I have decided to start this topic for a number of reasons. First and foremost, I discovered this when I was reading the Denver AFD this morning:
LATEST MODEL RUNS STILL SHOW UPPER RIDGE SHIFTING EASTWARD
ACROSS THE GREAT PLAINS...WHICH ALLOWS S-SWLY FLOW ALOFT TO ADVECT A
DEEP LAYER OF SUBTROPICAL MOISTURE UP FROM THE DESERT SOUTHWEST AND
SRN ROCKY MTNS ON MONDAY AND TUESDAY. MODELS ALSO SHOW A SHORTWAVE
EMBEDDED WITHIN THIS FLOW MOVING OUT OF SERN ARIZONA EARLY MONDAY
MORNING...ACROSS NRN NEW MEXICO BY LATE MONDAY AFTERNOON AND ACROSS
NERN COLORADO MONDAY NIGHT. MODELS ALSO SHOW A WEAK SFC COLD FRONT
DROPPING SWRD ACRS NERN COLORADO MONDAY MORNING AHEAD OF THE
SHORTWAVE. MODELS SHOW THIS FRONTAL BNDRY STALLING OUT ACROSS NEWRD
ACROSS NERN COLORADO...WITH A SFC LOW FORMING ALONG THIS FRONTAL
BNDRY IN THE PALMER DVD AREA. CIRCULATION AROUND THIS SFC LOW DRAWS
HIGHER DEWPOINT/THETA-E AIR UP FROM SERN COLORADO AND UP OVER THIS
FRONTAL BNDRY WHICH SHOULD PROVIDE THE FAVORABLE LIFT AND
INSTABILITY FOR STORM FORMATION ON THE PLAINS. AS PER THE WRF/NAM...
LAYER PRECIPITABLE WATER VALUES GO FROM 0.90 INCH AT DENVER AS OF
12Z/MONDAY TO 1.45 INCHES BY 00Z/TUE. GFS VALUES NOT AS GREAT...BUT
IT TOO SHOWS A SIMILAR UPWARD TREND. SLOW STORM MOTIONS AND
ABNORMALLY HIGH PRECIPITABLE WATER VALUES MAY PRODUCE HEAVY POSSIBLY
FLOODING RAINFALL MONDAY NIGHT.[/quote]
This last little bit especially caught my attention. As most of you know, tomorrow is the 30th anniversary of the devastating Big Thompson flash flood. I decided to go back and check the surface conditions for July 31, 1976 and compare them against the forecast for tomorrow evening. What I saw unsettled me: the forecasted surface and upper air features for tomorrow evening are nearly identical to those of July 31, 1976. Similarities include a stalled weak cold front oriented south/southwest to north/northeast across eastern Colorado, extremely moist easterly flow being pumped over the front and across the eastern plains and up against the foothills/mountains by a surface low forecast to set up over the Palmer Divide (dewpoints are progged by the WRF and NAM to be in the mid 60's by late afternoon, only slightly less by the GFS) and almost zero storm motions (500 through 850 mb winds are all going to be from the west under 10 kts through tomorrow night with weak surface flow out of the east around 15 kts, just enough to keep the storms from going anywhere if the forecast verifies). If a storm gets going in the mountains tomorrow, the 500-850 mb winds which would usually keep those storms moving east aren't going to be there, which is the same reason the Big Thompson storm remained anchored over the canyon for five hours.
Another disturbing similarity is that the spring and summer of 1976 had been dominated by heat and serious drought conditions. My mom, who was 16 at the time of the flood, says that the weather pattern so far this spring and summer of 2006 has been identical to the pattern preceding the flood in 1976. Now that is truly hair-raising. :blink:
Now I'm not trying to cry wolf here or say that we are going to have a Big Thomspon repeat tomorrow evening, but you cannot deny that the weather conditions are going to be eerily similar in nature. Based on what I'm seeing and what I know about flash flood causing weather patterns, there is a definite possibility that if the all the forecast surface and upper air conditions verify, we could see some sort of flash flood event somewhere in the foothills or mountains north of I-70 and east of the Continental Divide between 3 p.m. and 12 a.m. The severity of it would depend on how long any particular storm will stall over any particular drainage basin.
Another thing that worries me is a lot of people have gone up to the mountains to escape the heat wave down here. While there most likely aren't as many people up in the mountains as there were thirty years ago, I would imagine there are still quite a few people camping out up there. At least now, where Coloradoans learned their lesson from the Big Thomspon event, they now know to get up the mountainsides if the water starts rising. I think that fortunately since this is the flood anniversary weekend, people are going to be paying a bit more attention than usual to the skies.
This is something to keep a close eye on, most definitely.