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Update 10:30am: The Next Storm/Next Snow Forecast Discussion from the Cherrywood Observatory – June 19, 2018

This content was originally published by the Longmont Observer and is licensed under a Creative Commons license.

Forecast Discussion:

Update 6/19 10:30am

With yesterday’s severe weather as an indicator of how favorable the environment is, today will see even greater severe weather chances. The morning data shows a short wave trough timed well to cross the foothills and I-25 corridor in the mid- late-afternoon when maximum daytime heating is in place. Some models show just a few, isolated, severe storms, other model runs show multiple waves of storm initiation in the foothills moving east and becoming supercell storms with very large hail and a few tornadoes + strong damaging winds. The higher foothills are under a marginal risk (1 on scale of 1-5) including Estes Park. The rest of I-25 corridor is under a slight risk (2 on scale of 1-5). Southeast Denver and off to the east and south of Denver are under an Enhanced risk (3 on a scale of 1-5). (Figure 1 update). Figure 2 update shows that there is a 2% chance of a tornado within 25 miles of any given point in Longmont this afternoon and evening.  Figure 3 shows a 15% chance of hail larger than 1 inch in diameter within 25 miles of a point on town for this same time.

The only thing that will limit or hold off storm formation will be a possible persistent low cloud layer over the region. IF you see sunshine a good portion of the day, then storm chances will be better in town. Remember, lighting strikes (that will be frequent with these storms) can, and do kill many people every year.

Figure 1 update: the day 1 SPC thunderstorm and severe weather forecast made Tuesday (valid Tuesday).

Figure 2 update: the day 1 SPC tornado forecast made Tuesday (valid Tuesday).

Figure 3 update: the day 1 SPC large hail forecast made Tuesday (valid Tuesday).

End update 6/19 10:30m

The front and moisture paid off with severe weather and a slew of severe thunderstorm watches and warnings Monday Night. Figure 1 and 2 are a couple examples of hail that fell just south of Longmont last evening (thanks to twitter).

I promised to watch the moisture flow increase - Figure 3 is the sounding again with precipitatable water highlighted.

Going into Monday (Sunday PM), it was up at 1.24 inches of water.

It was 0.47 inches Thursday AM.

The Thursday PM vale was 0.67 inches.

Friday PM was 0.70 inches.

The tropical systems are out of the way (absorbed into the mid-latitude jet stream flow). The Gulf of Mexico fetch is being entrained up and around the cutoff low in the west that is also keeping us quite cool (below normal temperatures). Figure 4 shows the big spiral around the low. The dry notch of air that robbed Colorado of much of its expected rainfall is now over the southeast corner of the state and into Kansas and parts of the western Midwest. The 416 fire didn't get much water dumped on it, but it is up to 35% contained Monday PM thanks to calmer winds, cooler temperatures, and higher humidity levels.

With the low just to our west, and moisture still in place, we'll see another round of thunderstorms up and down the I-25 corridor this afternoon.  A marginal risk is forecasted for Longmont (1 on a scale of 1-5 in severity) for hail, high winds, and a few tornadoes (possible) - Figure 5.

Figure 1: Twitter posted picture of hail in Lafayette, CO Monday PM.

Figure 2: Twitter posted picture of hail in Boulder, CO Monday PM.

Figure 3: The sounding from Sunday PM with precipitatable water highlighted for comparison with prior dates.

Figure 4: the water vapor satellite image from Monday PM. Reds/Oranges are dry air, greys/whites are moist air regions. Upper level air flow are light blue arrow. Western upper level low is a red L, trough is a red line.

Figure 5: the day 2 SPC thunderstorm and severe weather forecast made Monday (valid Tuesday).

The longer range forecast:

The atmosphere begins to come to equilibrium as the western trough moves east and the severe chances move out of the state Wednesday. We still have a chance of afternoon thunderstorms as temperatures begin to rise again (but only into the 80'sF eventually).

The precipitation expected through Wednesday noon (Figure 7) is confined to the northeast and eastern portions of the state.  Longmont (pink dot, usually, on my maps) is sitting around 1/4 inch of rain by the middle of the week. So much for 1-2 inches of water!

Figure 6: the day 3 SPC thunderstorm and severe weather forecast made Monday (valid Wednesday).

Figure 7: total precipitation from the GFS between Monday PM and Wednesday midday. and weather5280.com.

Current Weather

Mostly cloudy

Mostly cloudy

62°F

UV Index
1 Low
Pressure
29.74 Falling
Visibility
8 miles
Dewpoint
23 °F
Humidity
23%
Wind
W 4.9 mph
Gust
12.7 mph
Wind Chill
62 °F

Hourly Forecast

Today
7 PM
58°F
Partly sunny
Today
8 PM
55°F
Intermittent clouds
Today
9 PM
51°F
Intermittent clouds
Today
10 PM
48°F
Intermittent clouds
Today
11 PM
45°F
Intermittent clouds
Tomorrow
12 AM
43°F
Intermittent clouds
Tomorrow
1 AM
41°F
Intermittent clouds
Tomorrow
2 AM
39°F
Intermittent clouds
Tomorrow
3 AM
38°F
Intermittent clouds
Tomorrow
4 AM
37°F
Intermittent clouds
Tomorrow
5 AM
36°F
Intermittent clouds
Tomorrow
6 AM
34°F
Intermittent clouds

7 Day Forecast

Partly sunny

Thursday

65 °F

Partly sunny


Intermittent clouds

Thursday Night

34 °F

Partly cloudy


Partly sunny w/ showers

Friday

53 °F

Cooler with times of clouds and sun; a couple of showers late in the afternoon


Showers

Friday Night

33 °F

A couple of showers in the evening; otherwise, cloudy


Mostly cloudy

Saturday

60 °F

Mostly cloudy


Intermittent clouds

Saturday Night

36 °F

Partly cloudy


Intermittent clouds

Sunday

62 °F

Times of sun and clouds


Mostly cloudy

Sunday Night

33 °F

Becoming cloudy


Showers

Monday

52 °F

Cloudy and cooler with occasional rain; windy in the afternoon


Intermittent clouds

Monday Night

29 °F

Windy in the evening; partly cloudy


Sunrise and Sunset

Sunrise
6:50 AM
Sunset
7:22 PM

Based on AccuWeather data