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FXUS63 KLMK 302341 AFDLMK

Area Forecast Discussion National Weather Service Louisville KY 641 PM EST Thu Nov 30 2023

Updated Aviation Discussion

Short Term
(This evening through Friday)
Issued at 330 PM EST Thu Nov 30 2023

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Key Messages

- Rain returns tonight; 0.25-0.75" expected in general

- Gusty winds continue on Friday

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So far this afternoon, mostly sunny skies and gusty SSW winds have helped temperatures surge into the low 60s across the area. Greater mid- and high-level cloud cover is approaching the region from the SW, currently overspreading our southern IN counties. While the bulk of the deepest moisture remains either west or north of the area this afternoon, a few light elevated returns have begun to appear on radar, though low-level scans reveal that much if not all of the precipitation is evaporating before reaching the ground.

Later this evening, upper level shortwave troughing over the southern Plains will eject east-northeastward into the mid- Mississippi Valley. Across our region, diffluent flow in the upper levels of the atmosphere will promote broad rising motion in the lower-to-middle troposphere. A 50-60 kt southerly LLJ will transport a fetch of higher theta-E air into the Ohio Valley, with the nose of the jet pushing into central KY and southern IN by just before sunrise on Friday. Since the antecedent moisture in the low levels will be fairly sparse, it will likely take some time before precipitation reaches the surface. Nevertheless, by around 06Z, rain is expected to begin across western portions of the area, overspreading the entire region by 10Z Friday morning. What will initially be a fairly broad area of stratiform rain should clear the area by the mid-to-late morning hours on Friday as drier mid-level air surges in. However, saturation in the sub-850 mb layer should lead to lingering isolated showers during the afternoon and evening hours Friday, especially north of the Ohio River and east of the I- 75 corridor. Expect rainfall totals to average between 0.25-0.75", with localized amounts up to 1" possible in the heaviest rainfall cores.

One more challenge in this forecast will be winds. While flow just off the deck will be very strong late tonight into tomorrow morning, a low-level inversion should keep much of the high momentum air aloft. However, in the heaviest rainfall cores, downward mass transfer may be sufficient for 30+ mph wind gusts to reach the ground, though confidence in this is not especially high. With that being said, HRRR guidance suggests a 70-80% chance of wind gusts greater than 30 mph along and south of the Parkways tomorrow morning, with 30-60% chances north of the Parkways. By mid-to-late morning Friday, the core of the LLJ will weaken and move off to the east, though sustained winds of 10-20 mph with gusts of 20-25 mph will continue through the afternoon hours.

Long Term
(Friday night through Thursday)
Issued at 257 PM EST Thu Nov 30 2023

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Key Messages:

- Scattered showers possible over the weekend w/above normal temps

- Northwest flow expected next week with a few disturbances with warm ups ahead and cool downs behind. Low end threat of some wintry weather possible.

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Friday Night through Sunday Night...

An area of surface low pressure will move through Indiana and Ohio Friday night while dragging a surface boundary through the region.
Will hold on to some scattered rain showers until the front passes and then we'll likely have some cold advection work into the region overnight resulting in a gradient of low temperatures. Blended guidance still looks good here with lower-mid 40s over southern Indiana with lower 50s across Kentucky.

For Saturday, we'll remain in a southwest flow aloft situated between a trough axis across the western Plains and a ridge over the southwest Atlantic. A weak wave within the southern stream will bring a batch of showers across much of the southeastern US with the potential for at least some scattered showers for areas south of the BG/WK Parkways. Overall coverage looks to remain scattered at best and it does not appear to be a washout. Temps will remain mild with highs in the mid-upper 50s across much of the region with a few lower 60s down south of the Cumberland Parkway. Cooler air will work in behind that wave with lows Saturday night int he upper 30s to the lower 40s over southern IN and northern KY. In areas of southern KY, lows will remain a little milder with readings in the upper 40s to near 50.

For Sunday, we'll need to make some changes to the forecast here as the models have trended a little more aggressive with a dampening upper level wave within the northern stream passing through the region. Based on the Euro/GEM solutions, a more widespread rain event looks likely here, while the GFS really isn't all that impressed with the system on its 12Z run. For now, will carry 40-50 PoPs across the area. Temperatures will remain mild with highs in the mid-upper 50s. By Sunday night, we'll need to watch for the potential of another perturbation coming through Sunday night and Monday morning. Thermal profiles here look to remain above critical thresholds with another cold rain possible. However, if temps were to cool by another 3-4 degrees, we could see a wintry mix move into portions of the area. Ensemble probabilistic data suggests the highest risk of wintry weather would be confined to area north of the I-71 corridor. Lows for Sunday night will be in the 34-39 degree range.

Monday through Thursday...

Monday looks to be a dry day as we'll be in between systems while remaining in a northwest flow aloft. Temps will remain mild with highs in the upper 40s to the lower 50s and overnight lows in the low-mid 30s. For Tue/Wed, we're continuing to see a considerable spread in the guidance regarding the next wave coming down. The Euro, and to some extent the GEM, is a bit faster here with a wave passing through Monday night and into Tuesday morning. The GFS is considerably slower and deeper bringing the wave through on Wednesday. For now, have sided closer to the Euro/GEM compromise which keeps the previous forecast intact. Though, we'll just have to continue to evaluate future runs to see if stronger forecast convergence takes place. Highs Tuesday will remain seasonal with highs in the 50-55 degree range. Lows Tuesday night should be a bit cooler with lows in the lower 30s. Cooler weather is expected for Wednesday with highs in the lower-mid 40s over southern IN and northern KY with mid-upper 40s over southern KY. Milder weather returns for Thursday with highs warming back into the lower-mid 50s.

Extended Forecast Discussion...

A warmer and drier period of weather is expected for late next week and into the second weekend of December as the MJO is forecast to rotate through the warmer 4/5 phases. In addition the EPO is forecast to be rather negative here. These two teleconnection patterns look to offset the -NAO/-AO pattern being forecast as well.
Signal analysis still suggests a period of unsettled weather in the 12/10-12 time frame. We're seeing this signal in the GFS along with the Euro. This feature looks to bring another round of rainfall to the region. Another brief shot of cold may occur behind this system, but with the MJO out in phase 5/6 the overall pattern should remain fairly mild. Where the MJO goes after the 12/13th is still a bit in question as the ensemble RMM plots show quite a few ensemble members taking a large orbit through phase 5/6. The CFS ensembles continue to have larger orbits and keep the MJO in phase 6 through at least the start of the Christmas holidays. These ensemble runs have been rather stable and we've seen the higher resolution ensembles gradually trend toward what the GFS shows. The bottom line here is that if the larger orbits occur, the trend to colder conditions will be delayed, while less amplified orbit and quicker arrival to phase 7/8 could allow colder conditions to arrive earlier.

Aviation
(00Z TAF Issuance)
Issued at 640 PM EST Thu Nov 30 2023

Impacts/Confidence: - Low to medium confidence in LLWS at BWG and SDF late tonight - High confidence in MVFR ceilings with rain onset - Medium to high confidence in IFR ceilings Friday morning - Low confidence in LIFR ceilings late Friday morning

Discussion...Terminals will start out under VFR conditions but weather will deteriorate later tonight as a low pressure system and attendant cold front approach the Ohio Valley. Showers and potent winds will move into the forecast area ahead of the storm system.
Model consensus is still medium, at best, for LLWS at BWG while decreasing for SDF. However, increasing wind speed between the 925- 850 mb layer and surface winds eventually rotating a bit east due south argue for a chance of LLWS at both terminals. Simultaneously, there will be some surface gusts around 15-20kt as the jet develops over the region.
In addition, ceilings will lower with rain onset and a solid MVFR deck is expected to move west to east during the pre-dawn hours with a moderate chance of IFR conditions by morning.
That being said, the probability of LIFR are non-zero but low enough to avoid their inclusion in the TAFs, for now. Rain will eventually diminish its intensity in the afternoon with only occasional drizzle given the continuing IFR ceilings. As of the latest guidance, conditions seem to improve late in the afternoon as cloud bases lift to MVFR thresholds.

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

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