Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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053 FXUS65 KBOU 132359 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 559 PM MDT Thu Jun 13 2024 .KEY MESSAGES... - Isolated severe storms with potentially damaging microbursts and large hail this evening - More numerous thunderstorms and a greater severe storm threat Friday afternoon and evening - A return to hot and dry weather this weekend into early next week. - Snowmelt producing minor impacts from high streams. Gradually diminishing flows next week. && .SHORT TERM /Through Friday/... Issued at 249 PM MDT Thu Jun 13 2024 Main concerns are for isolated severe storms this evening, and more numerous severe storms Friday afternoon. The "cold" front pushed back into the I-25 Corridor this morning, but as discussed earlier it didn`t have much impact on temperatures with highs in the mid 90s! Farther east, at least there was some cooling with current temps mostly in the upper 80s. The heating here has eliminated the CIN, with some high based moderate cumulus. SPC surface analysis shows a ribbon of 1000+ J/kg MLCAPE to the east and northeast of Denver, where post- frontal moisture depth is larger, with only around 500 J/kg closer to I-25. Thus, we should get additional convective initiation closer to the Front Range, and then intensify as it moves into the more unstable air just to our east. Enough instability and shear would be present for one or two severe storms with damaging winds and large hail the primary threats. Even outside of that area, microbursts will be possible closer to the I-25 Corridor where DCAPE values are in the 1500-2000 J/kg range - very impressive values which means isolated damaging gusts >60 mph will be possible. Also, if deeper moisture is able to push back (20-30% chance) can`t rule out a more traditional severe storm bringing a large hail threat. The bulk of storms should end as we head into the late evening and overnight hours as the airmass stabilizes. However, with further moisture/theta-e advection through 700 mb there will still be a slight chance of storms over the eastern plains overnight. On Friday, the airmass will destabilize while a shortwave trough approaches the region from the southwest. This times up well to produce a greater severe storm threat in the afternoon. MLCAPE will increase to 1500-2000 J/kg as surface dewpoints hold in the 50s. As southeast flow increases in the afternoon, a Denver Convergence Vorticity Zone (DCVZ) is expected to develop from the Palmer Divide/southeast sections of Denver onto the plains. That will help drive convective initiation (in addition to the elevated heating source of the mountains), and bring a greater risk of severe storms including a tornado threat. Large hail, damaging winds, and heavy rain will all be possible. HREF limited the amount of updraft helicity, while deep layer shear is a bit marginal. Nonetheless, the approaching shortwave typically can make up for that. Therefore, expect at least a few severe storms on Friday afternoon, continuing into the evening. && .LONG TERM /Friday night through Thursday/... Issued at 249 PM MDT Thu Jun 13 2024 A return to well above normal temperatures appears likely by this weekend as a large ridge builds across the eastern U.S. and gradually strengthens. Broad west-southwesterly flow will likely be in place by Saturday behind Friday`s shortwave, with considerable drying through the day. 700mb temperatures are projected to reach around 15C during the day with upper 80s to low 90s highs likely across the plains. There may be just enough lingering moisture for an isolated storm or two across the Front Range mountains/foothills but anything that does develop would likely produce more wind than precipitation. The mid-level thermal ridge is forecast to strengthen considerably Sunday into Monday. As a trough axis approaches to our northwest and the ridge out east continues to build, this will likely result in increasing mid/upper level flow and an added downslope component. Most deterministic, ensemble, and MOS guidance favor mid to upper 90s across the plains on Sunday and Monday as 700mb temperatures warm to near 20C. If we are as hot/dry as advertised, there would certainly be fire weather concerns at least one or two days this weekend/early next week. Heat Advisories may also be necessary if the higher end solutions pan out. At some point, guidance does bring a cold front into the region alleviating the heat for most of the region. The GFS is an outlier, bringing the front in on Monday, while most other guidance favors Tuesday. Though the aforementioned trough won`t bring much precipitation, it does appear that we`ll have a few days with temperatures near or below normal during the mid/late week next week. Unfortunately, this appears to be short lived, with a strong ensemble signal for the heat to return by next weekend. && .AVIATION /00Z TAFS through 00Z Friday/... Issued at 559 PM MDT Thu Jun 13 2024 Main concern continues to surround gusty outflow winds from isolated showers and storms. While strong easterly winds prevail, there have been interruptions of gusty microbursts as evidenced at KFNL with recent gusts 34-43 kts. Before the surge, we even had a gust to 53 kts at KAPA! We will continue the risk of these gusty outflows through TEMPO VRB winds in the TAF, with the greatest threat until about 03Z. However, still a small threat thereafter depending on when/if convection ever gets going in the metro area. Otherwise, deep and fairly strong easterly flow is expected through about 03Z, possibly enhanced by outflows of stronger storms to the east. Once convection ends this evening, winds should settle down to more normal southeasterly or southerly flow most likely in the 04Z-07Z timeframe. While low level moisture increases, this flow is not conducive to any stratus and airmass is quite warm - also limiting the stratus threat. On Friday, there will be a greater chance (>60-70%) of thunderstorms in the afternoon hours, so TEMPO TSRA is warranted. These storms will be stronger with potential for heavy rainfall, hail, gusty winds, and even visibility restriction. There is potential for even a couple severe storms nearby. The main threat time would be 21Z-24Z. && .HYDROLOGY... Issued at 249 PM MDT Thu Jun 13 2024 Thunderstorms Friday afternoon and evening will be capable of producing locally heavy rainfall. Precipitable Water (PW) values increase to between 1 and 1.5", with the highest amounts over the northeast plains. Storms will be more efficient rain producers than the last several days, as dewpoints on the plains climb into the 50s, with even some lower 60s over the northeast corner. Storm motion is expected to be around 20 mph, which isn`t terribly fast so given the intensity and ingredients above we think stronger storms will be capable of producing 1-2 inches in 30-45 minutes. Considering the low level wind fields, we believe the most focused area of heavy rain would be from the south/east sides of Denver metro and Palmer Divide in the mid/late afternoon, and then to the east and northeast across the northeast plains of Colorado through the evening hours. Moisture levels across the high country will be lower; however, there could be localized areas of heavy rainfall which could cause flooding issues across the burn scars. Cameron Peak would be most susceptible given somewhat richer atmospheric moisture profiles there. Elevated flows continue in the mountain streams, with Flood Advisories remaining in effect for the streams above Granby and Grand Lake. Remain alert of the dangers of water that is running high and cold. Flows are expected to gradually decrease over the weekend and through next week as the peak snowmelt is passing by. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...Barjenbruch LONG TERM...Hiris AVIATION...Barjenbruch HYDROLOGY...Barjenbruch