. It can't really be stated enough that all of this will fall as runoff and will exacerbate the already swollen rivers and streams. A surface warm front will lift northward during the afternoon, pushing the bulk of the heavy rain to our north by the early evening hours. Given the persistent moisture and weak waves moving through the area, numerous showers are still possible overnight Friday, but the rainfall amounts will remain on the lighter side. Focus then turns to the aforementioned trough and the impacts it will bring to the TN Valley. As the trough ejects into the Plains Friday night, surface cyclogensis will quickly occur and a cold front will trail to its south. This system will slowly pivot to the east- northeast through the day, taking on a negative tilt as it lifts into the Mid Mississippi Valley. Ahead of this, southerly flow will strengthen over the local area as the TN Valley becomes established within the warm sector of this approaching system. Dewpoints in the lower 60s will surge northward, reaching the area by the afternoon hours. Meanwhile, temperatures will climb into the lower 70s despite mostly cloudy skies. A line of storms will move ahead of the front, reaching the TN Valley by the late afternoon-early evening hours and then quickly pushing east of the area by the predawn hours on Sunday as the cold front crosses the area. Forecast models are in decent agreement, however the run to run consistency with regard to timing and strength is still on the lower side (which is typical at this point in the period). So, what are the potential impacts? Well, it's still tough to determine with high confidence just how strong thunderstorms will become over our area. The main upper level dynamics will push well north of the area as the storms move through. However, the low and mid level jets will strengthen quite a bit, with 850 mb winds approaching 60 kts. This will give way to plenty of shear, as 0-1 km bulk shear approaches the 30 to 35 kt range. Meanwhile, the surge of lower 60 dewpoints will combine with sufficient daytime heating to lend some destabilization over the area. The limiting factor, though, will be the northward movement of the upper low and thus the weaker height falls this far south. This will keep lapse rates on the weaker side and may limit the coverage and severity of thunderstorms. But, as I stated in the discussion yesterday, any breaks in the cloud cover and/or warmer temps in the afternoon will help overcome the weak mid level lapse rates and allow us to destabilize enough to give us an organized severe threat. The main coverage will come with the line of thunderstorms currently expected to move through late in the afternoon and into the evening hours. All threats are on the table (damaging wind gusts, hail and tornadoes), with the damaging wind gusts the highest threat. However, we will have to watch for any cells that develop ahead of this line, as these would provide the greater risk of severe thunderstorms, especially tornadoes. Heavy rainfall is possible within the strongest storms, which may lead to localized flooding. But given the quick movement of the line, widespread flooding is not anticipated. Even still, with the saturated soils, the flood watch may need to be extended through Saturday. The cold front will move east of the area between 06Z and 09Z Sunday, bringing an end to showers/thunderstorms. Dry air will quickly push into the area, with skies clearing through the early morning hours. Temperatures through the short term will be mild, with highs on Friday in the upper 50s to mid 60s and lows in the upper 50s. Temps on Saturday will depend on the extent of the cloud cover, but for now, lower 70s look reasonable. Overnight lows will fall into the upper 40s behind the front. .LONG TERM...(Sunday through Wednesday) Issued at 251 AM CST Thu Feb 21 2019 The next storm system and attendant cold front should be east of the cntrl TN Valley heading into the second half of the weekend period. Any rainfall associated with the frontal passage should also be clr of NE AL based on the latest blended guidance, as a strong ridge of high pressure out of the Plains states gradually builds southeast into much of the region. Cloud cover should also be diminishing over the area during the day Sun, finally providing a break in all of the rainfall over the previous week. Quiet and tranquil wx conditions look to prevail for the second half of the weekend, as temps Sun afternoon trend more seasonal, with highs mainly around 60F. The quiet wx pattern should continue into the new work week, as the strong sfc high settles more into the Midwest/OH Valley states. Drier air at/below H5 looks to spread east into much of the region, as a broad WSW flow regime becomes established over the srn Plains into the SE states. After a seasonably cool night, with early morning lows Mon falling more into the mid 30s, temps look to be quite mild later in the day, as highs climb predom into the upper 50s. Little change in the overall pattern will continue thru Tue/Tue night, as the sfc high translates more into the nrn/mid Atlantic Basins. Temps late Mon night look to trend more in the upper 30s/near 40F, before rebounding into the lower/mid 60s Tue afternoon. Rain chances may then return to the forecast Tue night into Wed, as a return flow pattern developing over much of the region ushers Gulf moisture back into the mid TN Valley. The latter half of the global model runs are hinting at another low pressure system out of the mid Plains lifting into the Great Lakes by Wed, with the attendant cold front stretching south into the Lower MS Valley region. As this next sfc boundary moves east, sct showers look to develop along/ahead of the front, as they spread more into the local area by mid week. && .AVIATION...(For the 06Z TAFS through 06Z Thursday night) Issued at 1107 PM CST Wed Feb 20 2019 IFR to LIFR conditions will continue overnight and early this morning from low ceilings, and IFR to MVFR fog will develop continue at each terminal thanks to the very moist boundary layer conditions. Some gradual improvements will occur at both terminals after sunrise, but a weak front will generate some -SHRA/SHRA (and maintain sub-1000 ft decks) across the region from the late morning through most of the afternoon. This activity will taper off by this evening as the boundary lifts north, with scattered VCSH remaining through the period. && .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...73 SHORT TERM...73 LONG TERM...09 AVIATION...AMP.24 For more information please visit our website at weather.gov/huntsville.