Marine Weather and Tide Forecast for Jacksonville, IL

DISCUSSION
Issued at 231 PM CDT Sat May 17 2025

An upper low was evident in satellite imagery northeast of the Great Lakes. Central IL remains in the cyclonic flow regime associated with this low pressure system, with breezy westerly winds gusting up to 35 mph. These gusts are right around the threshold at which we see blowing dust develop, and we received reports of localized blowing dust reducing vis along I-74 between Bloomington and Champaign between noon and 130pm. No other reports have been received as of this writing. A blowing dust advisory was issued for most areas east of the IL River and north of I-70.
The counties included in the advisory were based on the counties where tile drainage is most prevalent, which our local research has identified as being a factor that can increase the blowing dust potential. Wind gusts should diminish into this evening (and thus the threat of blowing dust will as well).

The remainder of the weekend should be pleasant across the region as a weak sfc high pressure shifts through, resulting in light winds and mostly clear skies on Sun. Temps tonight will be on the cool side, with lows ranging from the mid 40s in the north to mid 50s in the south. Typical lows for this time of year are in the low/mid 50s. Highs should be seasonable on Sunday, in the mid 70s.
Late Sun eve, CAMs suggest a back door cold front emanating from NE IL pushing southwest across the ILX CWA Current timing suggests this occurs between 9pm Sun and 1am Mon, but that could be adjusted slightly in subsequent forecasts. The main impact with this front would be a shift from light winds to northeasterly winds that could gust 20-25 mph.

*** MONDAY - TUESDAY SEVERE STORM POTENTIAL ***

Attention then turns to a slow-moving, long wave trough that is set to emerge onto the central Plains Sun-Mon. This system will result in an unsettled pattern locally, with multiple rounds of storms possible Mon-Tues including a chance for both severe storms and locally heavy rainfall. The severe storm hazards will be modulated in large part by the position of a warm front.

The first chance for precip is during the pre-dawn hours of Mon AM. At this time, the warm front is likely to be positioned south of the ILX CWA Between upscale growth of convection across the Plains as well as LLJ forcing into the warm front, most models bring precip into the ILX CWA by 2-3 AM Mon. There is a broad spread in the models in terms of how much elevated instability is present at that time, ranging from less than 1000 J/kg to upwards of 2000 J/kg depending on the guidance source, but given the synoptic placement of this precip the thinking is if a severe threat exists it would primarily be for hail. There may also be a marginal threat for strong wind gusts depending on how organized the incoming convection is. Eff inflow layer shear of 50+ knots could be sufficient for these storms to maintain an organized structure capable of those stronger wind gusts. The main takeaway is to expect scattered showers and storms very early Monday morning, with the highest chances across the southern half of the ILX CWA

As we progress through this event, the repeated rounds of convection leave uncertainty as to where the warm front will be positioned. There is good consistency in the storm evolution early Mon AM, and I suspect those storms could reinforce the warm front further to the south. Thus I lean more towards the 17.12z deterministic NAM solution which has the warm front further south (south of the ILX CWA during the day Mon) compared to the GFS solution. Guidance has a relative lull in precip activity during the day Mon, before becoming more widespread into the evening as the LLJ forcing returns. While precip coverage is less certain during the day Mon, guidance shows a plume of steep mid-level lapse rates advecting into the area, increasing the potential for large hail with any storms that do form. If we are correct in favoring the southern solution for the warm front placement, hail would continue to be the primary hazard into Mon night. Will note that the frontal placement will still need to be monitored closely as the environment appears quite favorable for tornadoes across the warm sector Mon night. The precip timing trends outlined above seem to be well reflected in the latest NBM PoPs: 60-80% chance Mon AM, 20-50% chance Mon afternoon, then a greater than 90% chance into Mon night.

On Tues, the sfc low is expected to track across the ILX CWA This would bring the warm front further north, and provide a window where portions of the ILX CWA are in the warm sector (best chance being east of I-55/south of I-74). Within this area, a volatile environment with strong instability (potentially over 3000 J/kg of MUCAPE), strong deep layer shear over 50 knots, two fronts (warm front and cold front) and a triple point could support severe storms capable of all hazards across east-central IL Tues afternoon, assuming sufficient atmospheric recovery occurs after any earlier storms. The placement of the SPC Day 4 severe thunderstorm outlook still looks well aligned with today's 12z model guidance, but probabilities may increase in later outlooks if current guidance holds.

*** MONDAY - TUESDAY HEAVY RAIN POTENTIAL ***

The other concern with this system is the potential for locally heavy rainfall. Guidance depicts PWAT values between 1.25-1.75", which is around the 95th percentile of climatology. Forecast soundings from the NAM suggest a warm cloud layer depth (cloud base to freezing level) of 10-12 kft for much of the period from early Mon AM through Tues AM. Warm cloud layer values above 10kft suggest efficient rainfall rates. The one limiting factor could be the kinematics (see the high deep layer shear values referenced above). The residence time of any individual cell will not be too long at any one location, so it would take multiple rounds of storms over the same area, or perhaps training storms, to drive higher rainfall totals. Currently, the NBM has a 70-90% chance of total rainfall exceeding 1" across most of the ILX CWA, and a 30-50% chance of 2" of rain. As CAMs start to encompass the full event, we'll get a better idea of what kind of localized, higher end rainfall amounts could occur. WPC has issued a slight risk of excessive rainfall for the southwest CWA on Monday, while the entire area is encompassed in a slight risk for Day 4.

*** REST OF THE WEEK ***

As the sfc low slides east by Wed, breezy northerly flow will persist across IL through the day, with forecast soundings suggesting mostly cloudy skies and perhaps some light showers.
Temps will be seasonably cool, with highs barely climbing into the low 60s.

The rest of the week features a weak, broad sfc high nudging into the region, which should result in gradually weakening wind speeds and mostly dry conditions during the day Thursday-Friday (less than 20% chance precip).

Erwin

AVIATION
(For the 00z TAFs through 00z Sunday Evening)
Issued at 623 PM CDT Sat May 17 2025

VFR conditions will prevail through the entire 00z TAF period. SCT diurnal CU at around 6000ft will drift over the I-72 terminals over the next hour or two before dissipating after sunset.
Westerly winds will continue to gust to around 20kt at all sites through about 01z/02z, then will become W/NW and decrease to less than 10kt. Winds will veer to N at around 10kt on Saturday under a mostly clear sky.

Barnes

ILX WATCHES/WARNINGS/ADVISORIES
None.