000 FXUS65 KBOU 210129 AFDBOU
Area Forecast Discussion National Weather Service Denver/Boulder CO 729 PM MDT Fri Apr 20 2018
.UPDATE... Issued at 729 PM MDT Fri Apr 20 2018
Precipitation has been rather disorganized so far. The first batch of convection this afternoon into early evening was less than anticipated especially in the mountains. On the plains, there has been a development of downslope flow from about 9,000 to 12,000 feet MSL per KFTG Vad Wind Profiler. That has really squashed the earlier convective development, but there are some trends showing up of lighter downslope and eventually this will turn around to weak upslope later this evening into the overnight/ early Friday morning hours when more widespread rain/snow should develop across the plains and I-25 Corridor. Overall, the orographic forcing is quite weak with this storm despite the favorable mid level storm track. Q-G diagnostics also show weak forcing so overall have cut back on precipitation amounts. Therefore, the Winter Storm Warning for the mountains has been downgraded to a Winter Weather Advisory, while snow accumulation numbers have been cut across the board for the mountains and foothills.
On the plains, we still see some potential for accumulation in the grass. In fact, the snow level in the lower elevations has actually been lower than forecast so far as mid levels cooled and saturated. It will still be a struggle to get surface wet bulb temperatures down since there`s little evidence of low level cooling or drying through the rest of this event. However, with nightfall we should still be able to get a mix or changeover across the higher elevations and ridges of the plains. Going forecast looks good there with light accumulations from Denver metro east and southeast toward Akron and Limon.
.SHORT TERM...(This evening through Saturday) Issued at 1218 PM MDT Fri Apr 20 2018
Current WV satellite shows the low center spinning over southern Utah with the def zone moving over the central mountains. Another branch of moisture moved over the eastern portions of the state bringing isolated thunderstorms and a mix of rain and graupel. Drying to the south of the vort max will pull the system to the SE over northern NM. At this time snow is increasing in coverage and intensity over Park county and will continue to through the evening hours. Coverage is still expected to increase into the Northern mountains as wind upslope improves through the evening and overnight hours. At the surface rain is expected to move into the Denver area between 4 and 5 pm and change over to snow around midnight and continue through the early morning hours.
By Saturday morning the system is expected to push ENE across CO with wrap around continuing to bring some rain and snow mix to the plains and light snow in the mountains that will gradually taper off. Models indicate another push of snow to the northern mountains and foothills with increased ENE flow Saturday late afternoon. Overall, did not make any major changes to amounts with 8 to 14 inches over the mountains with the higher elevation getting close to 16 inches. On the plains areas could see a trace to an inch on grassy surfaces with a few inches of wet snow possible over portions of the east. Temperatures started out warmer in the mid to upper 50s across Denver and cooler to the east but will continue to drop through the day with the precipitation. Lows will hover around freezing overnight with upper 40s to low 50s on Saturday.
.LONG TERM...(Saturday night through Friday) Issued at 1218 PM MDT Fri Apr 20 2018
The closed upper low will continue to move eastward away from Colorado Saturday evening with drying and more subsident airmass settling in behind it.There may still be some lingering showers over the mountains but any additional accumulations would be minimal. High pressure ridge aloft will be centered over Colorado by Sunday afternoon with continued dry and warming conditions, though could still be some showers over the higher mountains with lingering mountain level moisture. 700mb temperatures rise to +4c over the Front Range which will put temperatures back into the 60s across lower elevations.
Next fast moving disturbance and associated cold front will sweep across Northern Colorado on Monday night and Tuesday will cooler temperatures. Best chance of showers will be over the far northeast plains with some upward ascent noted and possible thunderstorms Monday evening with some low CAPES. Stronger subsident flow develops behind the trof by Tuesday afternoon with windy conditions on the plains. Could be some showers over east slopes/foothills on Tuesday in post front environment and some shallow upslope flow creating the showers.
The next fast moving system moves across the state on Thursday with the next chance of showers, windy conditions and cooler temperatures. Dry and warmer again by Friday as ridge of high pressure builds over the region.