Area Forecast Discussion
Issued by NWS Louisville, KY
Issued by NWS Louisville, KY
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064 FXUS63 KLMK 260741 AFDLMK Area Forecast Discussion National Weather Service Louisville KY 341 AM EDT Thu Sep 26 2024 .KEY MESSAGES... * Widespread moderate to heavy rain and windy conditions on Friday as the remnants of Hurricane Helene move through the Lower Ohio Valley. * Potential for 2 to 4 inches of rain from late Thursday night to early Saturday morning. Locally higher amounts are possible. * Wind gusts of 35 to 45 mph are possible during the day Friday as the center of Helene works into KY and the Ohio Valley. && .SHORT TERM /THROUGH THIS AFTERNOON/... Issued at 340 AM EDT Thu Sep 26 2024 Synopsis...Cut-off upper low will slowly move south across the Ohio Valley and meander over the western half of Tennessee Valley during the next 24 hours while upper ridging consolidates virtually across the rest of the CONUS. This stagnant pattern will allow Hurricane Helen to quickly move northward to make landfall later today under the increasing influence of the upper low to then become absorb by it. The increasing tropical moisture influx along with typical post- tropical transition dynamics will account for surging rainfall chances and strengthening wind gusts with worst impacts in our area expected on Friday and onwards (see Long Term section below). Today...Current surface analysis depicts a weakening frontal zone oriented north to south across eastern Kentucky. Radar and satellite imagery show shower activity well ahead of the front leaving central Kentucky and southern Indiana in the cool side of the boundary characterized by dry conditions, calm surface winds, and building low-level clouds/fog. As a result, expect reduced visibilities during the early morning commute along I-75 and southern KY highways with patches of dense fog close to river valleys and protected areas. Any fog/visibility issues should burn off by midmorning as daytime heating mixes the BL and surface winds slowly increase from the northeast with intermittent gusts of 15 mph or more during the afternoon. Rain chances will augment from east the second half of the day as the leading edge of the moisture influx clashes with the weakening front and starts to wrap around the upper low circulation. Considering PW values and synoptic lifting mechanisms, nature of today`s rainfall should be mostly light with a isolated chances of moderate rainfall along axes of stronger mesoscale/localized lifting. Tonight...Expanding moisture advection associated with the approaching 850-mb jet will take PW values over the 2-inch mark with surface winds gusting around 20 mph or more. Therefore, heavier and more persistent rain chances are reflected with perhaps a low chance of elevated thunder with the strongest cores. However, no flooding or severe concerns are anticipated at the moment. && .LONG TERM /TONIGHT THROUGH WEDNESDAY/... Issued at 340 AM EDT Thu Sep 26 2024 Friday - Sunday Night... The center of Helene is expected to travel from northern GA to western KY through Friday, with a transition to an extratropical state as it does so. Steady weakening is also expected to occur during this time, although current data supports a low pressure still sub 990 mb as it crosses into KY. That is still an anomalously strong low pressure system, and it will bring impacts to our region, most notably on Friday. Widespread rainfall is expected to be ongoing by sunrise Friday, along with increasingly gusty winds. Precipitable water (PWAT) values are expected to rise to the 2-2.25" range as the core of the system moves into the region, and not surprisingly, forecast soundings show a saturated and deep warm cloud layer. This will result in efficient rainfall processes through the day. Will also note that an extremely strong wind profile will also accompany this system, and low level helicity values will be very high. Looks like we will be lacking in the instability department, however at least have to be wary of potential for a brief spin-up tornado. CSU machine learning does show some low probabilities for tornadic activity 12z Friday to 12z Saturday. The more widespread concern with regard to the wind profile will be with how much wind energy aloft will make it to the surface? Wouldn`t have to mix very deep to get into a 50 knot layer of wind. While clouds and rain should limit that potential to mix very deep, do see some potential in some of the data. For instance, models are fairly consistently showing a "QPF shadow" on the NW side of the Appalachian spine through the morning hours of Friday that could be the result of dry air being entrained into the weakening system, and/or some downslope warming/drying of the low level airmass off the Appalachians. The low level jet does look to be oriented perfectly perpendicular at that time, and have seen this a time or two in the past with this type of setup. Should that occur, we could see some clearing across eastern/northeastern portions of our CWA through midday or so. Something to watch for that could result in higher gust potential in those area. Areas across our southern and western CWA are expecting more rain/cloud cover so thing the gust potential is a little lower there, although that area will be closer to the low pressure center. Right now, like the idea of messaging 30 to 40 mph with some locally higher gusts, but let`s keep that potential drying and any associated clearing potential in the back of our minds through midday Friday. We`ll be pretty close on Wind Advisory criteria, and the main focus in the long term will be Helene and how it impacts our region for the end of the week and into the weekend. Main impacts continue to be the potential rainfall as well as strong gusty winds associated with the system as it moves into the Ohio Valley. As of the 11am EDT update, Helene had become a category 1 hurricane with the National Hurricane Center forecast anticipating Helene to rapidly intensify into a potential category 4 hurricane over the next 36 to 48 hours before making landfall along the FL Panhandle Thursday evening. The current NHC track for Helene appears to be a bit further west and south compared to the 12z run of the deterministic models, which brings the center of the storm northward through GA then more into eastern KY before hooking it to the west and meandering over the area into the weekend as it weakens further. Our first impacts from Helene will be felt Thursday night into early Friday morning as moderate to potentially heavy rain work in from the south as the system quickly works through GA as it slowly weakens. Rainfall amounts could range from a quarter of an inch near and just south of the Parkways to nearly an inch along the KY/TN border. Rain will continue during the day on Friday as Helene works into the Ohio Valley then starts to hook and loop back to the west Friday night into Saturday. An additional 1 to 2 inches will be possible across our CWA from 12z Friday to 12z Saturday. Total rainfall from Helene is still anticipated to range between 2 to 4 inches with locally higher amounts possible. The second impact will be the potential for strong advisory criteria wind speeds and gusts during the day on Friday. NBM has continued to trend too low on its wind values for Friday and with the 12z HREF Ensemble probability of wind gusts equal or greater than 40 mph by 12z Friday is between 80-100% with EURO Ensemble showing the same during the day on Friday. While confidence is becoming high we will likely need a wind headline in the form of a Wind Advisory on Friday, locations needed in the advisory remain in doubt. Like the rain mentioned above, the wind forecast will be determined on how much Helene weakens as it works northward through GA and where the center of the storm will be located as it reaches KY and the Ohio Valley on Friday. Given this uncertainty, opted not to issue any weather headlines at this time. As Helene transitions into a post-tropical cyclone over the weekend, showery weather will linger as it merges with the upper low over the Ohio Valley. It will then open up and exit off to the east northeast as we start next week.pect we will likely need some sort of messaging at least for a portion, if not all of the CWA. The rainfall portion of the forecast has overall remained pretty consistent, and still thinking that a 2-4" range, with locally higher amounts, looks pretty good. Since the bulk of those totals is expected to fall over a 24 to 36 hour period, do think we`ll be able to handle most of it okay. For example, 6 hour FFG values are currently running from 2.5" to 4" across our area, and the latest 26/00z HREF data show probabilities of less than 20% for any 6 hour totals over 2". That alone would suggest that widespread flooding concerns are not likely at all. However, we also have to keep in mind that as the event unfolds and amounts pile up, these thresholds will drop, and flooding potential with any moderate or briefly heavy showers will increase. As we get deeper into Friday and Friday night, some flood concerns may begin to arise. WPC Slight Risk for Excessive Rainfall Friday into Friday night seems pretty reasonable. Possible we may need a Flood Watch, and will explore those possibilities as we near the event. River ensemble forecast still suggest that we will be able to handle this amount of rainfall, so not expecting any real issues on the basins unless something changes, or we get more rainfall than expected. The remnant low continues to weaken on Saturday, with central pressure likely rising above 1000mb, and a steady eastward wobble. While, rainfall won`t be as widespread and consistent, we`ll still see periods of showers through the day, with additional rainfall ranging between .33" and 1" for most. Continued weakening occurs on Sunday, with rain showers more spotty and weaker than Saturday. Overall, looking for small diurnal ranges through the weekend as one would expect with a saturated column/heavy cloud cover. Highs should generally range in the low to mid 70s for most, while lows mostly stay in the low to mid 60s for most. Monday - Wednesday... Lower confidence for the early week time frame as data starts to diverge on how quickly the lingering closed upper low exits to the east. Will keep some isolated to scattered shower chances (especially in the east) in for Monday and Tuesday, with a transition to fully dry by Wednesday. Highs look to remain in the 70s early week, possibly warming to around 80 by mid week. && .AVIATION /06Z TAFS THROUGH 12Z FRIDAY/... Issued at 147 AM EDT Thu Sep 26 2024 Impacts/Confidence: - Low to medium confidence in MVFR VIS this morning at BWG - Lowering ceilings, increasing rain chances, and enhancing wind gusts later this afternoon/tonight as TC Helen makes landfall. Discussion...Current surface observations and radar/satellite imagery reveal calm surface winds with some fog/low-level clouds building to the south and east, respectively. The most concerning trend towards sunrise appears to be the possibility of MVFR, or even IFR, VIS at BWG and to a lesser degree intermittent MVFR CIGs at LEX. Previously-mentioned restrictions should lift after 14z (at most) as daytime warming mixes the BL and northeasterly winds slowly augment. VFR conditions will predominate during the afternoon hours expect to the easternmost terminals LEX/RGA where an MVFR ceiling is likely to continue. Additionally, surface winds will keep increasing to the point that some gusts between 15-20 knot gusts could be reported. The last portion of the period features declining weather conditions as TC Helen approaches the Florida coastline and moisture/cloudiness spreads over the Ohio Valley. && .LMK WATCHES/WARNINGS/ADVISORIES... KY...None. IN...None. && $$ SHORT TERM...ALL LONG TERM...BJS AVIATION...ALL