209 FXUS65 KBOU 092005 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 205 PM MDT Tue Oct 9 2018 .UPDATE... Issued at 205 PM MDT Tue Oct 9 2018 Precipitation has been mostly in the form of snow, with the only mix or light rain noted on the northern fringe and in a couple valley locations. We've continued to increase the amount of snow on the eastern plains with TROWAL in play. Will have to watch this closely. UPDATE Issued at 1015 AM MDT Tue Oct 9 2018 Main concern through today is extent of widespread synoptic scale precipitation over the plains. Radar and satellite data show enhancement moving north across eastern Colorado. It is having a hard time moving north of the Palmer Divide, but we do expect it to fill in roughly along and south of the I-76 corridor as best upward QG forcing lifts across the plains. This also appears to coincide with the right rear entrance region of a strong upper level speed max. The precipitation is in the form of snow over the higher elevations (roughly above 5000 feet) as Limon is reporting snow and can be seen on local cams. Elevations as low as 4000-4500 feet could such as Akron should also see snow today as that area is in the deeper cold pool. We may start to see accumulations on the grass as surface temps appear to be holding near freezing. Denver may still have to battle surface wet bulb effects, but it is colder in that regard from yesterday so wouldn't be surprised to see some snow in the Denver area today, but only light QPF here so no accumulation expected outside of Douglas County. We'll also be lowering today's highs based on current trends. In the mountains, a few convective snow showers are expected to develop this afternoon and continue into the evening. Intensity may be limited by upper level cloud shield, but still enough for a couple inches of snowfall. Forecast for tonight looks on track as orographics improve late tonight ahead of our next disturbance. Temperatures turn cold enough to allow for a little snow accumulation especially over the northeastern plains. && .SHORT TERM...(Today and tonight) Issued at 355 AM MDT Tue Oct 9 2018 Have observed little change in the past several hours; low clouds blanketing the plains have been accompanied by areas of drizzle and fog in the presence of a moist, weak upslope flow. Furthermore, no organized areas of precip detected by radar within the CWA at this time. That included the high country. However, these conditions should change over the next several hours as broad circulation around the upper low nearing the Four Corners draws low/mid-level moisture up from the south-southeast. Models show a north-south band of light to moderate precipitation forming over the plains (east of the I-25 urban corridor) late this morning, its core centered over Lincoln and Washington Counties. This band of showers is progged to shift over the northeast corner of the state by this evening with the Four Corners low filling and the upper trough lifting northeast over the state. Temperatures are expected to remain just warm enough to prevent much if any snow accumulation on the plains, even in the vicinity of the heavier band of precip out east. By this evening, it will be cold enough for snow, but most of this precip should have shifted northeast of the area. Still, could see an inch or two of wet snow accumulate across the northeast corner of the state by midnight. Meanwhile, do not see this system having much impact on the I-25 urban corridor, other than keeping low clouds hanging around and producing areas of drizzle and/or light rain and patchy fog through this evening. Up around the Cheyenne Ridge, Palmer Divide and along the base of the foothills, temperatures may be cold enough for some light snow, and possibly some freezing drizzle. Accumulations here will be minimal. For the high country, should see a steady increase in cloud cover through the morning with moisture streaming from the south- southeast ahead of the upper trough now over western Colorado/ eastern Utah. Could see showers developing from south to north through late morning and afternoon hours with increasing QG ascent. However, it's not until tonight with passage of the 700-500mb trough axis and a shift to west-northwest flow ushering in colder air that the high country will see its best chance for measurable precip (all snow by then) during this 24 hour period. By morning, ridge tops and higher west facing slopes could see anywhere from 1 to 5 inches of new snow accumulation. Lastly, the Freeze Warning in effect for the plains will be dropped for the remainder of current nighttime period as freezing temperatures never materialized with the heavy overcast. However, the Freeze Warning will be allowed to continue for tonight (starting at 6 PM this evening) and run to 10 AM Wednesday morning. However, there is some concern that lingering cloud cover could keep temperatures from falling below 32 deg F until late tonight. .LONG TERM...(Wednesday through Monday) Issued at 355 AM MDT Tue Oct 9 2018 The continued unsettled weather pattern is expected to continue through much of this week with broad large scale troughiness across the Western United States. Temperatures will remain well below seasonal normals for this time of year with the continued threat of precipitation over the state, especially in the mountains. On Wednesday, the next upper level impulse will rapidly move into the mountains early Wednesday but weaken considerably as it does so. Still though, a rather moist, westerly flow with some orographic help for snowfall potential. 700mb temps around -2c but decent lapse rates of around 7C/KM. Trof quickly moves east during day with large scale subsidence taking over and some drying aloft after 00z. Westerly flow increases Wednesday night so will still see some light orographic snowfall Wednesday night and into Thursday morning. Across lower elevations, surface high pressure over the northern high plains will keep a moist, east to southeast low level flow in place on Wednesday and into Thursday. Expect some scattered rain/snow showers over the plains during the day as the upper trof/jet streak move over the area. Temperatures will remain cold in the mid to upper 30s. On Thursday, mean upper trof is still West of Colorado with only weak QG upward ascent but there is a jet streak which could result in more localized banded areas of showers. Low level flow becomes more southerly on the plains but could result in a cyclone development over the Denver area which could keep things socked in through much of Thursday along with more localized showers. Friday looks to be the driest and warmest day of the week as Colorado remains between storm systems and is under a drier, west to northwest flow aloft. May even see some sunshine on the plains by Friday afternoon with some weak downsloping winds and a drying airmass. More unsettle weather for the weekend forecast as both GFS/European solutions showing a strong upper level wave dropping out of the northern Rockies and into Western Colorado by early Sunday morning with moderate to strong QG ascent into central and eastern Colorado. There is also a strong associated cold front that sweeps over the plains by Saturday afternoon with enhanced upslope conditions developing and enough cold for all snow by late Saturday evening and into Sunday as 700mb temperatures drop to around -10c. The Canadian model is slower in the development and hangs the trof with storm split of energy. This could be the first widespread snow event for the plains if GFS/European solutions hold firm. && .AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon) Issued at 205 AM MDT Tue Oct 9 2018 IFR/low MVFR conditions are expected to prevail through early Wednesday morning. Precipitation shield has arrived and is mostly in the form of snow. (favoring KDEN & KAPA and points along/south of I-76. Could see IFR/LIFR conditions redevelop tonight with fog and areas of light precipitation. We may see a dusting of snow on the grass in the Denver area this evening, but nothing on the pavement. && .BOU WATCHES/WARNINGS/ADVISORIES... Freeze Warning from 6 PM this evening to 10 AM MDT Wednesday for COZ038>051. && $$ UPDATE...Barjenbruch SHORT TERM...Baker LONG TERM...Entrekin AVIATION...Barjenbruch