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The Next Storm/Next Snow Forecast Discussion from the Cherrywood Observatory – September 20, 2017

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

Forecast discussion:

In this dry time, waiting for fall rains and the fall chill to arrive, I thought it would be fun to watch the pacific upper air moisture arrive ahead of our dry cold front. Figure 1 shows the surface frontal position with the lift and moisture far out ahead of the front (in blue) at about 10am Tuesday. Figure 2 shows the same image at about 9pm.  That is a serious jump!  Winds were pretty hefty in the late afternoon in Longmont as well, but I didn't see anything up in the 40mph or higher range right in Longmont.  Figure 3 is the radar image for 10am Tuesday showing very little precipitation with the front, there was even less in the evening.

Figure 1: Water vapor satellite image showing approaching (dry) cold front in Utah and Wyoming at 9:45am Tue.

Figure 2: Water vapor satellite image showing approaching (dry) cold front in Colorado and Nebraska at 8:45pm Tue.

Figure 3: Radar image from 10am Tuesday.

The Long Range forecast:

The real excitement is concerning the approaching big upper trough, cold fronts, and the promise of precipitation. Figure 4 shows the graphical forecast with the front still rolling in later Friday.  Northeast winds and cooler air means upslope flow and chilly rain possibly all the way through Monday. Highs in the 50's Sunday and Monday will make everyone think Fall thoughts.

Figure 5 shows the real front on its way Thursday morning with snow and rain all over the West. Figure 6 shows the rainfall in Colorado Sunday  (with thunderstorms out in Kansas to Texas).  The upper air pattern for Sunday shows that big trough in Utah and Arizona in Figure 7.  This also shows us the pattern that might save the East Coast from Maria.  The remains of Jose should create a low pressure weakness to the northeast of Maria giving it a path to curve out squeezed between the big ridge on the East Coast and the Bermuda High to the east.  We'll see.

Speaking of the tropics, Figure 8 has the 5 day forecast for the remains of Jose - and it loops back to the west and (the ensemble models show) it heading back to the US - but in a very weakened state over the cold water. The 5 day forecast for Maria (a monster storm with 170mph sustained winds and gusts to 210mph headed right for Puerto Rico this evening) is shown in Figure 9.  Man, what a tropical system for the Atlantic so far!

Figure 4: Graphical forecast from Weatherunderground - 10 days out.

Figure 5: Thursday AM NCEP surface analysis forecast.

Figure 6: Weather.com rainfall map Sunday Sept 24.

Figure 7: GFS 500mb upper air forecast map Sunday Morning.

Figure 8: NHC 5 day Jose forecast.

Figure 9: NHC 5 day Maria forecast.

Current Weather

Partly sunny

Partly sunny

65°F

UV Index
4 Moderate
Pressure
29.81 Falling
Visibility
8 miles
Dewpoint
20 °F
Humidity
18%
Wind
SE 4 mph
Gust
10.8 mph
Wind Chill
65 °F

Hourly Forecast

Today
3 PM
63°F
Partly sunny
Today
4 PM
64°F
Partly sunny
Today
5 PM
63°F
Partly sunny
Today
6 PM
61°F
Partly sunny
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
44°F
Intermittent clouds
Tomorrow
1 AM
42°F
Intermittent clouds
Tomorrow
2 AM
40°F
Intermittent clouds

7 Day Forecast

Partly sunny

Thursday

64 °F

More sun than clouds


Intermittent clouds

Thursday Night

34 °F

Partly cloudy


Partly sunny w/ showers

Friday

54 °F

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


Showers

Friday Night

32 °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

36 °F

Becoming cloudy


Showers

Monday

48 °F

Cloudy and chilly 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