30-Day Weather Outlook for July 21, 2021, to August 6, 2021

  • Jul 21, 2021

BASIC PATTERN:

Large Scale Pattern –

The El Niño/Southern Oscillation (ENSO) has change to basically neutral phase along the equator. Some cold water continues along the California coast, but sea surface temperatures along Baja and Southern California have turned warmer than normal.  A stronger La Niña pattern is indicated to develop mid fall onward to the start of winter. 

Troughing continues near or off Northern California and Oregon-Washington at times. There is increasing troughing during the first part of August off the NORCAL and Pacific Northwest coast.  Some of this troughing and showers tries to reach into N California.  This is consistent with our current IVTinit pattern in the NE Pacific Ocean W of California as mentioned above. 

Sea surface temperatures off Ecuador and Peru continue warmer than normal mainly from the coast to 120W. This reflects the weakening of La Niña in the near term.

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.

There is a trend towards above normal rainfall in the central Rockies, Great Basin, and northcentral Sierras near Lake Tahoe for August.  

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, now extending from 135W out to the Dateline, mainly N of 40N.  Troughing continues to focus N and NE of Hawaii at 42-50N with active westerlies. Cyclogenesis in this region favors occasional downstream troughing at  the California coast. This combines with SE flow over Central and S California, to bring more tropical moisture and thunderstorms (TSTMS) into the Great Basin and Sierras to the NE Plateau.

High pressure and SE – S flow remain dominant for central and S California.

GENERAL FORECAST FOR 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 monsoonal pattern has developed, with upper high pressure over the southwestern US, and central to S California.  Moist easterly flow is in place across most of Mexico and southern Baja, with tropical cyclones continuing their normal July pattern.  Above normal showers and TSTMS are indicated in the CFSv2 monthly guidance for the mountains of N Arizona as well as SE and S Arizona.  Mainly normal rainfall for August is suggested for SOCAL, and a little above normal for the Sierras from summer thunderstorms. 

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 waves moving through western Mexico and reaching into Arizona. Tropical cyclones affect southern Mexico and far southern Baja California, but their moisture remains south of California. Monsoonal moisture will be having an increasing presence over California as we head into August.

Precipitation Trend – July 29-Aug 29

From the CFSDaily product that Fox Weather prepares from CFSv2… There are a few TSTMS or showers on 26th and 30-31 July to 1 Aug.  More showers in central Sierras occur 8-9 Aug and 23-24 Aug.  However, the main precipitation appears to focus in August 20-27 across mountains of northern and central California, and in S California. Showers and TSTMS are also suggested for SOCAL mountains and deserts during July 27 – Aug 3 and Aug 6. 

In S California – July 29-Aug 29

SOCAL Warm Spells, from models GFS and CFSv2 through CFSDailyAI: Warm to very warm across the state through Aug 21 as high pressure will continue to dominate the weather pattern.  Stronger high pressure with hotter temperatures Aug 6-12, 14-19 and 29-30.  Some cooling across N California from Aug 22-28 as a trough deepens into the region.  Some cooling but still warm across SOCAL with near normal to warm across CENCAL.

Sierra Nevada: July 29-Aug 29

Precipitation Trend: High pressure will remain but a gradual increase in monsoonal showers with some weak upper lows moving through will bring near normal rain to most of California.  Slightly above normal rainfall is expected across the SierraNV as a slightly higher risk of TSTMS are expected.  A cold front and trough will bring some rain to NORCAL Aug 8-9 (weak system with little rain. Better chances of NORCAL rain Aug 22-23 and 26 from a couple of troughs and cold front passages.

Fire risk July 29-Aug 29: 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 continue to increase in early Aug, especially as lightning incidence increases due to monsoonal TSTM periods.  These TSTMS usually peak in mid to late August through the region but still can bring decent showers and TSTMS even into the early and middle part of Sept.

The current eastern Pacific tropics are quiet but the forecast will develop a number of tropical systems in the region.  Through the middle of Aug these storms generally turn west away from Mexico and the SW US. By late Aug and especially in early to mid Sept these systems can have a more northerly track bringing potential enhanced moisture into northern Mexico, Arizona and even California.  Moisture from these storms may not affect California (especially in Aug), but will enhance the tendency for high pressure already over the region.  This results in hot and mostly dry conditions, plenty of occurrences of dry lightning, and continued high risk conditions for fire starts with the dry vegetation (fuels).

NORCAL and Central California will remain warmer than normal through Aug 21.  A series of troughs will bring some cooling at times.  A weak system will bring seasonal temperatures on Aug 8-9.  A deeper trough will bring seasonal to cool temperatures Aug 22-28.  Some light rain is possible from these troughs, especially with the frontal passage on Aug 22-23.  Some surges of moisture into the Sierras will occur.  An increase in moisture will migrate a bit north into Central California by the start of Aug, with better chances of showers and TSTMS by the middle of the month. Showers and some TSTMS will become more likely through the Yolla Bolly, Trinity Alps and Siskiyou ranges from the middle of Aug through the start of Sept.

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 29-Aug 29:

Highlights: Warm to very warm and occasionally hot through much of the period as high pressure remains near or just east of the region. Little change to this pattern is expected through the middle of Aug.  This will keep much warmer than normal temperatures across the region with a strong heat wave Aug 7-12, 14-19 and 29-31.  Monsoonal TSTMS occasionally will move into eastern areas but no showers of consequence are expected at the end of July.  A gradual increase in showers with some TSTMS in early Aug with the monsoonal flow moving more to the west into the SOCAL region by the middle of Aug.  We expect near normal precipitation across the desert region though brief higher than normal individual storms are likely on occasion.  This appears to be a typical monsoon with near normal SOCAL precipitation though eastern areas into Arizona should continue well above normal this season.  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.  With the much above normal sea surface temperatures across the Gulf of California some brief heavy rain events into SOCAL are possible.  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 which also takes place in mid to late Aug and early Sept.

Looking Ahead – Long Range Outlook Aug 19 - Oct 19

N and Central California:  La Niña should remain weak in a more neutral pattern through the start of Oct then redevelops, gradually strengthening from late Oct through the rest of the start of Dec.  Tropical storms and hurricanes near Baja will typically come NW near or off the Baja coast during Sept into early Oct.  These will tend to turn WNW in late Aug, then start recurving north into central or possibly N Baja during the first part of Sept.  These systems can pull moisture northward into central and N California in Sept or early Oct.  When the westerlies begin their seasonal re-strengthening and southward migration, typically in late Sept, the end of the summer monsoon generally occurs quickly.

High pressure near the west coast of the U.S. and California continues but starts to weaken, especially across NORCAL in late Sept and early part of Oct.  Continued warm to very warm and at times hot in middle Aug with some cooling with brief troughs into NORCAL in late Aug.  Sept should be typical with variations from seasonal to very warm across northern parts of the state and warm to very warm south areas.  Subsequent development of a stronger La Niña is indicated by the NMME model during Oct-Dec which will likely keep high pressure in place, across Central and S California.  A stronger La Niña will tend to bring wetter conditions to NW California and increased troughing into the Pacific Northwest and northern parts of NORCAL.

For SOCAL Aug 19-Oct 19: A late summer-type monsoonal pattern may become briefly active in SOCAL from mid Aug through mid Sept.  Monsoonal showers decrease quickly starting around Sep 20 with most showers ending by the end of the month.  Some tropical moisture is possible in Sept but usually ends by the early part of Oct.  Better chances of moisture from tropical systems are expected for SOCAL with less moisture into central and northern parts of the state.  Oct is usually a transitional month with a few monsoonal-type showers the first week and an increase in westerlies by the middle and end of the month.

Fire Risk……..Aug 19-Oct 19...: Fire risk and the potential for fire spread will be high this late summer and likely to continue through the middle of Fall in the Sierra Nevada due normal rainfall, somewhat higher than normal TSTMS and higher than normal temperatures.  The current pattern and flow of moisture is expected to produce above normal TSTM frequency, increased lightning and an increase in the fire risk in mid Aug through mid Sept. High fire risk is most likely in Aug with some lightning-induced fire starts.  Note that above normal TSTM frequency and dry lightning activity can occur concurrently with normal rainfall.  Note also that rainfall can be below normal for the season as a whole, but punctuated with an above normal rain events, associated with one tropical cyclone remnant.  From late Aug to late Sept we are more likely to see precipitation events from remnant tropical lows that move NNW into the region from near Baja and the Gulf of California.  Some of these systems can bring decent rain through SOCAL, and possible moisture 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 fire development, 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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