Area Forecast Discussion
Issued by NWS Central Illinois
Issued by NWS Central Illinois
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963 FXUS63 KILX 130433 AFDILX Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 1133 PM CDT Sun May 12 2024 .KEY MESSAGES... - In the Monday to Tuesday timeframe, there is a 40-60% chance for more than an inch of rainfall. This rain will contribute to area rivers and streams already running high, increasing the risk for localized inundation near riverbanks. This could perhaps also further delay planting and inhibit germination of agricultural seedlings. - Another system will bring rain to the area late Thursday into Friday, though at this time rainfall totals from that system are forecast to be lower than those with the first one. Chances for more than an inch are only around 10-20%. && .UPDATE... Issued at 754 PM CDT Sun May 12 2024 Fairly quiet as we approach sunset, with some mid and high clouds on the fringes of the forecast area. A more substantial increase in cloud cover is expected after midnight, ahead of the storm system currently over the central Plains. Our evening sounding is pretty dry below 10,000 feet, so any precipitation before sunrise should be rather sparse. More of a substantial increase in rain chances is expected after mid morning. Geelhart && .DISCUSSION... Issued at 133 PM CDT Sun May 12 2024 After yet another dry and pleasant spring day in central Illinois, precipitation chances will increase tomorrow with the arrival of a low pressure system from the west. Visible satellite at 130pm shows the cirrus shield associated with that system drifting eastward across Missouri, with a few gusty thunderstorms to our northwest across Iowa where mesoanalysis suggests over 1500 J/kg SBCAPE has materialized despite a rather dry and deep mixed layer. Deep mixing this afternoon across our neck of the woods has kept us even drier with dewpoints in the 40s, but low level moisture will increase overnight into tomorrow with 80-100% of the HREF ensemble bringing PWATs to over 1.3" (the 90th percentile from SPC`s sounding climatology) by tomorrow afternoon. This, together with tall, "skinny" CAPE profiles conducive to warm cloud processes, will increase potential for efficient rain producing showers and thunderstorms tomorrow afternoon/night, particularly north of I-70 where MUCAPE values (700-1200 J/kg, per the HRRR) are highest and the HREF`s LPMM is well over an inch in spots. The same ensemble shows spread in rain totals through Tuesday morning will be considerable given the convective nature of this precip, though; the ensemble max, a reasonable high-end amount which one or two locations could see if there`s any training of the heavier cells, is upwards of 3 inches in a few spots, while the 10th percentile (which should be representative of the many locations which get missed by the heavier showers) is only several hundredths. While the risk for widespread hydrological issues is negligible (less than 2%), many rivers and streams across the region are already running on the high side, so localized inundation near riverbanks could occur with the more flashy rivers (in addition to the ongoing flooding near Havana and Beardstown along the Illinois River). This rain could also delay planting even further for farmers, and could inhibit germination of already-sown seedlings. On the bright side, given weak shear (15-25 kt, per HREF mean) and weak instability (with low NCAPE values indicating slow updraft parcel accelerations), severe weather is not a concern at this time. Once we`re rid of this pesky system Wednesday morning, ridging will attempt to build back in but will be rushed out by another trough on its heels, southwest flow ushering in moisture and rain chances once again by late Thursday as an ill-defined surface low passes to our north. This system will be a bit more progressive than the Monday- Tuesday night one, so initial thinking is rainfall won`t be overly troublesome. However, a large component of shear in the cloud bearing layer will be parallel to the southeastward-moving cold front favoring some training of showers and storms, and EPS mean brings PWATs back up to 1.2 to 1.3 inches, so some locations could certainly see an inch or more of precip once again (NBM probabilities are around 10-20%) to once more delay seedling germination and raise river levels. Cluster analysis suggests there`s not a whole lot of model spread in the upper pattern this upcoming weekend and beyond, with middle of the road guidance suggesting 500mb flow turns rather zonal. This may be washing out potential shortwaves due to model timing/amplification differences, but there`s certainly not a strong signal for a large and dynamic system at the end of the forecast period. Bumgardner && .AVIATION... (For the 06z TAFs through 06z Monday Night) Issued at 1133 PM CDT Sun May 12 2024 VFR conditions to prevail into midday Monday, though ceilings will be steadily lowering as a storm system approaches. While a few showers are possible as early as 15Z, most will be during the afternoon and evening. Main period for potential thunder appears during the mid to late afternoon. A few storms may linger into early evening from KPIA-KCMI, though this is less certain, and will not be mentioned at this time. A rapid expanse of lower ceilings is expected in the afternoon, and most sites should be MVFR by about 21-22Z. Ensemble guidance suggests ceilings may dip below 1,000 feet late in the period, with about a 30-50% chance at this point. Geelhart && .ILX WATCHES/WARNINGS/ADVISORIES... None. && $$