Sunrise 5:38 AM   Sunset 8:13 PM Moonrise 5:39 PM   Moonset 3:57 AM

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FXUS65 KBOU 201143 AFDBOU

Area Forecast Discussion National Weather Service Denver/Boulder CO 543 AM MDT Mon May 20 2024

KEY MESSAGES

- Strong to severe storms are expected Monday afternoon and evening across most of the eastern plains, with all hazards possible.

- Another round of widespread 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

SHORT TERM /Through tonight/
Issued at 311 AM MDT Mon May 20 2024

A complicated convective forecast is in store for today, with the potential for scattered to numerous strong to severe storms across most of the plains from this afternoon into the overnight hours.

Water vapor satellite and 500mb RAP analysis data show a positively tilted trough axis extending from southwestern Montana into Nevada this morning, with a southern stream shortwave just off the coast of southern California. Broad west-southwesterly flow is in place in the region ahead of these features, and the upper level flow is expected to strengthen later today as the trough approaches. Meanwhile, the southern stream shortwave is expected to race across the Four Corners region and eject into eastern Colorado by late tonight, with accompanying lee cyclogenesis also expected later today. As this surface low develops, east/southeasterly surface flow should gradually increase with moisture advecting into the eastern plains through the afternoon hours.

With these larger scale features in reasonably good agreement across guidance at this point, our attention turns more to the finer-scale details. The first and most pressing question is how quickly the airmass recovers behind this morning's cool front.
Surface dew points this AM are generally in the upper 30s to upper 40s across the plains, though across the border into KS/NE Td's are closer to 55-60F. High resolution guidance has been relatively inconsistent with the westward advection of the higher theta-e airmass into our CWA The NAM/NAMnest are notably cooler across the plains today with pronounced capping... while most other guidance warms the plains into the low/mid 70s with a quickly eroding cap noted by 00z.

This leads us into the second important question... which is the timing of convective initiation across the plains. This generally looks to occur during the late afternoon or early evening hours as the shortwave approaches and height falls become more pronounced, along with increasing surface convergence along a stalled frontal boundary. If CI occurs closer to 00z, there would be a narrow window for surface based convection before transitioning to a more elevated supercell environment later into the evening. Along the aforementioned boundary, forecast soundings are quite impressive, with strong deep-layer shear with elongated/curved hodographs as the low-level jet strengthens. Assuming guidance is close on the timing, this would support a few supercells capable of a tornado, along with large (possibly significant) hail and damaging wind gusts. We may also see an additional round or two of supercells later into the evening that would pose a hail risk well into the overnight hours in addition to the initial storms.

All in all, the SPC Enhanced Risk appears appropriate, though there are certainly failure modes (lack of destabilization and/or moisture advection) that would shift the severe threat out of our area. As is always the case, observational trends will be important to watch through the day today. However, this does appear to be the best severe threat for the area this spring.

Meanwhile, across the I-25 corridor, today should be cooler with highs in the low/mid 70s. With the best instability located to the east, any showers or storms that form this afternoon would likely be sub-severe but could pose a brief gusty wind threat. Elevated fire weather conditions are possible across portions of the high country as very dry air advects into those areas.

LONG TERM /Tuesday through Sunday/
Issued at 208 AM MDT Mon May 20 2024

There will be a shortwave trough over the far northeast corner Tuesday morning that will continue to provide forcing for convection that continues from Monday night. These showers and storms should depart Colorado by around 9-11am. There will be plenty of dry hours especially over the urban corridor as the morning through the early afternoon should be dry. However, a secondary shortwave trough with colder air aloft will begin moving across our forecast area during the mid afternoon. This will provide strong forcing and widespread precipitation is expected to form. This will fall as snow above 9-10 kft and some accumulation is possible across the higher mountain ridges and slopes. Some mountaintops could be coated with 3 or 4 inches of snow during this event. Some of the higher mountain passes and Trail Ridge Road will likely receive accumulating snowfall. There could be travel impacts especially Tuesday evening across the mountain passes but overall, the travel impacts will be limited to small areas.

As a result of the rainfall Monday night, multiple cold fronts, and low clouds Tuesday morning, high temperatures will stay in the low 60s or even upper 50s across the urban corridor and plains on Tuesday. This will limit instability and most unstable CAPE will struggle to get above 300-500 j/kg. Therefore, storms that form over the higher terrain and move onto the plains will likely remain below severe limits. However, the strong forcing and shear along with good moisture could lead to strong storms that have hail and gusty winds. Rainfall totals across much of the area will be around a quarter to half an inch.

The shortwave trough will depart our area Tuesday night with ridging building in on Wednesday. This will lead to warmer temperatures and mostly dry conditions. There could still be a few isolated showers and storms that develop.

Another shortwave trough will enter northwestern Colorado on Thursday. There will be strong southwesterly flow ahead of this trough and that will lead to warm conditions and possibly the return of 80 degree highs across the plains. A cold front associated with this trough will move across the northeast plains Thursday night which could create a line of storms. There is a low but non-zero chance that these storms could become severe and this threat will need to be looked into over the next few days.

There will continue to be plenty of troughs that make their way across the western US Friday through the weekend. Isolated to scattered showers and storms are possible each day as a result of these troughs.

AVIATION /12Z TAFS through 12Z Tuesday/
Issued at 539 AM MDT Mon May 20 2024

Northerly push arrived close to on schedule this morning. Some stratus has developed north of the terminals but satellite trends don't suggest that this will cause too many issues this morning.
There could still be a brief period of near MVFR cigs but not high enough confidence to include in the TAF. Winds today will slowly turn more to the east/southeast by the late morning or early afternoon with a gradual increase in speeds as well.

Still quite a bit of uncertainty in the forecast late this afternoon into tonight. Storm coverage again looks highest east of the terminals but there's a brief period between 21-00z today where storms could develop this far west. PROB30 still seems appropriate for this potential. Beyond that, a wind shift back to the north is likely sometime this evening along with an increase in low cloud cover. Poor model agreement with the timing and amount of low clouds and the current forecast is close to a middle ground. Potential is still there for several hours of MVFR/IFR cigs overnight depending how moisture plays out.

BOU WATCHES/WARNINGS/ADVISORIES
None.

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