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FXUS63 KIND 070347 AFDIND

Area Forecast Discussion National Weather Service Indianapolis IN 1147 PM EDT Mon May 6 2024

KEY MESSAGES

- Isolated showers continuing this evening

- Severe storms expected Tuesday 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.

FORECAST UPDATE
Issued at 945 PM EDT Mon May 6 2024

A boundary stretching from northwest to southeast across central Indiana has been moving slowly north this evening. Near the boundary, isolated showers continue to develop and then diminish.

With loss of heating, expect even these isolated showers to gradually diminish over the next few hours. Kept some isolated showers near this boundary for a bit.

Dewpoints were much lower across the far northern forecast area, with lower 50s noted at Kokomo. Much of the remainder of the forecast area has lower to middle 60s dewpoints. Current low temperature forecast reflect this, with lows in the 50s north and 60s south, so made no significant changes.

With some partial clearing and light winds occurring in the southeast, and rain having fallen there this afternoon and early evening, added some patchy fog overnight. Will have to watch in case fog becomes locally dense in places.

Left slight chance PoPs in the far west very late tonight with the expected approaching line of convection. 00Z HRRR is slower though, so may have to adjust timing with later updates.

.SHORT TERM (This Evening through Tuesday)...
Issued at 305 PM EDT Mon May 6 2024

Convection has become more scattered this afternoon while the thicker clouds this morning have mixed out a bit and enabled some sunshine to filter through. This has introduced a weakly unstable environment over much of the area as low pressure passes by to the south. 18Z temperatures were in the 70s.

While some time will be spent on the scattered convective potential into the early evening...the primary focus for the short term will be on the initial severe weather risk focused on Tuesday afternoon and evening as a series of waves aloft kick out from a deep upper level low that will position itself over the northern Plains on Tuesday.

Rest of This Afternoon and Tonight

Weak surface wave near the Ohio River passing by just south of the forecast area this afternoon. Despite the lull in precip coverage referenced above over the last few hours...starting to note an uptick in convection across southern counties currently...likely being aided by the weak instability. Broader area of rainfall just northeast of KSDF may pivot into the far southeast counties over the next few hours before kicking out to the east of the region.

Will continue to see isolated to scattered convection into the early evening as the low shifts into northeast Kentucky. The presence of near 1000 j/kg CAPE values should be enough for a few rumbles of thunder as well. Showers will diminish shortly after sunset with the loss of heating and a stabilizing atmosphere. Have held onto low pops through mid to late evening but then expect dry conditions thereafter.

The rest of the overnight will see low clouds gradually increase with a focus turning to the west as the elongated robust convective line slated to develop over the central Plains later this afternoon steadily approaches the region from the west overnight. A strong 50+kt low level jet will help drive that convective line all night with the likelihood that the storms will be nearing the Wabash River shortly after 12Z Tuesday in a weakening state.

Tuesday Convective Round #1

Convection will be in the process of outrunning the better low level jet dynamics by daybreak and the jet will weaken even further during the morning as the storm line arrives into western Indiana.
Depending on cold pool depth from the overnight storms...convection is likely to be in a subsevere state by the time it arrives in western counties Tuesday morning. While overall instability profiles will be minimal across the forecast area in advance of the storms...an axis of stronger storm relative helicity within the 0- 1km range to enable the convection to maintain some level of intensity as it tracks across the forecast area through midday.

Storms are likely to have hail and gusty winds associated with them but model soundings do hint at the remnant of a nocturnal inversion which should mitigate some of the wind transport to the surface.
Even while the instability is weak as mentioned above...the CAPE profile is skinny and throughout the depth of the column with likely presence in the hail growth zone as well. To sum up...storms Tuesday morning will disrupt outdoor activities with brief heavy rainfall and lightning diminishing and at least some potential for small hail and gusty winds
But overall
the severe potential looks low through midday Tuesday.

Tuesday Convective Round #2

The remnants of the morning convective line will be moving away by early afternoon...leaving behind the warm front which will surge north into north central Indiana and potentially leftover outflow boundaries from the morning activity. With strengthening southerly flow and an approaching surface wave along an occluded boundary...the airmass will quickly moisten and destabilize with a rapid advection of theta-e into the region by mid afternoon. This will set the stage for renewed convective development after 18Z over Illinois and eventually migrating into central Indiana.

All signs are coming into form for severe weather from mid afternoon into the evening. CAPE values will rise to above 2000 j/kg with BL shear and 0-1 km SRH values remaining more than sufficient for robust convection initiation and development through the second half of the afternoon. Diffluent flow aloft on the lee side of the deep upper low over the Dakotas and the passage of an upper level wave through the region will further aid convective development and expansion. Model soundings become quickly favorable for all forms of severe weather with directional shear within the boundary layer present along with large right-turning hodographs.

While all of the above is strongly supportive of severe convection and supercell development as well...the eventual position of the warm front will become the wild card and could serve as a focal point for the best threat for severe and perhaps rotating storms carrying a tornado risk as well
At this point
that appears most likely to align across far northern portions of the forecast area during the late afternoon. With the strongest SRH values setting up just to the northeast of the region...expect the primary tornado risk late day Tuesday will be across northeast counties. Other than the tornado threat...large hail will be a big concern with an abundance of instability and stronger winds aloft within the hail growth zone. Stronger cores will also carry a damaging wind risk with drier air aloft and a near dry adiabatic flow through the boundary layer.

Have highlighted the severe risk through graphics and products shared on our social media platforms. Storms will shift east of the area by late evening Tuesday as the front pushes back south and weak ridging reestablishes. This is a temporary break in the storms with another round of severe weather slated for late Wednesday and Wednesday evening as the frontal boundary surges back north. More on this in the Long Term Discussion below.

Temps...expect lows into the upper 50s far north with 60s elsewhere tonight. Low level thermals support highs in the upper 70s and low 80s for much of the area Tuesday with a noticed uptick in humidity as well.

.LONG TERM (Tuesday Night through Monday)...
Issued at 305 PM EDT Mon May 6 2024

Late Tuesday night through Wednesday night...

A break in convection is expected to start late Tuesday night, with even weak ridging building into central Indiana from the subtropical surface high pressure to our southeast. Despite clearing skies, lighter southwesterly breezes will hold dewpoints in the upper 50s...resulting in low temperatures around 60F across the region.

Wednesday will bring another favorable set-up for convection and perhaps widespread severe weather with a warm frontal type boundary likely slowly advancing northward near our southern counties ahead of a strengthening surface low slowly approaching over the Middle Mississippi Valley. Much of the day Wednesday may be the quiet before the storm as the narrow ridging between the two systems continues to cross the CWA ..albeit eventually self-destructive with the bright skies bringing low 80s and light southerly winds holding dewpoints in the 55-65F range. Resultant CAPE by 21Z should range from 1000-3000 J/kg from north-south...including moderately strong 7- 8 deg/km lapse rates over most of the area. Widespread 40-55 kt 0-6 km shear will also support arriving/developing convection.

So far appears most likely storm pattern will be a mesoscale convective system initiating in the afternoon somewhere in the central/eastern Illinois region...before tracking eastward into Indiana during the late day/early evening hours. This would likely transition into more of a heavy rain/flooding threat later in the evening and into overnight hours, especially south of I-70 amid the system's warm sector's anomalous precipitable water values approaching 2.00 inches. While the environment would support discrete cells ahead of a later MCS arrival...organized severe weather's location would be dependent on warm frontal position, with timing likely determined by where/when storms begin to fire to our west.

All severe hazards are on the table for PM hours Wednesday with damaging winds and large hail the greatest threats during the afternoon/evening...and flooding the greatest concern for the evening/overnight. Although it is only early May, the set-up and potential are perhaps more indicative of a June pattern with the synoptic set-up able to effortlessly fuel ample deep moisture into central and especially southern Indiana. Rain chances will drop from west to east late Wednesday night as the supporting surface low crosses the state. Total 24-hour rainfall potential for Wed-Wed night will be 1.00-2.50 inches along/south of the I-70 corridor, with less than 1.00 inch expected for most locations north of I-70.

Temperatures will be above normal amid the S/SW flow and overall warm sector of the passing/approaching systems. Expect low 80s ahead of convection Wednesday...while lows Wednesday night amid decreasing chances of rain ranging from the upper 50s near Lafayette to the mid 60s south of the I-70 corridor.

Thursday through Monday...

The last five days of the long term will then trend to at least a couple northern stream, positively-tilted short waves cycling from the northern Plains into the Midwest. Not the greatest certainty with this pattern, with some guidance members originally hinting at the second wave plunging and inducing a strong baroclinic circulation near the Ohio Valley around the Sunday/Monday timeframe ...however, latest data now suggests amplification will be more modest, which should keep the pattern more progressive over the region thru the end of the long term.

This should translate to near to slightly below temperatures amid west-northwesterly breezes. Thursday will be marked by robust to gusty breezes as the gradient from the combination of departing low pressure and the passage mid-level supporting vort. The 5-day period will include several chances for at least stray showers under the troughy flow...with more organized showers expected both Thursday and Saturday per corresponding short waves. Widespread and/or heavy rainfall could be possible early next week should the weekend wave plunge and phase, although currently low chances in this set-up coming to fruition. The normal max/min at Indianapolis through the long term is 72/52.

.AVIATION (06Z TAF Issuance)...
Issued at 1147 PM EDT Mon May 6 2024

Impacts:

- Line of convection will reach the terminals after 11z-13z with MVFR ceilings and visibilities and briefly worse flying conditions possible

- Good confidence more PM thunderstorms will develop and move across

- SE winds less than 10 knots will pick up and switch to the S and SW after 11z-13z. Gusts to 20 plus 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 overnight and 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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