Area Forecast Discussion
Issued by NWS Knoxville/Tri Cities, TN
Issued by NWS Knoxville/Tri Cities, TN
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130 FXUS64 KMRX 270156 AFDMRX Area Forecast Discussion National Weather Service Morristown TN 956 PM EDT Thu Sep 26 2024 ...New UPDATE... .UPDATE... Issued at 943 PM EDT Thu Sep 26 2024 Key Messages: 1. Flooding risk remains overnight and into Friday morning. There is an increased risk for flash flooding near Hamilton, Marion, and Sequatchie Counties where a heavier band of rainfall is expected to train. 2. Strong 40+ mph wind gusts expected area-wide. Wind gusts over 60 mph will be possible in the Knoxville metro area and to the northeast across the Tri-Cities region and southwest Virginia. Higher elevations of the mountains and foothills may see wind gusts over 80 mph. 3. These strong winds, when combined with saturated soils, will likely result in widespread downed trees and power outages. Be prepared for prolonged power outages. Discussion: This is a historic setup for the Southern Appalachians that we haven`t seen in decades. Major Hurricane Helene will make landfall over the next couple of hours along the Big Bend region of Florida. Helene will quickly move inland and weaken, but its remnants will enter our forecast area only about 12 hours after landfall because of the quick forward speed. That means the strong pressure gradient and associated wind field will still be very intense across portions of the Tennessee Valley and Southern Appalachians. Due to the extratropical transition, the strongest winds will be on the eastern side of the track. This means the highest probability of 60+ mph wind gusts will be near the Knoxville metro and to the northeast. Even valley locations around Knoxville, Morristown, and the Tri- Cities may see wind gusts of 60 to 70 mph on Friday morning through the early afternoon. Locations near the low pressure center track that don`t see the strong 60+ mph wind gusts will likely still see wind gusts of at least 40 mph. Ridge tops will likely see the strongest wind gusts. Combine these winds with saturated soils with trees full of leaves, and widespread downed trees are expected. Widespread power outages will likely be the result with the potential for prolonged power outages. In addition to high winds, PW values are over 2.0 inches across the southern valley and southern plateau. There is a convergent region around Chattanooga that will result in heavy rain late tonight through Friday morning with a band of up to 3 to 6 inches of rain in a 6 hour period. Where this band sets up will have the potential for flash flooding on Friday morning. The flooding and river flooding risk continues area-wide with several rivers at flood stage or forecast to rise to flood stage overnight and into Friday. JB && .SHORT TERM... (This evening through Friday) Issued at 333 PM EDT Thu Sep 26 2024 Key Messages: 1. Major, near record flooding possible on French Broad and Pigeon rivers, with potential for life threatening flash flooding elsewhere. Some uncertainty in the far south and southwest though. 2. Widespread high wind event likely to unfold tonight into Friday morning. Expectations are that power outages, and downed trees will be very prevalent across the area. Discussion: A fairly historic situation is about to unfold across the southern Appalachian region over the next 24 hours as the remnants of Hurricane Helene move through. Synoptically speaking, Helene is rapidly gaining strength this afternoon and is expected to make landfall late this evening along the Florida coast. Afterwards, it will be quickly pulled northward through a gap between an upper low over western TN and a ridge over the western Atlantic. Ahead of the storm, tropical moisture will continue to be funneled into the southern Appalachian region, making for continued light to moderate rainfall across the forecast area before the last slug of tropical downpours arrive with the core of Helene late tonight into Friday morning. Flooding along with high winds continue to be the two main concerns with this event. Winds: The forward speed of this hurricane after landfall is expected to be on the extreme upper end of the spectrum. As such, the wind field will stay intact much further inland than normal. There remains some slight uncertainty in the center track of this storm, but the general consensus, along with the NHC track, brings the center across the southern TN valley around and shortly after daybreak tomorrow morning. Ahead of the system low level winds will turn southeasterly across the mountains, with winds in the higher terrain ramping up to warning levels by around 2 AM tonight. As the core of the system approaches the southern TN valley, winds in the lower elevations should ramp up considerably, especially down the TN valley as pressure gradients rapidly tighten. Those areas should see warning level winds arriving in the 5-7am time frame roughly. Believe that there will be some widespread areas of 55-65 mph winds in the TN valley, the plateau, and even areas up into our VA counties. Most trees still have their leaves on them, and these winds will occur with very saturated soils. As such, expect widespread power outages to occur tonight and tomorrow. Went ahead and upgraded the watch to a warning and now have our entire CWA is now covered by a high wind warning. This excludes our NC counties which are in a Tropical Storm Warning. I`m quite sure there will be some locations that do not see winds close to these values. But I think the likelihood of warning level gusts and resulting impacts warrants the upgrade. Rain & Flooding: Increasing southeasterly winds across the Appalachians will result in some rainshadowing effects and will cut down on additional rainfall north of I-40 relative to the southern valley tonight into Friday. However saturated soils mean that any additional rains will go immediately to runoff. Couple this with the immense amount of rains expected on the NC side of the border and some historic river flooding is expected in area rivers. The Pigeon and French Broad rivers in Newport will be very close to record crests with this event and that`s not to speak of the flash flooding potential across the area. The current forecast for the French Broad vier at newport is 23`, which is one foot shy of the record crest set in 1867! So, this is certainly an impressive event to say the least. Further south, additional rainfall amounts of 3-5" are expected in the southern valley and southern plateau with the approach/passing of the core of Helene. The uncertainty here is that those amounts closely match the 3hr and 6hr flash flood guidance values. Will the rainfall rates and amounts be enough on any time scale to overcome that and cause runoff and flooding issues? I`m not entirely sure. However, tropical events like this have a tendency to over perform, so will keep those areas within the flood watch and continue to advertise the possibility of flooding across the entirety of our forecast area with the remainder of this event. && .LONG TERM... (Friday night through next Thursday) Issued at 333 PM EDT Thu Sep 26 2024 Key Messages: 1. Unsettled weather will continue with seasonable temperatures into early next week. 2. Dry weather appears to return late in the period on Wednesday and Thursday. Discussion: Headed into Saturday, we will be notably on the downward trend from impacts from Helene. Her center is forecast to be over the state line of western Kentucky and Tennessee by that time. Southerly to southeasterly flow will continue to bring moisture from her remnants northward into our area. Although winds will be decreased across lower elevation spots, gusty conditions can still be expected across higher terrain of the southern Appalachians and Cumberland Plateau. Wind gusts in the 40`s to near 50`s is still possible there. The later we get into Saturday, the more the winds will diminish, especially over the typical windier spots. What`s left of Helene and a closed low aloft that will eventually envelop her, will keep cloudy and rainy conditions around at times through early next week. In the middle of the weekend, the closed low that phased with Helene will absorb into the mean flow, become elongated and then ripped off of the continent Monday into Tuesday. Upper level ridging across the lower tier of the US will then expand eastward, as well as surface high pressure from the High Plains that will reach the western Ohio Valley by Wednesday, bringing dry weather back to the area the last couple days of the long term period. High temperatures will mostly remain steady Monday through Thursday with upper 70s to low 80s for valley locale. && .AVIATION... (00Z TAFS) Issued at 722 PM EDT Thu Sep 26 2024 Flight conditions over the next 24 hours will be very poor across the region with widespread rain, vcts, very strong winds, and LLWS associated with the landfalling Hurricane Helene. IFR or LIFR conditions will setup overnight and are forecast to persist as the pressure gradient/winds increases and moderate to heavy rain overspreads the area. LLWS will also be a risk across the region with winds increasing at the surface by 12z Friday. Wind gusts on Friday will likely be over 35kt at all sites with some isolated gusts over 50 kt expected. && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 66 74 64 73 / 100 100 40 60 Knoxville McGhee Tyson Airport, TN 68 76 64 73 / 100 100 20 50 Oak Ridge, TN 66 74 62 72 / 100 100 40 60 Tri Cities Airport, TN 67 78 60 76 / 100 100 10 30 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...Flood Watch through Friday afternoon for Cherokee-Clay. Tropical Storm Warning for Cherokee-Clay. TN...Flood Watch through Friday afternoon for Anderson-Bledsoe-Blount Smoky Mountains-Bradley-Campbell-Claiborne-Cocke Smoky Mountains-East Polk-Grainger-Hamblen-Hamilton-Hancock- Hawkins-Jefferson-Johnson-Knox-Loudon-Marion-McMinn-Meigs- Morgan-North Sevier-Northwest Blount-Northwest Carter- Northwest Cocke-Northwest Greene-Northwest Monroe-Rhea- Roane-Scott TN-Sequatchie-Sevier Smoky Mountains-Southeast Carter-Southeast Greene-Southeast Monroe-Sullivan-Unicoi- Union-Washington TN-West Polk. High Wind Warning until 8 PM EDT /7 PM CDT/ Friday for Anderson- Bledsoe-Blount Smoky Mountains-Bradley-Campbell-Claiborne- Cocke Smoky Mountains-East Polk-Grainger-Hamblen-Hamilton- Hancock-Hawkins-Jefferson-Johnson-Knox-Loudon-Marion-McMinn- Meigs-Morgan-North Sevier-Northwest Blount-Northwest Carter- Northwest Cocke-Northwest Greene-Northwest Monroe-Rhea- Roane-Scott TN-Sequatchie-Sevier Smoky Mountains-Southeast Carter-Southeast Greene-Southeast Monroe-Sullivan-Unicoi- Union-Washington TN-West Polk. VA...Flood Watch through Friday afternoon for Lee-Russell-Scott VA- Washington VA-Wise. High Wind Warning until 8 PM EDT Friday for Lee-Russell-Scott VA- Washington VA-Wise. && $$ SHORT TERM...CD LONG TERM....KS AVIATION...JB