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FXUS63 KIND 200117 AFDIND

Area Forecast Discussion National Weather Service Indianapolis IN 917 PM EDT Sun May 19 2024

KEY MESSAGES

- Isolated showers and storms across parts of the northern forecast area this afternoon and Monday afternoon/evening.

- Unseasonably warm through Tuesday, highs in the mid to upper 80s.

- Increasing threat for at least 1-2 rounds of strong/severe t- storms late Tuesday through Wednesday.

- Pattern remains active going into the weekend with storm chances Friday night through Sunday.

FORECAST UPDATE
Issued at 917 PM EDT Sun May 19 2024

- Thunderstorms ending far northwest; Otherwise Mostly clear and warm.
- Lows in the low to mid 60s; Warm

Surface analysis late this evening shows a broad yet poorly organized area of high pressure stretching from NY State, across Indiana and Ohio and then to points southwest including LA and Mississippi. The remnants of a weak cold front was still found over far northwest Central Indiana. This front along with what is left of diurnal heating was still allowing some thunderstorms to be present over the Kokomo and Delphi areas. Weak southerly surface flow remained in place across Central Indiana and dew point temps were in the lower 60s.

As heating continues to be lost over the next few hours any lingering convection in the northern parts of the forecast area is expected to diminish and end. This should lead to dry weather across the entire forecast area by 11pm-12a. HRRR is on board with this, showing our forecast area precipitation free by midnight. Forecast soundings overnight continue to show a dry column overnight and dew point depressions are suggested to remain at 3F or greater. Thus the fog threat overnight does not appear to be as great as the previous nights. However, a caveat tonight will be area across the northwest part of our forecast area that have just received new rainfall.
Additional lower level moisture in these are may all for the development of fog with lower dew point depressions that might be suggested. Thus will include at least a patchy fog mention at those locations. Overall ,with light to calm southerly winds in place across the area lows in the lower to mid 60s appear reasonable.

.SHORT TERM (This evening through Monday)...
Issued at 256 PM EDT Sun May 19 2024

Weak ridging within a broad split flow pattern will promote rather benign weather through the period. Our one exception may be a few isolated showers or storms this evening near our northern CWA border where a weak boundary resides. There is little in the way of flow through the depth of the column. As such, storm-scale organization beyond single/pulse cell is not expected. Nevertheless, the column is quite dry so a core collapse leading to a brief burst of gusty winds and small hail is not out of the question. Overall the threat appears very low (under 25 percent).

Any convection is expected to diminish as night falls, leaving us with a few lingering mid to high-level clouds. Winds at the surface may go light and variable as the boundary layer decouples. As mentioned before, the column is quite dry...so any fog overnight will likely be very patchy in nature and mainly ground fog. With dew points in the low 60s and no advection our low temperatures likely remain in the 60s as well.

Monday will be quite similar to today, including a low risk for showers and storms in our far north/northwest. Shower and storm chances are again diurnally driven within a low-flow atmosphere.
Anything that develops will likely go through its life cycle within an hour. The thermodynamic profile also remains similar to today, so a low risk of a brief downburst (gusty winds and small hail) exists in our northwestern counties.

As for temperatures, there is little to no change at the 850mb level. However, displacement of the ridge to slightly east of us may allow for a more brisk SSW wind (10kt) to develop during the afternoon Monday. This may in turn lead to deeper mixing than today allowing temperatures to be a degree or two higher.

.LONG TERM (Monday night through Sunday)...
Issued at 256 PM EDT Sun May 19 2024

Monday Night Through Wednesday.

Another mild night is expected Monday night with lows only falling into the mid to upper 60s as southerly flow keeps dewpoints above normal for this time of the year. Winds will be weakest across the southern counties so will have to keep an eye out for fog as those areas are also where we expect the sparsest cloud cover, but with a few days of drying out fog will likely be limited to the river valleys. Focus then quickly shifts to the potential for strong to severe storms as a strong upper level low pressure system moves from the Dakotas into the Upper Midwest and a surface cold front stretches down into the Ohio Valley.

Storm chances begin as early as late Tuesday night as the surface cold front pushes into northwestern Illinois and a weakening MCS looks to potentially push out ahead and impact western Indiana.
There remains a fair amount of uncertainty as to if the MCS can survive that late into the night with a lack of robust instability, but if the cold pool matures it may last longer than the models suggest. Location of where this could impact is very uncertain but a best guess right now would be along the I-70 corridor.

The better severe weather threat looks to set up Wednesday evening into the early overnight hours along and just ahead of the frontal passage. With dewpoints near 70, surface based CAPE values may be well into the 2500-3500 J/kg range with models showing robust moisture transfer in the 850-700mb layer. Shear is a bit more marginal with the 500mb jet more towards western Illinois keeping the effective bulk shear more towards 30kts which would still be more than enough for severe weather, but may limit the higher end threat. Surface winds will also be generally southwesterly which will limit the curve in the hodograph. Another factor working against the overall severe threat is a weak cap at the top of the boundary layer, but lift right along the front should be enough to overcome the cap but would mean that the convection would generally be limited to just along and not ahead of the front.

With all that in mind, do think that there is a solid shot for isolated to scattered severe weather Wednesday evening into the early overnight, especially across the southern half of the state with a storm complex along the surface cold front. With several unfavorable factors there is a potential for the lower end severe threat, but lift with the front will help to overcome some of the more limiting of the factors.

Thursday Through Sunday.

The surface front is then expected to stall just south of the Ohio River Thursday into Friday with additional storm chances south of the forecast area. Confidence that the front settles to the south is fairly high so will limit the afternoon POPs compared to previous forecasts. By Friday afternoon there is a small chance that the front pushes just far north to impact the southern counties, but by then the front will be fairly diffuse and less impactful overall.
There does look to be a weak wave that may generate another storm complex, but again that is currently expected to be more of an issue along and south of the Ohio River.

Confidence in the forecast going into the weekend is fairly low with significant model spread on the broader synoptic scale, but the pattern looks to remain active with at least one round of storms likely between Friday night and Sunday.

.AVIATION (00Z TAF Issuance)...
Issued at 659 PM EDT Sun May 19 2024

Impacts:

- Thunderstorms near LAF dissipate quickly this evening; Otherwise VFR

Discussion:

VFR conditions expected through the TAF period. Thunderstorms have developed along a stalled frontal boundary just north of LAF. HRRR suggests these storms will dissipate as diurnal heating lost.

After the heating is lost, our weather will be dominated by a large area of high pressure stretching from NY State, across Indiana to the southern Mississippi Valley. Forecast soundings show a dry column tonight. Dew point depressions tonight are expected to be greater than 3F. Thus fog will not be as much of a concern overnight.

Forecast soundings on Monday show a dry column with unreachable convective temperatures as a bit warmer and drier air aloft arrives in the area. Thus will result in continued VFR and only some passing CI.



IND WATCHES/WARNINGS/ADVISORIES
None.

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