Large Scale Summary
- Support for troughing continues in the central N Pacific from about 142-150W (mainly N to NNE of Hawaii). After a few early season tropical cyclone related showers/TSTMs in early July, a hot dry ìdomeî of upper high pressure appears to persist over California during most or nearly all of August. Therefore, summertime hot conditions dominate in July-August.
- The near term precipitation guidance (GFS-model based) shows practically no rain during 18-31 July in California. The main exceptions are dry thunderstorms (TSTMS) over mountains in afternoons, e.g. Sierras, Siskiyouís. SOCAL mountains appear almost devoid of precipitation but that solution may be in question, given the increase in sea surface temperature anomalies near southern Baja.
- Monsoonal moisture and tropical cyclones over southern and southwest Mexico to southern Baja usually produce most of the rains this midsummer 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. Moisture is either carried N into Arizona, or WNW-ward out into the subtropical Pacific west of southern Baja. We note that periods of well above average temperature are suggested in the NOAA/NCEP/CPC guidance for 18 July to the end of month, and into the first part of August.
- Heat wave conditions occur in the Sierra Nevada west slope areas from Plumas Co south during rest of July and onward to mid August. Below average rainfall is expected in Sierras from a few thunderstorms (TSTMS). Most TSTMS will be generated by moisture from tropical cyclones or easterly waves affecting the Gulf of California and S Baja.
- Rain dates and areas: NE Plateau/Modoc/N Lassen Coís 27-31, central Sierra 28-31 and again Aug 17-18th. Currently rain appears unlikely for central Sierras/Yosemite NP area during most of the rest of July and in August.
Near Term Forecast
- Hot and dry throughout California except immediate coast. Risk of excessive heat to California coastal and intermediate valleys and Sierra west slopes.
Northern California
- Rain Dates in Siskiyou Mountains: NE Plateau/Modoc/N Lassen Coís 27-31. Rain Dates N to Central Sierras are: Rains (from TSTMS) occur July on 22-23, 28-29-30.
- NORCAL Hot/Dry Periods: in NORCAL and Central California as high pressure aloft over the central US drifts west into California. There is still some risk of mostly dry thunderstorms (TSTMS). TSTMS will tend to be accompanied by strong gusty winds 18-25 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 22-23, 28-30, Aug 11-12.
- Central Sierra Precipitation: TSTMS July 22-23, TSTMS 28-29-30, TSTMS Aug 11-12.
- CENTRAL CALIFORNIA HOT DRY PERIODS: Above average temperatures occur with a few thunderstorms (TSTMS) in mountains. Sparse showers are suggested for July 10-11, 22-23,28-30. High fire growth risk in Central and S Sierras during 10-11, 13, and S Sierras 14th are suggested by the prevailing airmass type. Strong fickle and gusty winds may occur together during the periods indicated for TSTMS and sparse showers.
Southern California
- SOCAL PRECIPITATION: The most likely precipitation is drizzle from early morning coastal fog/stratus. 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 sparse TSTMS July 22-23, TSTMS 28-29-30, TSTMS Aug 11-12. 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 east, bringing showers into SOCAL mountains and deserts on a few occasions: around 23-25 July with local cooling during rain periods.
- SOCAL WARMER PERIODS: Troughs will help keep the coastal areas less hot than the inland valleys (Escondido, Ramona, Hemet, and of course the eastern and southern DesertsÖall 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 the second half of July and first part of August. Inversion height Ö.lower than average in mid to late July and August due to persistent high pressure aloft.
- SOCAL Hot Periods: These will tend to precede and follow TSTM periods in late July and August.
Southern California Deserts Outlook.
- SOCAL deserts continue hot as tropical cyclones over S Baja and S Mexico induce development of hot and persistent upper high pressure, as we head to August. Shower dates near mountains or far SE SOCAL Deserts: 23-25 July.
Medium Term Outlook — 18 August — 18 September
- 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. 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 and keep the focus of stable upper high pressure about 38-45N in the western US as well as the ocean area west of California to 140W. Troughing and upper lows continue to recur in the mid Pacific, while tropical cyclones contribute additional support for upper high pressure for the West Coast and California. These argue for sustained hot weather in August, developing with fewer
than usual breaks in the sustained 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 already a turn to above average sea surface temperatures in southern and possibly central Baja California. be 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 and thru mid September.
Southern California:
- 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, but still continue above average temperatures for the most part in California. This would be interpreted to indicate a hotter than normal August, but with the trend for below average rainfall continuing.
- Flash flooding from tropical cyclone moisture doesnít just focus in the 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 TSTMS at 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 TexasÖconvection, upslope enhancement of rainfall, and colliding gust fronts.
- Well above average temperatures are expected to hang on from August into the first part of September. As tropical cyclones meander closer to the coast of Baja and California in August and September, there is a 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
- From the NOAA/CPC/NCEP monthly outlook maps, please note that September continues the trend in recurrently hot dry conditions (warmer than average). 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 winds and Santa Anas for SOCAL
Figures: Alan Fox, Text: Alan Fox, Fox Weather, LLC
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