Large Scale Summary
- Support for troughing continues in the central N Pacific from about 135-145W (mainly NE of Hawaii). During the period 29 July ñ 15 August, four tropical cyclones were suggested in the GFS forecast issued on morning 28 July, and currently 3 cyclones for 3-15 Sep. Recurrent, although irregularly occurring episodes of high pressure aloft enable hot conditions to occur during late July through August.
- The near term precipitation guidance (GFS-model based) shows mostly dry TSTMS at end of July in N and Central California, focusing on the Sierras, Trinity Co, and the Siskiyou Mountains. Dry lightning risk is indicated by NWS warning products, and we agree with their evaluation. The best chance for rains appears to be 3-7 September, as well as 28-30 August.
- Monsoonal moisture and tropical cyclones over southern and southwest Mexico to southern Baja usually produce most of the rains during this mid to late summer period. Sea surface temperature anomalies have continued to increase as we head towards August. There is no consistent pattern favoring flow of moisture from western Mexico into SOCAL, so showers continue widely-scattered in the nearer term. Moisture is either carried N into Arizona, or NW-ward out into the subtropical Pacific west of central Baja / SOCAL to Pt Conception. We note that periods of well above average temperature are suggested in the NOAA/NCEP/CPC guidance for most of August.
- Heat wave conditions occur in the Sierra Nevada west slope areas from Plumas Co south onward to late August. Below average rainfall is expected in Sierras from a few thunderstorms (TSTMS). Most TSTMS will be generated by circulations forced by tropical cyclones or easterly waves affecting the Gulf of California and S Baja.
- Rain dates and areas (Aug): NORCAL mountains and Plateau: Aug 21-23, 31st-Sep 3 and Sep 3-7, Central Sierra Aug 21-23, 29-Sep 3 and best chance for rains from TSTMS or tropical cyclone remnants 3-7 Sept. Currently rain appears unlikely for central Sierras/Yosemite NP area during most of Aug until 21-23rd.
Near Term Forecast
- Hot and dry throughout California except immediate coast. Risk of thunderstorms (TSTMS) to California coastal mountains
Humboldt, Trinity, and Mendocino Counties east of the coastal valley areas. These include Ukiah Valley, Larabee and intermediate valleys. In east portion: Sierra west slopes. - NORTHERN CALIFORNIA
- Rain Dates in Siskiyou Mountains: NE Plateau/Modoc/N Lassen Coís, N Sierras: Jul 30, Aug 17-18, 30, Sep 1-3,3-7. Rain Dates N to Central Sierras are: Rains (from TSTMS) occur Aug 17-18, 28-31.Sep 1-3, and Sep 3-7.
- Hot/Dry Periods: in NORCAL and Central California due to occasional subtropical lows forming near but W-WSW of SOCAL. There is still some risk of mostly dry thunderstorms (TSTMS). TSTMS will tend to be accompanied by gusty winds 15-20 mph, minimal showers, and local cooling during the TSTM periods. Watch for dry lightning in the brushy/dry grass areas, and fickle/sudden increases of wind speed.
- CENTRAL CALIFORNIA
- N Sierra Precipitation: July 30,Aug 17-18, 30-31.Sep 1-3, and 3-7th.
- Central Sierra Precipitation: TSTMS July on 30, Aug 17-18, 30-31.Sep 1-3, 3-7.
- CENTRAL CALIFORNIA HOT DRY PERIODS: Above average temperatures occur with a few thunderstorms (TSTMS) in mountains. Sparse showers are suggested for Aug 30-31-Sep 1-3. High fire growth risk in Central Sierras July 30 and Aug 1-3, and S Sierras 28,30,Aug 1-3 and 3-7th are suggested by the prevailing airmass type. Strong fickle and gusty winds may occur together during the periods indicated for TSTMS and sparse showers. Watch for dry lightning in the hills, fickle, sometimes sudden and strong gusty winds, and of course the dry grasses.
- SOUTHERN CALIFORNIA
- SOCAL PRECIPITATION: Early morning fog/stratus, will be followed by early burnoffs and dry days. Moderate fickle and gusty winds may occur together during the periods indicated for light coastal precipitation. The best chance for significant TSTM events with local brief small showers may occur in a few TSTMS July 30,Aug 17-18, Aug 25-31, Sep 1-3, Sep 4-7. San Diego Co, San Jacinto mountain areas, and the eastern San Bernardino Mountains are those mountain areas most likely to receive local TSTM events with brief showers. Tropical cyclone moisture may reach into Arizona, then flow will resume from the SE and E, bringing showers into SOCAL mountain and desert areas on a few occasions, with local cooling during rain periods.
- SOCAL WARMER OR HOT PERIODS: Troughs will help keep the coastal areas less hot than the inland valleys (Escondido, Ramona, Hemet, and of course the eastern and southern Deserts all of which are at risk for some extreme hot spells but minimal rains. We then return to hot, and minimal rains from TSTM-type conditions (mountains and deserts). Marine layer conditions will affect the coast but will irregularly contribute cooling as the marine layer briefly deepens. East winds will contribute some erratic clearings to the coast with some hot days during most of August. Inversion height Ö.lower than average in August due to recurrent high pressure aloft.
- SOCAL DESERTS.
- SOCAL deserts continue hot but changeable as tropical cyclones over S Baja and SW Mexico induce development of hot and recurrent upper high pressure, as we head to August.
- Shower dates near mountains or far SE SOCAL Deserts: 30 July and 13-17 Aug and end of Aug,
then 3-8 Sept.
Medium Term Outlook: September 1- 30
- The IVTinit pattern along with the Madden Julian Oscillation (MJO) both are showing a tendency for troughing in the subtropical mid and east N Pacific between SOCAL and Hawaii. In this pattern there is less tendency for troughing into Central and S California. Even N California appears quiet but subject to heat waves as atmospheric rivers try to develop in the central and southern Gulf of Alaska. These keep the focus of stable upper high pressure over ocean at 38-43N, high pressure aloft in the Utah Plateau area, in the western US as well as the ocean area west of California to 140W. Troughing and upper lows continue to recur in the subtropical mid Pacific between SOCAL and Hawaii, while tropical cyclones contribute occasional support for upper high pressure for the West Coast and California. These argue for periods of hot weather in August, interrupted infrequently by TSTMS and some breaks in the heat.
- There is not much overall change in the pattern as we drift from mid-August to early September. However, August often sees a turn of the upper winds to SE, and a more NW-ward drift of tropical moisture up the Gulf of California into SOCAL, California and Arizona. We note that there is continuation of above average sea surface temperatures in Baja California and into southcentral and S California. It would be a good idea to remain aware of the risk of a tropical cyclone development in the Gulf of California, moving NW-ward into N Baja, and possibly S California mountains and Arizona. TSTMS associated with tropical cyclone moisture, especially when above average sea surface temperatures are involved, presents an increased flash flood risk, as we have seen in recent seasons, both in August and September. Also, showers that develop in Arizona are more likely to find their way into SOCAL eastern deserts and into the Central Sierras in August and September.
- Northern California: For valleys and mountains of Northern California: hotter and drier than average in most of August, September and October, and into early November.
- South half of California: 1-30 September: Near average precipitation with above average temperature. Weíll see a trend toward a shallower and more erratic marine layer influence in August, and into September. Therefore, watch for interspersed hot periods accompanied by shallow coastal marine layer conditions at times. Hottest conditions are currently indicated for August, and erratic mild spells with briefly deeper marine layer as troughs occur.
30 Day Precip/Temp maps for August suggest below average rain from monsoonal TSTMS for Western Mexico and S Arizona as well as SOCAL in August. We continue above average temperatures for the most part in California. Other short term climate models (NMME) suggests above average rainfall for central and S Gulf of California for Aug, Sep, and Oct. - Flash flooding from tropical cyclone moisture does not only focus in afternoon hours when convective heating is greatest. It can occur at late evening or even 2-3 am, if there is sufficient moisture. The Gulf of California is a well-known source of moisture in an unstable atmosphere. Where there is moisture present, and an unstable atmosphere capable of supporting TSTMSÖat night as well as during the day, and there are colliding gust fronts
from adjacent TSTM areas, precipitation maxima occur, but the same physical principles occur in the Gulf of California that we saw recently in Texas convection, upslope enhancement of rainfall, and colliding gust fronts. - Well above average temperatures are expected to recur through most of September. As tropical cyclones meander closer to the coast of Baja and California in September, there is greater support for development of dry, hot east flow, as well as unusually moist and unstable conditions (Gulf of California to Arizona and S Deserts of California, and Sierras.
- From the NOAA/CPC/NCEP monthly outlook maps, please note that September continues the trend in recurrent hot dry conditions (warmer than average)in central and S California. The map for October suggests a possible rainy pattern for the Gulf of California Sonora/Sinaloa coasts. This suggests the possibility of heavy rains for Arizona and possibly SE California deserts in early October or late September. Otherwise we currently look for above average temperatures, presumably from recurrent east or southeast flow, and the usual Santa Anas for SOCAL.
- For September to early November: subtropical ocean areas west of California, and SOCAL NMME maps indicate a turn to wetter than average from about 130-135W westward to 160W. This is consistent with our discussions of active subtropical rain systems developing in the fall months, i.e. September, October, and into early November. This trend would be expected to increase as the subtropical westerlies activate during November and onward with the onset of mid fall to winter.
Figures: Alan Fox, Text: Alan Fox, Fox Weather, LLC
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