Area Forecast Discussion
Issued by NWS Chicago, IL

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537 FXUS63 KLOT 061210 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 610 AM CST Tue Dec 6 2016 .UPDATE... 437 AM CST Winds are slackening an fog is developing quickly. Temperatures also continue to fall into the 20s creating concerns of freezing fog...especially on elevated surfaces. Light ice accumulations could make for slick or locally icy conditions. An SPS has been issued but investigating the need for a dense fog advisory with the mention of freezing fog and possible icing. KMD && .SHORT TERM... 335 AM CST Through Wednesday night... Stratus is starting to erode from the southeast as a subtly drier air mass advects in, with the low cloud largely gone along the I-55 corridor southeastward. Farther west expect the stratus and fog to hold a bit longer being still removed from the drier air. Vsbys have largely been -1 mile in the denser fog and really have inched upward of late. These areas will remain under the inversion until a cold front approaches this afternoon, with other area s east still having a higher cloud deck. There are some indications that with a weakening wind field in the COL region of the approaching cyclone that vsbys could come back and become locally dense at daybreak or shortly thereafter. This is across north central Illinois including the Rockford area. Most locations in this area at freezing or so in the lower cloud deck that freezing fog would not be a huge concern but will need to monitor this first near term concern. Several additional features of interest in the near term ahead of our upcoming arctic air invasion. The first is a band of precipitation that is lifting northeast ahead of low pressure across the Arklatex region. This low will continue northeast into the Ohio and Tennessee valleys today, which its precipitation shield largely heading with it. Weak frontogenesis and isentropic lift across our far southeastern counties (east Central IL and several NW Indiana counties) will likely allow some echoes to move through in the coming hours. Temperatures have warmed to around 32 degrees across Iroquois county and into Benton/Newton/Jasper, with some slightly cooler temps farther north. Low level profiles suggest precipitation will be in the liquid form with a low level warm nose, though possibly transitioning to snow with wet bulbing. Expect there could be a window of light freezing drizzle/rain here a few hours on either side of daybreak. Much of this activity is not reaching the ground yet but radar returns are increasing. Model QPF is very low such that this does not appear to be a big problem, but could create some localized slick conditions as model reflectivity fields slow a slight uptick after 11z for a few hours before low level wet bulbs get above freezing. Next on the docket is an area of strengthening low pressure across the upper Midwest. Extending south of the low is a cold front where an area of light snow is falling. This front will move into north Central Illinois later this morning/early afternoon. In our area the front will become more removed from the upper forcing that will head north of the area, but model reflectivity showing some very light echoes across our far northern counties this morning into the afternoon. This could be a few flurries/drizzle as it comes through. For the rest of the area, this front will be the leading edge of a much colder air mass that will ooze in from the northeast in the coming days. Expect winds to pick up mid to late this afternoon ahead of and behind the front with gusts 25 to maybe 30 mph. Otherwise for most areas the front will come in with some clearing and little precipitation. The leading edge of the cold front will pass through the remainder of the area tonight. With some clearing and the cold advection temperatures will fall into the lower 20s in many areas. The cold air will continue to funnel in from the northwest on Wednesday as low pressure slides by well to our north. High temperatures look to remain in the mid 20s north to near freezing south. Cloud cover will increase ahead of a southern stream upper wave at the base of the large upper trough axis across the center of the country extending through Central Canada. No real surface reflection develops on the frontal boundary ahead of this wave such that models have trended toward very little precipitation if any Wednesday night except possibly in our southeastern areas (per only a few ensemble members) in addition to across the lake where the lake effect machine will ramp up. KMD && .LONG TERM... 400 AM CST Thursday through Monday... Very cold and somewhat cloudy conditions will greet us on Thursday as the cold upper trough migrates across the area. Some flurries are certainly possible as energy will rotate southeast through the trough axis. Highs peak in the lower 20s. While the focus of the lake effect machine will mostly be on the east side of Lake Michigan, the low level flow turns a bit more northwesterly such that lake effect snow may enter Porter and possibly Lake County Indiana later Thursday and Thursday night. More sunshine is possible Friday, but the core of the cold air will have arrived, making for another very cold day. Highs may not even reach 20 in north central IL. The coldest night area wide looks to be Friday night as a high pressure ridge will move overhead. Some of the drop off could be tempered as warm advective high clouds arrive, but still looking at teens. Warm air advection will continue Saturday ahead of strengthening upper level low pressure in the west. Of more significant attention in the extended period is the potential for another round of snow Saturday night into Sunday. The 0z GFS is amped up with all the classic signs of a powerful winter storm (deep upper trough becoming negatively tilted, strengthening upper jet and associated frontogentical circulations, strong moisture transport on a low level jet, deepening surface low) on Sunday while other guidance is a bit more muted, certainly reflective on how the upper trough ejects from the west and interacts with moist low level flow. All models show ascent developing ahead of western CONUS upper level energy with some lighter snow getting into the Mississippi valley by late Saturday afternoon then spreading into at least portions of the area Saturday night, then their evolutions diverge considerably with the EC some 18-24 hours later with bringing stronger upper level energy capable of developing a strong surface low. The low it develops is weaker as well but does still bring a snow/snow- rain mix in later Sunday/Sunday night. The Canadian model is sort of in between the two with a single system but more muted than the GFS. GFS Ensembles support a less amplified pattern not like the operational GFS, but predictability is somewhat low as expected. Regardless there are signs for another chance for snow during this period. KMD && .AVIATION... For the 12Z TAFs... Main concerns are in the near term with IFR and LIFR ceilings and with very low visibility around one quarter mile. Main stratus shield had progressed to the northwest of all the terminals with the exception of RFD overnight. However, dense fog and low ceilings have since developed in this clearing and are currently impacting RFD and DPA, while these lower conditions are right just north/northwest of ORD. Main stratus shield and now newly developed low conditions will continue to impact RFD and DPA, and then shift east impacting ORD and MDW over the next one to two hours. Do have ORD and MDW at 1 mile but it is possible for the vis to briefly drop below that. As low level flow shifts back westerly, this will then impact GYY with low ceilings and vis expected through mid to late morning. Vis in the fog will likely improve but do think ceilings will remain in place well into midday and even into the early afternoon. Much drier air will eventually help to raise ceilings and then completely erode them later this afternoon into the evening, with the arrival of a cold front and westerly winds. Rodriguez && .MARINE... 409 AM CST Main forecast concerns are with strong westerly winds and building waves for the nearshore waters later this afternoon and tonight, and then with possible gales over the north half of the lake. In the near term, southeast winds in the 15 to 25 KT range are occurring over much of the lake. However, surface high will swing through the southern end of the lake and allow for some diminishing winds in this location this morning but while winds stay elevated over the north half. This will be short lived though as a surface trough and frontal boundary are expected to move across the lake this afternoon into this evening. This will allow for a transition to a southwest and west wind, with generally west winds then expected later tonight into Wednesday. As this occurs, expect winds to quickly increase with hazardous conditions expected over the nearshore which will last into Wednesday morning and possibly into that afternoon. Gales will quickly be possible over the north half in the evening with the passage of the front, but then once again later tonight into Wednesday. Confidence with the duration of gales is low and so have only issued a watch at this time, despite this possibility within 24 hours. Will need to monitor later trends and guidance to see if the gales will prevail or if they will be shorter in duration. Also could see a brief window of gales over the south half this evening but have higher confidence that this would be short lived. Rodriguez && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011- ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022 UNTIL 11 AM Tuesday. IN...NONE. LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744- LMZ745...3 PM Tuesday TO 10 AM Wednesday. && $$ 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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