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Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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305 FXUS65 KBOU 181639 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1039 AM MDT Sun Aug 18 2024 .KEY MESSAGES... - Precipitation chances increase today, with scattered storms expected through next week. - There will be a limited threat of flash flooding over the burn areas today but a higher threat on Monday. - Above normal temperatures will continue through the week. A few days may see mid-90s highs across the metro and plains. && .UPDATE... Issued at 1039 AM MDT Sun Aug 18 2024 Shallow inversions have allowed a very fast warmup here in the Denver metro area. Farther east, over northeast Colorado, we have a cool surge enhanced by convection in Nebraska. That should act to cool temperatures a few more degrees than forecast over the northeast corner, while the I-25 Corridor will still be hot with highs in the lower to mid 90s. With regard to convection, it is interesting to see the delay in CAMs producing showers and storms today, as inversions are fairly weak. However, there is also another stable layer noted in the ACARS soundings near/just above 700 mb that is the likely culprit. The current forecast seems reasonable with convection developing over the mountains early to mid afternoon, and then spreading northeast across the plains late this afternoon and evening. Gusty winds and lightning will be the main threats as low level moisture is limited in most locations, although the plains could see a strong/severe storm along the axis of higher bouyancy/weaker shear. && .SHORT TERM /Through Tonight/... Issued at 418 AM MDT Sun Aug 18 2024 Current satellite pictures are showing plenty of mid and upper level cloudiness moving and up and over the ridge aloft. The upper ridge axis is over far eastern Colorado. The area radars indicate that convection has ended across the CWA early this morning. Current temperatures are still pretty warm for this time of night with mid 60s to lower 70s F still over most of the plains. 50s F are common over the mountains and foothills, with colder readings right along the divide. Models keep the upper ridge axis over the CWA today and tonight with 20 to 40 knot jet level westerly flow in place. The synoptic scale energy is mostly benign for the first two periods. Precipitable water values are progged to increase later day and tonight with values getting into the 0.80 to 1.30 inch range by tonight. The low level dew points increase this later this afternoon and evening with 50s to mid 60s F over the plains. The 60s F dew points are mainly progged over the eastern half of the plains. The best CAPE is progged over the eastern half of the plains later this afternoon and evening, with limited amounts over the rest of the CWA. Not sure how well the CIN will be able to erode away across the CWA this afternoon and evening. Cross sections are pointing to some downdraft producing thunderstorms for later today and this evening. The QPF fields from the various models are not very impressive with rainfall amounts for the first two periods, with the best rainfall to be over the southwestern quarter of the forecast area. For pops, will keep the 30-60%s going in the mountains with the highest over Park County. For the plains, 20- 30%s should suffice at this time. Still some brief heavy rain is possible, especially over the High County south of I-70. && .LONG TERM /Monday through Saturday/... Issued at 207 AM MDT Sun Aug 18 2024 The long term period will be dominated by the presence of a large upper ridge centered near the TX/OK panhandle. Monday should be one of the more active days of the period, with well above normal moisture in our region. Deterministic guidance suggests ~1.25" PWATs across the metro by the afternoon hours, which is roughly 150-175% of normal. The main question is how much instability will be available to work with, with substantial spread among the HREF and global ensemble members. While mean/median surface CAPE is less than 500 J/kg for KDEN Monday, there is a notable right skew to the data, with a handful of LREF members showing >2500 CAPE. While the answer is likely somewhere in between, there`s no obvious solution at this point. Weaker (stronger) instability would likely lead to less (more) organized/weaker storms and a lower (higher) threat of heavy rainfall. The ridge is expected to strengthen a drift a little closer to the forecast area on Tuesday, which should shove the plume of better moisture to the north and west of the forecast area. Enough moisture does linger across the high country for at least a few showers/storms, but it should be much drier across the plains. As we get towards the latter half of the week, the ridge is expected to gradually weaken and shift eastward, leading to a return of modest southwesterly flow aloft. This should allow better moisture to advect back into the region, with precipitation chances increasing... especially towards Thursday/Friday. We`ll have to wait and see how active the pattern looks as we get into next week. Eventually drier air should work back into the region as we get into next weekend. Meanwhile... as mentioned in the past several AFDs... the pattern broadly supports above normal temperatures across the region for the foreseeable future. Our forecast grids have 90F at KDEN every day in the long term period, with a couple of mid 90s highs forecast on Tuesday and Wednesday. It`s possible a day or two ends up a bit cooler from convection/outflows, but otherwise our streak of 90F days looks like it will continue on for the next several days. && .AVIATION /12Z TAFS through 12Z Monday/... Issued at 538 AM MDT Sun Aug 18 2024 Normal diurnal wind patterns are expected today at DIA with drainage winds kicking back in around 02Z this evening. Will again leave the TEMPO group in for -TSRA later this afternoon as models show some instability and limited measurable rainfall. Wind gusts to 35 knots are possible in and near the storms. There will be no ceiling issues. && .HYDROLOGY... Issued at 425 AM MDT Sun Aug 18 2024 Moisture availability is expected to increase later today and tonight. The highest pops are expected over the southwestern quarter of the CWA late this afternoon and this evening. The threat of flash flooding will be limited mainly to the newer burn areas and Park County. Better moisture is expected again on Monday, with PWAT standardized anomalies of 2-3 sigma noted on the GFS/ECMWF by Monday afternoon. There is still some question as to how much instability will develop across the Front Range and urban corridor despite the above average moisture content, as surface dew points only rise into the low 50s. Still, the combination of modest instability, abundant moisture, and relatively slow storm motions leads to a limited threat of burn area flash flooding, with an emphasis on the recent burn areas from this summer where rainfall thresholds are lower. The overall rainfall threat diminishes on Tuesday as the upper ridge expands and the better moisture is temporarily pushed out of the region. The pattern gradually turns wetter again for the latter half of the week, with slow moving thunderstorms becoming more widespread. At least a limited threat of burn area flash flooding is forecast through the rest of the forecast period, with the potential for a locally higher threat on a day or two as we approach next weekend. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Barjenbruch SHORT TERM....RJK LONG TERM.....Hiris AVIATION...RJK HYDROLOGY.....RJK/Hiris