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FXUS65 KBOU 210353 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 953 PM MDT Mon May 20 2024

KEY MESSAGES

- Severe threat continues mainly just over Washington continue for the rest of the evening.

- Another round of showers and storms is expected Tuesday afternoon and evening. Small hail and gusty winds are possible from these storms but they will likely stay below severe limits.

- Light snow accumulation will be possible across the higher mountain ridges. Some travel impacts could occur on mountain passes tonight and Tuesday night.

- Warming/drying trend for the second half of next week

UPDATE
Issued at 951 PM MDT Mon May 20 2024

Severe convection is continuing over the east central CWA at this time, mostly over Washington County. There is other convection over the rest of the eastern half of the plains. Hail up to 3 inches has been reported with the storms earlier this evening. Upstream over eastern Utah and western Colorado, there is a significant sized area of convection, mostly producing light rain associated with an upper trough axis moving eastward through Utah. I will make some pop, wind, sky and temperature grid adjustments based on current conditions and some of the high resolution models. I will make sure there are some pops over most of the CWA overnight. Models continue to show temperatures about 15 degrees F below normals for Tuesday's highs.

SHORT TERM /Through Tuesday/
Issued at 300 PM MDT Mon May 20 2024

SEVERE STORM POTENTIAL INCREASES THIS EVENING

While confidence grew earlier in the day regarding severe storm potential, there are still some uncertainties to deal with. First off, surface temperatures north of the warm front/developing Denver Convergence Vorticity Zone (DCVZ) were running a couple degrees cooler than forecast. Thus, CIN was holding on a little stronger, and instability was also weaker than expected, with MLCAPE as of 2 pm only reaching 250-500 J/kg from Denver onto the Palmer Divide. At the same time, it was noted very dry air aloft working in from the southwest, with single digit/lower teens dewpoints noted on the mountain tops through southwest Colorado.
This was serving as initial disruption of updrafts as well.

That said, we are now just starting to see convective development, and there will be further warming along/south of the convergence zone where there is more sunshine. Dewpoints were also starting to rise, with mid 40s dewpoints now back into southeast Denver, and near 50 dewpoints into pushing back already toward Fort Morgan, Akron, and Limon. Thus, with further heating and low level moisture advection (mainly along and south of the convergence zone) we should be able to see MLCAPES grow to around 1000, perhaps ~1500 J/kg. At the same time, shear is quite impressive, with HRRR general averages of an impressive 0-1 km helicity of 100-150 m2/s2 and 0-3 km helicity over 300 m2/s2. Updraft helicity swaths show high (>70%) probabilities of UH exceeding 150 once we move east into Washington County and points east and northeast.
Putting this all together would suggest potential for stronger, longer lived tornadoes than what we typically see in northeast Colorado, IF we get to those values mentioned above.

Meanwhile, north of the DCVZ/warm front, temperatures will remain cooler and we're starting to fill in with clouds. As expected, those areas roughly from Denver northward through Fort Collins will likely only see weaker isolated convection at least for this evening.

HRRR suggests not only that first round of storms firing along the developing DCVZ/warm front and then organizing east/northeast across the plains, but a second round of severe storms could also originate in southeast Wyoming and clip through the northeast plains late this evening in the 8 pm to midnight window. Those could be undercut by cooler air and thus have mainly a large hail threat, but still something to monitor closely.

For later tonight, there should be a round of showers and a couple storms with upper level support still present. Then, there will likely be a break of some sort tomorrow morning before another round of scattered showers and storms develops Tuesday afternoon across the mountains and spreads east over the plains.
This time, the threat of severe will be quite limited with MLCAPE less than 500 J/kg.

Finally, with each passing wave we'll see snow in the mountains above 8,500-9,000 feet. A couple inches and some slush will be possible on the high mountain passes with each passing wave.

LONG TERM /Tuesday night through Monday/
Issued at 300 PM MDT Mon May 20 2024

Ongoing scattered to numerous showers are expected to continue across northeastern Colorado Tuesday night. There a slight decrease in expected QPF amounts along the eastern plains but generally, most areas can expect totals between 0.30-0.80 inches. WPC Super ensemble indicates 0.20-0.50 over the high country which should lead to light snowfall for elevations above 9 thousand feet. Slick travel along higher mountains passes will continue through Wednesday morning for snowfall totals between 2-5 inches. A small amount of instability exist in the urban corridor thus isolated storms could produce small hail through Tuesday night.

Flow will return westerly overnight into Wednesday morning. Skies should clear by Wednesday morning but lingering low level clouds could occur depending on how much rain occurs. Afternoon highs on Wednesday should increase a few degrees compared to Tuesday. The urban corridor and plains increase between 63-72F. Mountains and valleys increase near 47-60F. Moisture becomes sparse along the eastern plains but GFS/NAM support MLCAPE values between 200-300 J/kg which could support an isolated shower or non-severe storm or two through early Wednesday evening.

There is strong agreement amongst model guidance on the next upper level trough arriving to southeast Wyoming Thursday afternoon but given this system is too far north, our region remain dry through the period. A cold front could sweep across the foothills and plains by Thursday night keep our low temperatures slightly cooler than normal. A series of shortwaves troughs from the northwest will impact the forecast area bringing afternoon scattered showers and storms Friday through this weekend. Temperatures will continue near normal through the period.

AVIATION /06Z TAFS through 06Z Wednesday/
Issued at 947 PM MDT Mon May 20 2024

Models show fairly strong northerly winds for DIA most of tonight.
Speeds are progged to relax Tuesday morning but continue upslope in direction. Normal drainage wind patterns are not expected tonight. With current observations and model cross sections in mind, will go with BKN-OVC010-020 for ceilings from 06Z into Wednesday morning around 15Z. There should not be any visibility issues overnight in spite of the Stratus.

BOU WATCHES/WARNINGS/ADVISORIES
None.

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