7/31/06 FCST: CO

[Mark Farnik]

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.

 

[Stan Rose]

Mark: i don't think its very useful to compare the two events. There was a negatively tilted ridge to the east for the Thompson event; this allowed easterly flow of 35-50 kts in the low levels to smack the mtns. i dont see that happening tomorrow: the NAM shows NE at 15-20kts for 800mb. I'm not sure what relevance the summer 'pattern' from 30 yrs ago has either. That being said, you have a good eye for picking out the fact that there was an uncoming trough and cold front for that big event (although, not an uncommon summer occurence). Tomorrow most definitely bears watching for the flood potential, but mainly due to the boundary and high moisture levels. The GFS and NAM are still in disagreement: the NAM has 500 mb flow from the south at 15kt while GFS is west (partly due to timing) Both models pick up the flow in the evening as the trough gets closer.

 

[Mark Farnik]

The flash flood potential for this afternoon/evening still seems to be right on track as the forecast appears to be verifying. The Boulder AFD reflects this:

STORMS WILL BE SLOW MOVING...AND PRECIPITABLE WATER VALUES SHOULD
INCREASE INTO THE 1.25 TO 1.5 INCH RANGE IF SURGE OF LOW LEVEL
MOISTURE REACHES THE FRONT RANGE BY THIS EVENING. THIS WOULD LEAD
TO AN INCREASED THREAT OF FLASH FLOODING ESPECIALLY SINCE STORMS MAY
ANCHOR AND TRAIN IN/NEAR THE FRONT RANGE WITH DEEP EASTERLY UPSLOPE
FLOW AND LIGHT SOUTHWEST FLOW ALOFT. IF THE ABOVE SCENARIO LOOKS
MORE CONFIDENT THEN A FLASH FLOOD WATCH WOULD LIKELY BE REQUIRED
LATER TODAY.[/b]

This mornings HWO is also worded fairly strongly:
MOISTURE IN THE ATMOSPHERE WILL INCREASE SIGNIFICANTLY TODAY. IN
ADDITION...STORMS WILL BE SLOW MOVING SO HEAVY RAINFALL WILL BE
POSSIBLE. DEPENDING ON THE AMOUNT OF LOW LEVEL MOISTURE AVAILABLE...
SOME STORMS MAY BE CAPABLE OF PRODUCING FLOODING RAINFALL BY THIS
EVENING ESPECIALLY IN AND NEAR THE FRONT RANGE.

RESIDENTS AND PEOPLE PLANNING TRAVEL ACROSS NORTHEAST AND NORTH
CENTRAL COLORADO THIS AFTERNOON AND TONIGHT SHOULD REMAIN ALERT FOR
CHANGING WEATHER CONDITIONS. STAY TUNED TO THE NATIONAL WEATHER
SERVICE OR YOUR LOCAL NEWS MEDIA FOR ADDITIONAL WEATHER UPDATES AND
POSSIBLE FLASH FLOOD WATCHES.

All the models show the deep easterly flow pushing high 50's/low 60's dewpoints into the Front Range by 4 p.m. It also appears that the latest NAM and GFS have found a compromise with southwesterly flow under 10 kts. They are also showing much deeper easterly surface flow than they had previously shown. I would expect initiation of storms around late afternoon when the surge of deeper moisture slams up against the Front Range. From that point it just depends on where the storms develop. If any storm gets stuck up over any of the mountain canyons north of I-70 and east of the Continental Divide, and with minimal southwesterly flow aloft allowing storms to anchor and train over the same area for as much as several hours, I think that it is not out of the question we could see a significant flash flood event somewhere along the northern Front Range this evening. I just watched the "It Could Happen Tomorrow" episode last night on the possibility of a Big Thompson-like flood on Boulder Creek and the potential for destruction and loss of life in Boulder was very frightening indeed. :blink:

 

[Mark Farnik]

Ok, not crying wolf here, but I'm starting to get more than a little uneasy. The front is in the process of pushing through right now. Winds are out of the northeast at about twenty miles an hour. A one hundred eighty degree view of the horizon from north to east to south is rimmed with a thick band of opaque moisture. The normally bright blue sky is tinged a pale, hazy white because there is so much moisture in the atmosphere. There is already a cumulus field developing and the cloud bases are the darkest I've ever seen on cumulus clouds. The birds are utterly dead quiet. Both my parents say they feel a storm coming, and my mom says this morning's weather is strange and eerily parallel in nature to the weather of thirty years ago today. I called my grandparents and right off the bat they asked me if we're going to have bad weather today because they felt something was strange about the air this morning. The Boulder WFO seems to be on edge as well, as can be seen in the serious nature of the text of this morning's HWO and AFD in regards to flash flood potential. All my internal alarms are shrieking that something is not right, that there is something terribly wrong and foreboding with this weather. I'm going to be going up to see the new flood memorial west of Loveland as well as attending a seminar on the flood in Berthoud this evening; if I see a big storm building and stalling over the mountains, I'm getting out of there. And I'm not going anywhere near Boulder today; I just have a gut feeling that I should stay away from there. God help them all if a storm stalls out over Boulder Canyon tonight.