479 FXUS65 KBOU 210353 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 953 PM MDT Mon May 20 2019 .UPDATE... Issued at 953 PM MDT Mon May 20 2019 Heaviest snowfall is expected to continue through the midnight hour per latest radar/satellite data, and latest totals and snowfall rates prompted the upgrade to an Advisory in the immediate Denver area (2-6", heaviest on the south/east sides of town), and an upgrade to a Winter Storm Warning over the Palmer Divide where 7 to 15 inch totals likely. We may see a decrease in intensity after midnight with some drying noted to the south, but there's still a lot of forcing later tonight through early Tuesday morning for additional accumulation in that time frame. Main impact for Denver will be broken tree limbs and scattered power outages in the areas that see the heavier totals, and some slush/snow on roads during heavier snow periods. More significant travel troubles over the Palmer Divide with much heavier totals of heavy, wet snow. UPDATE Issued at 845 PM MDT Mon May 20 2019 Radar has been showing several bands of precipitation passing north across the forecast areas, including I-70 from Bennett to Strasburg to Byers. In this area web cameras are indicating snow accumulating on grassy surfaces in locations just east of Denver, and has been heavy at times. Satellite and radar depict stratiform snow funneling up the east side of the Denver metro area tonight. For this reason the Winter Weather Advisory has been extended into eastern portions of Adams and Arapahoe Counties with snow accumulations of three to seven inches. Temperatures have trended colder as well, so we will add additional snowfall to the I-25 corridor from Denver north to Fort Collins. The upper level low is progressing towards the four corners region. Satellite imagery indicates a jet speed max sliding north up the eastern side of Colorado creating strongly diffluent flow and surface pressure falls. This will allow for more lift and heavier snow. The best time period for snowfall across the mountains, foothills, and Palmer Divide will be through the overnight period. The RAP and HRRR models have the moisture decreasing through the early morning, while the GFS and HiRes Euro keep precipitation through Tuesday. Basically, the I-25 Corridor and nearby adjacent plains and Palmer Divide area will have to be monitored closely to see how these waves of precipitation come to fruition, because at the intense snowfall rates and high water content, it won't take long to cause some problems with more snow/slush on roads, and possibility of broken tree limbs and scattered power outages where heavier snow falls. See below for current Winter Storm Warning and Winter Weather Advisory highlights. && .SHORT TERM...(This evening through Tuesday) Issued at 238 PM MDT Mon May 20 2019 The strong upper low which is currently approaching the 4 corners region this afternoon is providing extensive strong mid and upper level ascent and diffluence over eastern CO. A narrow dry slot over northern NM and the San Luis Valley has allowed for some surface based convection to fire in area of partly cloudy skies and slightly higher cape. This is being drawn northward into the central mtns and NE CO area early this afternoon. Will expect northward moving showers to continue into the evening along with batches of weak to moderate thunderstorms passing across the higher terrain and onto the Urban Corridor and plains. Can't rule out a few stronger storms containing brief moderate to heavy rain and small hail as they move into a supportive shear environment with a weak Denver cyclone currently in progress. Another area of convection and lift, closer the upper low near Flagstaff AZ is still behind this initial convective feature and will likely be the snow maker and for the mountains and a precip producer for the lower elevations once it arrives over the central CO. The stable drizzle-like sounding across the plains from this morning destabilizes to a degree in the mid and upper level for late and afternoon and tonight, however remains somewhat stable near the surface due to persistent and fairly deep northeast flow at the surface. Forecast cross sections and sounding become near saturated from the surface thru 400mb with lapse rates holding in the 6.5-7.5 C/km range thru the 03z to 12z Tuesday timeframe over the mountains. Can still expect warning criteria snowfall for the mountains as we typically see convective generated snowfall this time of year and especially under this type of synoptic environment. Snowfall totals of 15 inches plus will not be out of the question due to persistent showers and possible banding, especially on south and southwest facing aspects. For the lower elevations, slow levels still expected to fall near the 5000-5200ft elev as wet bulb 0C levels have come in lower on current runs. Will add Palmer Divide to the Winter Wx Advisory thru Noon Tue as 5-10 inches may be possible especially across the higher western portions of the ridge. Looking at Tuesday, will be watching for the timing and extent of drier surface downslope flow pulled off the higher terrain and onto the lower elevations of the NE plains. By 12z Tuesday, the upper closed low near the OK panhandle begins a fairly rapid exit out into NW KS by 18-21z. This will likely bring the beginning of the end to the best precip across the lower elevations as drying NW downslope flow fills in behind ejecting low. As upper low exits the region, NAM and GFS swing another trof axis across the CWA from the north for late afternoon Tue with another brief round of reinforcing cold air aloft. With this last push of cold air advection aloft this may keep the snow or snow showers going into early afternoon Tuesday across the mountains and higher foothills with weak convective rain mixed with snow showers thru afternoon across the lower elevations. By late afternoon into early evening Tue, strong QG subsidence begins to work into southern and central Colorado with rapid lowering of RHs across the CWA. .LONG TERM...(Tuesday night through Monday) Issued at 238 PM MDT Mon May 20 2019 For Tuesday night into Wednesday, the upper level low will be moving to the northeast into central NE. A deep surface low will be over western KS to south-central NE. The wrap around provided by the cyclonic flow of the surface circulation will keep strong NW flow moving into the region with high moisture. This will allow for continued showers over the plains into Wednesday morning and snow in the higher elevations. Warm air advection Tuesday will change over cold air advection as winds in the mid levels turn more to the northwest. Models are showing enough of a cool down in the mid and lower levels that some snow could mix in with the rain overnight into early Wednesday across the plains, mainly North near the Wyoming border and the northern foothills. Low temperatures for Tuesday night will be hovering around the freezing mark with values from 31 to 34 across the plains. The surface low will continue to track to the NNE through the day Wednesday pulling the energy and moisture north leaving CO in SW flow aloft. Conditions will dry over the plains with increasing subsidence but there is a slight chance of storms over the higher terrain with lingering moisture and some instability present. Highs on Wednesday will rebound slightly with temperatures in the 50s. For Thursday, the next upper level disturbance that will affect Colorado will be over Utah and is currently forecasted to push North into Wyoming and southern Idaho. At the surface, a deepening low pressure system over the southern portions of the state will move to the NNE pulling in some moisture in the cyclonic SSE flow. This could help to provide high enough dewpoints across the I-76 corridor for a chance of showers and thunderstorms Thursday afternoon. Elsewhere, lower chances of convection are possible with a chance across the higher terrain. Temperatures will continue to increase with highs projected to be in the lower 60s across the plains and 40s to 50s in the mountains. Friday and into the weekend, the upper low will continue to push North into Canada leaving Colorado in SW flow. Increasing subsidence from the SW with WAA in the mid levels will help to stabilize the plains and bring mostly sunny conditions. Temperatures will be closer to seasonal normals with highs back in the upper 60s to lower 70s with just a slight chance of convection over the mountains. This warmer and drier pattern will continue into the weekend with highs back into the 70s under just a slight chance of storms for the mountains. && .AVIATION...(For the 00Z TAFS through 00Z Tuesday evening) Issued at 845 PM MDT Mon May 20 2019 IFR conditions with snow will remain in the forecast through early Tuesday morning. Accumulations of 2-5 inches in the grass will be possible, along with some slush on runways. The heavier amounts would favor KAPA and KDEN. Ceilings are expected to slowly lift late Tuesday morning, with more of a mix of rain and snow toward 16Z-18Z. MVFR conditions should prevail thereafter with scattered showers around and only slight improvement of ceilings. && .BOU WATCHES/WARNINGS/ADVISORIES... Winter Storm Warning until noon MDT Tuesday for COZ041. Winter Weather Advisory until 9 AM MDT Tuesday for COZ040. Winter Weather Advisory until noon MDT Tuesday for COZ045. Winter Weather Advisory until midnight MDT Tuesday night for COZ036. Winter Storm Warning until midnight MDT Tuesday night for COZ031- 033>035. && $$ UPDATE...Barjenbruch SHORT TERM...Fredin LONG TERM...Bowen AVIATION...Barjenbruch