Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 241056 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 556 AM CDT Tue Apr 24 2018 .SHORT TERM... 253 AM CDT Through tonight... A mid/upper-level low continues to meander across the western KY/TN state line, with a notable trough axis and weak low to mid-level frontogenesis extending NW from the main low over central IL early this morning. This has resulted in an active region of light rain showers and drizzle arcing from near St. Louis to southeast Wisconsin. The region should remain active through mid-morning as it interacts with another mid-level trough axis shifting northward across Indiana. Short-term guidance supports this trend, placing the main focus for light rain across NE Illinois, including the Chicago metro, around daybreak. Patches of light rain should then continue through the morning before slowly ending from west to east as the upper low continues to shift east. Plenty of low-level clouds will inhibit some daytime heating, but with temps starting out the day around 50F, max temps just a few degrees cooler than Monday are attainable. A cold front will surge southward across the area this evening, reaching the WI state line around 9pm, and clearing the southern CWA during the early morning hours. N to NNE winds are expected to gust 25 to 30 mph through much of the night behind the front, with gusts up to 35 mph along the Lake Michigan shore. The main trough axis associated with the front will remain well north of the area, so precip is not expected through the night. Kluber && .LONG TERM... 241 AM CDT Wednesday through Monday... Fluctuating temperatures are in store throughout the extended period as a series of cold fronts move across the region through the latter half week and into the weekend. A big warmup may be in store by early next week, though, as a strong upper ridge builds across the Great Lakes. Cold front that sags through the region Tuesday night will usher in cooler temperatures on Wednesday. A modest north/northeast gradient will be in place early in the day ahead of an expansive ridge that extends from the High Plains into the Great Lakes. Ridge axis will gradually build south across northern Illinois and northwest Indiana by Wednesday evening/overnight allowing the gradient to gradually relax through the day. Temperatures will be cool on Wednesday, topping out in the mid 50s inland and 40s near the lake with persistent onshore flow throughout the day. Weather looks to be otherwise quiet Wednesday and Thursday under the influence of the building ridge. Temperatures should moderate behind the ridge axis on Thursday with afternoon highs near seasonal norms in the low to mid 60s. There probably will be a lake breeze on Thursday, but boundary layer flow increasing to around 10 kt out of the southwest may result in minimal inland progress, or a late day surge that won`t have much impact on high temps. Thursday night, another cold front is poised to sweep across the region. Guidance has been in good agreement indicating a line of showers and thunderstorms developing within a narrow corridor of instability near the front Thursday afternoon across portions of Wisconsin into Iowa. There has also been good agreement on frontal passage across the local CWA overnight with a rapidly stabilizing boundary layer which will limit the intensity of convection as it reaches northern Illinois. Would expect a decaying line of showers and not terribly confident that it will maintain through the overnight hours across the entire CWA. Behind the front on Friday, afternoon high temperatures drop back into the 50s with 40s near the lake. Another ridge axis builds across the region over the weekend bringing a dry spell and a warming trend back to seasonal norms by Sunday. Meanwhile, strong upper ridge will be translating east across the Great Plains over the weekend then across the Midwest early next week. A strong thermal ridge will move across the region with 850mb temps warming to 11-14C and 925mb temps of 16-20C Monday and Tuesday. Late April/early May climo has median surface high temps in the upper 70s given those temperatures aloft. Cloud cover could be a complicating factor, especially on Tuesday as another cold front approaches the region but both days look favorable to make a run at 80F. Deubelbeiss && .AVIATION...
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For the 12Z TAFs... Patches of -RA/-DZ and low-end MVFR ceilings have been observed across the area early this morning, but continue to become less common on the northwest extent on an upper-level low. Precip is expected to gradually diminish through the morning, though a broken MVFR strato-cumulus field should remain into mid afternoon. A cold front will usher in gusty N to NNE winds beginning mid- evening. Kluber
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&& .MARINE... 241 AM CDT Lake Michigan lies within a weak flow regime early this morning, between systems, but a cold front will move down Lake Michigan this evening and overnight with northerly flow strengthening in its wake. Wind gusts are expected to peak around 30 kt late this evening and overnight, then should very gradually come down through the day Wednesday as a ridge of high pressure across the region. A small craft advisory will be hoisted for the Illinois and Indiana nearshore waters for this evening through Wednesday evening. Winds flip to southwest behind the ridge axis on Thursday and ahead of another approaching cold front. This next front is expected to sweep across the Lake Thursday night bringing another period of north winds gusting to around 30 kt. BMD && .LOT WATCHES/WARNINGS/ADVISORIES...
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IL...None. IN...None. LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...10 PM Tuesday to 10 PM Wednesday. Small Craft Advisory...LMZ743-LMZ744-LMZ745...1 AM Wednesday to 1 AM Thursday.
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