Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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066 FXUS63 KLOT 031743 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1243 PM CDT Mon Jun 3 2024 .KEY MESSAGES... - Scattered thunderstorms expected this afternoon and evening, some of which could produce locally damaging winds and flash flooding. - Potential for additional showers and storms Tuesday. - Wednesday through midday/early afternoon will likely be the last day of the week with any meaningful thunderstorm chances. && .UPDATE... Issued at 1143 AM CDT Mon Jun 3 2024 Have made some refinements to the afternoon forecast as we continue to assess thunderstorm potential/coverage. Remnant circulation from earlier convection continues to track northeast over SW WI triggering thunderstorms just to the west/northwest of our forecast area. The few cells that did pop over the NW forecast area did not sustain themselves very long. Satellite imagery shows a large corridor of cu across northwest and west-central IL and much of eastern IA and eastern MO with an outflow/frontal boundary moving east toward the Quad Cities. This boundary looks to be more diffuse than it had been with cu not currently looking too impressive. It`s not readily apparent on satellite, but there looks to be some sort of wave tracking northeast across north-central or northeast MO which had earlier triggered a few cells but is not currently producing much. Between the cu, the boundary, the potential mid/upper wave, and a destabilizing atmosphere overhead, it still appears that isolated to scattered thunderstorms will still be possible this afternoon into early evening. High res guidance has shifted gears and shows little if any convection which does not necessarily help confidence. However, do agree that chances are not all that great so have reduced the eastward extent of chances through the afternoon with the main focus area being the NW quarter to third of the area or roughly west of a Woodstock to Mendota line with quickly decreasing chances to the east through the afternoon with some eastward spread of chances across the Chicago metro into the evening, though this may be a bit overdone. Isolated severe storms are still possible along with a flash flood threat given high precipitable water values. MDB && .DISCUSSION... Issued at 356 AM CDT Mon Jun 3 2024 Through Tuesday: Surface low pressure was analyzed over central MN early this morning, with a warm front extending south-southeast through IA and MO, and a cold front trailing southwest across the central Plains. The low will continue to lift northeast into western Ontario today, in association with a low-amplitude mid-level short wave trough propagating from the northern Plains into Ontario and the northern Lakes. As this occurs, the warm front will lift northeast across the forecast area and will bring a warmer and more humid summer-like air mass to the region. Within this higher Theta-E air mass, instability will increase diurnally during the midday and afternoon hours, with temperatures in the low-mid 80s and surface dew points into the mid-60s expected. Forecast soundings indicate MLCAPEs in the 1000-1500 J/kg range by afternoon, with little or no capping present by early to mid afternoon. While the primary short wave and associated surface low are tracking well north of the area, the Midwest radar mosaic depicts a well-developed MCV over northeast NE which evolved from an MCS earlier last evening. This feature is progged to continue to track east-northeast across IA this morning and southwest/central WI this afternoon. Though run to run and model to model differences persist, CAM guidance generally continues to indicate convective development in association with the MCV across southwest WI and northwest/northern IL this afternoon. Areas north of the I-88 corridor appear to be most favored, though details of timing and coverage remain of somewhat low confidence. Background deep-layer shear is relatively weak (~20 kts), though the MCV will likely enhance mid-level winds and increase deep shear to perhaps 25-30 kts for a time across southern WI and far northern IL this afternoon. The combination of diurnal peak instability and this period of somewhat enhanced shear may support a few severe thunderstorms with damaging wind gusts the main threat. Though low level flow is not very strong, subtle backing in the vicinity of the warm front or any outflow boundaries could support a low-end tornado threat. Storms would likely become somewhat messy with propagation tied to outflow boundaries by late afternoon and evening, but will likely spread east-southeast into parts of the Chicago metro area before weakening with the loss of diurnal instability this evening. Lingering storm threat may transition to more of a locally heavy rainfall threat during the evening as a southwesterly low level jet ramps up, and results in slow southward or even backbuilding propagation. Scattered thunderstorms remain possible on Tuesday as well, though with somewhat more subtle forcing as a slow-moving weak short wave lifts out of MO/downstate IL during the day. Better chances look to be Tuesday night however, as a cold front and more amplified short wave approach as outlined in the following section of the discussion. Warm and humid summer-like conditions will persist, with highs in the low-mid 80s. Ratzer Tuesday Night through Sunday: There is good ensemble member support for showers and thunderstorms affecting the region Tuesday evening and night, resulting in high PoPs, especially overnight (70-90%). The key large scale player will be a seasonably deep mid-upper level trough from the Canadian Prairies to the Upper Midwest. Strong surface low pressure deepening into the mid 980s mb over Manitoba will drag its cold front toward the mid-upper MS Valley overnight into Wednesday morning. While there is reasonable agreement in the details above, convective trends are quite unclear for various reasons. The strongest forcing will be well off to our northwest to start the evening. Thus, the presence of a convectively augmented impulse or MCV lifting northeastward will likely be the key to higher convective coverage during the evening hours to offset the decrease of instability with sunset. Otherwise, any lingering diurnally driven scattered showers and thunderstorms will likely weaken/dissipate with the loss of heating and lack of deep layer shear. The western half or so of the CWA may be in best position to be affected by the potential MCV early enough for convective maintenance, which is where the highest evening PoPs (~60-80%) are located. Farther east in the CWA, confidence is lower (only 40-50% PoPs), and there may very well be a path to a relatively quiet evening Mid-level height falls will maximize to the north of our area as the earlier mentioned deep trough axis ejects eastward, though we`ll still see appreciable height falls (6+ DaM/12 hours at 500 mb) overspread the area after midnight Tuesday night. Unfavorable diurnal timing suggests thunderstorm coverage may end up being scattered, though with the approaching front and height falls, shower coverage should be widespread enough to justify the 70-90% PoPs. Outside of the (uncertain) convective trends, Tuesday night will be fairly warm and muggy. Showers and embedded scattered thunderstorms should continue to translate eastward with the approaching cold front Wednesday morning. Activity will likely be focused over the eastern half or third or so of the CWA a few hours after sunrise. Unless convective coverage is greatly diminished overnight and precip and debris cloud cover ahead of the front is minimized Wednesday morning through midday, the potential for meaningful destabilization appears limited. This entails a low chance for any organized/strong convection prior to the frontal passage. By the late morning through midday on Wednesday, the cold front will likely be east of I-57 and quickly sweeping eastward, bringing any lingering showers and thunderstorms to an end for the far east/southeast CWA by or shortly after 1PM CDT. The rest of Wednesday will likely be drier, breezy, and increasingly sunny, with seasonable temperatures in the upper 70s to lower 80s. Deep upper level low pressure from the aforementioned strong mid-week short-wave will then become quasi-stationary from the northern Great Lakes across northeastern North America Thursday through the weekend. The extended stretch of northwest flow in this pattern will result in comfortable near to slightly below normal temperatures (highs in the 70s to near 80F away from any lake cooling, little chance of any meaningful rainfall, low humidity (daytime dew points in the 40s and 50s), and breezy daytime conditions. On this last note, Thursday will likely be a windy day for June, with west-northwesterly gusts up to 30-35 mph. Castro && .AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 1242 PM CDT Mon Jun 3 2024 Key Aviation Impacts for the current TAF period: - Non-zero thunder chances tonight, but confidence was low enough to remove from the TAF - Winds are expected to be prevailing 190, but occasional flopping to 170 is possible through Tuesday late morning - Leading showers may arrive at Chicago terminals late Tuesday afternoon Winds are slowly becoming more prevailing southwesterly; however, some flip flopping to southeasterly is still possible through the early afternoon. While there are storms just north of KRFD moving to the northeast at the time this discussion was published, the biggest change to the forecast was removing thunder from the TAFs. There will still be enough instability to generate a pop-up storm or two later today, latest guidance keeps the best areas for storms well north of terminals. With the probability less than 30 percent, it was decided to pull the thunder from the TAF and monitor chances through the night. There is low confidence in the exact wind direction overnight (SE vs. SW), though gusts are expected to come down before returning Tuesday afternoon. The next system that will bring showers and storms to the region will likely arrive after 00Z (and therefore outside the current TAF window). However, latest high-res guidance suggests a weak boundary drifting northward in the afternoon. Instability does not look impressive to add thunder, and while timing may be pushed back in later TAF packages, felt comfortable introducing -SHRA into the ORD/MDW TAFs. DK && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago