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FXUS63 KILX 231057 AFDILX

Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 557 AM CDT Sat Sep 23 2023

SYNOPSIS
Issued at 555 AM CDT Sat Sep 23 2023

Mostly sunny, warm, and dry today, with high temperatures in the low 80s. Scattered showers return for Sunday, and the best chance for any rain is west of I-55. Dry weather returns for Monday, then a few showers are possible Tuesday and Wednesday, primarily during the afternoon.

DISCUSSION
Issued at 240 AM CDT Sat Sep 23 2023

---------- Key Messages ------------

1. Warm and dry today, with highs in the 80s.

2. Light rain is possible (20-50% chance) during the day Sunday, with the highest chances west of I-55.

3. Significant weather impacts are not expected during the upcoming work week. Scattered showers are possible mid-week (Tues-Wed). Above normal temperatures are likely late in the work week into next weekend.

------------------------------------

Saturday will feel more like summer than fall, as the region sits beneath upper ridging between low pressure over the north-central Plains and Tropical Storm Ophelia over the Carolinas. Mostly sunny skies should prevail, although high clouds should start to stream in from west to east by the evening. High temps are expected to climb into the low to mid-80s, which is about 5 degrees above normal.

By Sat eve, the occluded sfc low over the northern Plains will have an occluded/cold front extending south along the Missouri River Valley, with a precip shield near/ahead of the front. As the system progresses east, a few showers could reach the IL River Valley during the pre-dawn hours of Sun morning, although most CAMs keep the region dry through sunrise. The best chance of rain from this disturbance is during the day Sunday (20-50% chance), although it's worth noting that forecast soundings show dry air in the lowest 4000 feet, along with meager instability, so any precip that does occur should be on the light side (less than 0.25").

It's worth noting the 23.00z run of the HRRR highlights some environmental factors that are supportive of tornadoes, showing a narrow corridor of low-level instability (0-3 km CAPE over 100 J/kg) and low-level shear (0-3 km SRH over 100 J/kg) just to the west of the ILX CWA Sunday afternoon. However, am skeptical of this for a few reasons. First, it is an outlier relative to other guidance. The NAM suite has all features further west than the HRRR, and has less low level instability, with virtually no sfc- based instability in the ILX CWA on Sun. Additionally, expansive cloud cover present on Sun should limit low-level destabilization.
Even if we take this HRRR solution at face value, the timing is such that diurnal heating is fading as the more favorable environment pivots into the western ILX CWA. It's worth keeping an eye on this threat, but concern is very low at this point.

Significant weather impacts are not expected next week, with some mid-week showers being the only notable forecast concern. The expectation remains that upper level ridging over Ontario/Quebec will strengthen into next week, resulting in an expansive 1034-mb sfc high centered near James Bay. Meanwhile, the upper low over the northern Plains weakens considerably, with 500-mb heights increasing from around 560-dam on Sun to around 575-dam by late Tues into Wed. However, that weaker upper disturbance is progged to meander south of the Canadian ridge, approaching Illinois on Tues. This will be enough to keep scattered precip mentioned in the forecast, primarily Tues-Wed, especially during the daytime hours as sfc heating combines with cool temps aloft to create marginal instability.

By late week, what's left of that weak upper low should shift east of the area, decreasing the rain chances. Through mid-week, high temps are forecast to be in the mid-70s. Late in the work week and into next weekend, an anomalous upper ridge is expected to develop, and temps should trend upward in response. The CPC 6- to-10 Day Outlook (valid Thurs Sep 28 - Mon Oct 2) strongly favors above normal temps across the Midwest (70-80% chance). Normal high temps for that period are in the mid-70s.

Erwin

AVIATION
(For the 12z TAFs through 12z Sunday Morning)
Issued at 555 AM CDT Sat Sep 23 2023

VFR conditions are expected through the period, along with light east-southeast winds. Mid-level clouds increase from west to east overnight. A stray shower is possible at KPIA/KBMI near the end of the period, but a better chance for rain will exist after 12z Sunday.

Erwin

ILX WATCHES/WARNINGS/ADVISORIES
None.

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