Marine Weather and Tide Forecast for East Peoria, IL

DISCUSSION
Issued at 215 AM CST Mon Dec 9 2024

While no watch/warning/advisory headlines are in effect, the forecast period is a busy one as a progressive pattern results in several systems and associated impacts to discuss.

*** MONDAY-TUESDAY ***

As of 1am/07z Mon, a swath of rain was ongoing between the IL River and I-72 corridors, gradually shifting ENE in time. This area of precip is well aligned with where the RAP depicts mid- level RH (700- 500mb) greater than 70%. Looking ahead, those RAP RH fields suggest a steady eastward trend with this rain, departing the ILX CWA by 4- 5am (10-11z). Followed a similar trend with the short-term PoPs, but kept a mention of drizzle in for a few hours after the rain ends. Forecast soundings show continued presence of a high RH layer (0-1 km RH > 85%) beneath an inversion, and model fields show weak omega (lift) within the high RH zone. While lift is weak, widespread IFR (500 to 1000 feet)
and LIFR (less than 500 feet) ceilings are a confidence builder that drizzle will occur. Similar to the rain, drizzle should taper off from west to east as drier air slowly filters in.

While we get off to a mild start today (sfc temps in the mid 40s to low 50s as of this writing), widespread cloud cover will prevent more than a few degrees of warming during the day.
Forecast highs are in the mid 50s. Sfc low pressure will remain positioned over the upper Midwest today, with an attendant cold front eventually pushing across the ILX CWA tonight. This will drop highs into the mid 30s for most areas on Tues. Guidance continues to depict a sfc low developing along the frontal zone on Tues, but this should occur far enough to the east to keep precip out of the ILX CWA, save for a low chance (15%) of rain in the far SE CWA

*** WEDNESDAY-THURSDAY ***

Wind speeds will increase on Wed as IL will sit between that deepening sfc low over the mid-Atlantic and an elongated sfc high pressure centered over the Canadian Prairie. Model wind fields depict 925mb winds increasing to around 30 knots, which forecast soundings suggest will mix to the sfc. The wind gusts from the NBM (~20-25 knots) have consistently appeared too low, and made adjustments to push peak gusts to near 35 mph, which is closer to the ensemble mean.

We've been noting that the synoptic pattern is favorable for flurries/scattered snow showers on Wed for the last few forecast cycles, and if anything the latest guidance is appearing more supportive of that. Another cold front is expected to push into the area on Wed while the long wave trough (and associated very cold temps aloft) pass overhead. In addition to DPVA from the trough providing broad lift, models like the NAM also depict a band of fgen forcing near the frontal boundary. NAM fcst soundings are now available for this period and they show much deeper saturation than the GFS soundings, clearly supportive of snow given the expected presence of multiple sources of lift. The NBM has finally caught on to this signal, now depicting a 20-45% chance for snow across much of the area. The combination of breezy winds, banded forcing, and non-zero SBCAPE values (~25 J/kg) are all favorable ingredients for snow squalls, and models do indicate non-zero values of the snow squall parameter present on Wed.
Given the progressive nature of the front, still anticipate any accumulations to be light (90% chance for less than 1"), but these conditions could result in brief but sharp reductions in visibility and quick snow accumulations.

Following Wednesday's cold frontal passage, sharply colder air will push into the region by Wed night. The latest forecast calls for lows in the single digits north of I-72 (50% chance near I-72, increasing to 80-90% chance north of I-74), and there is even a slight chance that we could see air temps fall below zero in the far northern portions of the CWA (15-25% chance north of Bloomington to Galesburg). The pressure gradient doesn't quite slacken by Wed night, either, so wind speeds remain elevated (10-15 mph) which will further exacerbate the cold. Wind chill values are forecast to fall below zero north of I-70, and could drop as cold as 10 degrees below zero north of I-74. (Note: We now issue Cold Weather Advisories, rather than Wind Chill Advisories. Our Cold Weather Advisory criteria for areas along and north of I-72 is -15 degF, so we're not quite forecast to reach advisory criteria). Very cold temperature persist into Thurs, as highs along the I-74 corridor may struggle to reach 20 degrees.
These high temperature values will be as much as 15-20 degrees below normal, but are unlikely to set record cold high temperatures. Those records are actually in the single digits!

*** FRIDAY-SATURDAY ***

Another system is progged to develop late in the work week as an upper wave shifts from Colorado towards the mid-MS Valley, and the expectation is that a warm front and associated WAA regime will lift into IL and provide a chance for precip. There are still differences in timing that need to be ironed out, with the GFS much faster (precip beginning Fri AM) than its ECMWF counterpart (dry locally until Fri night). One factor to keep in mind is that sfc temps are likely to have been below freezing for the two days preceding this system. Forecast soundings from the GFS suggest that strong WAA will result in a 850mb warm nose (temps a few degrees above 0 degC). Depending on the timing of this system and the speed at which saturation occurs, there is potential for a brief period of freezing rain before sfc temps warm above freezing. The best chance for this to occur would be near/north of the I-74 corridor, where it will take longer for the warmer (above freezing) sfc air to advect northward. From a probabilistic standpoint, the NBM has a 10-15% chance of exceeding 0.01" of ice accumulation through Saturday, and at this range confidence was too low to include a mention of freezing rain in the gridded forecast.

Overall, the NBM has a 40-60% chance of precip spread across Fri- Sat. While high temps are expected to climb above freezing during the day both Fri and Sat, nighttime lows near/below freezing will leave uncertainty as to the p-type of any precip during the overnight periods.

Erwin

AVIATION
(For the 12z TAFs through 12z Tuesday Morning)
Issued at 509 AM CST Mon Dec 9 2024

The main challenge this TAF period is in the short term as drizzle and low ceilings gradually shift east. Satellite imagery suggest PIA should be free of the low cloud by 12z. Most of the other terminals have improved to MVFR as of this writing, with CMI still stuck at LIFR. While the exact timing is uncertain, expect gradual improvement from west to east this morning. Winds will generally be out of the west today, then shift to northwesterly tonight between 03-07z as a front moves through the area.

Erwin

ILX WATCHES/WARNINGS/ADVISORIES
None.