30-Day Weather Outlook for June 29, 2021, to July 15, 2021

  • Jun 29, 2021

PURPOSE OF THIS OUTLOOK:

This extended weather outlook is intended to provide a general scenario for conditions to be expected over the next three months. This provides a general summary of trends in weather.  Therefore, we use such expressions as above or below “normal.” “Normal” in a changing climate scenario is a changing quantity, given the warming and trend towards a longer dry season that has occurred in California.  Although the discussion of climate change is beyond the scope of this weekly summary, we mention it so that you know it is a consideration to keep in mind.  As a reference, the PRISM Climate Group at Oregon State University issues their update of the PRISM climate 30 year normals every 10 years.  The latest version in general use is for the period 1981-2010.  Later this year, the PRISM Climate Group will be issuing the newly computed 30 year normals of temperature and precipitation valid 1991-2020. Although the 30 year updates are prepared each decade, it is important to monitor changes in the normals. Bottom line, “normal” is not a constant, it is linked to a moment (albeit comprising 30 years). Reference: The PRISM Climate Group (prism.oregonstate.edu). 

BASIC PATTERN:

Summary- There still is not much change to the basic pattern. The La Niña remains through summer but it seems to continue to weaken with some warming of water temperatures off the coast of Peru.  Still cooler than normal water temperatures off California but slight warming has taken place.  While still classified as “La Niña”, this SSTA pattern has some differences from a true La Niña pattern.

Some salient characteristics are: 

a) Warm SSTA’s are along the midlatitude Chilean west coast extend westward along 30-40S to the Dateline  (across the South Pacific).

b) Water temperatures off Ecuador and Peru are warmer than normal from the coast to 130W.  This represents  a significant short term warming at the equator.

c) Sea surface temperatures off the coast of Northcentral to Southern California continue below normal.  Sea surface temperatures NORCAL and Pacific Northwest are warmer than normal.

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

e) The tendency for troughing just west of California this week is related to a still active Madden Julian Oscillation (MJO) pattern at our longitude...near 120W.

f) Fox Weather's IVTinit(tm) analyses over the past week continues to show the active  IVTinit™ pattern, and support for cyclogenetic activity in the central N Pacific. Troughing continues to focus NW of Hawaii at 38-47N, 150-170W. Cyclogenesis in this region favors some downstream troughing, albeit dry, over N and central California coast and just west from the coast.

g) High pressure still remain the dominant weather feature with some start to the monsoonal season starting into Arizona.  Some of these showers are possible into SE California at times through the middle of July with better showers as the monsoonal pattern strengthens from the middle of July through the rest of the month.

h) The monsoonal season is starting about 2 weeks early and the SE moist flow that brings the moisture into Arizona and at times into California appears to be stronger than normal. Bottom line...a wetter monsoon seasonal is likely for Arizona with near normal or slightly wetter than normal across SOCAL and possible the rest of the state.

 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 becomes more moist.  The summer monsoon is setting up with an upper high pressure building across eastern Mexico and the western Caribbean.  This is an early than normal start to the monsoon season and appears to be stronger than normal this year.  Above normal showers and thunderstorms are likely across much of Arizona with some of these showers and thunderstorms likely bringing a wetter monsoon season to SOCAL this season (July through early September).  Models show the SE monsoonal flow will increase around July 1 with better showers reaching westward into western Arizona and possibly SE California and the southern and central SierraNV.

Precipitation Trend – June 29-July 28 - The CFSDaily product that Fox Weather prepares from the CFSv2 short term climate model is suggesting light showers into SE areas of SOCAL with some thunderstorms mainly east of the Colorado River on Jun 29-30. Showers and thunderstorms are possible across the central and southern Sierra on the same dates. Another surge of monsoonal showers are expected Jul 13-14 with (SOCAL & the SierraNV). Other surges of monsoonal showers/TSTMS: Jul 18-21 (SOCAL & SierraNV) and Jul 25-28 (SOCAL, CENCAL & Sierra mountains).  Showers and thunderstorms will move farther northward into central California and even NORCAL late in July.  Frontal rain is not expected as the storm track is well north near the Canadian border with high pressure dominating the weather pattern.

In S California – June 29-July 28 - SOCAL Warm Spells, from models GFS and CFSv2 through CFSDailyAI: Warm to very warm air will continue Jun 29-30 as high pressure will dominate the weather pattern. Hotter Jul 9-12 with a stronger ridge/high pressure over the region. Other hot days Jul 17-20, 24-27.  No cool spells are expected.

Sierra Nevada: June 29-July 28 - Precipitation Trend:  Dry through most of the end of June though a few showers into the SierraNV are possible Jun 29-30.  Other surges of monsoonal showers and thunderstorms are possible Jul 13-15, 18-21 and 25-28.  A gradual increase in showers with some TSTMS will increase with better chances of showers/TSTMS from the middle through the end of the July.

Colder dates during June 29-July 28:  Little or no cold air is expected. Temperatures will remain well above normal including warmer than normal nights through most of the forecast period.  As some clouds increase towards the middle of July with occasional showers and possible TSTMS, there will be slight cooling but still above normal temperatures.

Fire risk June 29-July 28:  Best chances for enhanced fire risk would be from high atmospheric instability in dry air (Haines Index).  Higher risk of fire increases July 13-15 as monsoonal showers and TSTMS with lightning, and above normal temperatures are expected.  Other surges of monsoonal moisture will bring higher fire risk later in 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 June 29-July 28:    

 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 end of July.  Some pre-monsoonal showers could creep into the southern desert region early in July with better and more active surges of moisture from the middle through the end of July.  An active monsoonal season is expected this season from July through August and likely into September.

 ***Looking Ahead – Long Range Outlook Jul 29 - Sep 26***

 N and Central California. La Niña will continue weak, but starts to redevelop in late September.  We are seeing some weakening of La Niña just this last week.  High pressure will continue near the west coast and across much of northern Mexico and Arizona.  This will bring an early start to the monsoon season with a weak monsoonal surge into SOCAL Jun 29-30 and an increase in monsoonal moisture into Arizona and SOCAL Jul 13-15.  Better and more frequent surges of monsoonal showers and thunderstorms late July through the middle of August.  We are now into the hurricane season with upper lows in the tropics worth watching as many will develop into tropical systems.  Hurricane Enrique developed but has weakened to a tropical storm as it moves towards Cabo San Lucas today reaching land near the southern tip of Baja likely Wednesday.  Higher than average tropical systems are expected in the eastern Pacific this year as some warmer than normal SSTs are across the region, especially in Gulf of California.

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. Showers and TSTMS from tropical cyclone remnants become more likely from mid Aug through mid Sep. Subsequent development of a stronger La Niña is indicated by the NMME model during Oct-Dec.  The start of the monsoon into southern portions of California appears to be starting early but wetter conditions usually start late in July and peak in mid Aug. Showers and thunderstorms are more likely around Aug 1.  August should see showers and thunderstorms reach a peak about mid-month.  In August-September, monsoonal moisture becomes more likely to reach north in the flow around upper lows and troughs.  Showers and TSTMS appear to be about 10 days to 2 weeks early into Arizona this year with some early start into SE parts of SOCAL.  This provides opportunity for more lightning and also showers into Northern California mountains as well as the Sierras especially later in July into Aug.  A normal or stronger than normal monsoonal pattern may also produce heavier showers and TSTMS in the southern and central SierraNV in mid to late July – mid to late August, with tropical cyclone remnant moisture during about 20 August through mid September.  As we know, dry fuels and lightning are the most likely contributors to fires in mid to late summer.

For SOCAL Jul 29-Sep 26:  A weak but early surge of monsoonal showers Jun 29-30. Better surges of moisture Jul 13-15 with an increase in showers and thunderstorms likely late July peaking by mid Aug.  The monsoonal season appears to be stronger than normal and a bit wetter than normal, especially for Arizona with some increase in showers and TSTMS are possible for SOCAL.  The monsoonal season for SOCAL will likely continue but decrease late Aug thru early Sep with the end of the seasonal usually by the middle of Sep.

Jul 29-Sep 26...: Fire risk and potential for fire spread will be very high this summer in the SierraNV due to below normal rainfall and higher than normal temperatures.  A more active monsoonal pattern is appearing to set up and could bring higher than normal lightning and an increase in the fire risk mid-July – Aug.  High fire risk is most likely in mid-July through Aug with some lightning-induced fire starts. From mid-August to mid-September we are more likely to see precipitation events from  tropical cyclone remnants arriving from southern Baja or the Gulf of California.

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

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