Area Forecast Discussion
Issued by NWS Louisville, KY
Issued by NWS Louisville, KY
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
221 FXUS63 KLMK 240659 AFDLMK Area Forecast Discussion National Weather Service Louisville KY 259 AM EDT Tue Sep 24 2024 .KEY MESSAGES... * Strong to severe storms possible today, especially in the afternoon and early evening. Gusty winds will be the main severe threat, with hail a secondary threat. A brief tornado is also a non-zero threat. * Drought-easing rainfall in the forecast as the remnants of a tropical system move into the region Friday. Stay tuned to latest updates from NHC on the forecast path/strength of this system. && .SHORT TERM /THROUGH TONIGHT/... Issued at 258 AM EDT Tue Sep 24 2024 Rounds of showers and thunderstorms expected today as a shortwave trough digs into the area, and a surface low gradually deepens across Indiana and the lower Great Lakes. There is a lead shortwave embedded within this larger trough that is currently responsible for driving a convective cluster mainly across southern IN at this hour. The overall environment that these storms are currently in isn`t that bad given 40 knots of deep layer shear, and around 1000 J/KG of MUCAPE. So, do expect these storms to maintain through the morning hours, and wouldn`t be surprised to see a few stronger radar signatures. Biggest limiting factor to anything severe is the notable near-surface inversion around 925 mb. This inversion is evident on forecast soundings, and observed AMDAR soundings early this morning. As a result, think this stable layer will mostly limit any stronger gusts from getting all the way down to the surface. Nevertheless will continue to monitor through the morning hours. Once this lead shortwave passes, do expect a short lull in activity as we remain in the subsident wake, and we haven`t had a chance to destabilize yet. As a result, looking for a fairly quiet window from around sunrise through mid to late morning. By midday into the afternoon, the main shortwave trough axis approaches with the surface low down around 1006-1007 mb over northern Indiana. The cold front trailing from this surface low will be poised to push through our area fairly quickly this afternoon into evening with scattered storms firing ahead of the front thanks to modest instability (1000- 1750 J/KG of MLCAPE), increasing forcing beneath the exit region of an intensifying upper jet, and moisture transport on a subtle low level jet. Overall, this setup could favor a few supercell structures given the exit region of the upper jet, and a fairly strong speed shear profile. The overall hodograph features mostly unidirectional shear, so the main threat should be damaging wind. That being said, can`t rule out a low end tornado threat should any localized surface winds become a bit more backed, and the low level hodograph becomes more looped. Like the addition of the 2% Tornado probs with this Day 1 Outlook. Did want to point out that although overall instability is fairly weak, and it may be somewhat hard to destabilize given cloud cover today, there is a good amount of forcing that could offset the weaker overall instability. As a result, do think we`ll get some strong updrafts. The Slight Risk looks good and happy with the expansion of it as well. Not much changes to the expected timing of the severe threat. 1 PM to around 7 or 8 PM looks reasonable at this time. The cold front only looks to make to around the I-75 corridor by tonight, and then will likely stall. This should shut most of our precipitation off from W to E as it does so, however will likely have to keep some lingering shower and storm chances across our east into the overnight. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Issued at 258 AM EDT Tue Sep 24 2024 Wednesday... The weather continues to look fairly active in terms of precipitation chances from midweek into the weekend. Wednesday is shaping up to be one of the quieter days, though still not rain- free. A closed upper low is forecast to spin southeast over SE Missouri on Wednesday, with a strengthening southerly ULJ over the TN Valley. A sfc cold front will begin the day draped roughly from Middle TN through east-central KY, with a plume of deep moisture (PW values > 1.6 inches) now east of the forecast area. The weak boundary will make slow progress through eastern portions of KY/TN. The increasing southerly winds aloft do draw slightly higher moisture content northward into the region with PW values near 1.4 inches along the I-75 corridor. PW values taper off to 1.2-1.3 inches further north and west. Skies look to remain mostly cloudy, with the exception of brief, partial clearing west of I-65. The cloud cover will limit surface heating, with afternoon highs only in the mid/upper 70s. Scattered shower activity looks possible, especially across the Bluegrass and Lake Cumberland regions. A few storms will also be possible near I- 75, but fairly weak in nature. QPF for the daytime period is light, only around a tenth of an inch or less on average. And the western half of the forecast area is likely to stay dry. Meanwhile, PTC Nine (currently named) is forecast to move north into the southeastern Gulf of Mexico by evening while strengthening to hurricane strength. Wednesday night - Thursday night... An upper level ridge of high pressure will drift slightly further east off the Southeastern US coast Wed into Wed night. The closed upper low just to our west is forecast to wobble westward over the Ozarks heading into Thu. All of the above allows PTC Nine to accelerate NNE over the eastern Gulf as a hurricane Wed night into Thursday. Locally, we will see increasing diffluence aloft and strengthening southerly to southeasterly winds through a deep layer. The strengthening southeasterly flow will provide increased forcing for ascent and moisture transport into the region. With even the potential for a lingering stalled boundary/inverted sfc trough draped somewhere through KY/TN, the synoptic setup suggests a predecessor rainfall event (PRE) is likely extending poleward from the tropical system. This would enhance rainfall totals through the Southeast and potentially into portions of eastern and central KY. Areas further south, such as GA and eastern TN, are likely to see higher rainfall amounts. However, we`ll need to retain elevated PoPs in the forecast well ahead of the arrival of the tropical system. Periods of scattered to numerous showers, moderate to heavy at times, will be possible during this time frame. Friday through the Weekend... The weakening tropical storm is forecast by NHC to arrive in KY as a tropical depression Friday evening. The system will then continue to weaken over the weekend, while possibly pinwheeling to the northwest around the remnant upper low and making the extratropical transition. Expect likely rain showers on Friday along with windy conditions. Cannot rule out at least a low-end severe threat given the wind fields surrounding the tropical depression, but this will largely depend on the east-west placement of the low when it traverses the Lower Ohio Valley. There is still some meaningful spread in the low track at this time range. Scattered showers will remain possible over the weekend, with details dependent on the evolution of the closed low pressure system. Total QPF (including rain in the Short Term) of 2-4+ inches is expected through the weekend. Some locally enhanced totals will be possible given the convective/tropical nature of the rain at times. Overall, this rain will be hugely beneficial given the ongoing drought. However, locally enhanced training would boost a short- fused flash flood threat. The risk for widespread, long-fused flooding is low. But given the unsettled pattern in place, instances of minor/nuisance flooding may become more common by the end of the week with tropical remnants moving through. && .AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 151 AM EDT Tue Sep 24 2024 A messy forecast for today as an area of low pressure moves just to the north of the TAF sites, and rounds of showers and storms accompany it. The first complex is entering the area now, with HNB/SDF the most likely candidates for a shower or storm in the pre- dawn hours. Any lingering low ceilings are expected to slowly improve as a warm sector tries to get a bit more established over the area. As we move toward midday and through the afternoon hours, scattered thunderstorms, some strong, are expected to develop ahead of a cold front and strong upper disturbance. Have included mention of t- storms with a Prob30 for the best expected time range. Conditions improve with winds starting to veer as cold frontal passage begins toward the end of this forecast window. && .LMK WATCHES/WARNINGS/ADVISORIES... KY...None. IN...None. && $$ SHORT TERM...BJS LONG TERM...EBW AVIATION...BJS