####018006032#### FXUS66 KMTR 130915 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 115 AM PST Sat Dec 13 2025 ...New SHORT TERM, LONG TERM... .KEY MESSAGES... Updated at 111 AM PST Sat Dec 13 2025 - Dense fog impacts are expected through the morning in the Interior North Bay and East Bay Valleys as well as coastal locations - Wet conditions return next week && .SHORT TERM... Issued at 111 AM PST Sat Dec 13 2025 (Today and tonight) Today's main impact will be a tale of two fogs: advection and radiation. Let's start with the persistence side of the forecast, or what has been happening. Portions of the region will continue to be affected by tule fog (a type of radiation fog) as it seeps out of the Central Valley through neighboring gaps and passes which will continue to facilitate below normal temperatures. There is also a southerly surge underway which will bring more traditional advection fog to coastal locations. This will not only be a change to the warm, sunny afternoons that have been observed the last week, but also to the cold overnight conditions that have been made possible by clear skies which allow for radiational cooling. We will continue to monitor the evolution of both of these through the morning to see if the Dense Fog Advisory that is in effect through 11AM for North Bay and East Bay Interior Valleys needs to be extended in area, time, or both. Consider delaying unnecessary travel until visibilities improve. If travel is necessary, make sure to use your low-beam headlights, increase following distance between vehicles, and allow extra time and patience. Elsewhere, sensible weather will yield temperatures up to 15 degrees above normal as the region remains under the influence of upper-level longwave ridging. Offshore flow will also continue with high pressure across the Great Basin and a coastal trough off the West Coast. && .LONG TERM... Issued at 111 AM PST Sat Dec 13 2025 (Sunday through Friday) The aforementioned upper-level longwave ridge will exit to the east Sunday, giving way to zonal flow for most if not all of the long term forecast. Passing disturbances to the north and an increase in precipitable water and integrated vapor transport values will renew rain chances as early as Monday with no clear cut dry day beyond that. Rainfall in the beginning of the week is expected to be light and beneficial for those who are lucky enough to get any. This will also act to disrupt the pattern that we have been in that has allowed for relentless tule fog. Towards the tail end of the long term forecast period, a relatively more significant system is forecast as a surface low pressure system and its attendant cold front encroach the California Coast. It is too soon to delve into the specifics, but expect a change in the pattern and a return to wet conditions next week. This weekend would be a great time to clean out gutters and trim trees away from your home and power lines. But wait, there's more! Beyond the long term forecast of seven days, it continues to be advertised that there's a moderate risk of heavy precipitation and high winds Saturday, December 20th through Friday, December 26th. This will not only be important for daily life, but also holiday travel. && .AVIATION... (06Z TAFS) Issued at 949 PM PST Fri Dec 12 2025 Fog returns across the Bay Area and stratus returns to the Central Coast. For the Bay Area, followed a persistence forecast with fog arrival and dissipation timing for STS, APC, LVK, and SJC with fog not reaching SFO or OAK. Current obs show dense fog developing across the the Bay Area so have made the TAFs more pessimistic with more widespread 1/4SM visibilities. A southerly surge of stratus is bringing stratus south to north across the southern coastal waters with stratus to reach HAF, MRY, and SNS overnight. There is some potential for fog to develop, at least temporarily, at MRY and SNS overnight. Another round of stratus and fog is likely again tomorrow night. Vicinity of SFO...VFR. Currently not anticipating stratus or fog to reach SFO tonight. Confidence is increasing that stratus will redevelop along the coastline by tomorrow night and will likely reach SFO early Sunday morning. Winds remain light and generally offshore through the TAF period but may become more variable at times. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...IFR-LIFR overnight. Kept the TAFs slightly more pessimistic with dense fog expected at least temporarily early tomorrow morning. The NBM is most supportive of dense fog developing and seems to be doing a good job with the arrival of the southerly surge. Winds generally stay light and offshore through the TAF period but may shift more onshore during the afternoon/evening hours. && .MARINE... (Tonight through next Thursday) Issued at 844 PM PST Fri Dec 12 2025 Dense fog is possible across the southern coastal waters tonight as a southerly surge of marine stratus moves northward. Gentle northeasterly winds will gradually shift southerly on Saturday morning before moderate northerly winds return again on Monday. Seas continue to subside with slight waves continuing through the weekend. Unsettled weather returns by the middle of next week with seas to build and light rain possible across the coastal waters. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...Dense Fog Advisory until 11 AM PST this morning for CAZ506-510. PZ...None. && $$ SHORT TERM...Sarment LONG TERM....Sarment AVIATION...Kennedy MARINE...Kennedy Visit us at www.weather.gov/sanfrancisco Follow us on Facebook, X, and YouTube at: www.facebook.com/nwsbayarea x.com/nwsbayarea www.youtube.com/nwsbayarea ####018007469#### FXUS63 KLSX 130917 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 317 AM CST Sat Dec 13 2025 .KEY MESSAGES... - Snow is very likely (60 to 80%) to cause minor impacts to travel in parts of northeast Missouri and central/south-central Illinois today. A Winter Weather Advisory remains in effect for these areas. - There is a slight chance (20%) for patchy freezing drizzle today which could cause slippery conditions on untreated roads south of I-70 in Missouri and along and south of I-64 in Illinois. - Wind chill values will drop to dangerous levels tonight into Sunday morning across much of eastern Missouri and Illinois. A Cold Weather Advisory has been issued for these areas. Temperatures will remain very cold through Monday morning. Temperatures warm rapidly Monday and continue warming through midweek. && .SHORT TERM... (Through Late This Afternoon) Issued at 309 AM CST Sat Dec 13 2025 Going forecast looks on track for a period of snow across northeast Missouri into west central and southwest Illinois today. The leading edge of an Arctic airmass is nosing into northeast Missouri this morning. Short range guidance continues to show strong low to mid level frontogenesis back in the cold air across Iowa and Illinois, stretching into northeast Missouri. The HREF LPMM shows the heaviest accumulations in a band stretching from Ottumwa IA to Lincoln IL where the strongest frontogenesis is forecast. However, it also shows 1-3 inches in far northeast Missouri into west central and south central Illinois. HREF Snowfall probabilities continue to suggest there will be a tight gradient across these areas. For example, Quincy IL is showing a 70-80% chance of 1 inch accumulation or more, but Hannibal MO drops to around 30%. Increase to 2 inches or more and both Hannibal and Quincy drop to virtually 0%. With that in mind, f-gen bands are tough to pin down and small shifts in the position of the snow bands can make a big difference in how much snow ultimately falls. Regardless, the current Winter Weather Advisory looks good, so will continue it without changes at this time. New to the forecast this morning is a low chance (~20%) for some mixed precip, generally south of I-70/I-64 in Missouri and Illinois respectively. RAP soundings are showing a moist layer near the surface with weak lift in the layer and no cloud ice above it. Drizzle is the preferred precip type with this setup. The RAP and NAM both show this signal, but the GFS is drier in the low levels and doesn't show as much lift. The NAM is the most bullish with simulated reflectivity developing very light returns around 15Z and continuing until 22-23Z, presumably when the cold dry Arctic air builds far enough south to dry the low levels out. Other CAMs are dry except for a brief, fast moving band of very light precip moving southeast in the afternoon. The RAP can be good at forecasting drizzle, and with temperatures at or below freezing there is the potential for a light glaze of ice on untreated surfaces. Have therefore added mention of freezing drizzle to the forecast, but limited to 20% due to lack of model consensus. This will need to be monitored closely this morning for a potential Winter Weather Advisory if the freezing drizzle does indeed develop. Whether it's snow to the north, or potential for drizzle/freezing drizzle to the south, precip should end by 00Z or shortly thereafter as the cold/dry Arctic airmass pushes farther south and dries out the low levels. Carney && .LONG TERM... (Tonight through Friday) Issued at 309 AM CST Sat Dec 13 2025 Cold Weather Advisories also look on track for Saturday night/Sunday morning. The 1040+mb Arctic high continues building southeast and temperatures dip into the single digits across most of the area to 1 to 3 degrees below zero in northeast Missouri and west central Illinois. The pressure gradient remains fairly tight due to the strength of the high, and the wind remains in the 10-14kt range until almost 18Z Sunday. It will be bitterly cold no matter where someone walks outside on Sunday morning with wind chill values ranging from -5 in southeast Missouri to -20 in west central Illinois. With the coldest values of -10 to -20 in eastern Missouri and Illinois, the Cold Weather Advisories will continue unchanged. Temperatures Sunday afternoon will struggle into the teens in most locations despite what looks like a full-sunshine day. After another cold night Sunday night, temperatures warm up rapidly on the western side of the Arctic high which will move into the Southeast U.S. by Monday afternoon. Highs rise into the upper 30s to low 40s in most locations Monday, the upper 40s to low and mid 50s on Tuesday, and mid 50s to near 60 on Wednesday in persistent south to southwest flow. The next significant chance for precip is still thursday as a cold front moves through the Mississippi Valley. Deterministic GFS and ECMWF are still differing on the amplitude of the trough driving this cold front through the region. While temperatures should cool off behind the front in either case, the magnitude of the cool down is uncertain. To illustrate, LREF temperature IQRs increase from around 8 degrees on Thursday to 12+ degrees behind the front on Friday. One way or another, the precip associated with the front looks to be all liquid at this time. Even the colder solutions end the precip on Thursday/Thursday night before temperatures can cool enough for more wintry weather. Carney && .AVIATION... (For the 06z TAFs through 06z Saturday Night) Issued at 1022 PM CST Fri Dec 12 2025 Ceilings will remain below 2000 feet AGL through 16-18Z at all of the terminals except UIN, before climbing above 2000 feet. At UIN, snow will likely move into the terminal by 13Z with the best chance (>90%) for accumulating snow between 16-20Z. IFR conditions are likely during this time with potential for 1-3" of snowfall during the day. Snow will move out of UIN by 00Z leaving ceilings above 2000 feet. VFR conditions will be possible at the terminals after 06Z. West to northwest winds will slight shift to the north by the end of the period. Britt && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Cold Weather Advisory from 9 PM this evening to noon CST Sunday for Knox MO-Lewis MO-Lincoln MO-Marion MO-Pike MO-Ralls MO- Saint Charles MO-Saint Louis City MO-Saint Louis MO-Shelby MO. Winter Weather Advisory until 4 PM CST this afternoon for Knox MO-Lewis MO. IL...Winter Weather Advisory from 10 AM this morning to 6 PM CST this evening for Fayette IL-Greene IL-Macoupin IL-Montgomery IL. Cold Weather Advisory from 9 PM this evening to noon CST Sunday for Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-Marion IL- Monroe IL-Montgomery IL-Pike IL-Randolph IL-Saint Clair IL- Washington IL. Winter Weather Advisory until 4 PM CST this afternoon for Adams IL-Brown IL-Pike IL. && $$ WFO LSX ####018006211#### FXUS63 KFSD 130919 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 319 AM CST Sat Dec 13 2025 .KEY MESSAGES... - Another round of snow quickly moves across the region through midday. Moderate snowfall rates at times near-south of I-90 will result in quick accumulation of 2-4 inches in 6-8 hours. A Winter Weather Advisory remains in place through noon. - A brief influx of arctic air will lead to cold temperatures and even colder wind chills (as low as -25 to -30F) by Sunday morning. - Quieter conditions with mild temperatures return next week, though fresh snowpack may impact how quickly temperatures warm early in the week. && .DISCUSSION... Issued at 318 AM CST Sat Dec 13 2025 TODAY-SUNDAY: Snow has developed as anticipated over the past 2-3 hours, mainly along/south of I-90 across our portion of SD as of 3 AM CST. Greatest lingering uncertainty is centered on the northern extent of the more impactful snowfall this morning, with a slight southward shift noted in the latest models. This, in part, would be in response to a northerly flow of drier air which is limiting initial development north of I-90 early this morning. Satellite trends also show indications of enhanced lift with a relatively narrow band of cooling cloud tops south of I-90/toward Missouri River Valley. This enhanced lift along with soundings showing a deep nearly isothermal layer within the dendritic growth zone will support moderate snowfall rates around 1/2" per hour or a bit higher for a few hours this morning, which will result in a quick 2-4 inches of accumulation, mainly south of I-90. With upstream visibility still as low as a mile at times in eastern Montana, and 06Z NAM/latest RAP/HRRR showing a secondary band of moderate snow toward KHON/KFSD around daybreak, am hesitant to trim the northern extent of our existing advisory just yet. Models are consistent in timing bulk of the snowfall out of our forecast area by 16-18Z, so scheduled expiration of the advisory at noon CST still looks good. Weak cold advection along with widespread clouds/snow will limit temperature recovery today, with highs only in the single digits to around 10F. A modest northerly breeze 10-20 MPH will keep wind chills below zero through the day, leading the way to a very cold night. Clearing skies and light winds are expected as high pressure slides over the region. Fresh snow cover will complete the trifecta for optimal radiational cooling with lows in the teens below zero near/east of the James River Valley. At these temperatures, even a slight breeze under 5 MPH will push wind chills into the 20s below zero. values may be marginal in some locations, but overall the existing Cold Weather Advisory covers the area/timing of greatest concern. Sunday will again bring limited temperature recovery most of the day, especially given high albedo as sunshine prevails over a fresh layer of snow. That said, a strong warm front pushes into western portions of the area by 00Z Monday, with late day/evening highs from the single digits above zero east to the 20s west of the James River. Non-diurnal temperatures will rise throughout Sunday night in response to the very strong warm advection and breezy south-southwest winds overnight. MONDAY ONWARD: Northwesterly flow aloft continues over area Monday as upper level ridge axis builds in the Rockies and trough moves off the East Coast. This is short lived as ridge breaks down Tuesday with zonal mid level flow through next week. Guidance is quite variable on the timing/strength of any waves, but most guidance brings a wave through the northern Plains Monday and another (or two) Wednesday into Thursday. Details on where and how much precipitation may fall are unclear, but much of the week looks to be dry outside of Wednesday/Thursday. Periods of breezy to windy conditions are expected, especially early Monday and again mid week. Southerly low level flow brings much warmer temperatures to the region early next week, although uncertainty exists in how much we'll warm due to our fresh snowpack. Above average temperatures are expected with high (over 80%) chances of temperatures above freezing area wide on Tuesday and Wednesday, and high chances for over 40F in south central SD both days. South central SD even has moderate (up to 60%) chances of exceeding 50F by Wednesday - although we'll have to see how quickly we melt our snowpack. We'll see a slight decrease in temperatures for the latter half of the week, but still within 5 or 10 degrees of seasonal averages. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1046 PM CST Fri Dec 12 2025 MVFR stratus and the next round of snow are still expected to move into the area overnight, moving southeast through the morning and early afternoon hours. Beginning to see some light returns on radar as well as some snow on the DOT cameras across south central SD. Once VFR conditions deteriorate, we should remain MVFR to IFR with the snow. MVFR stratus may linger through the late afternoon, with some improvements and return to VFR through the evening hours. Northerly winds remain light through much of the period, with gusts around 20 knots during the afternoon. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...Winter Weather Advisory until noon CST today for SDZ038-050- 052>054-057>071. Cold Weather Advisory from 6 PM this evening to 10 AM CST Sunday for SDZ038>040-053>056-059>062-065>067. MN...Cold Weather Advisory from 6 PM this evening to 10 AM CST Sunday for MNZ071-072-080-081-089-090-097-098. IA...Winter Weather Advisory until noon CST today for IAZ001-012- 013-020>022-031-032. Cold Weather Advisory from 6 PM this evening to 10 AM CST Sunday for IAZ001>003-012>014-021-022-032. NE...Winter Weather Advisory until noon CST today for NEZ013-014. && $$ DISCUSSION...JH/SG AVIATION...SG