Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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347 FXUS65 KBOU 010146 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 746 PM MDT THU JUN 30 2016 .UPDATE... Issued at 717 PM MDT Thu Jun 30 2016 The first batch of convection is moving into goodlands area this evening...and there is a large area of subsidence behind it. Surface temperatures in the upper 60s appear to be too cool for continued convection. The only new thunderstorm development we see is brewing along the boundary in southeast Elbert County. The strong outflow boundary that swept through DIA had no convective development when it swept into the foothills and palmer divide...although there may be some recent activity trying to get going along the foothills of Larimer and Boulder Counties. The evening Denver sounding had 1.07 inches which was less than what had been forecast. There is an apparent wave in northwestern Colorado that may have enough lift to develop more convection. This was suggested in the HRRR two hours ago...but later version of the HRRR is not nearly as robust with thunderstorm development. Right now there are enough questions in my mind to leave the flash flood watch in place...mid shift will be able to drop it if additional storms fail to develop. In the grids I lowered pops through the night...especially this evening behind the first wave of thunderstorms. && .SHORT TERM...(This evening through Friday) Issued at 349 PM MDT Thu Jun 30 2016 Convection is breaking out as expected. Upper winds remain about the same with 10 knot winds at 500 mb. Storm motions so far as expected, around 10 knots with some variability based on storm inflow/outflow. Still expecting some slowing/more variability as they move east and southward on the plains overnight. The main threat of flash flooding should be across east central Colorado where the upper winds are lighter and there may be better surface inflow. However even in the north the storms will be slow moving and efficient rain producers so there is a flash flood risk from the foothills eastward. Models hint at convection eventually focusing on an area of convergence across east central Colorado around midnight and what we are seeing now may be the beginning of the evolution of that feature. Threat should be diminishing further west by midnight but there are several hints of outflow pushing westward and redevelopment nearer the foothills. As a result we decided not to get fancy with the watch and left it for all areas until 2 am. Tonight`s main convective blob should be moving out by morning, but there is decent agreement on another band of upper level focing coming over in the morning, so I increased morning POPs. Probably cool enough for showers but still some thunder threat. Then in the afternoon there is a little drying aloft which could lead to some sun and a bit more instability. By that time there should be a bit more west wind aloft and low level winds will hopefully not be opposing the upper levels anymore, so we should see a bit faster storm motions, maybe 10-15 mph. Still a favorable heavy rain environment, but with the faster storm motions it should be a more typical flooding threat, mainly intense rain but for less than an hour for most of the storms. There may be more of a hail/wind threat as the wind profile will be better for tilted and/or rotating storms. .LONG TERM...(Friday night through Thursday) Issued at 349 PM MDT Thu Jun 30 2016 Friday night and Saturday...weak mid level qg ascent will be over the region as a slow moving upper trough makes its way across the state. NAM12 does show good capes at 00z Saturday...around 1100 j/kg at kden and adjacent plains...with precipitable water values still around 1.3 inches. Storm motions around 10 kts. Main issue will continue to be heavy rainfall with the stronger storms...with the potential for localized flash flooding. A marginal severe threat as well with the best directional shear and highest surface based CAPE wl be over the Palmer Divide. On Saturday...should have another good chance of thunderstorms producing heavy rainfall. Forecast soundings do show some mid level drying and lower precipitable water values at kden. By 21z those values are progged to be around 0.80 inches...but still closer to one inch east of the Urban Corridor. Overall those values are still pretty healthy. Mid level drying may help increase the severe potential with a few of the storms... especially over the Palmer Divide. Forecast sounding yield capes 1500-2000 J/kg at 21z Saturday. By Saturday evening...the upper trough moves to the east with weak downward qg descent as an upper level ridge moves into CO fm the west. No significant changes in the grids for Sunday through Thursday. A broad upper ridge will remain over the southwest and south central U.S. Subtropical moisture under the ridge will continue to advect into the state. It will be warmer but there will be at least a slight chance of aftn/evng thunderstorms each day...especially over the higher terrain and far eastern plains. && .AVIATION...(For the 00Z TAFS through 00Z Friday afternoon) Issued at 349 PM MDT Thu Jun 30 2016 Expect to see a minimum of thunderstorm activity through 10pm...then scattered thunderstorms will be possible. Any storms over the airpoprt will produce local mvfr condtions and winds gusting to 40 mph. storms may also affect the arrival gates and KDEN traffic may need instrument approaches due to the ceilings and showers. After a relatively quiet morning on Friday expect Scattered showers and thunderstorms to develop by afternoon. KDEN instrument approaches may be needed at times, but VFR conditions are expected to prevail. && .HYDROLOGY... Issued at 349 PM MDT Thu Jun 30 2016 Radar indicated storms had produced up to 3 inches of rain this evening...but the highest reported rainfall was 1.4 inches. With some subsidence behing the first wave of thunderstorms... we should see a lull for a few hours...but we expect more storms to develop. Again overnight...stronger storms will be capable of rainfall rates up to 2 inches in an hour. On Friday storms should be moving faster, but a few storms could be able to produce 1-2 inches of rain in an hour. There will still be a small risk of flash flooding with the strongest storms. Friday night and Saturday...high precipitable water values around 1.3 inches will continue through Friday evening so the storms will likely produce very heavy rain with the potential for flash flooding. Storm motions will be west to east around 10 kts. On Saturday...some drying but still relatively high precipitable water amounts close to one inch. As a result...this may lessen the rainfall rates slightly and the threat potential for flash flooding. Threat will remain the highest in the burn scars and in areas where the soil is already saturated from heavy rain which is yet to be determined at this time. && .BOU Watches/Warnings/Advisories... Flash Flood Watch until 2 AM MDT Friday for COZ035-036-038>051. && $$ UPDATE...RTG SHORT TERM...Gimmestad LONG TERM...Cooper AVIATION...RTG HYDROLOGY...RTG/Gimmestad/Cooper

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