In Brief:

A cold front arrived Thursday and will hang around for a day and a half. It will allow some moisture to return and may trigger severe weather (a Marginal Risk for Friday) with large hail. For Labor Day weekend, the front leaves and we bake again in the 90’sF Saturday-Monday. Go take advantage of that pool!

Forecast Discussion:

The cold front that arrived Thursday afternoon is draped down the Front Range foothills but the air in place is still very dry (Figure 1). This will get a bit of a reinforcement Friday with a bit more moisture returning. A short wave ripple in the upper level flow (Figure 2) will pass Friday increasing storm chances late morning through the evening (Figure 3).

The SPC has issued a Marginal Risk (1 on a scale of 1-5) for the eastern half of Colorado for Friday. Super cellular storms with large hail will be the main threat (Figure 4). Still, looking at the precipitation for the next 5 days, not much is expected to fall over a large region according to the GFS (Figure 5). Most people won’t get wet.

The Longer Range Forecast:

Our regularly scheduled summer returns on Saturday and things dry out for the long weekend (Figure 4). We might see some moisture return and temperatures go back to normal next week with a hint of a big cold front at the end of next week (that is a LONG way out though, so don’t get too excited).

What you can get excited about is Hurricane Dorian that is approaching the Bahamas and Florida for the weekend and start of next week (Figure 6). I’ll watch this closely (I like Walt Disney World and Universal Studios – so I hope they fare well ). This might make it to category 4 out of 5. It will be a very significant storm. It might also make it into the Gulf of Mexico and endanger the Gulf coast from Louisiana to Florida again.

The Month End Glimpse into September:

The climate models are trying to figure out the mean pressure patterns for September. If this output from the weather channel is correct, we’ll be near normal to a bit above normal for the month in temperatures (Figure 7). Precipitation is expected to be above normal for the month (Figure 8) – but what’s new looking back at this last year? I like it.

Figure 1: The forecast surface map for Friday midday from NCEP.
Figure 2: The 500mb forecast upper air analysis for Friday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 3: the graphical forecast for the next 10 days for Longmont, CO from weatherunderground.com
Figure 4: The severe storm weather forecast for the U.S. from the Storm Predication Center in Norman, OK. Made Thursday for Friday.
Figure 5: the forecast accumulated precipitation map from the GFS and weather5280.com for Colorado, for the next 5 days.
Figure 6: The hurricane Dorian 5 day forecast map from the NHC and weatherunderground.com
Figure 7: The September temperature outlook for the U.S. from weather.com
Figure 8: The September precipitation outlook for the U.S. from weather.com
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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 – www.strategies.org) . He is in his 14th year running this review. He is an astronomer (from the 2nd grade onward) and became a meteorologist (in the 5th grade) when a thunderstorm in Arizona rained on his telescope when the weather service had only forecasted a 10% chance of rain. He has college degrees in physics and astronomy and climatology and a graduate degree in meteorology and earth science. He lectures at the Little Thompson Observatory in Berthoud, the Estes Park Memorial Observatory in Estes Park, and for a number of online universities. He built and runs a backyard observatory near Pace and 17th in northeast Longmont where he has lived for 8 years with his wife, daughter, son, and two cats. Invitations to open house nights at this observatory, LTO, and EPMO will be posted with future discussions when they are scheduled. Forecasting severe weather and snow amounts via text lead to this column. He began texting friends about the weather right after the September 2013 flood. The readers of this column will, hopefully, keep him honest in what he ‘thought’ he had forecasted for ‘the most recent’ storm.

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