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FXUS63 KLMK 030713 AFDLMK

Area Forecast Discussion National Weather Service Louisville KY 313 AM EDT Wed Jul 3 2024

KEY MESSAGES

* A heat advisory is in effect this afternoon and early evening for areas west of a line from Frankfort to Campbellsville to Tompkinsville. Heat indices within the advisory area could be as high as 109 degrees.

* Scattered thunderstorms are expected to develop by mid to late afternoon and continue into tonight, with the greatest coverage over southern Indiana and north central Kentucky. Some of the storms this afternoon and evening will be capable of strong gusty winds and torrential downpours. Overnight the severe threat will diminish but the threat of local flash flooding will continue with additional heavy downpours possible, especially in southern Indiana and north central Kentucky.

* Unsettled weather will continue through Friday with daily chances of showers/storms. Strong storms will be possible each day with the potential for isolated flash flooding. Isolated damaging wind gusts, torrential rainfall, and intense lightning will be the primary hazards.

SHORT TERM /THROUGH TONIGHT/
Issued at 313 AM EDT Wed Jul 3 2024

Overview: South southwest winds between high pressure over the East Coast and a cold front stretching from the Great Lakes to Southern Plains plus a healthy upper ridge over the Southeast will provide us with a hot and humid day today. Scattered afternoon storms are expected to develop in the warm, humid, unstable atmosphere this afternoon/evening and continue into tonight as the cold front approaches the Ohio River.

Heat: Though there is some concern that widespread cumulus development, scattered showers/storms, and convective debris clouds from upstream convection may hamper insolation, it will still be a hot and muggy day. With 850 temps nudging up a degree or two, good warm advective flow, and a persistent air mass in place, will go a few degrees warmer than yesterday resulting in highs in the lower and middle 90s. Dew points in the 70s are already in place over much of the area, and will become more widespread today as two corridors of moisture -- one from the central Gulf Coast to the Ohio Valley and another over the Mississippi Valley -- combine. This combination of heat and humidity will lead to heat index readings over 100 degrees west of I-75 and into the 105-108 degree range along and west of I-65. A Heat Advisory is already in place and will remain.
Added a few counties in the northeast after chatting with ILN.

Severe: Scattered storms are expected to develop mid-late afternoon, especially in southern Indiana and north central Kentucky, as the atmosphere destabilizes, convective temperatures are met, the cold front approaches from the northwest, and there may be outflow boundaries and/or differential heating boundaries present. With weak mid-level lapse rates, weak deep layer shear on the order of 20- 25kt, high freezing heights, and fairly straight hodographs, the main severe threat today will be strong gusty winds associated with the heaviest downpours. Sounding progs suggest convection may become elevated after midnight tonight.

Flash flood: With dew points in the low to mid 70s and precipitable water amounts around 2.1 to 2.3 inches there will be plenty of available moisture for torrential downpours. Soundings continue to show tall, thin CAPE and some slight veering of low/mid-level winds, further supporting the idea of locally intense rain. 850-300mb flow is parallel with the surface boundary and lines up with the height gradient on the periphery of the upper ridge to our south, indicating the possibility of training storms with forward motion around 15-25 mph and possible regeneration on outflow boundary interactions. Looking at model 24-hour QPF, the highest amounts are in a band coincident with the above features from Missouri through southern Illinois and southern Indiana to Ohio. WPC QPF also favors this area though brings it into north central Kentucky as well, and WPC has a Slight Risk for excessive rain along and several counties either side of the Ohio River. At this time it looks like a general 1-2" of rain will be possible tonight in southern Indiana, interestingly right where D1 Moderate Drought currently is located.
However, while the rain may be needed, heavy rain and flash flooding are particularly dangerous at night.

Summary: Hot and humid today with scattered thunderstorms popping up by mid/late afternoon, continuing into tonight. The main threats will be locally gusty winds and heavy downpours. Localized flooding will become the greater threat than severe weather overnight.

LONG TERM /THURSDAY THROUGH TUESDAY/
Issued at 313 AM EDT Wed Jul 3 2024

Independence Day...

A stalled, weak sfc boundary is forecast to be in the vicinity of southern IN and northern KY on the morning of the Fourth of July.
Rich moisture will already be in place, straddling the frontal boundary. Precipitable water of 2.0-2.3 inches is forecast for Thursday, which is right around the daily max PW in the BNA sounding climatology for the first week of July.

A small mid-level shortwave impulse is forecast to lift ENE from Missouri to Indiana Thursday morning and will bring enhanced WSW flow aloft - 40-45 kts at 500 mb and 25+ kts at 850 mb. Enhanced lift and low-level moisture transport associated with this wave will result in increasing rain and thunderstorm chances Thursday morning.
Detailed convective evolution remains uncertain, but some HRRR members point to a cluster evolving east across the Wabash River Valley early Thursday with increasing precip chances in southern IN and north-central KY as early as mid-morning. This does seem probable given the arrival of the mid-level shortwave trough and associated WSW LLJ. Should storms evolve east across the northern half of the forecast area during the morning to early afternoon time frame, a lull would then follow for the heart of the afternoon.
Isolated or scattered redevelopment would then be possible late in the day after a period of airmass recovery. South-central KY has a better chance to start off the day dry with increasing rain/storm chances by early to mid-afternoon.

While detailed timing remains problematic, showers and thunderstorms are likely on the Fourth of July. It just won't be an all-day rain.
Cloud cover will help limit sfc heating and destabilization. Fcst soundings show a very moist profile characterized by tall, skinny CAPE. Deep-layer shear could be as high as 30-35 kts, but the main hazards will be heavy rainfall and localized flash flooding. Any training of storms will significantly boost the local flash flood risk. The anomalously moist environment will be supportive of extreme rainfall rates of 2+ in/hr over relatively short time periods. Gusty winds and intense lightning will also be possible with thunderstorms. Cannot rule out very isolated damaging winds, as any stronger updraft would pose an increased wet microburst threat.

Outside of scattered storms, expect a warm and humid day with afternoon highs ranging from the mid/upper 80s north of I-64 to the lower 90s in south-central KY.

Thursday night - Friday night...

Additional scattered showers/storms are possible Thursday night, though a chunk of the overnight hours may stay dry. Rain chances do increase heading into Friday as a stronger wave of low pressure swings over the Upper Midwest and Great Lakes. This low pressure system will drag a stronger cold front through the region late Friday, and additional showers and storms are likely along and ahead of the cold front. Depending on convective evolution on Wednesday and Thursday, think the main hazard on Friday will be flooding.
There is at least a Marginal/low-end risk for isolated severe wind gusts with the presence of stronger forcing and modest deep-layer shear. Strong destabilization looks unlikely with clouds and precip limiting high temps to the mid/upper 80s.

We'll dry out Friday night with temperatures falling into the 60s in the wake of the cold front.

This Weekend and Early Next Week...

The weekend looks dry with a much drier airmass behind the front and weak sfc high pressure building across the region. Look for a mostly sunny Saturday with highs in the low to mid 80s. Dewpoints are forecast to be in the mid 60s, which will actually feel decent after the Wed-Fri stretch. Sunday will be a bit warmer with highs in the mid/upper 80s. Deepening troughing over the central CONUS and MS River Valley will bring rain chances back to the forecast for early next week.

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/
Issued at 1252 AM EDT Wed Jul 3 2024

The pre-dawn hours will be quiet with mostly clear skies and a light breeze out of the south. A low level jet is expected to develop from BNA to CMH but should remain weak enough to stay below LLWS threshold.

Today will be a hot and muggy day as a cold front approaches from the northwest. Scattered summertime thunderstorms are expected to develop by late afternoon and continue into tonight. Very heavy downpours and gusty winds will be possible with the storms.
Confidence is low in the exact location of convection, but high enough to include PRO30s at all sites, and went ahead with a VCTS at SDF late tonight as the front approaches the Ohio River. Winds may be somewhat chaotic tonight with thunderstorms and various outflow boundaries.

LMK WATCHES/WARNINGS/ADVISORIES
KY...Heat Advisory from 1 PM EDT /noon CDT/ this afternoon to 8 PM EDT /7 PM CDT/ this evening for KYZ023>034-038-045-053- 061>063-070>074.
IN...Heat Advisory from 1 PM EDT /noon CDT/ this afternoon to 8 PM EDT /7 PM CDT/ this evening for INZ076>079-083-084- 089>092.

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