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Forecast Discussion:
Welcome to September. We cool down starting today, but, for reference, the latest we've hit 100F was around September 13th and the latest we've broken 90F was around October 17th. The heat is still possible. The normal high for today is 83F and the normal high for September 30th is 72F.
The upper air pattern is changing, but we are going to be plagued by quite dry surface air. The trough in the west takes over as the upper air pattern today (Figure 1). Moisture is moving in from the Gulf of Mexico and the Pacific ahead of the trough (green arrows Figure 2).
The surface forecast map for today does show a front that has entered northern Colorado (it is not as far south as it should be in Figure 3). Rain chances have returned today for most of the state. The problem is that even with upper air moisture in place (Figure 2) the surface remains quite dry. Figure 4 is the forecast dew point map (and wind direction) for this morning. The dew point is the temperature you need to cool the air down to to get condensation - dew or fog. Lower numbers are very dry (greens and blues) while higher numbers (yellows and oranges) are very moist. A rule of thumb cutoff for healthy storm chances and rainfall this time of year is around 50F. Longmont is in the upper 30's F for dew points. Storms that do form will see much of their rain evaporate before hitting the ground. Stronger winds and lightning would be more likely except where the stronger cells pass.
Figure 5 is the severe storm risk today. There is a marginal (1 on a scale of 1-5) just to the east, where the higher dew point values are.
The longer range forecast:
For Sunday, the first trough passes drying us out a bit more on the western side of the trough (Figure 6). Dry air begins to push further east (Figure 7). By Sunday night, it is a bit drier yet in Colorado (Figure 8).
Way off in mid week, the next trough in the southern branch of the jet stream is approaching making things unsettled (and cooler) - Figure 9. I've highlighted the (somewhat low) chances of rain in the afternoons Saturday, Sunday, Monday and Tuesday. Our next, pretty warm day, is out at next Saturday.
