Sunrise 6:42 AM   Sunset 8:38 PM Moonrise 3:47 AM   Moonset 3:51 PM

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FXUS63 KLMK 041440 AFDLMK

Area Forecast Discussion National Weather Service Louisville KY 1040 AM EDT Sat May 4 2024

KEY MESSAGES

* Isolated to scattered showers and thunderstorms possible this afternoon. Slow moving storm motion and moderate to heavy rain expected with any storm.

* Isolated showers and thunderstorms are expected on Sunday before widespread rain chances return Sunday night into Monday.

* Increasing potential for multiple rounds of strong to severe thunderstorms next Tuesday through Thursday, though confidence in details is still low.

* Elevated flooding potential will exist where multiple rounds of heavy rain from strong storms occurs over the next week.

UPDATE
Issued at 1039 AM EDT Sat May 4 2024

Focus for convective development next couple of hours continues in the same 2 corridors mentioned earlier, along a line from Scottsburg to Cincinnati area and then a weaker corridor in southern KY. Still expecting a brief pause in activity before afternoon heating allows for new development. Seeing more breaks in the clouds west of I-65.
It's interesting to note that the HRRR and NAM3k do not have a lot of convective activity this afternoon, whereas the FV3 shows scattered coverage along and east of I-65. Thinking with moisture in place and some breaks we should get that scattered coverage, so keeping the forecast in that direction. Not looking at severe potential, but with high precipitable waters and relatively slow movers, we'll have to watch for training cells and localized flooding.

Issued at 801 AM EDT Sat May 4 2024

Hiding under the mask of high clouds moving across the region, we have some lower-topped storms developing in a couple of patches this region, one nearing the KCVG terminal and another across south central KY. Have bumped up thunder chances and pops in general this morning to account for current radar trends. SPC mesoanalysis does show a corridor of MUCAPE running from south central KY NE to the Bluegrass. This axis has been weakening over the past few hours, so would not be surprised to see thunder chances take a brief pause before returning later this afternoon with some heating. Updated forecast already out.

SHORT TERM /THROUGH TONIGHT/
Issued at 331 AM EDT Sat May 4 2024

========== This Morning ==========

A weak, stalled frontal boundary remains draped across portions of the region this morning. Temperature/Moisture spreads on either side of the boundary remain diffuse, but weak low level convergence has aided in sporadic shower development early this morning. ACARS from SDF have indicated poor low/mid level lapse rates, so thunderstorms are highly unlikely in most of this activity.

While it's been slow to develop, stratus is expected to become gradually widespread toward dawn along with some patchy fog. These clouds will gradually lift after sunrise and transition to more of a strato-cu by the afternoon. Should the stratus not be as widespread as anticipated by dawn... dense fog would then become an issue this morning in areas of clearing as winds will be light and soils/ground very moist from recent rains. Will continue to evaluate the cloud/model trends going through the morning hours but something to monitor closely.

========== This Afternoon ==========

Surface heating will begin to erode a weak low level cap so that by early afternoon we should be CIN-free across the region. Atmosphere will turn more unstable as models indicate 1,000-1,500 J/KG of MLCAPE to support thunderstorm potential, though deep layer shear will be very weak so storms that do develop today shouldn't pose much of a severe/organized threat. Coverage of storms looks to be mainly limited by a lack of a defined lifting mechanism as the frontal boundary currently in place this morning will become diffuse and 'washed' out by the afternoon. Convection that develops today will generally be along mesoscale/differential heating boundaries, and most of the CAMs point to those being east of the I-65 corridor.
As such, generally carrying 20-30% chances for rain/storms west of the I-65 corridor, with 30-50% chances east of the I-65 corridor.

Given weak deep layer shear, storm motions will be on the slower side today (~10mph), and forecast soundings and environmental parameters support heavy rain in thunderstorms today. Areas that see training (repeated) storms over a short timeframe could have some localized flood issues.

========== Tonight ==========

Showers/Storms from the afternoon should steadily decrease overnight as a weak nocturnal inversion builds in. Most rain chances drop considerably after midnight. Should be another mild night as dewpoints remain elevated and lows by Sunday morning hover in the low/mid 60s.

LONG TERM /SUNDAY THROUGH FRIDAY/
Issued at 331 AM EDT Sat May 4 2024

Synopsis...

At the beginning of the extended forecast period on Sunday, two areas of upper level troughing are expected across North America.
First, a lower amplitude trough axis extending from Hudson Bay down into the upper Great Lakes will quickly eject eastward toward the Canadian Maritimes during the first half of next week. A sfc cold front will approach the Ohio Valley from the west on Sunday but is expected to dissipate as it loses support from the progressive upper trough. In the wake of the first cold front/upper trough, a subtle mid-level disturbance is expected to develop across the southern Plains on Sunday before moving northeast into the mid-Mississippi and lower Ohio Valleys Sunday night into Monday. Ahead of this feature, low-mid level moisture return is expected across the Ohio and TN Valleys, supporting enhanced chances for rainfall as this wave crosses the region late Sunday night into Monday.

The more impactful large-scale feature in the long term will be an amplified upper trough which is expected to eject eastward from the intermountain west into the central and eastern CONUS during the middle of next week. With the trough progged to broaden considerably in medium-range ensemble guidance, its eastward transition will be fairly slow, leaving our region in deep-layer SW flow as embedded shortwaves transit from SW to NE over much of next week. This will allow for multiple chances of showers and thunderstorms, as support from upper-level shortwaves will combine with a moist, unstable air mass south and east of a broad baroclinic zone. By the end of next week into early next weekend, the large-scale trough is expected to settle across the eastern half of North America, bringing a reduction in instability, and by extension, shower and thunderstorm chances across our region.

Sunday through Monday Night...

As mentioned above, a decaying cold front will approach on Sunday, which will primarily be identified by a low-level moisture gradient, as drier air tries to push in behind the front from the north and west. With the front expected to hang up/wash out over our area, low- level moisture will still remain elevated, with HRRR mean dewpoints Sunday afternoon in the mid 60s. This will allow for 1000-1500 J/kg SBCAPE to develop (HRRR 25th-75th percentile), which when combined with the subtle forcing from the front, should be enough to initiate isolated to scattered showers and storms Sunday afternoon. Current PoPs range from 30-50%, but this may need to be adjusted downward if subsequent hi-res model runs increases confidence in more isolated convection. Coverage of showers and storms should decrease initially after sunset Sunday as instability decreases.

Sunday night into Monday, increasing southerly flow and moisture advection downstream of the approaching mid-level shortwave will support increasing coverage of showers and a few embedded thunderstorms. There is still some disagreement in the models on the exact track of Monday's system, although enhanced rain chances from most solutions, with variations in track mainly affecting precipitation totals. The severe potential on Monday looks to be limited, as a deep warm and moist layer creates sounding profiles which more closely resemble the "tall, skinny CAPE" paradigm, which is generally unsupportive of severe convection. By Monday night, precipitation should begin to clear the region to the east, assisted along as the deeper trough over the western CONUS pushes its way into the central CONUS. Monday night into Tuesday morning should see a relative lull in precipitation coverage and intensity as brief ridging aloft moves across the Ohio/TN Valleys.

Tuesday through Late Next Week...

Confidence continues to increase in the potential for multiple waves of strong to severe thunderstorms and flooding during the middle of next week. While there is relatively good agreement on the large- scale features during the middle of next week, there is lower confidence in timing and expected intensity of individual waves of showers and thunderstorms, which will be driven by transient shortwaves ejecting along the southeastern flank of the larger trough. Of particular note is continued elevated values of the CAPE and CAPE-Shear fields within the ECMWF Extreme Forecast Index, with values exceeding 0.8 in both fields from Tuesday through Thursday.
This suggests that there is good agreement within the ECMWF ensemble members in elevated values of CAPE and wind shear during this time period. Colorado State Machine Learning severe weather probabilities show an area of greater than 30% chances of severe weather across the mid-MS, OH, and TN valleys during this time period. In fact, there is relatively good agreement in extended rage severe convection proxies of multiple kinds (Neural Network/ML, analogs, ensemble-derived) during the middle of next week.

While this whole Tuesday-Thursday time period exhibits elevated concern, the expected combination of kinematics and thermodynamics appears most concerning next Wednesday into Wednesday night.
Ensemble guidance generally shows a sfc low passing north of the I- 70 corridor, with destabilization taking place across central KY and southern IN within the warm sector Wednesday afternoon. There are certainly failure modes out there (residual effects from antecedent convection, to name one), but taking model sounding progs at face value, there would be ample instability and shear for storms to work with, in addition to a well-defined forcing boundary in the form of a descending sfc cold front. The combination of multiple rounds of showers and thunderstorms over the next 3-6 days will mean that river levels as well as areal flooding will need to be monitored, especially the further we head into next week.

By the end of next week, the large-scale trough is favored to sink southward across the eastern CONUS, with low-level NW flow helping to clear out the moist and unstable air mass. However, exactly how quickly this transition takes place is a source of lingering model disagreement, so we'll continue to leave PoPs in the forecast into Friday. On the whole, a quieter period of weather is expected across the region next weekend, as the post-frontal environment will have limited instability and cooler temperatures.

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/
Issued at 629 AM EDT Sat May 4 2024

Complicated and low confidence forecast for the TAF period. MVFR to IFR cigs with localized LIFR conditions have developed this morning, along with some patchy fog. While cigs will bounce around some through the remainder morning hours, expect a gradual improvement trend where by the early afternoon, all sites should be at VFR cigs with 4-5kft strato-cu decks.

Isolated to scattered storms be possible in the afternoon, with the highest chances for precipitation generally east of the I-65 corridor. For now, have VCTS mention at KLEX/KRGA, but PoPs elsewhere are not high enough to warrant thunder mention. Forecast will be monitored closely in the near term and thunder chances for KSDF/KHNB/KBWG may be introduced in subsequent forecast updates if confidence increases.

LMK WATCHES/WARNINGS/ADVISORIES
KY...None.
IN...None.

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