The Next Storm/Next Snow Forecast Discussion from the Cherrywood Observatory – October 1st, 2018

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Figure 1: The forecast surface map for Monday PM. From NCEP.

Forecast Discussion:

Enjoy the last three days of late-summer weather.  It all ends this week.

Figure 1 is the forecast surface map for today. Tropical Storm (soon to be downgraded) Rosa is mushing (yes “mushing”) ashore in Mexico with rain pouring into Arizona and southern California. We have ‘some’ moisture already making it into western Colorado.  Models are showing the precipitatable water increasing every day this week until we are very moist by Wednesday.

The face of change is the trough on the west coast. The ridge out in the Pacific will grow to become a giant, giving Alaska record heat, and transition us into late Fall/early Winter like weather (Figure 2).  Once you see the upper air pattern (Figure 2) – take a look at the water vapor image (Figure 3) and you can see the same pattern in the flow of dry and moist air.

Over the next 5 days, the GFS “STILL” keeps us quite dry (only up to about 1/4 inch of rain) through Friday (Figure 4).  (More below in the longer range forecast…)

Figure 1: The forecast surface map for Monday PM. From NCEP.
Figure 2: The 500mb forecast upper air analysis for Monday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 3: The visible satellite image from Sunday PM from NOAA
Figure 4: The total precipitation between Sunday PM and Friday noon from the GFS and weather5280.com

The longer range forecast:

That trough in the west will grow and expand to dominate the western 1/2 of the nation (Figure 5). This means much cooler weather and greater rain chances. Looking at the GFS for the next 10 days (Figure 6) we approach 1 inch of rainfall over the next week plus.

You can see the big picture in Figure 7.  We stay in the lower 80’sF through Wednesday when another, strong, front approaches and turns on the cold upslope-flow machine. Rain and thunderstorm chances return.  This model (different from the GFS) gives us a similar, roughly 1/10th inch of rain by the weekend. We go off the cliff Wednesday night.  Each day gets colder and rain chances slowly increase through next week.  And, yes, Virginia, there is snow in the forecast for NEXT Monday/Tuesday.  Nighttime temperatures drop below freezing about 9 to 10 days out as well.

Figure 8 is a final look at Rosa.  It should still be a tropical depression in central AZ Tuesday night.  The moisture that heads to the northeast will join our front later this week.  Where did you put your coat, gloves, boots, and hat last year…?

Figure 5: The 500mb forecast upper air analysis for Monday AM. Pink dot is Longmont. Red lines are troughs, blue lines are ridges.
Figure 6: The total precipitation for the next 10 days between Sunday PM and Wednesday 10/10 noon from the GFS and weather5280.com
Figure 7: the next 10 days of the graphical forecast for Longmont, CO from weatherunderground.com
Figure 8: The 5 day forecast track and intensity map for Hurricane Rosa from the NHC and weatherunderground.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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