Thursday, July 30, 2015
Our warm days and cool nights will continue through the first half of the weekend. By later in the weekend, moisture to our south moves northward in a weak monsoonal surge and combines with a weak wave moving through the Great Basin and over our area late Sunday or Monday. As a result, storms will threaten our region by Sunday afternoon, with the threat persisting through Sunday night and into Monday.
The wave is east of us by Tuesday allowing drier air to spread into the the area. However, moisture left behind by the late-weekend surge will be recycled under the western ridge, leading to a slight chances of storms each day through the workweek.
By late in the workweek, though models have a strong Pacific storm approach the northwest coast, there is model disagreement on when this storm actually makes landfall. The American GFS is far more aggressive than the European ECMWF in bringing the storm eastward, advertising a strong monsoonal surge of moisture ahead of the storm that moves over our area as soon as Friday. The ECMWF, on the other hand, still has the storm off the coast by mid-weekend, leading to an uncertain forecast for next weekend.
Thursday, July 23, 2015
Dry air from the west has invaded the area leading to classic dry summertime weather with warm days and cool nights for the next week, except for possibly late Sunday and Monday. A storm crossing the northern half of the West Coast Sunday will back our winds to the southwest and allow some moisture to creep into Colorado by late in the weekend. There is some model disagreement on the northward extent of this moisture, but there is a chance for Sunday afternoon storms.
The West Coast storm will approach our area on Monday, dragging a strong, for this time of year, cold front across the region later Monday or Tuesday, as models have been trending slower with this storm. There may be showers ahead of and behind the front, but dry air quickly re-establishes itself in the post-frontal westerly flow by later Tuesday or Wednesday.
There is a fair bit of model uncertainty for next weekend as models are struggling with the evolution of northwest storms that may bring some cool air over the region and the re-establishment of the persistent monsoonal tap from the south.
Thursday, July 16, 2015
Our weather for later this weekend will be impacted by the evolution of two anomalous storms; an unseasonably strong and cold storm over the northern Rockies and hurricane Dolores presently located off the southern tip of the Baja peninsula.
The previous persistent monsoonal plume of moisture from the south has been severed by westerly flow ahead of the approaching storm to our north. Some drier air will move over the area in this westerly flow and lead to a decreased, but not insignificant chance of afternoon storms for Friday and Saturday.
The northern Rockies storm will shear as it drops southward tomorrow, leaving some energy in northern California and keeping most of its energy to our north and then east. Some cool air filtering into the region from the passing storm will reinforce our seasonably cool summer weather for the weekend.
There is a fair bit of model uncertainty as to how the energy left behind in northern California will progress. Current models eject some energy westward on Saturday, with this energy tapping some moisture from Dolores, but it is not clear whether this moisture will move far enough north to affect our area for Sunday.
Even though it appears remaining energy over northern California will further interact with the remains of Dolores, the moisture may stay south of us as some more dry air moves over our area in westerly flow. By around midweek, another strong storm will approach the Pacific coast and will back our winds to the southwest, allowing the monsoonal moisture tap to reestablish itself over our area.
Thursday, July 9, 2015
A storm currently located over northern California will eject a wave this afternoon that will travel over our area tonight and allow for another round of possibly heavy rainfall this afternoon and overnight. Significant drying is then forecast to begin Friday as dry air is dragged over Colorado by the storm as it moves northeastward across the Great Basin. However, residual moisture will fuel the possibility of afternoon storms tomorrow, though they will be much weaker and drop much less rain if they do form than the storms earlier this week.
Just in time for Rainbow Weekend and Art in the Park, a beautiful weekend looks to be in store as the dry air remains over our area through most of the weekend. Saturday and Sunday morning look to be rain-free, but a flat ridge in the southeast will move westward and draw moisture from the south over our area as soon as Sunday afternoon.
Unlike the pattern last week which I described as monsoonal-like, southerly flow around this ridge is representative of our classic monsoon pattern. If not by Sunday afternoon, then certainly by Monday the chance of afternoon storms once again producing locally heavy rainfall will return.
Another storm crosses the West Coast late in the weekend and stays to our northwest, but we will still feel its effects as bits of energy move over our area triggering or enhancing storms in our moist atmosphere around Tuesday.
The classic monsoonal pattern is forecast to persist into next weekend, leaving high chances for locally heavy rainfall each day. The forecast becomes uncertain by next weekend as some models show not only another strong storm approaching the West Coast, but also a hurricane moving northward along the Baja peninsula that may or may not interact with that storm.
Saturday, July 4, 2015
Similar to the last few days, there will be a chance of afternoon storms today, though with increasing atmospheric moisture there is a greater chance of wetting rains. As discussed in last week’s forecast, a strong cold front for the summer season is forecast to cross the area around sunset Sunday. Coincidentally, a weak wave from the southwest is forecast to cross over our area around that time or even as early as noon. The end result is showers should increase through the day as temperatures drop, especially later in the day, with some storms producing periods of heavy rain. Showers should continue overnight and into Monday morning.
The cold front will keep temperatures quite cool on Monday with continued showers. Again, some of these showers could produce locally heavy rainfall, though the cool air will moderate the strength of these storms.
Moisture will remain over our area for the following workweek, allowing for the possibility of storms each day though Friday that may produce continued locally heavy rainfall. Storms on Tuesday may be enhanced by another much weaker cold front moving though the area that day.
The forecast pattern is monsoonal-like, though the northward moisture transport in this case is not solely caused by the usual mechanism of upper level flow rotating around the eastern side of a strong western ridge. Instead, waves of energy moving over our area, including the cold fronts on Sunday and Tuesday, appear to be the primary driver of our wetter pattern.
Dry air is currently forecast to return to the area for next weekend as a storm from northern California drags dry air over our area as it passes to our north. This looks to end our wet week as a dry ridge is then forecast to build over the west, bringing a return to hot and dry weather.