Area Forecast Discussion
Issued by NWS Des Moines, IA

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000 FXUS63 KDMX 082353 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 553 PM CST Sun Dec 8 2019 .UPDATE... Issued at 548 PM CST Sun Dec 8 2019 Have issued Dense Fog Advisory for northwestern to far northern tier of counties... specifically highlighting the potential for freezing fog. Confidence high that ongoing 1/4 mile visibilities will continue and expand until precipitation arrives... which looks to be around 09z or so. A non-zero chance may need Winter Weather Advisory after 09z across northern counties for mixed precip... transitioning to snow. Timing of mixed precip is during AM commute. Icing, followed by a snow may amplify adverse travel impacts. Will monitor closely over next few hours. && .DISCUSSION.../Tonight through Sunday/ Issued at 312 PM CST Sun Dec 8 2019 The primary weather highlights through the period will be fog this evening, wintry mix to light snow and wind tomorrow, and then a surge of colder temperatures Tue into Wed. This will be followed by a return to near normal temperatures and several minor precip windows as we translate from the mean MS Valley trough early this week to faster, lower amplitude Pacific flow into next weekend. At onset the 20z surface analysis shows a weak front/pressure trough SW-NE through the area with stratus oozing in from the north and south. The front wont move much into the evening, but will be reinforced by Plains cyclogenesis ahead of the approaching MT PV anomaly. This may lead to areas of fog north and south this evening with some separation possible. Would not be surprised to see a Dense Fog Advisory needed, but will hold off for now until geography and magnitude confidence increases. Precip wise, patchy drizzle may form overnight either side of the boundary and could be of the freezing variety north until mixing with and then changing entirely to snow into the morning as moisture from the bottom up and top down begins to erode an appreciable 1-2km dry layer. Much of the QG forcing will remain to our north ahead of the aforementioned anomaly, but models are in good agreement with a trailing lobe of lift across northern IA from the predawn hours into at least mid morning which will be immediately followed by increasing NW winds. High resolution model simulations suggest the window for snow at any particular site may not last more than 1-3 hours, but could be moderate+ and quickly accumulate around an inch of snow north. The question is whether a Winter Weather Advisory is warranted. Road surface temperatures are currently above freezing well into MN even though air temps are below freezing due to the blanket of stratus, so am unsure how impactful freezing drizzle will be to travel. Snow amounts will not approach advisory criteria, but will likely occur during the morning commute. So at this point, rather than issue a mess of two marginal headlines, have opted to leave decisions for later shifts which will have a better handle on trends and potential impacts. The snow should end by midday tomorrow with wind the primary story. Sustained speeds will be 25 mph+ with gusts around 35 mph and temperatures falling area wide by afternoon heading toward lows in the single digits and teens Mon and Tue Nights, and only a minor rebound during the day Tue and Wed. Some below zero temps are possible north. These temps are certainly below normal, but not to an extreme degree with sensible cooling more a reflection of our recent mild stretch. There could be a few periods of flurries Mon into Tue with the cool down resulting in stratus at <= -12C temps, but moisture will be of marginal depth so it has been omitted for the time being. Any occurrence would likely be brief, insignificant and difficult time. The cold won`t last long however with near normal temperatures back in the forecast by Thu. Heading into that, have added a chance for light snow north late Tue Night as a short wave traverses the baroclinic zone and start of warm advection before heights begin rising in earnest. Another potential window would be around Wed Night when there is decent lift, but as of now moisture seems insufficient to introduce PoPs. After this the fast Pacific flow buckles, and as is often the case, is not handled well by the models with uncertainties regarding how and when individual short waves will intensify. Thus the forecast has a fairly broad-brushed period of intermittent low end PoPs from Friday into the weekend. This low confidence, low amplitude flow will keep seasonal temps going through the period. && .AVIATION.../For the 00Z TAFS through 00Z Monday evening/ Issued at 548 PM CST Sun Dec 8 2019 High confidence in IFR to LIFR TAFs across Iowa through tonight and into tomorrow. Will be watching KMCW for potential for dense fog to cause LIFR before current 10z forecast... At this time it looks like vsbys reductions should just miss the site. Gusty northwest winds develop Monday afternoon, which may lead to blowing snow. CIGs to gradually improve. && .DMX WATCHES/WARNINGS/ADVISORIES... Dense Fog Advisory until 3 AM CST Monday for IAZ004>007-015-016- 023-024. && $$ UPDATE...Kotenberg DISCUSSION...Small AVIATION...Kotenberg

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