30-Day Weather Outlook for July 15, 2021, to July 31, 2021

  • Jul 15, 2021


La Niña remains this week with some cooling of the water temperature off the coast of California. La Niña is expected to continue through the summer with some minor fluctuations in strength. A stronger La Nina pattern is indicated for mid fall onward to the start of winter.

Troughing continues in Northern California, and has been inducing deep marine layer with drizzle or light drizzly rain along the north coast. Otherwise, there is some of the usual morning drizzle along the central and S California coasts as well.

Sea surface temperatures off Ecuador and Peru are slightly warmer than normal from the coast to 130W. This represents some temporary weakening of La Niña in the near term (through most of September).

The NMME forecast model suggests subsequent strengthening of La Niña, reaching near maximum intensity and areal coverage in Dec 2021 – Feb 2022. La Niña winter seasons are typically dry in central and southern California, but wet for the NW California coast northward to Washington State.

Sea surface temperatures off the coast of Northcentral to Southern California continue below normal. Sea surface temperatures NORCAL are slightly cooler than normal.

The MJO continues in the positive (inactive) phase which supports upper high pressure in the Pacific near California, and over the SW U.S. We will continue to see heat waves in California, but continued lack of support for the usual monsoonal TSTMS that occur in mountains and deserts in California in summer.

Fox Weather's IVTinit™ analyses over the past week continue to show the active IVTinit™ pattern, and support for cyclogenetic activity in the central N Pacific. Troughing continues to focus N of Hawaii at 38-48N, 165-140W. Cyclogenesis in this region favors occasional downstream troughing in the Great Basin-Rockies areas. These conditions are typically dry and breezy for northern and central California.

High pressure remains the dominant weather feature for central and S California.


Above normal sea surface temperatures continue through the Gulf of California. This supports more moisture and risk of thunderstorms (TSTMS) as the airmass over and west of western Mexico continues to moisten. The summer monsoon is setting up with an upper high pressure building across eastern Mexico and the western Caribbean, as well as the southwestern US and central and S California. Above normal showers and thunderstorms are likely across W Mexico, and southern and southeast Arizona. A mainly normal rain season with this summer’s monsoon is indicated for SOCAL, and the Sierras. The precipitation anomaly map shows above normal rainfall for August in the northern Arizona Mogollon region, including the Salt River Watershed.

The driving factor for rains into NW Mexico and southern Arizona is not only moisture from the Gulf of Mexico and Caribbean, but active easterly troughs moving through western Mexico and reaching into Arizona. Tropical cyclones affect southern Mexico and far southern Baja California, but their moisture remains well south of California.

Precipitation Trend – July 15-Aug 15: 

The CFSDaily product that Fox Weather prepares from the CFSv2 short term climate model is keeping all of California dry through Jul 17. There are a few TSTMS or showers on 20th, 24th, and 31st. However, the main precipitation appears to focus in mid August, i.e. 14-19th across mountains of northern and central California, and in S California. CFSv2 temperature guidance suggests stagnant upper high pressure remains over all of central and S California during July 23 – August 2. Strong, stagnant warm upper high pressure tends to discourage TSTMs and showers near its center where the airmass is drier and more stable aloft.

In S California – July 15-Aug 15:

SOCAL Warm Spells, from models GFS and CFSv2 through CFSDailyAI: Very warm Jul 15-21 with hot conditions except immediate coast Jul 22-24. Warm to very warm Jul 25 - Aug 2. Very warm to hot Aug 3-7. Well above normal temperatures are expected through the middle of Aug as a strong high pressure will dominate the weather pattern. The very dry pattern will continue through most of Jul 23 with some showers into the SE corner of the state Jul 24-26 and a better surge of showers with some TSTMS Jul 27-28. Dry Jul 29 - Aug 5 with an increase in showers with some TSTMS Aug 6-15.

Sierra Nevada: July 15-Aug 15:

Precipitation Trend: A mostly dry pattern is expected through Jul 23. A surge of moisture will bring some showers and TSTMS Jul 25-28 into the Sierras. Dry Jul 29 - Aug 5 with another surge of moisture into the Sierra crest Aug 6-15. Showers and TSTMS are expected to increase around the middle of the month. Well above normal temperatures will continue through the region with high pressure over the area.

Fire risk July 15-Aug 15:

Best chances for enhanced fire risk would be from an increase of instability in a predominantly dry airmass (high Haines Index). Fire risk and potential fire severity increase late in July through mid Aug, especially as lightning incidence increases do to monsoonal showery periods.

GFS forecast guidance is showing an increase in tropical cyclone activity near southern Baja California during late July and early August, as many as three tropical cyclones near Baja California in 2-3 weeks. These storms moisture will likely not affect California but will enhance the high pressure that is already over the region. Monsoonal moisture will tend to remain south of SOCAL. This results in hot conditions, plenty of occurrence of dry lightning, and continued high risk conditions for fire starts and unusually dry conditions.

NORCAL and CENCAL will remain well above normal for temperatures with still very dry conditions through the start of Aug. Some surges of moisture into the Sierras were mentioned previously. An increase in moisture will migrate a bit north into CENCAL by the start of Aug with better chances of showers and some TSTMS by the middle of the month. Dry through most of NORCAL well into Aug with a few mountain showers in the Yolla Bolly, Trinity Alps and Siskiyou starting around the middle of the month.

The listing of dates normally included for hot and cold spells, and precipitation are based on our CFSDaily and CFSDailyAI forecast products, and present generally expected trends in precipitation (both products) and temperature (CFSDailyAI) to 4km. Our system gives some consideration of terrain and coastal influence. We consider the CFSv2 as one of the better ways to represent basic weather down in the sub-monthly time scale beyond the 15 day GFS or monthly maps from CFSv2 or NMME.

 Southern California Deserts Outlook for July 15-Aug 15:

Highlights: Very warm to hot through much of the period as high pressure will remain near or over the region. Little change to this pattern is expected through the start of Aug. Monsoonal TSTMS occur here and there across the S and E deserts with the first decent surge Jul 24-27. Another surge of moisture could bring some showers and TSTMS to the desert area Aug 6-8. Heavy rain events usually associated with the summer monsoon, will tend to be more localized than usual, with dry lightning being the primary risk condition. Per the normal seasonal trend, this dry TSTM pattern should change as we move into mid Aug, and as tropical cyclones are directed farther north. However, with sea surface temperatures below normal off the Baja coast, and west toward Hawaii, the dry trend in the monsoonal activity may be difficult to break in central and S California. It appears that the transition to more normally moist monsoonal conditions may occur mid to late Aug into Sep.

Looking Ahead – Long Range Outlook Aug 5 - Oct 5:

N and Central California: La Niña recurs intermittently Aug-early Oct. We expect La Niña to re-develop gradually late in September through the end of Fall and into the start of Winter. Tropical storms and hurricanes from Baja will typically come NW near or off the Baja coast. These will tend to turn WNW for the most part in Aug, then start recurving north into possibly N Baja during the first week or two of Sept. If this season turns active for tropical cyclones in Sept, we could see some showers coming north through SOCAL as troughs and cutoff lows begin to develop again during that period. These systems can pull moisture northward into central and N California. When the westerlies begin their seasonal re-strengthening and southward migration, typically after Sep 20, the end of the summer monsoon generally occurs quickly.

High pressure near the west coast of the U.S. and California continues as the dominant weather feature with very warm to hot temperatures across the state. Subsequent development of a stronger La Niña is indicated by the NMME model during Oct-Dec which will likely keep the high pressure in place, especially across SOCAL.

For SOCAL Aug 5-Oct 5: The monsoonal season will likely become briefly active in SOCAL. The most active period is usually between mid Aug – mid Sep. Monsoonal showers decrease quickly starting around Sep 20. In Sep there is a humid hot transition, with a chance for tropical cyclone remnant rains/showers/TSTMS to reach into the region.

Fire Risk, Aug 5-Oct 5: Fire risk and potential for fire spread will be very high this late Summer to early Fall in the SierraNV due to below normal rainfall and higher than normal temperatures. The current pattern and flow of moisture is expected to bring above normal thunderstorm frequency, increased lightning and an increase in the fire risk early in Aug through mid Sept. High fire risk is most likely in Aug with some lightning-induced fire starts. Note that above normal thunderstorm frequency and dry lightning activity can occur concurrently with below normal rainfall. Note also that rainfall can be below normal for the season as a whole, but punctuated with an above normal rain event associated with one tropical cyclone remnant. From mid August to late September we are more likely to see precipitation events from such tropical cyclone remnants. These arrive from the south, including southern Baja or near the Gulf of California, pass through SOCAL, and arrive into NORCAL and NE California, with localized heavy thunderstorm rains. Also, Santa Ana-type downslope wind events become more likely from mid Sept through Oct. These dry winds greatly enhance fires and cause further drying of vegetation.

Alan Fox & Zane Stephens...Fox Weather, LLC
Copyright © 2021, Fox Weather, LLC, Used by permission.

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