Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 01/12/22

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
527 PM CST Tue Jan 11 2022 .SHORT TERM... Issued at 300 PM CST Tue Jan 11 2022 Through Wednesday night... The main winter weather concern, albeit a low confidence one, is the potential for a narrow area of heavier precip rates mid Wednesday morning through the early afternoon, primarily for far northern Illinois. If the precip rates are heavy enough, the concern is that this would be a burst of snow causing at least some travel impacts. In the near term, our windy moderation coming off a few very cold days is well under way, with south-southwesterly wind gusts up to 30-35 mph. These gusty winds will continue through this evening while veering to more southwesterly despite the lingering glaciated snow cover for most of the area as a very impressive west- southwesterly low level jet of 50-60 kt between about 2kft and 3kft AGL. Expect temperatures to continue to climb past sunset and then probably level out by the mid to late evening. Weak cold front or moreso surface trough passage and departure of the low level jet off to the east after midnight will cause winds to quickly ease and temps to drop back to the mid to upper 20s. Our lower amplitude short-wave of interest for tomorrow is currently over the Canadian Rockies and will quickly cruise in tomorrow morning and interact with a fairly tight baroclinic zone over the western Great Lakes and upper MS Valley. Even at this near term lead time, there is plenty of uncertainty on exact trajectory of the wave and overlap of forcing and sufficient saturation for precip. Note that the NAM and GFS have been generally north of the IL/WI state line, while the ECMWF (and its ensemble/showing likely probabilities of measurable precip) and the Canadian favor the state line area and points just south. While confidence is low due to lingering uncertainty in the guidance, concern is elevated for a 2-4 hour burst of heavier precip in form of snow due to the presence of transient strong f-gen as well as steep mid-level lapse rates near/over 7C/km. These can be "gotcha" set-ups that can put down a quick inch or two of snow and cause travel impacts. Even though deterministic forecast is only coating to several tenths of snow, will message a limited snow risk in the HWO. The thermal profile is also in question, if precip occurs, as initial warm nose may pose some threat for brief freezing rain at onset, but will let later shifts assess this. A trailing area of light precip could sag south during the afternoon, though confidence also low on this aspect. Aside from the low-confidence but potentially impactful period mid-late AM through early afternoon for portions of far northern Illinois, tomorrow will feature above normal temps ranging from mid 30s I-80 and north to upper 30s to around 40 south. Another weak cold front will move across the area on Wednesday night ahead of next period of interest Thursday morning, covered in the long term AFD below. Low temps will be above normal in 20s to around 30. Castro && .LONG TERM... Issued at 237 PM CST Tue Jan 11 2022 Thursday through Tuesday... The forecast points of interest for the latter half of the week into the weekend continue to center on the series of disturbances (i.e. wave train) that bring snow chances. Predictability with these drops off quickly, due in part to their low amplitude nature as well as the recent trend in guidance (particularly with Friday-early Saturday). The first of these disturbances in this period will be dropping southeastward over the region on Thursday morning. This will be traversing on the periphery of the polar lobe extending over Hudson Bay, riding the left exit region of a 140+ kt jet. Guidance is in decent agreement of the 500 mb vorticity center tracking over/very close to northeast Illinois and northwest Indiana, and with a focus for ascent from the pre-dawn hours through the morning. The elevated lapse rates are noteworthy with this wave, being around 7.5 C/km from 800 to 600 mb. The saturation is a little less impressive as is the depth of the dendritic growth zone. The extended portions of the RAP and HRRR show a few hundredths to even a tenth of an inch of QPF for parts of the CWA, and a fairly good signal that some part of the CWA should see at least brief light snow. Have nudged up PoPs above a blend of the latest guidance, and will be a period to watch with its intersection of the Thursday morning commute. Temperatures look to be in the mid-upper 20s during the possible brief period of snow. In the wake of the stronger forcing, there is a marginal signal for some drizzle after, but that would be late morning into afternoon, with temperatures nudging up to or a little above freezing. For the rest of Thursday into early Friday, north-northwest surface winds will veer to northeast. Cold advection will be limited for that wind direction this time of year, but may still be enough for lake effect clouds and some low quality light snow into northeast Illinois/far northwest Indiana. The forecast inversion heights are only 3,500-5,000 ft AGL, so not notably high for anything robust if it were to materialize. The next digging system for Friday afternoon and night into early Saturday has been one that has consistently been shown to be a potentially stronger one. The 12Z deterministic guidance suite largely showed a westward shift with this, and to a lesser degree the GEFS and ECE runs. This wave is still traversing the Gulf of Alaska region and is low amplitude. The hard right turn that it is forecast to take in northwest Canada is key, as just slight variance in that will dictate its track into the U.S., the eventual phasing time with another wave into the Plains, and overall where an accumulating snow track is. The WPC Cluster Analysis, while based on 00Z ensemble data, did show about 30% of the ensemble members still having a solution that brings QPF fairly far east from the mean and smears the gradient into and over our area. For now, the guidance provided blend of PoPs...from basically 30 percent east to 50 percent west...provides a meaningful representation of our lower-end certainty. So from the previous forecast, this is only a slight edge downward -- basically a lesser shift than the 12Z guidance did from the 00Z. It should be noted too, that through Friday into early Saturday there continues to be some non-heavy lake effect snow possibility into northeast Illinois as the east-northeast winds steadily increase around that digging system. Inversion heights never really come up much, but the temperature profile is forecast to cool later Friday night into Saturday morning. No changes made beyond that in the forecast. Looks like another couple waves Sunday through Tuesday that could introduce a hearty shot of cold air by the mid to latter part of next week. MTF && .AVIATION... For the 00Z TAFs... South to southwest winds will likely remain gusty well into the evening hours, particularly at ORD/MDW. Recent ACARS flights out of ORD show about 45kt at 2000ft AGL with some increase in winds just off the deck expected this evening before winds through the low level shift more westerly later this evening and subside. VFR CIGS expected through Wednesday, likely lowering some to FL080-100 by midday Wednesday. Some potential for lower CIGS with stratus toward the end of the 30 hour MDW/ORD TAFS, but confidence isn`t high whether it arrives within the 30 hour TAF or more likely just beyond, so kept it out for now. - Izzi && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Gale Warning...nearshore waters until midnight Wednesday. && $$ Visit us at Follow us on Facebook, Twitter, and YouTube at: