083 FXUS63 KLOT 261155 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 555 AM CST Fri Jan 26 2024 .SHORT TERM... Issued at 341 AM CST Fri Jan 26 2024 Through Saturday... Key Messages: * Widespread rain diminishing this morning, though additional showers and occasional drizzle can be expected into this evening especially across far northern IL. * Fog has thinned with rain overnight, though visibility will likely deteriorate again later this morning as rain diminishes. Areas of dense fog will be possible into tonight. * River flooding threat persists into the weekend as runoff from melting snowpack continues, and ice jams break up. Early morning GOES vapor imagery depicts a pair of mid-level short waves tracking across the region. The first is a negative- tilt wave lifting across the forecast area at this time, with a surface low pressure reflection over northern OH. Widespread area of light to moderate rain associated with this disturbance has lifted mainly across far northeast IL/Lake Michigan as of 3 AM. Farther to the west, another weaker mid-level vort was across central IA, with another area of more organized rain over east/northeast IA. This vort will shear eastward across the forecast area through this afternoon, with additional light rain/showers tracking across northern IL after a brief lull early this morning. Otherwise, occasional drizzle likely into this evening as well. Arrival of rain last evening had allowed fog to thin and visibilities to improve, though the expectation is that fog will fill in again with deterioration of visibilities across portions of the area this morning as rain eases. Confidence is fairly low in how quickly this occurs and the footprint of dense fog today, though various model guidance does support fog thickening again especially across Lake Michigan and far northeast IL by mid-morning. Guidance also depicts the potential for a more widespread expansion of dense fog across much of the area this evening. Weak mid-level ridging is progged across the area late tonight into Saturday, with drier conditions expected. An upper trough currently over the southwestern CONUS is progged to track south of the Ohio River Saturday into Saturday night, and looks to bring some additional rain as far north as the southeastern parts of the forecast area Saturday evening as detailed in the long range portion of the discussion. Cloud cover is expected to linger through the period, helping to maintain milder than average temperatures (lows in the low-mid 30s and highs mid-30s to near 40) for late January. Recent rainfall, rapid melting of our snowpack and frozen/saturated ground conditions due to the recent sharp cold spell have all combined to increase runoff into area rivers. This runoff, along with several ice jams especially across our southern cwa, has led to river flooding in several locations especially on main-stems south of I-80. The chance for flooding along ditches, streams, creeks, rivers, and poor drainage areas will continue to increase as snow continues to melt and shifting river ice maintains a risk of ice break-up jams. Ratzer && .LONG TERM... Issued at 341 AM CST Fri Jan 26 2024 Saturday night through Thursday... Key Messages: * Clipper system brings chances for precipitation Monday night into Tuesday * Blocking patter forecast to develop late next week (possibly through next weekend) * Above normal temperatures forecast through the period The broad upper trough, currently sprawled across the eastern Rockies and central CONUS, is forecast to be gradually moving eastward into the deep south and Great Lakes Saturday night into Sunday. As this occurs, a shortwave disturbance is expected to traverse through the eastern edge of the trough resulting in the development of a broad area of precipitation. While the latest guidance trend is for the bulk of this precipitation to remain south and east of our area across the Ohio and Tennessee River Valleys, there remains a 20 to 30 percent chance for some precipitation to brush the southern third (mainly south and east of I-55) of our forecast area. Regardless, forecast soundings do show a stout mid- level warm nose which should keep precipitation in the form of rain. Heading into Sunday afternoon, the broad trough is forecast (and embedded shortwave) is expected to move into the Mid-Atlantic region which should allow modest mid-level ridging to build into our area. As a result, rain is forecast to conclude Sunday afternoon but lingering low-level moisture and periodic vorts traversing the backside of the trough may produce spits of sprinkles and/or drizzle. Drier air is expected to advect into the western Great Lakes Sunday night into Monday which should dissipate any lingering drizzle and possibly even generate some breaks in the cloud cover. However, the break in precipitation looks to be short lived as guidance is in remarkably good agreement on a robust shortwave diving out of Canada into the Great Lakes late Monday into Tuesday. In fact, ensemble probabilities of at least 0.01 inches of QPF are in the neighborhood of 50 to 80 percent which is notably higher than the 15 to 20 percent POPs offered by the NBM. Given this signal I did decide to increase POPs during this period into the 30 to 40 percent range areawide to trend more inline with virtually all other ensemble and deterministic guidance but further revisions may be needed going forward. In terms of precipitation type, forecast soundings do show a large warm layer in the mid-levels which I suspect will keep p-types mainly in the form of rain. Though, there are some indications that temperatures could cool just enough to allow a couple of snow flakes to mix in briefly around daybreak on Tuesday in spots. Since this part of the forecast is still 4 to 5 days out I maintained the splotchy mentions of rain/snow offered by the NBM but will look to refine these details with later forecasts. While this shortwave moves through Monday night/Tuesday, broad ridging is forecast to develop across the western CONUS and gradually shift east into the central CONUS towards the middle and later portions of next week. As this occurs guidance shows the aforementioned shortwave stalling over the northeast while another trough stall over the Pacific. The resulting blocking pattern looks to generate an extended period of well above normal (10 to 15 degrees) temperatures to close out January and start February. However, it should be noted that guidance does tend to struggle with the longevity of blocking patterns which means that it still uncertain as to exactly how long these conditions can be expected so stay tuned. Yack && .HYDROLOGY... Issued at 254 PM CST Thu Jan 25 2024 Key messages: - Warm temperatures, high dewpoints and frozen near-surface soil is leading to efficient runoff from snowmelt across the area. - Rainfall tonight will contribute more runoff, including the possibility of flooding along ditches, streams, rivers, and poor drainage areas. - Significant ice cover exists along most area rivers. Rising water levels may break up ice cover. Snowpack across portions of northern Illinois holds up to 2 inches of water equivalent, with snowmelt underway. The highest values generally exist along and north of a line from near the Quad Cities east to near the western Chicago suburbs. Soils are saturated and frozen near the surface. Significant ice cover exists along most area rivers, with a few ice jams ongoing. Temperatures are forecasted to remain above freezing for the next several days, which will cause continual snowmelt. Rainfall is expected to arrive later by early evening, and will continue overnight. With saturated and frozen soils, this will cause additional runoff combined with ongoing snowmelt. Temperatures will also remain above freezing with upper 30s and low 40s expected Friday through Sunday. Even the impacts of cooler overnight lows will be mitigated by continuous cloud cover and fog. The chance for flooding along ditches, streams, creeks, rivers, and poor drainage areas will increase over the next few days, especially in areas with considerable snow cover. Shifting river ice along area rivers will lead to a risk for break-up ice jams. It is for these reasons that the Flood Watch was expanded to include areas north of I-88 from Southern Cook County to Lee County. Additional flood warnings have also been issued for areas along the Kankakee and Illinois rivers where rivers continue a steady increase. A Flash Flood Warning was issued for the lower Kankakee River downstream of the ice jam near Wilmington as the imminent break up or shifting of ice could cause rapid-onset flooding from the city of Wilmington downstream to the Phelan Acres area. DK/WSL && .AVIATION... For the 12Z TAFs... Forecast concerns for the terminals... * Persistent IFR/LIFR ceilings through the period * Periods of drizzle and fog resulting in very low visibilities through this afternoon, possibly under 1/2 SM at times * Another round of fog tonight into Saturday morning A pair of upper-level disturbances continue to pivot overhead resulting in some rain and drizzle at the terminals. The rain has helped to improve visibilities with most locations reporting values in the high MVFR to low VFR range; however, there continues to be a strong signal that fog will redevelop later this morning. Exactly when the fog will redevelop and how low visibilities will go continues to be points of uncertainty. While guidance continues to support the idea of visibilities falling into the 1/4 SM to 1/2 SM range at times this afternoon, the timing of when this occurs has trended later with each new model run. As a result, I have pushed back the arrival of the lowest visibilities at the terminals but suspect that these times may still be too soon given the current observations. Periods of rain and drizzle will also persist through this afternoon which may allow for fluctuations in the aforementioned visibilities especially with any true rain showers. The rain and drizzle should begin to taper this evening as the disturbances exit the area but lingering low-level moisture and light winds will once again set the stage for patchy dense fog tonight into Saturday morning. Given uncertainty in coverage in the dense fog have decided to maintain the PROB30s for this potential. On the bright side, there is a signal that visibilities should improve Saturday morning as drier air moves in and begins to erode the fog and possibly lift ceilings a bit as well. Otherwise, IFR and LIFR ceilings are expected through the bulk of the forecast period with light northerly winds generally around 5 kts. Yack && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Flood Watch...ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021- ILZ023-ILZ032-ILZ033-ILZ039-ILZ105-ILZ106-ILZ107-ILZ108 until noon Monday. IN...Flood Watch...INZ001-INZ002-INZ010-INZ011-INZ019 until noon Monday. LM...Dense Fog Advisory...IL and IN nearshore waters until 10 AM Friday. && $$ Visit us at weather.gov/chicago Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/NWSChicago www.twitter.com/NWSChicago www.youtube.com/NWSChicago