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FXUS63 KILX 291122 AFDILX

Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 522 AM CST Wed Nov 29 2023

SYNOPSIS
Issued at 522 AM CST Wed Nov 29 2023

Dry and seasonable conditions are expected today. Things turn unsettled late this week when a system lifting from the southwest brings widespread beneficial rain Thursday afternoon through Friday.
Occasional chances for additional precipitation exist this weekend through early next week as the large-scale weather pattern stays in a progressive state.

DISCUSSION
Issued at 306 AM CST Wed Nov 29 2023

Key Messages:

* Seasonable temperatures return today, sticking around through the weekend.

* Widespread beneficial rain arrives Thursday afternoon and continues into Friday. There is a 50-60% chance for at least 1" of rain south of I-72.

Surface ridging has pushed east of here as high pressure settles into the southeast US. Winds have shifted to the south, which has acted to increase temperatures by a few degrees or hold them steady since around midnight. Winds will veer back to the west later this morning as a surface trough works across the area. This will quickly be replaced by surface ridging once again which will turn winds back to the south-southwest by later today. High temperatures will be the warmest we've seen in several days with values topping out in the 40s to low 50s.

Temperatures will remain seasonable through late week, but our dry weather will be replaced by widespread soaking rain. Water vapor imagery as of 3 am captures our next system starting to move onshore the northern coast of California, with it forecast to dive into the Fours Corners region tonight. The upper trough will then lift through the southern Plains and eventually the middle Mississippi Valley later this week. A surface low will spin up in west Texas and eject northeastward reaching southwest Missouri by 12Z Friday morning. Dry low to mid-level air on Thursday morning will quickly erode as strong WAA and moisture transport ahead of the low overspread the area (including PWATs approaching 1"). Precipitation chances become likely (60%+) Thursday afternoon into evening as the low tracks into far southern Illinois. Deterministic guidance has deviated some with the exact track of the low. The GFS/Canadian show the low tracking closer to I-70 and the ECMWF along the Ohio Valley.
A track closer to I-70 would suggest higher QPF being pushed further northwest, which is now reflected in the latest NBM. Probabilities remain favorable (50-60%) that at least 1" of QPF will be seen near/south of I-72. If the low does track closer to I-70, lower QPF amounts would be seen over southeast Illinois than currently advertised. As of now, QPF ranges from 0.50" over northwest counties to around 1" over the southeast with most of this falling late Thursday night into Friday morning.

Although most of the area will only see rain late this week, there is a chance those in northwest Illinois could see a wintry mix. An upper shortwave sliding just north of the Great Lakes Region on Thursday will push a cold front southeast into the Midwest. The front will move into northwest Illinois Thursday evening, becoming stalled out roughly from a Macomb to Lacon line. Temperatures will cool near or below freezing behind the front late Thursday night into Friday morning with upper 30s to 40s temperatures remaining out ahead of it. Forecast soundings show an isothermal layer up to ~800 mb near where the front will stall out, which should uniformly cool and transition to or mix with snow. Wintry accumulations are low in our forecast as of now, but it is worth noting that a few ensemble members of the ECMWF and CMC show measurable snow (>0.10") in Galesburg Thursday night-Friday morning.

The surface low will move into Indiana later on Friday, but precipitation chances may linger a bit longer than previously expected. A deformation zone will set up somewhere from southern Iowa into northwest Illinois, largely staying north of here.
However, another subtle shortwave in the southwest flow aloft will lift through central Illinois Friday night and interact the deformation zone. PoPs near and north of I-72 range from 20-40% Friday night to account for this change, which seems reasonable.
Additional QPF with this should remain on the lighter side.

Additional chances for precipitation exist through early next week, but the details are still quite murky. Flow aloft will transition from southwest to northwest early next week, bringing somewhat cooler than normal temperatures. Several fast moving shortwave disturbances will work through the flow aloft during this time, which will open the door for additional precipitation chances. Left NBM PoPs in place for now due to lower confidence, which highlights occasional 20% chances through early next week.

NMA

AVIATION
(For the 12z TAFs through 12z Thursday Morning)
Issued at 522 AM CST Wed Nov 29 2023

VFR conditions are expected through 12Z Thursday. Southwest winds will veer to the west late this morning and then back to the south tonight. Speeds will be between 10-15 kts during the day, falling below 10 kts overnight.

NMA

ILX WATCHES/WARNINGS/ADVISORIES
None.

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