wxgeek's weather-Potential Spring storm Later This Wk-Update 4/17

Reports generally show 1-2 feet of snow in the foothills from our Spring storm, with 16-20 inches on Conifer Mountain. Upper low currently centered near Durango, and models in decent agreement in moving the upper low slowly to the northeast during the day today and into Saturday. Track by Saturday is somewhere between Denver and Pueblo. The farther south the low remains the more additional snow we will receive. Satellite imagery shows a dry slot across the eastern plains, but moist upslope flow remains along and west of I-25. Radar shows additional moderate to heavy precip moving into the foothills this morning. Latest WRF model shows precip continuing into early this afternoon for our foothills, then precip band begins to move north in response to upper low moving farther north, so we may see a break in precip from this afternoon into tonight, with just some showers at times. Snow level looks to remain in the 5500-6500 ft level possibly up to 7000 ft this afternoon. Looks like an additional 2-6 inches possible through this afternoon.

For Saturday, models suggest mostly showery conditions across eastern CO. Depending on the eventual track of upper low, some areas could see some wrap around precip on the back side of the upper low as it moves east Saturday afternoon and evening. Additional accumulations would be on the light side however with snow level remaining in the 6000-7000 ft range Saturday.

Models still suggest a weak upper trough moving down from the northwest Sunday afternoon into Monday morning, so some additional precip looks likely, with an additional 1-4 inches possible above 6000-7000 ft.

For next week, temps look to return to above seasonal normal values by Tuesday, but models suggest a chance for afternoon and evening showers and isolated thunderstorms Tuesday and Wednesday as atmosphere remains rather moist and slightly unstable with sow level in the 9000-10,000 ft level. By next Thursday, models suggest another upper level trough will move across the Desert Southwest and into CO from Thursday into next Sunday. Some significant precip looks to be possible across CO at times, with snow level lowering to 5000-6000 ft Friday into Sunday, so more wet and heavy Spring snow will be possible. Details on amounts will emerge next week. The good news is that we will not be talking about high fire danger for a while longer.

_________________
"Climatology is what you expect, Weather is what you get".

"It is better to be roughly right than precisely wrong".

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Upper low still centered just west of 4 Corners area, and latest 12Z models move it very slowly east next 24 hours, then begin to move it farther north during the day on Friday into central CO. Heaviest precip this morning has been mostly north if I-70 where up to 7 inches of snow has been reported in Larimer county, around 5 inches so far on Conifer Mountain. Radar showing a bit of a lull in the action locally, but latest NAM and WRF models suggest some very heavy precip will be possible this evening and tonight, specifically between 6 pm this evening and 6 am Friday morning. If models are correct in moving upper low into central CO during the day on Friday, then our area would likely see a break in the weather. However, models in general do not do a very good job of forecasting the movement of upper level cut-off lows. Because of their dynamic preference, models like to move things along, but cut-off lows sometimes like to just hang out in place for extended periods. We will see how our upper low moves tomorrow. Between now and Friday morning, expect some very heavy precip with snow level in the 5500-6500 ft range this evening and tonight. Roads will likely become snow and ice packed above 6500 ft, making the commute Friday morning difficult. We may see a break in the weather Friday, but if upper low remains stationary or farther south than models currently suggest, moderate precip will continue across eastern CO. There is also the chance for thunderstorms, with some reaching severe limits across the eastern plains this afternoon and evening, as well as farther east into western KS, OK and TX. Snow totals through Saturday morning still on track for 1-2 feet locally, and it appears amounts in the 2-3 foot range will be possible in the northern foothills.

_________________
"Climatology is what you expect, Weather is what you get".

"It is better to be roughly right than precisely wrong".

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- It has begun.

Upper level low currently spinning just west of the 4 Corners area. Models in general agreement on track of upper level low over the next 48 hours. Upper low is forecast to move slowly east along the CO/NM border through Friday morning, then models suggest center of upper low will drift northeast into central CO Friday afternoon and reside over southeast CO Saturday morning. this track will allow for pretty much continuous precip from the Divide east through Saturday morning. Heaviest precip looks to occur from this afternoon through Friday afternoon, with lighter precip from Friday afternoon through Saturday morning. Snow level this morning near 5000-5500 ft, but is expected to rise to 6000-7000 ft this afternoon. Snow level looks to remain in the 5500-6500 ft range through Saturday morning. WSW in effect through Saturday morning, and snow amounts still look to generally be in the 1-2 foot range for foothills and mountains east of the Divide above 6500 ft. There could be areas that get up to 3 feet of snow from this system, and models point to northern foothills of Boulder and Larimer counties as highest snow amounts. Roads remaining mostly wet this morning up to 8000 ft due to warm recent temps, but snow and ice packed roads will become likely by this afternoon down to about 7000 ft and through Saturday morning. Travel in the foothills and mountains will become difficult to impossible by later this afternoon through Saturday morning, so unless you absolutely have to travel, safer to stay put. Power outages may become common by Friday into Saturday as wet heavy snow accumulates on trees and power lines. If northeast winds remain steady enough, it may help prevent large accumulations on power lines and trees, but best to be prepared. Precip should begin to diminish by Friday evening becoming more showery into Saturday afternoon. System should finally move east by Saturday night and create some dynamic severe weather across the southern plains late Saturday into Sunday.

For CO, we won't see much of a break as next system moves in from the northwest Sunday into Monday, so more snow will be possible above 6000-7000 ft, with 2-5 inches of additional accumulation possible over the mountains and foothills.

Tuesday looks to be a break day with mostly dry conditions and temps near seasonal norms. Then an unsettled pattern will move into CO from late Wednesday into next weekend as an upper level trough moves slowly across CO. Snow level looks to be from 7000 to 9000 ft during most of this period. Models suggest additional precip chances through the end of April, which is very welcome news to diminish fire danger across the state.

_________________
"Climatology is what you expect, Weather is what you get".

"It is better to be roughly right than precisely wrong".

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Upper level low currently near Salt Lake City. Associated cold front is moving through the mountains of CO this morning and will move through eastern CO this afternoon. Some showers and isolated thunderstorms will accompany cold frontal passage, with best chances over the eastern plains this afternoon and evening. Temps will begin to lower post frontal passage, and some gusty northwest winds will be present in the wake of cold front.

The GFS has rejoined the consensus of model ranks today, so there is actually pretty good agreement now on track of upper level low over the next 72 hours. Upper low is forecast to move into the 4 Corners region by Thursday morning, then track very slowly east along the NM/CO border Friday into Saturday morning. This track is slightly farther north than past tracks and presents an optimum track for snow across eastern CO. Heaviest snow is forecast from the Divide to the foothills, and mostly north of the Palmer Divide into southeast WY. With this type of situation, what usually happens is upper low center moves very slowly, but lobes of energy (vorticity lobes) rotate around the upper low. The proximity of the vorticity lobes represent where the heaviest precip will occur. Then there still is the matter of the snow level. Latest models in general have slightly warmer air with this system, so snow level looks to drift between about 5500 and 7000 ft during the duration of this system, with lower levels overnight, rising during the day. So snow accumulation below 6000 ft will depend on when heavier snow bands move through. Systems like this also tend to have waves of moderate to heavy precip. There is also some indication by Friday into Saturday morning some Gulf of Mexico moisture could be advected into eastern CO. Models in general output from 1 to 2 inches of liquid precip over eastern CO from this afternoon through Saturday morning. Some models have up to 3 inches, so this now looks to be a solid snow event for the mountains of CO east of the Divide and foothills above 7000 ft. Lesser precip expected west of the Divide, and mostly a rain or rain/snow mix event for the Urban Corridor and plains. Either way precip falls, it is greatly appreciated for a parched state. So here is how I see the forecast unfolding:

Showers and isolated thunderstorms possible today into this evening as cold front moves across the state. Snow level about 8000-9000 ft through this evening. Overnight, precip will move into western CO as upper low moves towards the 4 Corners, so majority of precip will remain west of the Divide overnight with snow level lowering to near 6000 ft. During the day on Thursday, surface low forms over northeast NM and northeast upslope flow develops across eastern CO, so precip intensifies across the state including over eastern CO with snow level in the 6000-7000 ft range. Snow becomes moderate to heavy at time from late Thursday afternoon into Thursday night, especially in the foothills and mountains east of the Divide. Snow level lowers to 5500-6500 ft Thursday night. Models show a break in the moderate to heavy precip during the day on Friday, but I believe light snow will persist in the foothills and mountains as good upslope flow continues with snow level in the 6000-7000 ft range. Models then bring another surge of moderate to heavy precip into eastern CO Friday evening into Saturday morning with snow level again lowering to 5500-6500 ft. Precip looks to linger across eastern CO into Saturday afternoon, then begins to diminish Saturday evening and night. For snow amounts, here is what I see:

Western and Southwest Mountains above 7000 ft: 6-14 inches
Mountains East of Divide and Foothills above 7000 ft: 10-28 inches
Palmer Divide and Foothills below 7000 ft: 5-12 inches
Urban Corridor and plains: 3-7 inches with highest amounts across higher elevation areas west and south of the Metro Area

With this forecast now having better confidence, time to be prepared. Travel in the mountains and foothills will become difficult by Thursday afternoon and potentially impossible at times into Friday morning. Expect much longer travel times through Saturday morning. Power outages may become an issue as very wet and heavy Spring snow accumulates on trees and power lines, so be prepared for extended power outages.

Models still indicate more precip chances Sunday afternoon into Monday afternoon with snow level 6000-7000 ft. An additional 2-6 inches look possible across the mountains and foothills. Models then indicate more precip chances will be possible from next Wednesday through next Sunday as another upper level trough from the northwest interact with upper low from the southwest over CO. Areas of the mountains and foothills could see significant snowfall during this period.

_________________
"Climatology is what you expect, Weather is what you get".

"It is better to be roughly right than precisely wrong".

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Warm and breezy day statewide with much of CO under a Red Flag Warning today. Southerly winds of 15-30 mph common with highest winds across the eastern plains and western slope, combined with RH values below 15%. Foothills are not included in the RFW, but fire danger is high today. A chance for isolated showers late this afternoon and evening mostly from the Divide to about I-25.

On to the potential storm. Nealy all models (NAM, ECMWF, Canadian, GEM, UK Met) are in decent agreement now, except the GFS, which is mostly being ignored. Majority of models bring an upper level trough into the Great Basin today and then develop a closed and cut-off circulation along the CO/UT border Wednesday. System remains closed and cut-off near the 4 corners region Thursday, then slides into north central NM on Friday and into the TX panhandle Saturday. This is a favored track for a Spring snow event in CO. At the surface, cold front will move across CO Wednesday afternoon with showers developing along front, and some isolated thunderstorms will be possible. Snow level Wednesday afternoon will be near 8000 ft, but lower to 6000 ft by evening. Any accumulation of snow Wednesday expected to be light, although brief heavy showers will be possible. Precip is expected to continue Wednesday night into Thursday morning as surface low develops near the TX panhandle with northeast upslope flow across eastern CO. Snow level lowers to near 5000 ft by Thursday morning. Precip is then expected to persist across much of CO through the day on Thursday as well as Friday. Snow level is expected to hover between 5000 and 6000 ft during this period, so snow amounts below 6000 ft will be questionable and variable. Heaviest snow looks to be from Thursday afternoon through Friday evening. Snow may persist into early Saturday morning then diminish by Saturday afternoon. As for snow amounts, models provide a wide range of possibilities. The GSF being the outlier and lowest amounts of 2-6 inches. I will use the NAM as my preferred amount guidance as it is the middle of the ranges. NAM outputs 1-2 inches of liquid equivalent from Wednesday afternoon through Friday night, which would roughly translate to 1-2 feet of snow above 7000 ft. The ECMWF model, which has remained the most consistent through this event, outputs from 2-4 inches of liquid equivalent which would provide 2-4 feet of snow. The eventual outcome will of course depend on the actual track and intensity of the upper level low. Confidence is higher in the suggested track above, but given the GFS is in such stark disagreement, still some uncertainty. My instinct is to use the NAM model for guidance at this point and suggest the foothills will receive 1-2 feet of snow from Wednesday afternoon into Saturday morning, with the potential for up to 3 feet of snow. Heaviest snow will be from the Divide to the foothills above 7000 ft, although most of CO will receive some precip from this system. Once an upper level system does become cut off from the main flow, models have a difficult time determining where they will go and how long they hang around. This was the case in the March 2003 system when an upper low became cut off and remained nearly stationary for 48 hours over southwest and south central CO. Best at this point to be prepared for the worst case scenario, as extended power outages can occur during Spring snow events due to the wet and heavy nature of the snow. Travel may also become difficult to impossible in places. Below 5500-6000 ft this event will mostly be a rain or melted snow event, so impacts expected to be less.

Another system is expected to move across CO Sunday into Monday with some precip likely and snow level 7000-8000 ft, although any snow amounts would be light.

It is hard to put much faith in extend range models this time of year, but they suggest the possibility of another fairly significant precip event across CO late next week. Will have to wait and see how models handle that.

_________________
"Climatology is what you expect, Weather is what you get".

"It is better to be roughly right than precisely wrong".

---

Lovely day across the state today, unless you consider heightened fire danger a problem. Temps remain well above seasonal norms, but winds remain relatively light today. That is expected to change on Tuesday as system approaches from the northwest. Strong southerly winds are expected to develop across much of CO Tuesday, along with very warm temps and low RH values, thus Red Flag Warnings have been issued for Tuesday afternoon and evening for much of CO. Currently foothills are not included in the RFW, but fire danger is high.

On to the upcoming system for later this week. As expected, models beginning to vacillate and provide a plethora of possible solutions. GFS today has the upper trough splitting and elongating and moving through quickly. NAM has begun to lean towards the GFS, but since it is initialized from the GSF, not surprising. The ECMWF and UK Met as well as Canadian models have kept the track similar to past 48 hours, developing a cut off low near the 4 Corners and northern NM through Saturday. Consensus in the local NWS office is to discard the GFS solution for the time being, but the range of possible solutions makes a definite forecast rather difficult, hence forecast confidence is quite low at this time. If the GFS and NAM verify, CO would receive very little precip. If the ECMWF model verifies, we would see a significant snowstorm across eastern CO. I am tending to favor the ECMWF model since it has been the most consistent of models on the track of this system, and cut off upper lows tend to be favored during Spring months. However, we will have to wait and see how future model runs handle the track of this system before we have better confidence in the solution. Even if the system tends towards the ECMWF solution, if the track goes too far south as some ensemble members suggest, precip could remain too far south in NM. So, lots of variables with the upcoming system. Up until Wednesday morning, models are in good agreement, so forecast through Wednesday morning would have a chance for showers Tuesday afternoon and evening, mostly over the mountains, but foothills could see a stray shower. As cold front moves across CO early Wednesday, showers likely to develop along front with precip spreading into eastern CO by Wednesday afternoon and evening, as snow level drops to 5000-6000 ft by Wednesday evening. From there, lots of possibilities. GFS and NAM would have some precip into Thursday afternoon, but very limited. ECMWF would have moderate to heavy snow develop across much of CO Thursday and persist at least into Friday night, possibly Saturday. We will have to wait and see what transpires. Hopefully better consensus tomorrow.

Both models have another chance for precip Sunday into Monday as a system drops out of the northwest across CO with snow level 7000-8000 ft.

_________________
"Climatology is what you expect, Weather is what you get".

"It is better to be roughly right than precisely wrong".

---

Mild and breezy day for most of the state today. Weak cold front skirted across northern CO early this morning, so temps a bit cooler than yesterday and moderate westerly winds. Red Flag Warning is in effect across much of northeast CO today.

It could be an interesting week. The week will start out dry and mild Monday into Tuesday. Temps should remain above seasonal norms both days. Some showers and isolated thunderstorms may be possible Tuesday afternoon and evening from the Divide to about I-25. Some gusty southerly winds also likely to develop Tuesday afternoon, which will increase fire danger once again.

Wednesday will potentially be the beginning of en extended and significant Spring snowstorm across much of CO. Both GFS and ECMWF models from today have the track of the upper level system coming over UT Wednesday, then system becomes a closed circulation over the 4 Corners region Thursday morning, then moves to northeast NM Friday morning with system remaining nearly stationary over northeast NM through Saturday morning, before moving into western TX Sunday. At the surface, a cold front will across CO during the day on Wednesday, with a surface low developing over southeast CO. This will create much cooler temps and begin northeast upslope flow by Wednesday evening. Precip begins Wednesday afternoon and evening across northeast CO with snow level around 7000-8000 ft initially. Precip expands and intensifies over most of CO Wednesday night with snow level lowering to plains level overnight. Precip then continues from Thursday morning through about Saturday morning to afternoon with snow level hovering between 5000 and 6000 ft. There could be periods of moderate to heavy snow during much of this period. Models continue to output from 2 to 4 inches of liquid equivalent from Wednesday afternoon through Saturday afternoon. Using a Spring ratio of about 10:1 for snow to liquid, you can see this has the potential to be a significant event. However, as with all model forecasts, the track of the system could change between now and later this week. If this system either tracks farther north or south, CO could see substantially less precip. The good news is that models have been somewhat consistent with this system the past 48-72 hours and the GFS and European model are in good agreement. I will continue to monitor and provide updates as we get closer. If this system validates what models currently suggest, travel could become very difficult across much of CO, and the potential for extended power outages would exist.

Models then suggest a chance for some additional precip next weekend Sunday into Monday, but that will depend on ultimate track of the system this week.

_________________
"Climatology is what you expect, Weather is what you get".

"It is better to be roughly right than precisely wrong".

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