First fall cold fronts move through the area Saturday and again next week
Thursday, August 21, 2014
Currently, mid and high level clouds from tropical storm Lowell off the Baja cost have overspread the area, keeping temperatures cool in the wake of a storm system that moved through the area late Tuesday and Wednesday. Cool temperatures should minimize the threat of showers today before a complex area of low pressure currently located off the northwest coast affects our area later Friday and through Saturday.
As this low pressure system moves west, another area of low pressure left behind from the Tuesday night storm in southern California will be forced to move northeastward and will travel over our state by late in the day Friday. There is some model uncertainty with regards to the exact track of this system and that will affect whether the rain comes earlier or later Friday, but periods of heavy rain appear likely sometime Friday afternoon through early Saturday morning.
Following closely on the heels of this system, the main fall-like storm from the northwest will first produce showers later Saturday and then produce rapidly falling temperatures and likely heavy rain again as the main front moves through late in the afternoon or evening. Rain is forecast to persist for a significant portion of the night before clearing early Sunday morning, and there may even be a dusting of snow at the highest elevations of the Colorado Rockies.
Sunday should be unseasonably cool in the wake of our first fall front, with lots of sun early and perhaps a stray shower late. At this point, further model disagreement about the evolution of a trailing wave makes the forecast very uncertain. The European ECMWF hangs this wave back before finally moving it over our area mid to late week, while the American GFS keeps this wave progressive, moving it over our area around Tuesday.
In either case, another rainy and cool day looks to occur sometime during the next workweek. After this storm passes, the forecast points towards dry weather and seasonably warm temperatures as westerly flow cuts off the very persistent and long-lasting monsoon, which has been producing steady rains since the beginning of July.
Very wet Wednesday and Thursday before weekend drying
Tuesday, August 12, 2014
As discussed in last Thursday’s forecast, the monsoon pattern has been reinvigorated by southerly flow around the west side of the strong western ridge and will produce very wet conditions for our area later Wednesday through Thursday. Before that, another beautiful summer day is on tap for today with some more clouds than yesterday, especially in the afternoon, but with any precipitation staying likely staying south of us until tomorrow.
A subtropical wave currently over the Arizona - Mexico border is forecast to move first northward and then northeastward as it approaches our area tomorrow. This wave will be accompanied by very moist air and rain showers will likely begin ahead of this wave around noon or early afternoon tomorrow. Showers will increase and transition to periods of very heavy rain by later in the afternoon as this first wave moves over our area sometime Wednesday evening.
Coincidentally, a large area of low pressure currently over the northwest coast will move eastward, and it appears some of this energy will be absorbed by a second subtropical wave from the south, keeping the likelihood of continued heavy rain over our area for Thursday.
Conditions improve considerably by Friday as the subtropical wave moves to our east, but plenty of moisture left behind will fuel afternoon storms on Friday. By Saturday, the northwest low, still well north of us, moves eastward, bringing a much drier westerly flow over our area. There will be a chance of storms Saturday and Sunday afternoons as there will still be lingering moisture, but the westerly flow is forecast to end the monsoonal pattern for a while, bringing classic beautiful late summer weather to our area for the following workweek.
Less moisture this weekend before a weaker monsoon is re-established for next week
Thursday, August 7, 2014
Lingering moisture from the monsoonal surge this past week will keep the threat of storms for this afternoon, though the wet pattern will be reduced for much of the weekend as some dry air from the desert southwest moves over the area.
However, subtle waves in the mean flow will keep some threat of showers over the area, especially in the afternoons. Additionally, a weak wave moving along the Canadian border will increase the forcing Saturday afternoon and suppress the drier air southward for some possibly stronger storms then.
A piece of this wave will be left behind along the central west coast by late in the weekend, re-establishing the moist monsoonal pattern as southerly flow ahead of the wave drags up more moisture. This lingering wave is forecast to move over us sometime around Wednesday and Thursday, increasing the chance of more wetting rains.
This wave then phases with another wave well to our north as it moves east of us last in the workweek, carving out a longwave trough in the Great Plains and leaving us with a rapidly building ridge to our west. This scenario should lead to dry conditions heading into the following weekend, with seasonably cool temperatures as we will be in close proximity to the much cooler airmass to our east.
Temperatures warm for the weekend before another surge of monsoonal moisture for next week
Thursday, July 31, 2014
The moisture and cool air discussed in the Monday morning forecast verified nicely, and there is a slight chance of afternoon storms today as the main push of monsoonal moisture retreats south. However, the remnants of former hurricane Herman off the coast of Baja will keep some moisture over our area tomorrow, allowing for a greater chance of the typical afternoon storm then.
Most of the weekend is forecast to be considerably drier and warmer, however, the monsoonal pattern is re-energized late Sunday and through much of next week. This occurs as an easterly wave present in the east to west tropical flow over Mexico is entrained into the clockwise flow around the western ridge, moving moisture and upward forcing over our area, similar to what occurred this past Tuesday and Wednesday. Interestingly, this wave is forecast by some models to split into two pieces, with the first piece moving over our area on Monday leading to the threat of more heavy rain.
The second piece is forecast to merge with a separate wave moving into the Pacific northwest and move over our area midweek, again increasing the likelihood of heavy rain that may last until the end of the workweek. Finally, a different and further south Pacific wave is forecast by some models to cross the central west coast sometime around next weekend, with drying and warming expected as the flow veers from the south to the southwest ahead of the wave and drags some warm and dry air over our area.
Heavy rain likely for Tuesday and possibly Wednesday
Monday, July 28, 2014
After a typical summer day today, the remnants of a sub-tropical wave currently to our west and the monsoonal moisture plume already in place will conspire to make heavy rain likely over most of Colorado tomorrow. Since the wave is moving slowly and is composed of several pieces of energy, the threat of heavy rain will also be present for Wednesday.
An interesting development is that this sub-tropical wave looks to drag some cool air over our region on Wednesday as it interacts with the STILL present Hudson Bay vortex. This frontal boundary is forecast to persist for part of the day in our vicinity and may lead to more organized storms with heavy rain and possible hail on Wednesday. Some dry air behind the front should quickly end the storms after the front finally moves through the area, currently forecast for later that day.
Thursday should be a nice day, but another weaker wave is forecast for Friday which will likely increase the threat of heavy rain again. There may be some interaction between this wave and the currently weakening tropical cyclone Herman off the southern coast of Baja, so the details for Friday are uncertain. Again, this wave is forecast to be followed by some drier air which should make for a nice weekend if it occurs, though the monsoon looks to be re-established early in the following workweek.
