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
302 FXUS65 KBOU 191640 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 1040 AM MDT Fri Jul 19 2024 .KEY MESSAGES... - Widespread showers and storms are expected Friday afternoon into the early evening hours, with a few strong to severe storms possible across far northeastern Colorado. Locally heavy rain is also possible with a limited risk of burn area flash flooding. - A cooler and more moist pattern will exist from Saturday into Monday. There will be a limited threat of flash flooding, mainly on the burn areas. - Sunday favored to be most active day - greater afternoon thunderstorm coverage expected, with locally heavier rain possible. - Turning warmer and generally drier Monday onward. && .UPDATE... Issued at 1025 AM MDT Fri Jul 19 2024 A cold front is moving across the far northeast corner of Colorado this morning. A line of storms has already formed in the panhandle of Nebraska and these storms will soon make their way to Sedgwick and Phillips counties. PoPs were increased to account for these storms. By the early afternoon, this cold front will trigger storms in Washington and Lincoln Counties. These storms could be strong to severe with strong winds the primary threat. Morning cloudiness across the mountains and foothills is limiting the instability. There are still expected to be showers and storms this afternoon across all of our CWA but storms to the west of a line from Fort Morgan to Lincoln are not expected to be severe. Gusty winds could still occur with gusts up to 45 mph possible. && .SHORT TERM /Through Saturday/... Issued at 237 AM MDT Fri Jul 19 2024 The ridge of high pressure at 500 mb continues to move very little through Saturday morning. The position over Arizona with clockwise flow places Colorado under northwest flow aloft. During the midday hours, some cooling at 700 mb works its way in from the northwest, and with it instability increases across the area. The timing of the cooling is pretty good with the heating of the day, though the combination looks to initiate convection a little sooner that is typical on an average late July day. Storms should initiate over the higher terrain of S. WY and northern Colorado as early as 11 AM, and steering-level winds of 25-30 mph should move storms to the southeast throughout the early afternoon. Instability and deep layer shear are not ideal for severe potential other than across the far northeast corner (more on that in a minute). However, a broken line/lines of storms should move off the terrain and across the plains during the early afternoon hours. The first wave of storms should be the strongest with lightning, small hail, brief heavy rain, and wind gusts perhaps exceeding 40 mph. The stronger storms should be east of metro Denver by 3-5 PM, and out of the area by 8 PM. Coverage of the storms should be higher than on Thursday given the better instability and PW values approaching 1" east of the Divide. There is a slight risk of severe thunderstorms across the northeast corner of Colorado, generally east of Sterling. Tds are expected to be in the low 60s, and with plenty of clear skies during the early afternoon hours, surface-based CAPE looks to be in the 1200-2000 J/kg range. Deep layer shear is about 30, maybe 35 kts. Using DESI (which is a fantastic ensemble viewing tool) and the HREF, there is greater than an 80 percent chance of > 1200 SBCAPE and 30 kts of 0-6 km shear to occur together in the late afternoon hours over the northeast corner. CAMs initiate convection in the NE panhandle, and those storms move southeast across the northeast corner of Colorado. Hail up to golfball size could occur with the strongest storms sometime between 3-7 PM. In terms of temperatures today, the ridge is still close by, and there will be plenty of heating before the storms move through. High temperatures should be in the low 90s across all of the plains of Colorado below 6,000 ft elevation. Relief from the seasonably warm temperatures will be quick and early, especially in the high country and along the I-25 corridor where clouds and cooling outflow (along with a 30-60% chance of measurable rain) will occur by mid afternoon. In the high country, the mountain valleys and eastern foothills above 7,000 ft will warm into the mid to upper 70s before cooling off via convective outflows. Additional showers with a few rumbles of thunder are possible in the late evening hours across much of our area. Coverage will be isolated and rainfall amounts will be very light, if there indeed is another round of showers. In any case, by midnight all shower activity should be over or moved southeast of our area. Midlevel clouds may be stubborn to clear across a good chunk of our area overnight, but the convective cooling and enough radiational cooling should result in lows in the 50s to near 60 across the plains, low 60s in the urban heat islands, and low to mid 40s in the mountain valleys. && .LONG TERM /Saturday night through Thursday/... Issued at 237 AM MDT Fri Jul 19 2024 The shortwave across the Midwest will remain slow-moving, with additional slight digging Sunday sustaining relatively cool and moist northeasterly flow at low levels. Temperatures will thus remain below average for the date for a second consecutive day, and some ensemble members suggest much of the lower elevations might even struggle to break 80 degrees. Cloud cover should be a little more abundant in the morning and, with ample cold air aloft and healthy dewpoints, instability looks sufficient to support the highest precipitation chances of the week Sunday afternoon, particularly for the plains and urban corridor. Shear profiles are much less impressive however, so the severe weather threat will be tapered some, but slower storm movement could result in some more efficient rainfall and heftier accumulations for favored spots. Although we`ll remain anchored between mostly stagnant high pressure to our west and a troughing pattern to our east, Monday looks generally drier as flow aloft turns slightly more northerly and warmer air begins to fill the upper levels, reducing instability region-wide and also slightly limiting PWAT values to below 1" for most areas. Best coverage of afternoon showers and thunderstorms should thus be focused across our higher terrain and in the vicinity of the Palmer Divide. The ridge is then slated to gain more prominence Tuesday into Wednesday, with more noticeable eastward expansion. This will maintain mostly north/northwest flow aloft, but certainly limit both moisture and instability. Temperatures will warm modestly each day and we`ll likely be closer to seasonal normals or perhaps slightly above by Wednesday. Isolated to scattered afternoon convection can still be expected, but will favor the high country with much spottier coverage elsewhere. Come Thursday, the ridge axis is progged to continue it`s eastward amplification, pushing temperatures back above average and well into the 90`s for the lower elevations. Across our higher elevations, precipitation chances will persist or even increase as flow aloft becomes increasingly westerly and contributes to improved moisture advection. Elsewhere, drier conditions will be favored, but will at least a slight chance (~20%) of some afternoon convection still. && .AVIATION /12Z TAFS through 12Z Saturday/... Issued at 538 AM MDT Fri Jul 19 2024 VFR through Saturday. CAMs from overnight still like convective chances this afternoon at the terminals. Before then, expect north winds around 10 kt to begin after 15/16Z. Storms should get going by 18Z across the high country, then move southeast at about 25-30 mph. One change since the 06Z TAFs has been increased confidence that 1. -TSRA will probably impact the terminals in some fashion and 2. Only one round of storms is expected, and the window should be reasonably short. As it looks now, the best bet for -TSRA is between 20-22Z at DEN and APA, and an hour earlier at BJC. Even if the -TSRA doesn`t get to the airports, there will almost certainly be a wind shift with gusts somewhere between 30-40 kts for a short period of time. Best guess on direction is probably northwest but it`s much too early to say anything with confidence about the direction. Overall the chances of -TSRA at the terminals is between 20-40%, thus a TEMPO group should cover it. There does not look like much of a chance for another round of showers this evening, so will just plan on some E to NE winds in the wake of the broken line of showers, roughly from after the storms pass to the east until after 00Z. Drainage winds should commence after 09Z (SSW at DEN and APA, W at BJC). For Saturday, there are hints at a scattered cloud deck between 060-080. Less chance of convection in the afternoon (but will still need a TEMPO group for gusty winds and perhaps -TSRA when we get that far), and the prevailing wind direction should be NE. && .HYDROLOGY... Issued at 237 AM MDT Fri Jul 19 2024 Today, PW values look to be between 0.8" and 1.0" along the Front Range Foothills. Flow aloft is out of the northwest, and steering winds should be around 20 mph during the afternoon, which should help move storms along. There is pronounced cooling in the 700 mb layer which should help with convective initiation, earlier in the afternoon than the last few days. Over the burn scars, convective initiation could be as early as 11AM. The best chance for heavy rain would be between noon and 2 PM, with instability decreasing substantially after that through the afternoon. Overall, looking like a limited risk of flash flooding on the burn scars. Across the northeast corner of Colorado, severe storms are very heavy rainfall are possible during the late afternoon hours. Wind shear and storm motions will be stronger, so even with very heavy rain for 20-30 minutes, flash flooding is not expected to be an issue across northeast Colorado. On Saturday, the atmospheric column dries out just a little, and instability is much less over the burn scars. While rainfall is still likely in the Front Range and Western Slope, it`s not likely to be heavy enough for any flooding concerns. Storms will be moving 25-30 mph on Saturday as well, with stronger northwest flow aloft. On Sunday, deeper moisture and relatively slow steering-level flow will promote slightly greater potential for localized flooding, particularly if any training of cells occurs. This threat will carry over into the burn scars, where some potential will exist for localized hourly rainfall rates of 1-1.5"/hr. The pattern will turn slightly drier starting Monday, with lesser thunderstorms coverage and a tapered potential for heavy rain (but still non-zero in the high country). && .BOU WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Danielson SHORT TERM...Schlatter LONG TERM...Rodriguez AVIATION...Schlatter HYDROLOGY...Schlatter/Rodriguez