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Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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259 FXUS65 KBOU 121645 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1045 AM MDT Mon Aug 12 2024 .KEY MESSAGES... - Flash flood threat increasing with a round of afternoon and evening thunderstorms. Marginal risk for severe storms as well. - Chance of severe storms continues on the plains for Tuesday, along with locally heavy rainfall. - Gradual drying trend expected from Wednesday through the end of the work week with near to slightly above normal temperatures. && .UPDATE... Issued at 1044 AM MDT Mon Aug 12 2024 Ingredients are about as good as we`ve seen all year for potential for flash flooding. Precipitable water values are expected to rise near the maximum for the date, or at least close to the 99th percentile. Current dewpoints were in the lower 60s across much of northeast Colorado, stretching back as far as Fort Collins, with upper 50s dewpoints across the Denver metro area. Moisture so far is a bit limited in the higher terrain, but low/mid level flow is expected to turn more east/southeast through the afternoon and aid deeper moisture advection, while mid/upper level water vapor also shows increasing moisture from the west as a shortwave extends from central Wyoming into eastern Utah. This feature should contribute to both lift and moisture for additional thunderstorm development. Taking these parameters, as well as all the ingredients and relative slow storm motions into account as discussed in the early morning forecast discussion, we`ve hoisted a Flood Watch for flash flooding for the Front Range Foothills, Denver and eastern/southern suburbs, and the Palmer Divide area into east central Colorado this afternoon and evening. We still think the heaviest totals (localized amounts 3-4" or more) are likely to occur across Elbert, Lincoln, and Washington Counties, but rainfall rates of 1-2 inches in 30-45 minutes will also be possible closer to Denver and Douglas County. We`ve seen pretty good sunshine and thus that brings the risk up for Denver metro as we`re almost certain to reach convective temperatures. There`s probably the most uncertainty farther west into the foothills as deeper moisture there is initially lacking. However, we still think this will be one of the higher threat days we`ve seen this year mainly for the recent burn areas. Thus, we`ve opted to include them in the Flood Watch for flash flooding as well. Finally, enough instability (MLCAPE) exists for a threat of wet microbursts and marginal hail in the stronger storms on the plains. && .SHORT TERM /Through Tonight/... Issued at 419 AM MDT Mon Aug 12 2024 The main features of today`s weather are increased moisture at all levels and lighter winds aloft. That adds up to an increased threat of heavy rains and flooding. As usual when the moisture increases, cooler temperatures and increased cloud cover will likely be limiting factors. We will likely have to wait for diurnal convection and then lift from what looks like the short wave/jet streak of the day which is now entering eastern Utah and is probably well time for late afternoon mountain and evening plains convection. Once we do get storms going though, they should produce outflow boundaries that create slow moving convergence lines in a moist unstable environment. Late afternoon/evening soundings show 1000-2000 J/kg CAPE and precipitable waters up to about 1.3 inches. Storm motion will ultimately depend on the strength/organization of outflows, but sounding winds suggest something around 10 mph, perhaps even slower early in the afternoon. By evening, there`s potential for backbuilding/training as southerly low level winds will create inflow on the back side of cells. It`s probably not enough for long-term stationary storms, but it will slow the movement of storm clusters. We should also have a pretty productive environment, with little shear or drying aloft to produce entrainment and warm cloud depths ranging from about 5000 feet near the mountains to as much as 9000 feet on the plains. We should be able to have some storm clusters that produce up to 3 inches of rain in a couple of hours. We considered a flash flood watch, but plains FFG is relatively high and we`ll likely have enough organization for some storm motion with outflow winds coming off the mountains, generated by the rain, or from the shortwave. For the burn areas, the early storms are the most likely problem as they won`t have the organization to create storm motion at the beginning. So the mid afternoon hours are the most likely problem time there, shifting to the plains in the evening. Forecast temperatures looked alright. We upped the cloud cover and adjusted PoP timing a bit later due to the temperatures and shortwave timing. && .LONG TERM /Tuesday through Sunday/... Issued at 419 AM MDT Mon Aug 12 2024 Ridging will amplify and move eastward on Tuesday placing Colorado on its backside. This will allow for southwesterly flow aloft to transport moisture into the forecast area from the southwest. Southeasterly surface flow is expected to advance low-level moisture northward, keeping dewpoints in the 50s and 60s across the plains. An upper-level shortwave disturbance mixed with steepening mid-level lapse rates will help promote scattered showers and storms over the high country in the afternoon, shifting eastward across the plains into the evening. PWATs are expected to be ~150-160% of normal for the high country and eastern plains. With this heightened moisture content, some storms may be possible of producing heavy rainfall at times. Storm motion is expected to be slightly faster than Monday`s, but there will likely still be an elevated threat for localized flash flooding across sensitive areas, especially our newest burn scars. As the storms move eastward into the 50 and 60 degree dewpoints across the plains, forecast soundings show MLCAPE values between 1800-2000 J/kg, and 43kt sfc to 8 km shear that would help promote more organized thunderstorm development. There will be an isolated threat for hail and severe winds in the stronger storms that form, mainly east of the I-25 corridor. Afternoon high temperatures are forecast to be near normal with 80s expected for the plains, except for areas in the far northeastern corner where low level clouds are likely to keep solar heating minimized. The mountains are forecast to be in the upper 60s- mid 70s, and the foothills will be slightly warmer in the 70s to low 80s. Overnight temperatures will be in the upper 50s for the plains, low 50s for the foothills, and 40s for the mountains. Models look to come into more agreement that a shortwave trough is expected to move across the PNW Wednesday that will help flatten out the ridge bringing a more zonal flow aloft over the forecast area. This will bring drier conditions as PWATs look to decrease to 110- 120% of normal, however, there will still be enough moisture and enough synoptic support from the shortwave to the north to allow for afternoon scattered storms and showers to develop over the mountains and move east across the plains. High temperatures are expected to be within a few degrees of normal across the forecast area. A cool front will move across the CWA overnight Wednesday that will drop overnight low temperatures 3-5 degrees from Tuesday`s. A drying trend looks likely for Thursday through Saturday as Colorado looks to be on the front side of a 500 mb ridge bringing NW flow to the CWA, cutting off our monsoonal moisture supply. PWATs look to drop to 80-90% of normal, and temperatures will rebound to the 90s for Friday and Saturday. Some scattered afternoon showers and storms look possible for the high country on Saturday afternoon, but Sunday looks to see a more significant moisture return with more widespread chances for precipitation with southwesterly flow returning on the backside of an amplifying ridge. && .AVIATION /12Z TAFS through 12Z Tuesday/... Issued at 419 AM MDT Mon Aug 12 2024 Low clouds with IFR ceilings have moved across KDEN. There is less certainty about whether they will get to KBJC (70% chance) and KAPA (50% chance). LIFR ceilings are possible for a few hours, until about 15z, but dense fog is not expected. After that, VFR conditions will prevail. Thunderstorms are expected to be in the area between 21z-03z. The main impact will be a few wind shifts with gusts up to 35 knots, but localized IFR visibilities and gusts up to 50 knots are possible with the strongest storms. The threat of fog is lower for Wednesday morning. && .HYDROLOGY... Issued at 419 AM MDT Mon Aug 12 2024 ...FLOOD WATCH FOR FLASH FLOODING FOR THE FRONT RANGE FOOTHILLS...DENVER AND EASTERN/SOUTHERN SUBURBS...PALMER DIVIDE...AND ADJACENT EAST CENTRAL PLAINS OF COLORADO... Meteorological reasoning is above...but there is an elevated risk for flash flooding in the more sensitive burn areas mainly this afternoon. Elsewhere, some of the storms will produce heavy rainfall of 1 to 3 inches, with the heavier rains presenting a risk of at least minor flooding mainly on the plains this evening. If one of the heavier storms moves over an urban area in the late afternoon/early evening that could also be a flood risk. Tuesday will bring another day of heightened threat for heavy rains across the forecast area. Above normal moisture will still remain in place that could lead to localized heavy rainfall in stronger storms that develop. We have left the Alexander Mountain Burn scar in an elevated threat with a limited threat for older scars. There will be another day of afternoon showers and storms across the high country that could create a limited threat for our burn scars as moisture content begins to decrease. By Thursday and into the weekend, drier conditions are expected. && .BOU WATCHES/WARNINGS/ADVISORIES... Flood Watch until 9 PM MDT this evening for COZ035-036. Flood Watch from 2 PM this afternoon to 9 PM MDT this evening for COZ040-041. Flood Watch from 3 PM MDT this afternoon through this evening for COZ045>047-049. && $$ UPDATE...Barjenbruch SHORT TERM...Gimmestad LONG TERM...Bonner AVIATION...RPK HYDROLOGY...Bonner/Gimmestad