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FXUS63 KIND 071307 AFDIND

Area Forecast Discussion National Weather Service Indianapolis IN 907 AM EDT Tue May 7 2024

KEY MESSAGES

- Severe storms expected today, especially during the afternoon and evening. All forms of severe hazards are possible.

- Additional severe storms are possible Wednesday afternoon and evening. All forms of severe hazards are again possible.

- Heavy rain is possible at times through Wednesday night. Localized flooding will also be possible.

- More chances for thunderstorms mainly Thursday and Saturday.

- Normal to slightly below normal temperatures from Thursday into early next week.

MESOSCALE UPDATE
Issued at 907 AM EDT Tue May 7 2024

A few showers and weak thunderstorms have developed in a line extending from about Lafayette to Indianapolis, moving northeastward. This development is occurring on the nose of a low- level jet that is beginning to enter the region. Behind these showers, ACARS soundings out of IND have shown an increase in MUCAPE as warm buoyant air advects northward. The CAPE profile is elevated, which makes sense, as warm advection at the surface hasn't quite begun yet.

Further west, a line of thunderstorms extends north to south across the eastern portions of Illinois. These should be entering Indiana shortly, and are likely to maintain somewhat due to the increase in CAPE immediately ahead of the line. Severe weather (associated with the line of storms this morning) does not appear likely at the moment, though northern portions of the line which are in better shear-cold pool balance may be capable of stronger winds (up to 40kt).

Going forward, the line should begin a weakening trend before exiting Indiana. This is because it is outrunning larger-scale lift (a strong vort max and jet streak lifting northeast into the Great Lakes). Obs and CAM guidance suggest that strong theta-e advection continues immediately behind the line, which should negate the presence of a modest cold pool. Additionally, satellite shows clearing across western Illinois behind the line. As such, rapid destabilization is likely as the line exits to our east. This will set the stage for convective re-development later this afternoon.

FORECAST UPDATE
Issued at 617 AM EDT Tue May 7 2024

With the main line of convection still well to the west over west central Illinois, slowed down PoPs a bit. The ETA of the line into the Wabash Valley has been pushed back to 13z per AWIPS distance speed tool. Expect the line to weaken substantially with little to no instability to work with per LAPS data.

.SHORT TERM (Today and tonight)...
Issued at 245 AM EDT Tue May 7 2024

Tuesday Rd #1 | Strong but Mostly Sub-Severe | Between 9AM and 12PM

An ongoing MCS over MO will progress eastward overnight, propelled by a strong LLJ. As this reaches the Ohio Valley this morning, the LLJ will begin to wane, remaining stronger further west. Still, sufficient 0-3km CAPE between 100-150 J/kg, and a sustained cold pool should be enough to keep the MCS ongoing despite weaker LL dynamics. The expectation is for this MCS to be over Indiana between 9AM and 12PM.

Atmospheric disorganization and a lull in thermodynamic availability will be the main mitigating factors for severe convection within this initial round. An elongated low to the north will keep deep layer shear elevated over the northern half of central Indiana, but mid level lapse rates and theta-e advection ahead of the MCS will be much weaker. The opposite will be true for the southern half of central Indiana (Elevated Instability, but Weak Shear).

This should cause the storms over northern central Indiana to become more shear dominant, with convection outrunning the UDCZ. For this reason, the northern section of the line may briefly be near severe criteria as it nears/enters Indiana, but should quickly weaken as it progresses eastward.

The southern half of the storm then becomes the greater concern depending on how quickly the cold pool moves eastward. Its possible the cold pool becomes dominant on the southern half, and the main concern then shifts towards initiation along the outflow boundary.
However, if balance does remain, there is high enough 0-3km CAPE, and theta-e ingestion for potentially severe wind gusts along the UDCZ for this first round.

Tuesday Rd #2 | Scattered Severe Storms | Between 3PM and 10PM

Even though this is round 2 of convection for the day, the storm mode and overall hazards will likely not be the same over the entire 7 hour threat period. For that reason, I am going to split this portion of the discussion into 2a (3P-7P) and 2b (6P-10P).

Round 2a (3P-7P): The remnants of the morning convective line will be moving away by early afternoon with a capped lower level for portions of the late morning and early afternoon. Any cap that does form will quickly erode as rapid theta-e advection pushes a warm, moist layer over central Indiana this afternoon. Broad diffluence of the mid/upper level jet streaks along with this rapidly destabilizing atmosphere should lead to scattered CI over central Indiana around mid-afternoon.

Early on, multicellular clusters will be the more common storm mode as overall parameter spacing remains moderated by sufficient, but weak 800-700mb winds, and weak anvil level flow. Current expectations for this initial time period are for 0-1 SRH to be around 100, with CAPE beginning around 1500 J/kg. Effective shear will still be plenty strong for updraft organization within multicells, and an isolated wind and hail threat is likely. Poor precip dispersion aloft within weak anvil level flow, may lead to precip loading in the updraft, creating more sub-severe and melting hail, but confidence is not high in this solution given robust effective bulk shear.

Round 2b (6P-10P): The second half of this round looks to be the most potent portion of the entire day. A pressure trough will begin to develop across central Indiana late this afternoon into the evening providing another source of lift and convective organization. Shear vectors along this pressure trough remain orthogonal keeping the storm mode mostly discrete. During this time period, mid level lapse rates ramp up even further, pushing SBCAPE values near 3000 J/kg. This along with better anvil level flow, and a strengthening LLJ will likely lead to the formation of supercells.
This time period will be most concerning for areas along and east of the I-69 corridor including the cities of Muncie, Indianapolis, Bloomington, Shelbyville, Columbus, Rushville, Winchester, and Seymour.

With the expectation of supercells during this time period, all severe hazards are on the table. Strong 0-1km lapse rates, 0-1km shear and low LCLs also intensify the tornado threat for this time period, including the potential for a few strong tornadoes (EF2+).
Giant hail (2.5"+) will also be possible within strong updrafts of supercells.

.LONG TERM (Wednesday through Monday)...
Issued at 245 AM EDT Tue May 7 2024

Wednesday and Wednesday night...

A severe weather outbreak followed by heavy rain and flooding are possible Wednesday afternoon and night as a surface low heads northeast towards central Indiana in the fast southwest flow aloft around the base of the High Plains upper low. This will allow a warm front to lift northeast from southern Indiana to near the I-74 corridor by late afternoon. By Thursday morning, the associated surface low will be somewhere near Muncie with a cold front sweeping through from the west. Out ahead of the cold front and in the warm sector to the south of the northeastward moving warm front, the atmosphere will destabilize during the afternoon with CAPEs in excess of 1000 J/kg. The combination of the instability, strong mid level shear in excess of 50 knots and the synoptic forcing will likely lead to rapid thunderstorm development during the afternoon with all modes of severe storms possible. Model soundings are afternoon curved hodographs with 0-3 km SRH near 200 and 0-1 km SRH slightly over 100.

The biggest caveat to severe weather potential across central Indiana will be how far northeast the warm front gets during the afternoon and if discrete cells can form ahead of a likely MCS approaching from the southwest. If the front doesn't it make it as far northeast, and the 00z runs are hinting at this scenario, severe weather will likely be limited to our southwestern counties. This has prompted a further southwest shift in the SPC Day2 severe outlook, which has the Enhanced Risk south of Terre Haute and Greensburg and the Slight Risk only as far north as Cicero and Anderson. FWIW, CAMs favor the bulk of the MCS to move south of I-74 with little to no discrete cells ahead of it. Still, with uncertainty, will continue with 80+ percent PoPs over all central Indiana Wednesday night with 50 percent PoPs Wed afternoon over southwestern counties tapering to 20 percent northeast.

By nightfall, the focus will shift more towards heavy rain and flooding as PWATS increase to 1.5 inches or higher which is the max moving average for this time of year. In addition, coupled jet structure from jet maxes over the Great Lakes and southern states, could enhance rainfall totals. DESI suggests an inch or more 24 hour QPF is possible over south central Indiana and this looks to be on the low side. Will make mention of heavy rain and flooding potential in the HWO and in web site and social media graphics.

Thursday night through Tuesday...

A pair of upper waves in northwest flow aloft will bring decent chances for convection to the area Thursday and again Saturday.
Otherwise, signal lacking for anything other than low chances for convection for the rest of the weekend due to general troughy flow aloft. Meanwhile, models are hinting at an approaching upper ridge early next week which would tend to bring dry weather.

Temperatures look to be more on the cool side than recently with normal or slightly below normal afternoon highs in the 60s and 70s.

.AVIATION (12Z TAF Issuance)...
Issued at 617 AM EDT Tue May 7 2024

Impacts:

- Line of convection will reach the terminals at or shortly after issuance time with MVFR ceilings and visibilities and briefly worse flying conditions possible

- Good confidence more PM thunderstorms will develop and move across before coverage decreases or ends and flying conditions improve

- SE winds less than 10 knots will pick up and switch to the S and SW soon after issuance time. Gusts to 25+ knots by this afternoon and possibly briefly with the morning gust front.

- Gusts to 50+ knots and large hail possible in severe storms this afternoon and evening

- Brief MVFR and worse fog can not be ruled out until the line of thunderstorms gets close

Discussion:

A line of dying thunderstorms will move quickly ENE across central Indiana this morning. MVFR and possibly briefly IFR conditions are possible along with gusty winds.

The atmosphere will reload this afternoon and and upper trough and approaching cold front along with increasing instability and moisture will result in rapid thunderstorm re-development this afternoon. Strong and severe thunderstorms are possible with this activity this afternoon.

Winds will become gusty and switch to the S and SW during the day with gusts to 20+ knots away from convection up to 50+ knots in severe storms.



IND WATCHES/WARNINGS/ADVISORIES
None.

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