Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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183 FXUS63 KLOT 221156 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 556 AM CST Fri Nov 22 2024 .KEY MESSAGES... - Lake effect rain showers to prevail in northwest IN today, especially in Porter County and points east. - Moderating temperatures for Sunday, then turning colder again Tuesday through the Thanksgiving Holiday. && .DISCUSSION... Issued at 315 AM CST Fri Nov 22 2024 Through Saturday: The upper-level low that brought us the snow yesterday continues to spiral over the Mid-Atlantic and eastern Great Lakes this morning. While most of the precipitation has come to an end across northern IL, some pockets of drizzle continue to be observed near between I-39 and I-355. The drizzle is expected to linger through daybreak before tapering by mid- morning. However, a mid-level vorticity maximum is expected to pivot overhead this afternoon and interact with the persistent low-level moisture which may lead to another bought of patchy drizzle this afternoon. Since the coverage is somewhat lower confidence for this afternoon have decided to forego a formal mention in the forecast at this time, but did boost POPs around 10% to account for this potential. Regardless, temperatures are expected to remain above freezing (in the upper 30s to mid-40s) today which will keep the any impacts low. Aside from the drizzle, there is also the ongoing lake effect showers in northwest IN to discuss. The lake effect is currently riding along a north-south oriented convergence axis that will continue to drift eastward into northern IN as winds become more west-northwesterly later today. So while lake induced instability will remain sufficient (ELs in the 5000 to 6000 ft range), the coverage of showers should focus into far eastern Porter County and points east by midday. In terms of precipitation type, forecast soundings continue to show that virtually all of the effective moisture to be underneath the DGZ (-12 to -20 C layer) which will keep the showers in the form of rain. However, a couple of snowflakes may mix with the rain prior to daybreak where temperatures are still hovering near freezing. The lake effect showers and any drizzle today will conclude this evening as the aforementioned upper-low progresses east and allows modest upper ridging to build overhead. Therefore, dry conditions can be expected for tonight and through the day on Saturday with otherwise seasonable conditions with highs in the 40s and overnight lows in the low to mid-30s. However, lingering low-level moisture trapped beneath the ridge does look to maintain mostly to completely cloudy skies through Saturday as well. Yack Saturday Night through Thanksgiving: Focus in the extended forecast period is on a brief warm-up to start the upcoming week before a cold front early Monday ushers in a period of slightly below normal temperatures through the Thanksgiving holiday with additional wintry-precipitation chances. Much colder temperatures are then potentially in store over the extended holiday weekend. Heading into Sunday morning the surface ridge axis will begin to translate east of the area bringing a return to southerly low-level flow (aka warm advection) with continued dry conditions. While there will likely be a fair amount of cloud cover overhead, temperatures should still manage to warm back above normal with forecast highs in the lower to mid 50s. Be sure to get out and enjoy it if you can! A cold front subsequently moves through the area early Monday ahead of an approaching weak mid-level shortwave. This will bring the next chance for rain to portions of the area; however, questions remain as to whether sufficient moisture will be available locally given the displacement of these two features. In fact, a faster frontal passage could keep most areas dry. Opted to not make large changes to the previous forecast for now with the best potential (20-40% chance) mainly near the lake and east of I-55 during the day on Monday. This cold frontal passage then ushers in a period of slightly below normal temperatures for late November with highs generally in the mid 30s to lower 40s and lows in the 20s to lower 30s forecast through Thanksgiving. The upper level pattern also turns more quasi-zonal toward midweek next week as the jet stream sags a bit further south latitudinally. This would place us just to the north of the favored system track, which could bring additional wintry precipitation chances toward midweek, including the Thanksgiving holiday. Stay tuned. Looking ahead there remains a fairly strong signal for a period of much colder temperatures over the long holiday weekend into the first week of December with the Climate Prediction Center forecasting a 70-80% chance of below normal temperatures across much of the region. Petr && .AVIATION /12Z TAFS THROUGH 18Z SATURDAY/... Issued at 555 AM CST Fri Nov 22 2024 Key messages: - MVFR stratus through the period with occasional breaks to VFR possible. - Breezy NW winds today with 20-25kt gusts. MVFR stratus prevails across the area early this morning mainly in the 1500-2500ft range. A small patch of drizzle with associated IFR ceilings and visibility in SW WI and NW IL could briefly graze RFD and will be closely monitored this morning. Occasional breaks in the stratus are also evident via GOES nighttime microphysics imagery over Wisconsin which suggests at least short periods of VFR conditions will be possible today as the broader cloud shield gradually rounds the departing upper low. Northwesterly winds will turn breezy after sunrise today with gusts in the 20-25kt range expected through this afternoon and potentially into the evening. Winds then gradually ease overnight, turning westerly on Saturday. Hi-res guidance continues to suggest this stratus could persist overnight and into the day on Saturday. An earlier return to prevailing VFR can`t be ruled out, but it is also possible that ceilings instead gradually lower to near IFR before scattering out later in the day on Saturday. Petr && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory until 4 AM CST Saturday for Winthrop Harbor IL to Gary IN. Small Craft Advisory until 10 AM CST Saturday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN. && $$ Visit us at weather.gov/chicago