After a fairly quiet start to September, a sudden recent change in the weather pattern has now put most of eastern Colorado in the above normal precipitation category for the month (see below). In fact, some locations in the Southeast Plains have accumulated 2-3 times their normal precipitation. The vast majority of the precipitation has fallen in relatively orderly fashion and flooding has largely been avoided (just like in the summer, thankfully). West of the Continental Divide<\/span>, the San Juans have fared well with close to normal amounts, but other locations are below normal for September precipitation.<\/p>\n As shown in the Precipitable Water<\/span> (PW<\/span>) composite anomaly for the past 7 days, a large area of above normal moisture has existed in the Central and Northern Great Plains. Anomalies of up to 0.2-0.4 inches (the units below need to be multiplied by 0.04 to arrive at inches) have been found across OK, KS and NE. With even weak return (easterly) flow, pulses of this moisture have found their way into the Arkansas River valley, supporting the widespread<\/span> generous precipitation.<\/p>\n As shown in the water vapor image, below, we expect a continuation of a very active pattern over the next week or so. The current disturbance, presently located over the Four Corners, will trek slowly NNE within the large scale trough<\/span> in the subtropical jet stream<\/span>. It will continue to provide mainly light to perhaps moderate precipitation through this weekend (Event #1). However, overall, this large-scale trough<\/span> is not expected to move anywhere fast and a new shortwave<\/span>, currently in the Gulf of Alaska, will help keep the western US in an unsettled weather pattern. This is identified as Event #2.<\/p>\n Unfortunately, the forecast for Event #2 has a very large amount of uncertainty with it. This can be easily seen in the forecasted PW<\/span> plumes for Denver and Grand Junction, below. At both locations, not the sudden increase in spread beginning on Wednesday, 10\/4. This arises due to uncertainty in the southern extent of the new trough<\/span>, as well as how long of a return flow will exist from the Gulf of Mexico. In short, the range of PW<\/span> forecasts is from well below normal to near October record levels approaching or exceeding 1 inch at both locations! Due to the continuation of much above normal PW<\/span> in the Central Plains, a brief Elevated threat is warranted for eastern Colorado during Event #2. However, it is essential to stress that this may change. However, if a flood threat appears, we will be doing special Flood Threat Bulletin(s) as warranted.<\/p>\n Below we describe the two identified precipitation events in more detail.<\/p>\n No Apparent Flood Threat <\/strong><\/span>as precipitation coverage gradually steps down from Friday through Sunday<\/p>\n With enough moisture available, along with favorable dynamics, we expect pulses of rain and snow shower activity to continue through Sunday. The highest coverage will be Friday, favoring the upslope regions of the San Juans and Northern Mountains. Activity will subside by Saturday, though isolated<\/span> to scattered<\/span> showers will still be possible mainly over the higher terrain. By Sunday, there is a chance of enhanced precipitation coverage in far eastern Colorado as a warm sector is expected to setup here. At this time, the highest instability and low-level moisture is expected to remain in KS. Thus, while hourly rainfall rates up to 0.8 inches could occur along the KS border, this would not be enough to cause flooding concerns.<\/p>\n![\"\"](\"https:\/\/archive-coftb.dewberryanalytics.com\/wp-content\/uploads\/2017\/09\/threat_timeline-5.png\")
<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\nEvent #1: Friday (9\/29) through Sunday (10\/1)<\/strong><\/h3>\n
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