WxWatcher007

D1 High Risk (Flooding) added: Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1144 AM EDT Wed Sep 25 2024 Day 1 Valid 16Z Wed Sep 25 2024 - 12Z Thu Sep 26 2024 ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHEAST GEORGIA THROUGH FAR WESTERN SOUTH CAROLINA AND SOUTHWEST NORTH CAROLINA... ..LIFE-THREATENING FLASH FLOODING INCREASINGLY LIKELY OVER THE SOUTHERN APPALACHIANS.. Deep layer south to south-southwest flow will allow for the advection of increasingly moist, unstable air to funnel poleward ahead of Helene thanks to a closed upper low/trough centered over the Mid-Mississippi Valley. Afternoon destabilization within the budding warm sector along with sufficient left-exit region (LER) jet dynamics from the trough will translate to more widespread convective coverage across the Southeastern U.S into the Central/Southern Appalachians as we move into the late-afternoon and evening time frame. Areas across the Escarpment located over far Northeast GA into the Western SC/NC border have seen 2" to as much as 5" of rainfall in the last 24-36 hrs leading to a priming of the top soil layer as indicated by the latest NASA SPoRT soil moisture percentiles approaching 60-70% over the areas affected. Recent CAMs and ensemble output have increased the QPF signal across the Southeast with the axis of heaviest rainfall bisecting Eastern AL up through Western GA into North GA and the Southern Apps/Smokies bordering TN/SC/NC. HREF blended mean QPF indicates widespread 3-6" of rainfall from this predecessor rainfall event (PRE) that correlates with the strong upper forcing and deep tropical connection to north of Helene as it wanders into the Eastern Gulf. Probability fields are also very aggressive with the signature for enhanced rainfall rates and totals with the EAS signal for 3" settling at 50-80% within the Escarpment area into Northeast GA. Neighborhood probabilities of >5" between 70-90+% and >8" between 20-35% are more than sufficient for a very impactful event expecting to unfold across the complex terrain in-of the Southern Appalachians. Given the signals expressed via the hi-res ensemble, along with ample support from the various ML output for the heaviest rainfall, as well as coordination with the local Greenville-Spartanburg WFO, a targeted High Risk was added for the D1 period.

27 was fine and my guess is 98 was good too. The difference here is the wind field is likely to be so much bigger that I think it’s going to be tough on state roads. Ideally for me, I’d end up closer to I-10. If the models hold that would be the case, and with the forward speed we’d still get big winds inland.

We have our first VDM with data on a nascent eyewall trying to form, along with FL winds slightly above 64kt. Product: Air Force Vortex Message (URNT12 KNHC)Transmitted: 25th day of the month at 13:54ZAgency: United States Air ForceAircraft: Lockheed WC-130J Hercules with reg. number AF99-5309Storm Name: HeleneStorm Number & Year: 09 in 2024 (flight in the North Atlantic basin)Mission Number: 11Observation Number: 11 ( See all messages of this type for this mission. )A. Time of Center Fix: 25th day of the month at 13:12:50ZB. Center Fix Coordinates: 21.33N 86.16WB. Center Fix Location: 46 statute miles (74 km) to the ENE (75°) from Cancún, Quintana Roo, Mexico.C. Minimum Height at Standard Level: 2,941m (9,649ft) at 700mbD. Minimum Sea Level Pressure: 982mb (29.00 inHg)E. Dropsonde Surface Wind at Center: From 360° at 22kts (From the N at 25mph)F. Eye Character: Open from the northeast to the southeastG. Eye Shape: Elliptical (oval shaped)G. Orientation of Major Axis in Elliptical Eye: 360° to 180° (N to S)G. Length of Major Axis in Elliptical Eye: 50 nautical miles (58 statute miles)G. Length of Minor Axis in Elliptical Eye: 30 nautical miles (35 statute miles)H. Estimated (by SFMR or visually) Maximum Surface Wind Inbound: 47kts (54.1mph)I. Location & Time of the Estimated Maximum Surface Wind Inbound: 46 nautical miles (53 statute miles) to the NE (36°) of center fix at 13:00:00ZJ. Maximum Flight Level Wind Inbound: From 128° at 51kts (From the SE at 58.7mph)K. Location & Time of the Maximum Flight Level Wind Inbound: 94 nautical miles (108 statute miles) to the NE (42°) of center fix at 12:46:30ZL. Estimated (by SFMR or visually) Maximum Surface Wind Outbound: 60kts (69.0mph)M. Location & Time of the Estimated Maximum Surface Wind Outbound: 29 nautical miles (33 statute miles) to the SSW (204°) of center fix at 13:21:30ZN. Maximum Flight Level Wind Outbound: From 284° at 68kts (From the WNW at 78.3mph)O. Location & Time of the Maximum Flight Level Wind Outbound: 39 nautical miles (45 statute miles) to the SSW (205°) of center fix at 13:24:30ZP. Maximum Flight Level Temp & Pressure Altitude Outside Eye: 10°C (50°F) at a pressure alt. of 3,049m (10,003ft)Q. Maximum Flight Level Temp & Pressure Altitude Inside Eye: 16°C (61°F) at a pressure alt. of 3,049m (10,003ft)R. Dewpoint Temp (collected at same location as temp inside eye): 6°C (43°F)R. Sea Surface Temp (collected at same location as temp inside eye): Not AvailableS. Fix Determined By: Penetration, Radar, Wind, Pressure and TemperatureS. Fix Level: 700mbT. Navigational Fix Accuracy: 0.02 nautical milesT. Meteorological Accuracy: 2 nautical milesRemarks Section: Maximum Flight Level Wind: 68kts (~ 78.3mph) which was observed 39 nautical miles (45 statute miles) to the SSW (205°) from the flight level center at 13:24:30Z

I’d wait to see more longer term motion/center fixes. That’s a better way to characterize it, thanks. Here’s the illustration for others. Will try to add those in future posts. 18z yesterday 00z 06z

The west trend has continued on the globals, with the 06z Euro and 06z GEFS ticking west. More in line with the spaghetti models.

Long post looking at the various hazards.

First light is beginning to reveal a much more organized TC near the Yucatán Channel, a harbinger of things to come. Recon, radar, and satellite all show a modestly organizing and gradually intensifying TC that will likely be a hurricane by the 11am full advisory. With shear reduced over the system itself and dry air most likely out of the way as an inner core forms in the next 12-18 hours, it looks like the runway is primed for Helene to take off in intensity late tonight into Thursday provided the center does not traverse the Yucatán. You can tell dry air—something that has plagued systems across the basin all season—is not a factor at this time because of the expansive outflow and deep CDO convection that has persisted all night. I would watch the SE quadrant in case some of the drier air to the south wraps in. We have a system that’s already sub-980mb as a result, but that’s only part of the story. Generally speaking, the wind field is still broad. A smaller system would be a hurricane by now with a tighter center. Helene is poised to become one of the largest named storms since 2000. This will have tremendous implications for the forecast and scale of region wide impacts. First, the wind. This is extraordinary to look at—tropical storm warnings from the Keys to coastal GA for a Gulf storm and hurricane warnings well into southern GA. The storm already has TS winds expanding 175 miles out and that’ll grow substantially over the next day. In fact, you can already see how large Helene is using zoomed out satellite. It’s going to take time for the RMW to tighten enough for major hurricane wind speeds to be found. What we’ll likely see are very low pressures with an expansive area of “modest” hurricane force FL then surface winds before RI takes place and tightens the inner core at that point. That said, I don’t believe the HAFS depictions of a small core and large TS field. This may limit peak wind potential some. That’s not to say the potential for something on the order of a category four isn’t there because it is, but we’ll just have to see how the storm reacts to falling pressure. Second, the surge. What may be lost in peak wind potential will likely be more than made up for in surge, which may be catastrophic in the Big Bend region and major in other locations. The difference in category will matter little given the surge potential. While a stronger or weaker peak wind could do significant damage inland, the coastal impacts are already baked in. Third, the rain. If wind and surge weren’t enough—and both have the potential to be high end—rainfall looks to be a major concern well inland. A Predecessor Rain Event looks likely, and a HIGH risk for excessive rainfall has been issued. Extreme rainfall is possible and inland folks should not let their guard down just because Helene is currently well south. Fourth, severe potential. Finally, we have severe potential on the east side of Helene. I think we need to see how banding sets up tomorrow and Friday, but a D2 enhanced risk is pretty significant for a TC. This is a long post that didn’t even go into track (though I’m closely watching potential shifts west, possibly worsening wind impacts into the Panhandle and southern GA) But for the general public the center will matter less than usual given the size of the wind field and surge potential all along the coast.

Spaghetti models are a touch west too. I do think there’s a limit to how west it can go given the steering pattern and how deep I think this’ll get, but TLH could be the bullseye.

Thanks. Ominous and maybe a touch west too. This is going to be an extraordinarily impactful storm for the region, and it’d actually be a bit easier for me if it did force me into the panhandle—easier highway access.

This is an absolutely exhausted WxWatcher007 reporting live from Perry, FL. I just got here so I can’t give much of a lay of the land, but I know from the logistical challenges already that this is a serious one. I may need to reposition, as it’s unclear what, if any, places will be open here in the region Thursday. In a sturdy ride out spot at the moment. The goal this time is (hopefully) no need to move (I’m hedging east) and a very good pressure trace of the center. Will be reporting on conditions too of course. For now though, first food of the day and a little rest. Wish us all luck down here.

New D2 SPC outlook highlights a pretty significant tornado risk in coastal GA. @GaWx Off to bed. Long day of travel to the Big Bend region. May need to reposition tomorrow.

The 18z hurricane models (HWRF, HMON, HAFS-A&B) show just how critical the next few 12-18 hours may be with the track of the LLC near the Yucatan. Each run misses the landmass and all develop a formidable major hurricane as a result.

Good stuff, @40/70 Benchmark. Our thoughts are aligned. If there is a legitimate inner core completed or near completion at the Yucatán Channel, I think it’ll be all systems go for a high end wind and surge system, even if an ERC levels things off later. If not, it may just slow walk its way along in terms of intensification until Thursday. I do think this will end up a large system and will be much more resistant to shear and/or dry air as a result, with the increasing forward speed being a secondary factor in limiting any window for weakening. I think the floor is probably around 100mph and the ceiling is still fairly high, around 140mph. Right now I think LF is probably around 120 imo.

The thread gets renamed, and I try to keep the latest advisory info in the title until landfall.

Much like in the Gulf, shear vector and storm motion matter a lot in how much shear a TC actually encounters. It looks like a shift in shear direction has now given this the space to organize. Just look at visible satellite from yesterday vs now. Tail Doppler shows that the LLC and MLC are pretty well aligned. That likely means the organizational trend (not necessarily intensity, yet) is poised to pick up the pace—if alignment holds.

A couple things also stand out on that GFS run. The first is the presence of a pretty significant Predecessor Rain Event (PRE) in north GA/eastern TN/western NC. The second is a significant wind field of TS winds, even along the southeast coast. That kind of onshore flow could cause some flooding issues as well.

I uninstalled this week. It’s not like nights are 75/71 now. I haven’t had a low above 63° since 9/1.

I don’t think dry air is likely to be a major inhibitor (but watch for it still given the theme of the season) because I don’t think we’re going to get enough shear in the Gulf given storm motion to impart much before there’s a well organized inner core. Unlike other storms, even if there’s a last minute increase, which didn’t weaken storms like Beryl and Francine, this one will be moving quite fast near the coast. Not much time to weaken imo. To me, the things that could keep this in check wind wise (and currently I think “high end” 2 seems to be the floor) are: 1) too much time organizing the next 48 hours and establishing an aligned vortex, which could be done with the current southwesterly shear in the Caribbean and/or 2) Early ERCs that promote the storm to expand substantially and become inertially stable, but with a much broader wind field. In fact, I think the earlier RI happens the more likely 2 becomes. To be sure still potentially higher end in this scenario, with surge impacts being enhanced along the west coast of Florida and peak wind impacts blunted some in the Big Bend/Panhandle. Still a lot of uncertainty for something less than 72 hours out.

Really high end RI. Honestly it’s impressive that both the EPAC and Gulf could see rapidly intensifying hurricanes just a few days apart.

It may be trying to get to the surface but I think it’s going to be hard to get a read without recon, and it may take a while given the southwesterly shear. Cayman Islands radar could be helpful tonight as the process takes its first shot at it. https://www.weather.gov.ky/radar

Thanks for reminding people, and not that anything I say matters, but for folks reading—this could be a high impact event, and we have not just posters but probably a lot of lurkers that will be reading this. Not that anyone should be making decisions based on what’s said here lol (always follow official sources, etc) but for people posting I just ask that we set aside any wish casting, down playing, or non-contextualized posts (for example, posting an image of a hurricane model without analysis) as this comes together. Obviously stuff happens but let’s try to stay on track. There’s an opportunity to learn a lot here the next few days about tropical genesis, the development of inner core structures, and how TCs interact with troughing/deep layer shear, and that discussion gets lost in poor posts. tl;dr I sound like a prude lol but let’s be responsible and thoughtful in our posts as people will likely be seriously impacted by this one.

PTC Nine will be designated at 11am.

The NHC in the 8am outlook is telegraphing a PTC designation by 11am to issue watches/warnings for Cuba and Mexico.

This was the first cone for what became Michael—also a system that came from a CAG. But I should also add that Michael (obviously) was the exception to the rule. Few Atlantic CAG TCs become major hurricanes, for many of the reasons discussed here.

I think the ceiling for this is less related to shear and more to how organized it can become before reaching the Gulf. While the environment in the Gulf doesn’t look pristine from an intensification standpoint, I think some of the higher end potential (not necessarily the GFS solutions but rather the middle ground between GFS/Euro) can be realized if this isn’t a broad gyre type system in that timeframe. The loop current in the Gulf is awfully impressive and I don’t know if moisture is going to be a huge inhibitor. The impact of shear could also be lessened if it’s aligned with storm motion. Just my .02.