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Monday Update: Longmont weather forecast: Less rain, slow warm up

In Brief:

We start the weekend with dry heat and begin the week wet and chilly, very wet.

Monday update:

Rain kept falling all day (after 7am) but Figure 2 update shows what fell at first.  Longmont received about 1/2 inch of liquid while most of Boulder broke the 1 inch mark and approached the 1.5 inch levels.

The forecast beyond today still looks good with smaller rain chances through Wednesday afternoon. There will be dry periods and a slow warm-up. Stay dry!


Figure2u_05032021Figure 2 update: the rainfall reports as of 7am Monday (rainfall fell all day- after this) from CoCoRaHS.


End Monday update.


Saturday update:

Before a second front arrives in the evening, we'll have some instability over the region from early afternoon to late afternoon. The Storm Prediction Center in Norman OK has painted us with a Marginal Risk of severe weather for the afternoon - with the primary risk being strong damaging thunderstorm winds and large hail (Figure 1 update). There is a slight risk (an elevated 2 out of a scale of 1-5) for severe weather just to our east.  Tornadoes are not a great risk this time, but not impossible. The GFS and other models have backed off on rainfall totals.  Longmont is now in the 1 to 1.5 inch zone by Thursday (Figure 2 update).

Overall, not much of a change.  Get ready to watch your lawn grow.


Figure1u_05012021Figure 1 update: the severe weather outlook for Sunday from the SPC in Norman, OK



Figure2u_05012021Figure 2 update: the rainfall total through Thursday noon from the GFS and


End Saturday update.

Forecast Discussion:

The two big stories are the extreme warmth Saturday then the potentially significant amount of rain we'll pick up early in the week. Figure 1 shows us reaching the upper 80s on Saturday with downslope winds, surface high pressure (Figure 2) and sinking air ahead of our next system, the trough seen to the west of our state in Figure 3. Temperatures across the state may be up to 20 degrees above normal. Then next cold front arrives Saturday evening, the first blue dashed line in Figure 1.


Figure1_04302021Figure 1: the 10 day graphical forecast for Longmont from weatherunderground


Figure2_04302021Figure 2: the surface forecast map for Saturday from NCEP.


Figure3_04302021Figure 3: the 500mb upper air map valid Saturday noon from the GFS and tropicaltidbits


Figure4_04302021Figure 4: the surface temperature departure from normal map for Saturday from



The Longer Range Forecast:

Saturday evening's cold front is followed by another wind shift and more significant north winds (Figure 1 - second dashed blue line). A cut-off low forms to the west of Colorado, slows down and pulls significant moisture toward us (Figure 5) as it takes about three days to drift across the state. Figure 1 shows rain chances picking up rapidly between 9 a.m. and noon Sunday. They taper off some Tuesday, but some rain will keep falling through Wednesday, with some thunderstorm activity thrown in.

The weatherunderground model in Figure 1 shows about 1.5 inches of rain. But the GFS is hitting us with 2.5-3 inches of water (Figure 6). The mountains may see a foot of snow in the higher elevations. We could see a dusting of snow late Monday night when temperatures cool enough (Figure 7). The GEM model (Figure 8) splits the rainfall difference with about 1.75 to 2 inches of water.




Figure5_04302021Figure 5: the 500mb upper air map valid Monday AM from the GFS and tropicaltidbits


Figure6_04302021Figure 6: the precipitation forecast from the GFS and through Thursday noon.


Figure7_04302021Figure 7: the 10:1 snowfall forecast from the GFS and through Thursday noon.


Figure8_04302021Figure 8: the precipitation forecast from the GEM and through Thursday noon.


About the Author: John Ensworth

John Ensworth works from Longmont as the Principle Investigator for the NASA Science Mission Directorate Earth and space science education product review through the IGES (The Institute for Global Environmental Strategies –
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Current Weather

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UV Index
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10 miles
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SSE 10.4 mph
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Wind Chill
79 °F

12 Hour Forecast

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7 AM
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7 Day Forecast

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84 °F

Partly sunny and warmer

Partly cloudy

Friday Night

50 °F

Partly cloudy

Partly sunny w/ t-storms


70 °F

Partly sunny and cooler; a stray afternoon thunderstorm


Saturday Night

39 °F

Mostly cloudy and cooler with a couple of showers and a thunderstorm, mainly early



54 °F

Cloudy and cooler; a thundershower in the afternoon


Sunday Night

41 °F

A few evening showers; otherwise, cloudy



47 °F

Chilly with periods of rain and a thunderstorm; there can be some wet snowflakes mixed in

Rain and snow

Monday Night

36 °F

Overcast; a mix of snow and rain in the evening followed by a rain or snow shower in spots late

Rain and snow


47 °F

Low clouds and chilly with a bit of rain; mixed with snow early in the day

Intermittent clouds

Tuesday Night

31 °F

Partly cloudy

Sunrise and Sunset

5:53 AM
8:01 PM

Based on AccuWeather data