949 FXUS65 KBOU 260606 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1106 PM MST Mon Feb 25 2019 .UPDATE... Issued at 1102 PM MST Mon Feb 25 2019 Issued a dense fog advisory for the areas of Jefferson, Boulder counties below 6000 ft as well as Denver into Douglas counties. This is to account for the persistent dense fog that is seen across these areas. Freezing fog is expected to continue into the morning commute hours, hopefully ending between 8 and 9 am with increased SSW winds off the palmer. Drivers should give themselves extra time and drive slowly when fog is encountered. UPDATE Issued at 736 PM MST Mon Feb 25 2019 The GOES 16 nighttime microphysics channel clearly shows the low clouds that have rushed westward toward the foothills this evening. Visibilities at KDEN have reached as low as a quarter mile with very low ceilings across north-central and northeast Colorado. Despite the fog at KDEN, there are very few locations under the clouds that are actually reporting fog and low visibilities. Plenty of locations are reporting light flurries and with such a cold airmass in place over the eastern plains, it is possible that the ice formation within the clouds is diminishing the water droplets and may be responsible for the lack of fog. A dense fog advisory is still being considered but we will hold off on issuing it now due to the lack of observations meeting the criteria. Little to no snow accumulation is expected from these flurries. The low clouds and cold air will stick around well into Tuesday morning. Models show way too much mixing of the boundary layer and lower atmosphere during the day tomorrow. Therefore, I lowered the high temps by 5 to 8 degrees over the urban corridor and eastern plains from the previous forecast. It looks like we will do this whole thing all over again tomorrow night as the shallow cold and moist air that remains over far eastern Colorado during the day tomorrow pushes westward toward the foothills tomorrow evening. && .SHORT TERM...(This evening through Tuesday) Issued at 322 PM MST Mon Feb 25 2019 A wave can be seen on GOES-16 IR imagery sliding onto northeast plains. Observations of cooling temperatures and a slight increase in winds indicate a frontal passage has moved over the extreme northeast plains, reaching Pine Bluffs, WY, Sterling, and Akron as of mid afternoon. A synoptic high over western Montana has setup the northeast flow behind the front, allowing Arctic air to slide onto the plains. As the surface high pressure surge moves to the southeast, the low level flow will eventually turn more southeasterly overnight, aiding the development of a Denver Cyclone. The easterly upslope flow, combined with the colder arctic air running into an airmass with higher dewpoints, will support fog this evening and overnight across the eastern plains to the I-25 Corridor, from Denver north through Fort Collins. Dense fog is possible in this regime, with expected overnight lows about 4-8 degrees below current dewpoints. There will probably be lighter fog over the far northeast corner where surface dewpoints advecting into that area are much lower (below zero in north central and Central Nebraska). Moisture is quite shallow with this surge, but there appears to be a small threat of freezing drizzle or very light snow over the northeast plains, mainly along and east of a line from Sterling to Limon where the moisture is slightly deeper. In the mountains, winds are expected to increase overnight with a bump in the cross mountain component to around 40 knots. We don't see much amplification in the winds due to a marginal mountain top stable layer, but some peaks and higher foothill locations could see a few gusts into the 55-65 mph range. .LONG TERM...(Tuesday night through Monday) Issued at 322 PM MST Mon Feb 25 2019 Westerly flow aloft is going to be the story of the week. Temperatures will moderate through the first half of the week, but as the week comes to an end, cold air that has settled over the northern Great Plains will surge back into eastern Colorado for the weekend. Then, as the weekend continues, disturbances embedded in the westerly flow will produce periods of snow in the mountains and then out onto the plains. Wednesday and Thursday will be dry across the plains with high temperatures warming to the lower and mid 50s along the I-25 corridor, but only the 30s to lower 40s further to the east, where snow cover will hold temperatures back. In the mountains, periods of gusty winds will be possible due to the moderate to strong flow aloft interacting with the high terrain. Cross sections show 40-60 knots of westerly flow at mountain top levels, so wind gusts of that magnitude will be possible. Varying amounts of moisture will affect the amplitude of mountain waves, and the presence of cool air on the plains will keep the gusty winds up at higher elevations in the foothills. On Wednesday night, cross sections also show cool air moving into far eastern Colorado which could produce areas of fog from northeastern Weld County down to Lincoln County. Wednesday afternoon will also see an increase in moisture over the mountains, which will bring more light snow back to the Colorado high country Wednesday night. Snow amounts are expected to be light at this point. From Friday through the weekend, northerly surface flow will begin bringing colder air back over the plains. Mountain areas will continue seeing the chance of snow as Pacific moisture continues moving in and jet stream flow increases. Continuous easterly and northeasterly winds on the plains through the weekend will begin producing light upslope precipitation over the weekend. As this week progresses, later model runs will help determine how heavy the snowfall amounts will eventually be. For now, no highlights are necessary for the next few days. && .AVIATION...(For the 00Z TAFS through 00Z Tuesday evening) Issued at 736 PM MST Mon Feb 25 2019 A push of cold and moist air is arriving from the northeastern plains this evening and the LIFR ceilings have already reached DEN and BJC and will reach APA by 04Z. It is likely that these LIFR ceilings and visibilities will continue through much of the night and into tomorrow morning at all three airports. However, satellite shows a couple breaks in the clouds over eastern Colorado this evening so brief periods of better visibilities and higher ceilings are possible. As is normal with these shallow fog and low cloud events, there is low confidence on when the ceilings and visibilities will improve in the morning. DEN will likely be the last to clear out and it is possible it will take until noon to do so. The afternoon and early evening will have VFR conditions with light winds at the airports. Another round of fog and low clouds is a good possibility tomorrow night. && .BOU WATCHES/WARNINGS/ADVISORIES... Dense Fog Advisory until 8 AM MST Tuesday for COZ039-040. && $$ UPDATE...Bowen SHORT TERM...Sullivan LONG TERM...Dankers AVIATION...Danielson