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Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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149 FXUS65 KBOU 261728 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1128 AM MDT Wed Jun 26 2024 .KEY MESSAGES... - Scattered to numerous thunderstorms today. - Some storms will be strong to severe with heavy rain, large hail, and strong winds. - Scattered-numerous thunderstorms return Thursday, some strong to possible severe, especially in the plains. - Turning drier into the weekend, with some moderation in temperatures Saturday. - Potential for significant heat will rise again next week. && .UPDATE... Issued at 1128 AM MDT Wed Jun 26 2024 Still a good deal of uncertainty about convective initiation and evolution. The best low-level moisture and thus instability remains over the northern quarter of Colorado, westward all the way to the Larimer County foothills. Given this, the best be for initiation would be across the Larimer County foothills where there is a nice cu field that has already developed, and then the storms would move move east across the plains during the late afternoon. Lightning, small hail, gusty winds are possible from 1-5 PM. GOES-16 also showing better instability across the Mosquito and Front Range mountains/ridges near and south of Berthoud Pass. Storms should get going there and also move east, but when the get to metro Denver instability is less vs Larimer and Weld Counties. Gust winds and brief showers may be the norm this afternoon for much of metro Denver from 2-6 PM. However, when the storms from the north and make it east of Denver, CAMs want to develop a broken line of strong to severe storms that moves across much of the plains of northeast Colorado. Those storms could contain severe hail up to golfball size. If that happens, it`s most likely to occur after 6 PM. Storms should move east of our area by 10 PM. The existing forecast has these trends well handled. && .SHORT TERM /Through tonight/... Issued at 243 AM MDT Wed Jun 26 2024 Upper level high that`s been centered over the Central and Southern Rockies will slowly sink south today. At the surface, a weak cold front drops south across eastern Colorado this morning. Temperatures today drop to near normal with highs in the upper 80s to lower 90s across northeast Colorado. We see a good increase in moisture today. Water vapor satellite imagery showing a plume of moisture across the Four Corners region. This moisture will rotate northeast around the upper level high today. Also, low level moisture will increase behind the cold front this morning with dew points in the 50s to lower 60s across the Front Range and plains. This will result in precipitable water values of 125-150% of normal east of the mountains and over the higher terrain precipitable water values near 200% of normal. Heavy rain will be possible with the stronger storms. Mid level winds of 20-30 knots will help to keep storms moving along. There are small differences on the amount and placement of instability and shear today. Models generally agree we will see MLCAPE of 1000-1500 J/kg over a good part of northeast Colorado. The main difference is how far west the better instability will be. As far as shear goes, where southeast surface winds reach 15-20 knots, bulk shear (0-6km) reaches 30-40 knots. This is enough instability and shear for severe storms. Hi-Resolution models show scattered storms forming over the higher terrain this afternoon. The storms then progress eastward across the Front Range. Once this convection reaches the plains (east of the metro area), it is expected to increase in intensity and coverage with a broken line tracking across the plains. Main threats will be hail up to golf ball size and strong damaging winds. && .LONG TERM /Thursday through Tuesday/... Issued at 243 AM MDT Wed Jun 26 2024 Of the extended forecast period, Thursday certainly carries the higher potential for severe weather, together with more expansive coverage of late day convection. Ample moisture will remain in place, only negligibly lower than today`s values, with precipitable water peaking between 1.10-1.40" over the urban corridor and plains, the bulk of this above 600mb. The weakening 500mb ridge will continue to flatten, but a 700mb shortwave becomes more noticeable in advance of broader troughing to our northwest. Sustained warm temperatures into the lower 90`s for much of our lowlands will make for drier near-surface conditions, but with dewpoints in the 50`s along the urban corridor and 60`s in the eastern plains, and steepening lapse rates as cold air advection increases aloft, we`ll have sufficient instability to support scattered to numerous afternoon showers and thunderstorms. Rainfall will likely be more efficient in the high country relative to today given richer low-level moisture, becoming less productive due east of the Front Range, especially along the I-25 corridor where precipitation should be more limited. As far as timing is concerned, a strengthening upper-level jet will help provide for some enhanced dynamic support in the evening, with a few showers/storms likely lingering in the eastern plains and high country. The trough axis traverses overhead on Friday with robust westerly flow prevailing aloft, resulting in a marginal moderation of temperatures and breezier conditions across our higher elevations. Subsident flow will keep drier conditions in place below 600mb, and help reduce thunderstorm coverage and intensity east of the mountains. Nonetheless, a chance of afternoon storms (35-60%) will persist. A cold front Friday evening/night will usher in milder temperatures for Saturday, with highs descending into the low to mid 80`s for the lower elevations - most likely the coolest day of the forecast period. More stable conditions will also develop as a ridging pattern begins gaining prominence, with notably lower precipitation chances region-wide. ML CAPE will be much more marginal, outside of perhaps the southernmost row of our plains counties and Palmer Divide where chances for afternoon/evening convection will be highest. Warming and drying then accelerates into Sunday under the expanding ridge, quickly returning us to the 90`s for highs and keeping precipitation chances quite low outside of the mountains. Come Monday, there are some signs of troughing over the northern plains with enhanced west-northwest flow along its leading edge, which carries potential for some fire weather concerns depending on just how dry we stay. Regardless, expect the heat to linger and precipitation chances to remain on the low side. Depending on the timing and amplitude of the trough, we could see a slight reprieve in temperatures for Tuesday, but moisture still appears rather lacking and any cooling is likely to be short-lived, with hotter temperatures favored to take hold for the mid to late portions of the week. && .AVIATION /18Z TAFS through 18Z Thursday/... Issued at 1128 AM MDT Wed Jun 26 2024 VFR conditions expected through Thursday. Thunderstorms are expected to develop across the high country after 12 PM, and then move east. There is uncertainty about how well the storms can stay together across metro Denver and out to DIA. There are enough ingredients in place for severe storms east of DIA, but storms will be less intense if they move over any of the terminals this afternoon. Given the expected spottiness of the storms, TEMPO groups seem to be the best way to handle the potential of -TSRA. Showers and storms should clear the terminals by 7 PM. Gusty winds to 35 or maybe 40 kts are possible with the strongest storms this afternoon/evening. In the wake of the showers/storms, southeast winds may briefly gust over 25 kts, but by midnight drainage winds at the terminals should take over (SSW DEN and APA 8-12 kts, WNW at BJC 5-8 kts). Tomorrow winds should shift to the north by midday at 10 kts or less, and there will be another round of thunderstorms that either impact the terminals (40-50% chance) directly or generate gust fronts that result in winds shifts and gusty winds. && .HYDROLOGY... Issued at 243 AM MDT Wed Jun 26 2024 With an increase in moisture, thunderstorms will be capable of producing heavy rainfall today. Mid level is expected to be strong enough to push storms along. However, where storms train or remain anchored, flash flooding will be possible. Also, flood prone areas, such as burn scars could see flooding if heavy rain is slow to leave the area. Conditions will be similar Thursday with ample moisture remaining in place. Precipitation efficiency could even be slightly higher in the mountains given a more favorable low-level moisture profile, although storm motion should still be decent enough to keep most storms moving fast enough to limit the scope of flash flooding concerns. Expect the pattern to turn notably drier heading into the weekend. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Schlatter SHORT TERM...Meier LONG TERM...Rodriguez AVIATION...Schlatter HYDROLOGY...Meier