Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
368 FXUS65 KBOU 271713 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1113 AM MDT Thu Jun 27 2024 .KEY MESSAGES... - Numerous showers/storms for the mountains/mountain valleys. Heavy rainfall the greatest risk with a slight chance for a severe storm. - Scattered showers/storms foothills, urban corridor, and nearby plains. Marginal (<20%) chance for a severe storm here. - Scattered/numerous thunderstorms far northeast Colorado this evening, some may be severe with large hail and damaging winds. - Thunderstorms return Friday afternoon, strongest over our northern/northeast zones. - Turning drier starting Saturday, with heat rebuilding Sunday onwards. && .UPDATE... Issued at 1113 AM MDT Thu Jun 27 2024 Minor updates have been made to this afternoon/evening`s forecast. The updates were primarily based on observational trends and surprisingly excellent agreement between consecutive runs of the HRRR and the NAMnest, and consistencies between other members of the HREF. The latest model data indicates that weak convection should get going early afternoon across Larimer County and the Palmer Divide. This convection is not likely to be severe given limited instability, but could produce moderate rain and small hail, along with lightning. It should move quick enough to preclude most flooding concerns across Cameron Peak. Models want to generate convection this evening across W/C Colorado associated with the trough axis. This is a reasonable solution, but all CAMs significantly weaken the convection as it crosses the Divide due to rain shadow/downslope effects. We feel the drying/weakening is a little overdone, but nevertheless impacts should be minor and rainfall amounts look to be light after crossing the divide. Have lowered PoPs accordingly but not too far because light rain and a few rumbles of thunder could still occur, looking at 20-40% for areas east of the Divide out to Fort Morgan. The line of showers/storms could drop a quick 0.5" on the East Troublesome burn scar, and areas of Jackson-Grand-Summit Counties, maybe as far as South Park, but overall impacts look low from round two this evening. Finally, there continues to be a slight risk of severe thunderstorms east of Sterling this evening. Moisture is in place but it looks less unstable across the northeast corner of Colorado than it did 12 hours ago. Have thus lowered PoPs and will message that the threat of severe weather is a little less, but any storms that form could generate severe hail and gusts to 60 mph. Finally, we took a degree or two off the highs given current temperature trends and expected partly to mostly cloud skies across much of the area, but still expect lows in the lower 90s across much of the plains and urban corridor. && .SHORT TERM /Through tonight/... Issued at 227 AM MDT Thu Jun 27 2024 Upper level trough located over the Great Basin at this time is producing night time showers and thunderstorms across Utah. This system will progress east-northeastward today around the upper level high centered over the Southern Rockies. Ahead of this system, surface analysis shows a surface trough (dry line) forming over eastern Colorado and northward into central Wyoming. The forecast for the Front Range and eastern plains will be challenging today because we will see lift from the trough this afternoon and evening, but the surface trough over eastern Colorado will help produce a drying westerly downslope flow. For the mountains and mountain valleys today, lift from the approaching trough is expected to produce numerous showers and thunderstorms. Precipitable water values will be 150-200% of normal. Heavy rain, small hail, and gusty winds will accompany the thunderstorms. Can`t rule out a severe thunderstorm or two with SBCAPE values of 500-1000J/kg. Across the foothills, urban corridor, and nearby plains, westerly downslope flow will produce subsidence and some drying. Still enough moisture, instability, and lift from the approaching trough that there will be a decent chance (30-50%) for showers and storms this afternoon and evening. The strength of the westerly winds will likely play a role in the amount and intensity of the thunderstorms. Some of the hi-resolution models overdue the westerly downslope winds (especially the HRRR). If this happens, and we see weak westerly winds or just variable winds we could see more showers and thunderstorms than the models are advertising. The far eastern plains, east of the surface trough will see the highest threat for severe weather. Here, southerly winds will keep a moist (dew points 55-65) and unstable (SBCAPE 1000-2000 J/kg) airmass in place. Storms that form, along and east of the boundary may become severe with large hail and damaging winds. Main threat area is expected to be over far northeast Colorado and northward. Shear decreases to the south and thus lowers the severe threat. && .LONG TERM /Friday through Wednesday/... Issued at 227 AM MDT Thu Jun 27 2024 A 700mb shortwave beneath mostly zonal flow aloft will push across Colorado Friday, with an associated surface front descending north to south through our lower elevations from mid to late afternoon. Although at first glance many parameters appear somewhat less favorable for strong thunderstorms relative to previous days (more marginal MLCAPE of a few hundred J/Kg, PW under 0.80-1.00", lower dewpoints), the front combined with upper-level jet support should provide for efficient lift. Deep-layer shear 30-45 kts, highest across our northern/northeast counties, would also be sufficient to support a few stronger to severe storms in the afternoon, especially for areas from Weld County eastward. With steering level flow between 25-40 kts, storms will move at a pretty good clip, helping to taper the heavy rain/flood threat. The fairly dry downslope flow will likely reduce coverage and intensity over the urban corridor, with stronger storm clusters developing later in the afternoon across the northeast plains. SPC`s Slight Risk area does a good job of highlighting the corridor of higher threat, with large hail and severe wind gusts the primary concern. Most of the lower elevations will likely still break 90F, assuming the front indeed holds off until mid afternoon or later for most. A more stable airmass will be in place Saturday, with moderated high temperatures in the mid 80`s east of the mountains. However, isolated to scattered afternoon thunderstorms can still be expected, being more favored for the southern Front Range mountains/foothills under southwest flow aloft, as well as our southern plains later in the afternoon, with returning light southeast winds at the surface contributing to slightly higher dewpoints in a post-frontal regime. Low-level warm air advection kicks up a notch on Sunday and that will bump temperatures up around 10 degrees across the plains, but with little if any change for the high country. Warmer conditions aside, the overall setup will be similar to Saturday, with mostly southerly flow favoring our southern mountains and Palmer Divide for the highest chance of afternoon showers/thunderstorms. Ensemble guidance has pushed back slightly with the magnitude of the heat next week, hinting at a more robust trough developing to our north which should keep largely zonal flow in place over Colorado early to mid-week. That`s not to say it won`t be hot - highs in 90`s for plains/urban corridor appear a reasonably safe bet for now each day. Still time for that to change, especially if we end up with a stronger front on Monday than currently advertised. In any case, precipitation chances will be lower for this period, with isolated to locally scattered afternoon convection largely confined to the higher elevations and perhaps eastern plains. Finally, worth keeping an eye on humidity and wind trends for early in the week. Could see pockets of elevated to near-critical fire weather conditions by Monday as pre-frontal winds pick up and subsident flow sustains lower humidity in the lee of the Continental Divide. && .AVIATION /18Z TAFS through 18Z Friday/... Issued at 1113 AM MDT Thu Jun 27 2024 VFR conditions are expected through Friday. WNW winds should be the norm this afternoon, 10-15 kts with some gusts to 20 kts. The convective potential looks a little less for this afternoon/evening than it did 12 hours ago. For now, it looks like the best bet for showers/thunderstorms impacting the terminals should be after 00Z, maybe as late as 04Z. Chances are 20-40% during this time window, which would include a wind shift or at least a short period of strong/gusty winds as storms approach the terminals. Will handle this potential with a TEMPO in the TAFs, with lightning and gusty winds the main impacts. The best bet for wind direction associated with the convection/convective outflows would be WNW given the movement of the storms and the prevailing flow, so about the same as before the storms, just more wind and more gusts. Storms should be east of the terminals after 06Z for sure, and drainage winds (SSW at DEN and APA 8-12 kts, W at BJC 4-8 kts) will kick in all night long through 14Z Friday. On Friday, there will again be chances (~30%) of afternoon thunderstorms, and it`s safe to assume even without direct thunderstorm impacts, 1-2 wind shifts with strong/gusty winds are expected roughly between 2100-03Z. We will introduce a PROB30 group in the TAFs to handle that. && .HYDROLOGY... Issued at 227 AM MDT Thu Jun 27 2024 Over the mountains and mountain valleys, heavy rainfall will be possible from thunderstorms with precipitable water values of 150- 200 percent of normal. Storm movement should be quick enough to keep the flash flood threat low today. However, flood prone areas, such as burn scars will see a greater risk for flash flooding today. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Schlatter SHORT TERM...Meier LONG TERM...Rodriguez AVIATION...Schlatter HYDROLOGY...Meier