Area Forecast Discussion
Issued by NWS Denver/Boulder, CO
Issued by NWS Denver/Boulder, CO
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177 FXUS65 KBOU 070248 AFDBOU Area Forecast Discussion National Weather Service Denver/Boulder CO 848 PM MDT Mon May 6 2024 .KEY MESSAGES... - Very active weather pattern through the next couple days, with Winter Weather, Fire Weather, and High Wind highlights in effect - Periods of snow and blowing snow through Tuesday with some travel impacts, especially across the Park Range/Rabbit Ears Pass. - Widespread gusty winds continue through at least Tuesday, with the strongest winds expected across the higher Foothills. - Cool and unsettled weather along with much lighter winds Thursday through Saturday. && .UPDATE... Issued at 848 PM MDT Mon May 6 2024 We mainly just updated the highlights that were getting close to expiration this evening. First, the RFW for southern Lincoln County was ended given RH is over 25% in the area, and winds have come down significantly. We also opted to cancel the high wind warning for the adjacent plains below 6 kft MSL in Larimer, Boulder, and Jefferson Counties. The only remaining places that are gusty per latest observations are near the foothills in a few pockets and at best are gusting a little over 40 mph. Over 98% of the remaining area in that part of the warning have much weaker winds, more like 20-25 mph gusts. There are still ingredients in place for a mountain wave to develop now through 3 AM, but none of the CAMS bring the winds out of the foothills overnight. Still think there is enough signal for wind gusts 40-50 mph in the wind prone areas along CO-93 and north of Fort Collins, and the existing forecast reflects this, but even if mountain wave enhanced winds occur they should remain well below warning criteria. Strong winds should continue and possibly pick up in the foothills with the approach of the next weak trough, so will keep the HWW in place there until 6 AM. Overall the details of the forecast were not updated much, mainly just to clean up (i.e. remove) some of the PoPs before noon where measurable precip isn`t expected in the face of strong subsidence and strong downslope flow (i.e. below 6500 ft elevation) until potentially after noon Tuesday. && .SHORT TERM /Through Tuesday/... Issued at 135 PM MDT Mon May 6 2024 High winds will continue to be the main concern for impacts across northeast Colorado this afternoon through Tuesday. Based on surface observations, the stronger winds are across the Front Range mountains/foothills and the adjacent plains with gusts 40-60 mph and locally higher for the Front Range windy spots. Radar shows snow continuing across the mountains with CDOT cameras shows minor impacts to the higher passes. A band of showers remains over northern Weld County; however, large dewpoint depressions will make it difficult for any liquid to hit the ground. Starting with the highest impact..wind. Strong winds continue through Tuesday. Toward the early evening, winds decrease some across the urban corridor with high winds still across the Front Range mountains/foothills. More dry subsident flow moves in overnight. Model cross sections show a period of decreased moisture in the high country before the next wave approaches. There is also indication of a wave mountain wave set up during this time with cross sections showing a stable layer with increased flow aloft, and decreased moisture. This supports the potential for wind enhancement across the Front Range mountains and foothills in the 03-09z timeframe. If this were to set up, those areas could see gusts 60-75 mph and locally up to 80 mph in the windy spots, further justifying the ongoing High Wind Warning. Moisture increases again in the high country by sunrise and the pressure gradient relaxes across the Front Range which will decrease the higher magnitude winds Tuesday morning. Tuesday will still be windy, although not as windy as Monday with HREF showing mean gusts 40-60 mph for the Front Range and 30-45 mph elsewhere. Regarding snow. Snow continues in the mountains through early this evening with minor impacts to travel possible. Later in the evening, moisture decreases along with lapse rates promoting a lull in snow for a period overnight. The next wave increases and deepens moisture across the high country with steepening lapse rates early to mid AM Tuesday. This will increase snow showers across the mountains. Tues AM through late afternoon will be the best timeframe. Can`t rule out minor impacts, although May sun angle will make it difficult to accumulate on most trafficked roads. Mountain passes will have the higher chances of any impact. With windy conditions, patchy blowing snow is possible. From tonight through tomorrow, we`ll see closer to advisory level amounts. Held off for now on issuing any additional headlines with hesitation on actual travel impact. Could see up to another foot in the Park Range in spots and 4-8 inches for the Front Range mountains and mountains of Summit County with some locally higher amounts. && .LONG TERM /Tuesday night through Monday/... Issued at 135 PM MDT Mon May 6 2024 Winds will gradually subside Tuesday evening as gradients relax slightly and we lose daytime heating and mixing. There is still a moderate gradient aloft, however, so gusty winds could linger off/on overnight into early Wednesday morning in/near the foothills but to much less magnitude than tonight. With less wind, clearing skies, and a very dry airmass, there is a greater than 50% chance of sub-freezing temperatures on the plains. With the growing season beginning, we`ve opted for a Freeze Watch in all but the northeast corner where winds will stay up a bit more. There`s a greater chance we could remain above freezing in some of the western suburbs depending on wind, but opted to keep it simple for now with a contiguous area across the plains and I-25 Corridor. It`s also been quite some time since it`s frozen in the western suburbs. For Wednesday, generally lighter breezes are expected in most areas, with temperatures still averaging a few degrees below normal. We`ll be mostly caught in between moisture sources (departing to our east and another staying to our north) during the day, so only a few mountains snow showers expected. That will start to change Wednesday night and Thursday as an elongated trough slips southward into our forecast area. And yes, that will be the remnants of the deepening (current) storm system moving through the western Dakotas tonight and Tuesday. Given the increase in moisture and instability associated with the upper trough, at least scattered showers are expected Thursday and Thursday night. Temperatures will be several degrees below normal, and in fact snow levels could hover around 7,000 feet during the day, and a little lower than that at night. We don`t see any big precipitation amounts at this time, but perhaps a couple more inches for the northern mountain snowpack. For Friday into Saturday, there is still considerable uncertainty regarding how the upper trough further elongates and even shears south/southwest. The EPS members are showing strong agreement with the shearing/deeper trough into the Desert Southwest, while the GEFS is almost entirely opposite showing the elongated trough essentially getting squashed to our south and building a ridge quicker into our area. Even the EPS, however, is a little too far south and west so overall a drier weather pattern is expected to develop sometime around Friday, and then hold through most of the weekend. It`s hard to buy the EPS solution of lifting the low back to the northeast and across us toward Sunday, but definitely worth keeping an eye on as that could slow our warming trend and drier weather forecast. At this point, it`s hard to argue with the ensemble forecast which shows gradual moderation and above normal temperatures by Sunday and Monday. Low odds (~20%) that we would stay cooler with more numerous showers. && .AVIATION /00Z TAFS through 00Z Tuesday/... Issued at 535 PM MDT Mon May 6 2024 Not an overly complicated forecast through Tuesday but operationally there will continue to be impacts via strong west winds much of the day Tuesday. First, VFR conditions will prevail through Tuesday. Winds out of 280/290 should finally weaken through 01 to 02Z at DEN and APA, with winds coming down to 10-15 kts out of the SW after 02Z. From there, they should remain around 10-15 kts until 15Z. Winds out of 280 increase across the terminals 20G30KT after 15Z, and should remain that way all the way until weakening around 03Z Wednesday. There is a low chance (25%) that winds are 5-7 kt less than we have in the TAFS during the afternoon, but for the most part the CAMs all have sustained west winds 20 kts or higher from 15Z Tue - 03Z Wed. There is an additional thing to mention regarding BJC. There are several ingredients in place for a short-lived mountain wave to develop after 03Z. This would keep the winds up at BJC through about 08Z. For now, have just kept the 28020G30KT going until 08Z because of the strong chance of a mountain wave keeping the winds elevated. CAMs actually keep the stronger west winds in the lower foothills during this time and away from BJC, but we can`t ignore the favorable ingredients. If the ingredients come together we`d likely see gusts 40-50 kts late tonight. But after 08 or 09Z those ingredients weaken significantly, so we are pretty confident there will be a lull at BJC from 08-15Z tomorrow with winds 10 kts or less. && .BOU WATCHES/WARNINGS/ADVISORIES... High Wind Warning until 6 AM MDT Tuesday for COZ035-036. Freeze Watch from late Tuesday night through Wednesday morning for COZ038>049. && $$ UPDATE...Schlatter SHORT TERM...Mensch LONG TERM...Barjenbruch AVIATION...Schlatter