As we receive the remnants of post-tropical Storm BETA, we are expecting rainfall anywhere from 2-4″ on Thursday and Friday. A Flash Flood Watch is currently in effect for northern Alabama/southern-middle Tennessee to account for this.
URGENT - IMMEDIATE BROADCAST REQUESTED Flood Watch National Weather Service Huntsville AL 239 PM CDT Wed Sep 23 2020 ALZ001>009-016-TNZ076-096-097-240345- /O.NEW.KHUN.FF.A.0011.200924T0900Z-200925T0000Z/ /00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/ Lauderdale-Colbert-Franklin AL-Lawrence-Limestone-Madison-Morgan- Marshall-Jackson-Cullman-Moore-Lincoln-Franklin TN- Including the cities of Russellville, Winchester, Red Bay, Cullman, Town Creek, Cowan, Estill Springs, Huntsville, Sheffield, Arab, Sewanee, Decatur, Lynchburg, Athens, Muscle Shoals, Decherd, Fayetteville, Albertville, Guntersville, Boaz, Tuscumbia, Moulton, Florence, and Scottsboro 239 PM CDT Wed Sep 23 2020 ...FLASH FLOOD WATCH IN EFFECT FROM LATE TONIGHT THROUGH THURSDAY EVENING... The National Weather Service in Huntsville has issued a * Flash Flood Watch for portions of Alabama and southern middle Tennessee, including the following areas, in Alabama, Colbert, Cullman, Franklin AL, Jackson, Lauderdale, Lawrence, Limestone, Madison, Marshall and Morgan. In southern middle Tennessee, Franklin TN, Lincoln and Moore. * From late tonight through Thursday evening * Periods of moderate to heavy rainfall associated with the remnants of Tropical Storm Beta will impact the Tennessee Valley on Thursday. Efficient tropical rainfall rates will likely overpower absorption rates into the ground, leading to flash flooding across the area. PRECAUTIONARY/PREPAREDNESS ACTIONS... A Flash Flood Watch means that conditions may develop that lead to Flash Flooding. Flash Flooding is a very dangerous situation. You should monitor later forecasts and be prepared to take action should Flash Flood Warnings be issued.
Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 903 PM EDT Wed Sep 23 2020 Day 1 Valid 01Z Thu Sep 24 2020 - 12Z Thu Sep 24 2020 ...A MODERATE RISK OF EXCESSIVE RAINFALL IS IN PLACE FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY... ...Lower MS Valley and portions of the Mid-South... Post-T.C. Beta this evening is moving steadily off to the northeast across central LA and will soon be advancing into southwest MS as thee system gradually continues to accelerate ahead of a mid to upper-level trough over the mid-MS Valley. A fairly large shield of moderate to heavy rain continues across the lower MS Valley, and is seen streaking northeast toward the mid-South. Generally a large portion of this rainfall is stratiform in nature owing to the strong isentropic ascent over top the relatively cool/stable boundary layer that is in place to the north and northeast of Beta's circulation. However, closer into the low center from areas of southwest to central MS, there is a substantial area of very heavy rainfall associated with strong convection that is being aided by enhanced mid-level ascent/dynamics and which is working in tandem with focused deep layer moisture convergence, and a nose of moderate instability that is lifting up over a warm front extending east from Beta's low center. MLCAPE values ahead of Beta are as high as 1000 to 1500 j/kg aiming in across much of southeast LA and toward far southwest MS, and with a south-southeast low-level jet of 30 to 40 kts. Areas of the lower MS Valley involving especially southwest to central MS and perhaps to a lesser degree downstream across areas of northeast MS and northwest AL are expected to see the focus of the heaviest rainfall overnight as Beta continues to advance northeast and drives the ongoing heavy rainfall swath off to the northeast with it. Generally, the thermodynamic environment will tend to wane just a tad given the loss of daytime heating, but the presence of very favorable jet-aided dynamics aloft and focused low-level moisture transport/convergence near and ahead of the low center (generally along and just north of the aforementioned warm front) is expected to maintain a focused area of heavy showers and thunderstorms. PWs across the region are on the order of 2 to 2.25 inches, and with strong deep layer forcing, the rainfall rates within the stronger convective cores are likely to exceed 2 inches/hr. The latest HRRR/HRRRexp guidance and 18Z HREF suite of guidance still supports locally an additional 3 to 5 inches with isolated storm total amounts of 5 to 7 inches going through 12Z/Thursday. However, they still collectively appear to be a tad too far north with their axis of heaviest rainfall, and so some modest southward adjustments were made to the Slight and Moderate Risk areas were made to account for the latest radar trends. Numerous instances of flash flooding are expected, and potentially some locally significant flash flooding within the Moderate Risk area. Adjacent areas of the mid-South involving western and central TN should see notably lower rainfall totals with a relatively diminished threat of flash flooding since the rainfall here should tend to be more of a longer duration moderate to locally heavy warm-advection rainfall evolution with minimal (if any) convection expected. Farther back to the west over the remainder of the lower MS Valley, the excessive rainfall/flash flooding concerns will gradually be diminishing from southwest to northeast, as the back edge of the rainfall shield moves to the northeast, and the latest ERO update reflects this with adjustments made based on the latest radar trends. ...Upper Mississippi Valley... Ahead of a shortwave trough moving currently through the Dakotas, showers and thunderstorms are expected to develop shortly after 00Z across north-central Minnesota just north of a surface front oriented from WSW to ENE. Precipitable water values of 1.0 to 1.3 inches in the pre-convective environment and MUCAPE values in the 1000 to 2000 J/kg range are forecast across south-central to north-central Minnesota. As southwesterly 850 mb winds of 30+ kt, ahead of a surface trough extending into the Central Plains, ascend over the front, mean storm motions are expected to parallel the frontal boundary supporting training. There is a relatively brief window between 03Z and 09Z when flash flood potential will be greatest, before instability weakens to the north and forward propagation increases toward the south, ahead of the surface low and in the wake of the surface trough axis. Rainfall rates locally as high as 1 to 1.5 in/hr are expected which may cause some isolated flash flooding or runoff concerns. A Marginal Risk area remains highlighted across portions of north-central MN and extending east across far northern WI and the far western portion of the U.P. of MI. Some modest adjustments were made to the area based on the 18Z HREF suite of guidance. ...Pacific Northwest... A rather strong atmospheric river for this time of the year is impacting the Pacific Northwest with IVT values locally in excess of 750 kg/m/s just ahead of a cold front which is currently analyzed near the coastal ranges of western WA. Southwesterly 850 mb winds of 40 to 50 kts continue to focus strong moisture transport, and there is also some weak instability just ahead of the front and a few convective elements moving onshore across western WA. Expect locally an additional 2 to 3 inches of rain over especially the higher terrain of the Olympics and the WA Cascades overnight, with some hourly rainfall rates in the 0.5 to 0.75 inch/hr range possible. Lesser amounts of as much as 1 to 2 inches with isolated heavier amounts are expected farther south over areas of western OR as the cold front moves inland. There will be some concern for some of the burn scar areas of especially northwest OR and southern WA to see some enhanced runoff problems from these locally heavier rainfall rates that get up into the 0.50+ inch/hr range, and as such the Marginal Risk area is maintained at this time.
Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 356 PM EDT Wed Sep 23 2020 Day 2 Valid 12Z Thu Sep 24 2020 - 12Z Fri Sep 25 2020 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE TENNESSEE VALLEY INTO THE SOUTHEAST... ...Lower Mississippi Valley/Tennessee Valley/Southeast... ...20Z Update... Modified the Marginal and Slight Risk areas based on the latest model trends/WPC QPF and after coordination with the local offices. Models are coming into better agreement with respect to the axis of heaviest rain as the ingredients look to overlap just along and north of the surface low track/instability gradient. And when comparing the previous forecast amounts and the overall footprint, did increase values and nudged the higher rainfall totals northward at this update. The swath of heaviest rain stretches from northeast MS into northern AL and much of southeast TN with on average 2-4+ inches, with locally higher amounts possible. In addition to these changes, the guidance is spreading the heavy rain quickly to the northeast across portions of the southern/central Appalachians with some trough interaction. Given that much of the Deep South/TN Valley has received little to no precipitation over the past week, decided it was best to retain the Slight Risk category for flash flooding. However, if QPF values come up substantially, then a more focused Moderate Risk area may be necessary. ...Previous Discussion... As remnants of Beta get picked up by a positively tilted long wave trough extending from the OH Valley into the Lower MS Valley during Day 2, deep moisture and instability are transported across the TN Valley and Southeast states. There is a fair amount of spread concerning just how far north the remnants are drawn northward, and how much instability follows, particularly into the TN Valley. The 00z NAM/ECMWF look too far north with the weakening surface system, apparently allowing too much instability to reach northern MS/AL. The 00z GFS looks too fast with the system, as it lifts out the surface low into the eastern TN Valley. The 00z UKMET looks like a fair proxy for the expected rainfall distribution, so it was used as a template for the WPC QPF and Excessive Rainfall Outlook. With the mid level trough lifting to the northeast from the OH Valley into the Lower MS Valley, the remnants of Beta gets lifted northeast. However, as mentioned earlier, there is quite a bit of spread with respect to how quickly this occurs, and how far north the surface system gets. Since the model guidance has been too quick to lift Beta with the trough over the past couple of days, it seems as though the track of the system will be further south than much of the guidance. With a track further south with the surface system, the northern edge of the instability gradient is expected to lie from central MS into central and possibly northern AL during the afternoon and evening hours (when instability peaks). During this time, a 50 knot jet streak passing just to the north across the OH Valley provides synoptic scale ascent for banded rainfall to extend from central and northern MS across northern AL into far south central TN. Much of the high resolution guidance showed a band of 4.00/6.00 inches of rainfall along the northern edge of the instability gradient. While placement looks reasonable, the amounts seem too high, as these models appears to be bringing too much instability into northern MS/northern AL. The moisture plume with Beta is transported along the isentrope toward an elevated boundary over this area, so heavy rainfall is expected, but lacking deeper instability, the high resolution values look too high. Hourly rainfall rates could top out near 1.50 inches, with total rainfall amounts of 3.00/4.00 inches possible in spots. This area was spared the highest rainfall associated with Sally, and three hour flash flood guidance values are generally above 2.50 inches. Further south, instability is better across southern MS/southern AL into the western FL Panhandle, with MUCAPE values between 1000/2000 J/KG. Inflow of 2.00/2.25 inch precipitable water air within the instability axis could support bands of convection tracking north into these areas from the northern Gulf of Mexico. Banding should be fairly progressive as the mid level flow becomes more westerly at the base of the mid level trough to the north. Still, hourly rainfall rates here could peaking near 2.00 inches, with local 3.00+ inch rainfall amounts possible, despite relatively modest model QPF amounts.