Area Forecast Discussion
Issued by NWS Chicago, IL

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416 FXUS63 KLOT 210336 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 936 PM CST Mon Feb 20 2017 .UPDATE... 925 PM CST No real big changes to the going forecast tonight, with showers still likely, and even a few thunderstorms still possible. Radar imagery showing precip axis still staying just to the west of the I-39 corridor across northwest Illinois. A slow eastward trend has occurred with these showers, and anticipate this slow trend to continue overnight. Outside of the western third of the CWA, most of the CWA will remain dry through around the midnight time frame. Then expect these showers to spread further east reaching other areas in northeast Illinois and northwest Indiana very late tonight into early Tuesday morning. Although instability remains limited, do think a slight chance of thunder is still warranted as the narrow axis of MUCAPE slides east with this precip. However, would anticipate only a few rumbles of thunder. Rodriguez && .SHORT TERM... 147 PM CST Through Tuesday... Record breaking warmth continues today across northern Illinois and northwest Indiana ahead of low pressure currently over northern Minnesota and a trailing cold front that stretches south across the mid Missouri Valley. Main forecast challenge for this evening and overnight will be timing and strength of line of showers and thunderstorms associated with the cold front. Precipitation is ongoing across much of Iowa this afternoon within a broad area of mid level WAA which is expected to lift NNE across WI this evening. Meanwhile, an advancing cold front is expected to kick off additional showers and possibly a few thunderstorms this evening moving east across the Mississippi Valley around 00Z and gradually spreading into the local forecast area through the mid to late evening hours. Surface based instability is expected to diminish after sunset, but forecast soundings do indicate that very weak instability will persist rooted just above the nocturnal boundary layer. Meanwhile, forcing is also expected to weaken, especially after midnight as the surface low lifts NNE and pre-frontal convergence weakens. All this should amount to a decaying line of showers moving across the area with precip chances winding down completely from the west late tonight into tomorrow morning as the surface cold front pushes across the area. Depending on the progress of the front, there could be a few spotty showers that redevelop tomorrow afternoon in the far southeastern portions of the CWA, but better chances will be off to our east. Despite passage of a cold front, temperatures behind the front quickly rebound. Clearing skies and lower dewpoints will help surface temps respond to insolation tomorrow, and expect highs once again well into the 60s with near record warmth. Deubelbeiss && .LONG TERM... 258 PM CST Tuesday night through Monday... A record warm stretch will start the extended and then a shift will occur on Friday and Friday night, which given how anomalously warm we have been, this windy transition will likely be accompanied with active weather. A short-lived dose of winter is then expected on Saturday with at least light snow looking more favored for part of the area. The weather pattern on Wednesday and the regime leading into it continues to support the now likely of reaching or exceeding all time February high temperatures during the afternoon. In the wake of tonight`s cool front, the air mass will have dried out, especially aloft limiting clouds. A quasi-zonal flow pattern across northern states will send a 1003 mb low eastward across Iowa into Wisconsin Wednesday afternoon and evening, providing focused low-level warm advection into the inherent anomalously warm air mass. The 925mb temperatures are forecast by almost all available guidance to be 15C, which is basically what is occurring today where sunny areas are reaching low 70s. Thus see no issues with the entire CWA into the 70s, with plenty of mid and even some upper 70s expected. Please see the climate discussion below for more on the records presently forecast to be broken. Regarding fire weather/dry concerns Wednesday afternoon, it remains forecast below that, but will need to keep an eye on it. Wind gusts are forecast to be a little above 20 mph Wednesday afternoon. There has not been much afternoon mixing of dew points during this warm stretch, but Wednesday may be a little more favorable. At this time have afternoon humidity forecast between 30 and 35 percent. While not to what we consider red flag concerns, it could be to still elevated fire risk levels. The system to our north will steer an initial cool front southward into the area Wednesday night. The air mass is forecast to moisten some, but not enough for rain. Cooler air will gradually ease into the area Thursday morning, especially near the lake as winds turn onshore with little to no temperature recovery. Where exactly the boundary stalls is a bit challenging and becomes noteworthy for Friday. This stalling is most favored across central Illinois on Thursday afternoon and retreating slowly northward as a warm front Thursday night with strong lee cyclogenesis across western Kansas. Rain will expand quickly at some point late Thursday into Thursday night across the region as forcing for broad ascent increases as well as within a narrow corridor of strong isentropic ascent and frontogenesis north of the warm front. Some bands of moderate to heavy rain with totals in excess of one inch are certainly possible with precipitable waters over an inch and the focused forcing. Also the likelihood of scattered elevated thunder should increase into the night. Overall model guidance is in good agreement with Thursday night, so this portion of the forecast has high confidence for three days out. A southern portion of the broad western NOAM upper trough will shift eastward Friday, with global guidance trending the past 24 hours in closing off the mid-level circulation by Friday afternoon, resulting in a slower but still strong surface low. The general slowing over the past 48 hours in guidance solutions supports very warm temperatures across much of if not the entire CWA. However, east winds north of the warm front may keep some lakeside areas much cooler. The effective warm sector...that is where the highest dew points and low-level instability will be...varies some in placement on guidance. The southern and eastern CWA remain most likely on GEFS spreads and deterministic solutions to be into this, but this low-level instability of 500-1000 J/kg could be all the way up to the warm front. Given the kinematics with a near 990 mb low in February, this continues concerns for some strong to severe convection Friday afternoon as the dry slot moves in. Synoptically this too continues to show some similarities to cool season severe weather events. Will have to watch if model guidance is a little generous in veering the surface winds in eastern areas, as they could be more backed than forecast. Even without that, the potential for isolated strong to severe winds looks reasonable with any quick-moving convection if the evolution pans out as presently shown. The surface low should progress northeast across the area with some signs it may be a double-barreled low on the 20.12 GFS and GEM, which both provide better wraparound potential for rain showers transitioning to snow late Friday night and Saturday morning. This however will depend on the low track and speed, as will how long duration the gusty winds are. There is potential for gusts in excess of 40 mph, including during Saturday morning when it very well could be snowing too. At the least this would provide a completely different picture outside than what we have been seeing this past week. Light accumulation is possible, especially if this double-barreled signature pans out, slowing the system and providing a more southerly surge to the system TROWAL. We are not expecting surface temperatures to climb much if at all on Saturday with them holding steady in the lower to mid 30s with a little warmer on Sunday. Confidence in specifics really drops off in the forecast after that time period. The primary low-level baroclinic zone will be located to our south with the active jet pattern that has been pummeling California to then be across a good part of the southern/central U.S. latitudes. So it certainly looks active into the last days of February into the first week of March, and CPC 6-10 day outlook of better chances for above normal precipitation is reasonable. MTF && .CLIMATE... 258 PM CST A highly anomalous warm stretch that has already broke daily records will continue to threaten records this week, including the all time February warm records for any time of the month on Wednesday. Here are daily records. Chicago: Rockford: High: High Min: High: High Min: 2/21: 67 (1930) 47 (1930) 64 (1930) 42 (1930) 2/22: 68 (1922) 52 (1930) 61 (1922) 48 (1930) All-time February Record Highs: Chicago: 75 2/27/1976 Rockford: 70 2/25/2000 For more, including consecutive days of such warmth this time of year, please see our web top news headline. && .AVIATION... For the 00Z TAFs... Main TAF concerns are with gusty south-southeast winds through this evening, and a period of showers with IFR/MVFR cigs spreading east across RFD by mid-late evening and Chicago terminals mainly after midnight. Rain looks to end early Tuesday morning, with a gradual improvement to VFR conditions by midday. Surface low pressure was lifting from northern Minnesota into western Ontario late this afternoon, with a trailing cold front extending across western Iowa. Atmospheric column will moisten from the mid/upper levels on down as the cold front advances, with gusty south-southeast winds helping to draw low-level moisture into the region. Guidance is in good agreement in general timing of the band of precipitation expected with the approach and passage of the cold front overnight, and with a period of IFR/MVFR cigs lingering early Tuesday morning. Winds shift west Tuesday however, with forecast soundings depicting strong drying and improvement to VFR by midday. Tight surface pressure gradient ahead of the cold frontal trough will support fairly strong winds 15-20 kts with gusts potentially in excess of 25 kt at times, though shallow mixing may keep those higher-end gusts more sporadic. Gradient gradually weakens late this evening ahead of the cold frontal trough and wind shift to the west-southwest. Ratzer && .MARINE... 258 PM CST A more active period of the lake is likely, especially late week. An approaching cold front will help tighten the pressure gradient with increasing southerly winds this evening. The stable air over the lake may limit full gust potential, though could be some occasional gale force gusts in the northern to central part of the open waters. Conditions will be near small craft advisory criteria in the Illinois and Indiana nearshore zones. The cold front will pass across the lake by Tuesday morning with a limited cold push behind it. Thus winds should not increase too much. Another low will scoot across the lake on Wednesday night, with a much deeper low at the end of the week. This deep low still looks to favor a period of gales, possibly both ahead and behind the low passage. Behind the low is most favored as the cold advection sweeps over the lake. MTF && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...Small Craft Advisory...NEARSHORE WATERS UNTIL 3 AM Tuesday. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSCHICAGO WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO

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