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FXUS63 KLMK 300253 AFDLMK

Area Forecast Discussion National Weather Service Louisville KY 953 PM EST Wed Nov 29 2023

Forecast Update
Issued at 950 PM EST Wed Nov 29 2023

Water vapor imagery reveals a subtle surge of upper-level moisture moving across the region ahead of a deamplifying mid-level ridge.
Such clouds will not prevent surface temperatures to fall to the 30s tonight amid a good radiational environment with the more sheltered valley locations reading in the upper 20s already. On the other hand, winds will remain light and from the south with a notable increase tomorrow morning and afternoon as daytime mixing allows momentum transfer from the southern extent of the LLJ. Rest of the afternoon forecast is on track and grids have been update to reflect latest observation and trend.

Short Term
(Tonight through Thursday evening)
Issued at 315 PM EST Wed Nov 29 2023

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Key Messages

- Mostly clear and milder temperatures tonight

- Mix of sun and high clouds Thursday, clouds increase late in the day

- Breezy conditions return on Thursday; SSW winds 12-18 mph with gusts 20-30 mph

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Across the region this afternoon, sunny skies abound with temperatures considerably warmer than yesterday thanks to a combination of ample sunshine and milder SW winds. These SW winds, which have at times gusted over 20 mph, are thanks to a strong low- level pressure/height gradient between a high over the Deep South and a low over Hudson Bay. As we head later into the afternoon and evening hours, winds will gradually weaken thanks to boundary layer decoupling limiting downward transfer of higher momentum air aloft.

Tonight, mostly clear skies are expected with upper level moisture providing scattered high clouds mainly after sunset. In spite of aforementioned boundary layer decoupling, temperatures will still be considerably warmer than the past few nights, with lows expected to range from the upper 20s in the coldest valleys to the mid-to-upper 30s in urban heat centers and elevated ridge tops.

Tomorrow should be another breezy day across central Kentucky and southern Indiana as a strong gradient in low level pressures/heights continues. A transient northern stream wave will move across the upper Midwest and Great Lakes region, with a trailing southern stream shortwave lagging behind over the southern Plains. The combination of these two upper level systems will be associated with a surface low and northeastward-extending cold front stretching from the lee of the southern Rockies up to the upper Peninsula of Michigan. East of this system, broad low-level SSW flow will continue to bring warmer air into our region, and temperatures tomorrow afternoon should be 5-10 degrees warmer than today across the area. Moisture, especially in the lowest layers of the atmosphere, will be slow to return over our area as the influence of the low-level high keeps the deepest moisture over the Mississippi Valley and points west. Expect the daytime hours to remain dry over much if not all of the area tomorrow, with morning sunshine/high clouds gradually giving way to more cloud cover late in the day (especially west of I-65). If any one area did see a light rain shower Thursday afternoon, the best chance (5-15%) would be north of the Ohio River.

Long Term
(After midnight Thursday night through Wednesday)
Issued at 310 PM EST Wed Nov 29 2023

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Key Messages:

- Widespread rainfall Thursday night into Friday. Expected rainfall amounts 0.25-0.75"

- Breezy conditions expected on Friday

- Above normal temperatures expected through this weekend, closer to normal values early next week

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Thursday Night through Friday Night...

Southern stream system still on target to affect the region Thursday night and Friday. Model cross sections continue to show a good amount of Gulf moisture being advected north into the region courtesy of a 55-60kt low-level jet axis. Model proximity soundings still show PWAT values of 0.90-1.00 inch so widespread moderate rain showers are expected. Will continue to keep PoPs >90% from late Thu through Fri morning. Rainfall amounts of 0.30 to 0.75" still look likely, though a few spots may be able to squeeze out an inch of rainfall. Lows Thursday night will be in the lower 40s with highs on Friday warming into the upper 50s to lower 60s with breezy conditions expected.

By Friday night, surface low will move off to the northeast with a decent deformation band of moderate to heavy precipitation extending from northern IL thorugh Michigan. Locally, our moisture will get shut off fairly quickly with rain ending from west to east during the evening. Lows Friday will feature a gradient with lower 40s across our SW Indiana counties with upper 40s/lower 50s across central/SE KY.

Saturday Through Wednesday...

For the weekend, we'll remain in a broad southwest flow aloft and temperatures will stay above normal. Saturday should feature mainly dry conditions across much of the region, with the exception of a few showers down across the far south/southeast (Lake Cumberland area). Highs on Saturday will be in the mid-upper 50s across southern Indiana and northern KY with lower 60s in areas south of the Cumberland Parkway. On Sunday we should remain fairly dry though we'll have a weak northern stream system that will pass through late in the day and could produce a few rain showers. Highs on Sunday will be similar to Saturday with upper 50s across southern IN and northern KY with lower 60s across southern KY.

By late Sunday and into Monday, our southwest flow aloft will transition over to more of a trough with the core of the cold going into the NE US. A couple of small perturbations within the flow will pass through and mainly skirt just to our northeast. This could bring some rain chances to our northeast sections on Monday.
With the cold trough in place, we'll see colder temps on Monday with highs in the lower 50s and overnight lows in the mid 30s. Latest 29/12Z model suite still is in agreement with a stronger clipper system dropping southward and affecting the region Tuesday night into Wednesday. Thermal profiles will warm up well above critical thresholds ahead of the approaching clipper so this one will bring a cold rain to the region with the bulk of wintry weather staying across WI/MI/northern IN/northern OH. Behind that clipper, another surge of colder air will infiltrate the region.

Extended Forecast Discussion...

Looking into the extended, the cool blast behind the above mentioned clipper will be rather short lived as a warmer pattern is likely to redevelop across much of the CONUS. Much of this will be driven by the MJO which will be heading into phases 3/4/5 during this period.
While the NAO and AO are forecast to remain negative, the MJO and the expected +EPO pattern will likely help sustain the warm pattern.
Updated signal analysis from last week suggests the next signal passage coming in the 12/10-12 time frame. Given the MJO and teleconnection pattern the main storm track looks to be from the southern Plains into the Great Lakes. Another bout of moderate rainfall looks likely here with another possible cool down behind that.

Longer range data continues to be in pretty good agreement with a colder pattern developing toward the late part of December as those models suggest that a ridge will develop across the western US with deeper troughing setting up across the east. The timing to this pattern remains uncertain due to uncertainties on how large the upcoming MJO pulse will be. While the ensemble mean from the GFS and Euro push the MJO into phases 4/5 and then collapse it rather quickly, there are some ensemble members that have larger orbits through phases 4/5/6 and do not get over to phase 7/8 until late month. Additionally, the Pacific Jet will play a major role in the possible pattern change for late month. As of this writing, the Pacific Jet is forecast to remain extended and strong through the month which will greatly diminish the probability of a west coast ridge popping up. On the other hand, if the Pacific Jet were to retract a bit more to the west, then the possibilities of the west coast ridge developing will increase.

Aviation
(00Z TAF Issuance)
Issued at 623 PM EST Wed Nov 29 2023

Impacts/Confidence: - High confidence in gusty SW winds on Thursday - Medium confidence in LLWS at HNB and SDF tonight

Discussion...VFR conditions will continue through this TAF cycle at all terminals. As stated in previous discussions, an enhancement of the low-level wind field is expected tonight as a disturbance moves across the northern portion of the Great Lakes. Winds at 925-mb will be around 30-35 knots from the SSW while surface flow will be weak (around 5-6 knots) and from the south. Therefore, the combination of speed and directional shear will warrant the inclusion of a LLWS group for HNB and SDF from the second half of tonight to the first morning hours. As for model guidance, the RAP, NAM and to a lesser extent the HRRR support the such inclusion. For tomorrow, still anticipating gusty (18-22 knots) SW winds from 15 to 22Z with calmer winds by sunset. The cloud deck will stay in the mid to upper levels with increasing low-level, VFR clouds after sunset.

Extended Outlook...There is a non-zero chance of MVFR ceilings and another LLWS event Thursday night.

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

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