LARGE SCALE SUMMARY
- For NORCAL coastal mountains, valleys and foothills from Mendocino Co north into Oregon and Washington: Dry and warm 24-25. Rain Dates in NW California: Mar 27,30, 31, Apr 1-4. Another subtropical system comes through California with showers on 7 Apr. Per the CFSv2 model, showers Apr 11-12, 16-18. Showers also NW California- and west portions Oregon-Washington (ORWA).
- Support for troughing continues in the central N Pacific from about 130-145W (mainly NE to NNE of Hawaii), and at 160W to near the Dateline to N to NW of Hawaii. CFSv2 suggests moderate rains Apr 17 in California mountain areas (including Sierras).
- A warm/mostly dry period appears to develop (CFSv2) about Apr 19-30th. Will keep an eye on this, it may develop into hot spells with dry downslope winds. Some rains in late March (29 and 31) may include Southern California as fronts move across SOCAL.
- For forestry, the next wet spells appear for following dates — rains and mountain snows: Dry 25-26, rains 27-28, 30-31, Apr 1-4. Watch for a warm dry breezy period with downslope winds again about Apr 19-22 and 28-29.
- Watch for a hot June preceded by a dry May in California.
NEAR TERM OUTLOOK — March 25 – April 25
- NORCAL precipitation dates in coastal valleys are : Mar 27, 28, 30, 31, Apr 1-4, 7, 12, 15, 16, 17, 18. Mild/Dry Periods: Cool to near seasonally average temperatures. There is a general seasonable warming from Apr 5-Apr 19.
- NORCAL cool with occasional low snow levels: North Sierra Precipitation: Mar 27, 28, 30, 31, Apr 1-4,l ate Mar / early Apr. Cool periods with showers on 7, 12, 15, 16, 17, 18.
- Central Sierra precipitation: Mar 27, 28, 30, 31, Apr 1-4, 7, 12,1 5, 16, 17,1 8.
- CENTRAL CALIFORNIA Precipitation: Mar 27, 28, 30, 31, Apr 1-4. Mild dry periods alternate with cold and (brief or small snow in mountains), only brief frosts/freezes end of March to mid Apr.
- SOCAL precipitaiton about March 27-28, 30-31 with showers Apr 1-4, 7, 12, 15, 16, 17,1 8. Several nights may have frosts in the normally colder valleys, on nights in which clearings occur for a time overnights/early mornings
between fronts/rain events. Early hot period Apr 24-25 with dry/warm Santa Ana type events: Apr 24-29. - Southern California Deserts colder than average during a few showery periods. Rainfall amounts below average. Shower dates in SOCAL Deserts: Rain sparse in April, occurring mainly 7,12,15,16,17,18 in sparse light showers.
MEDIUM TERM OUTLOOK — April 25 - May 25
- Northern California: Precipitation decreasing to near average by first week of May, with temperatures trending near average about mid-Apr, and above average during Apr 20-30 and above average away from coastal marine influence in May. The few mid Apr rains are focused in central California. Rainfall may still average near normal in April for Central California, occurring over several days. Temperatures below average during mid Apr, otherwise warmer than average during warmest periods (last week of Apr) through most of May..*** except cool or mild days in the marine
influence zone. Frosts: below average number of frosts for Apr in the coastal valleys, depending on depth of the marine layer. Watch for onshore flow and deep marine layer conditions extending from Marin Co to Napa Co-Solano Co into the Sacramento Delta. Drizzle or drizzly rain will be most likely in Marin-Sonoma-Napa-Solano Co's in mornings. Mild breezy periods in the Delta to Sacramento-Stockton-north San Joaquin Valley still possible in early May. Most of frost if any is suggested for the period 21-22 Apr, followed by days progressively warmer than average through end of Apr and beginning of May. - Southern California: Near average precipitation with temperatures trending above average about first week of Apr, then turning dry in last week of Apr to mid May. SOCAL will most likely receive low clouds in Apr and through mid May (stratocumulus) and cool conditions, with morning to midday drizzle/drizzly-rain during the mid Apr and May periods. Temperatures: cooler than average during mid Apr, otherwise shifting quickly to very warm or hot conditions (away from marine influence) during warmest periods (first week of Apr and late Apr-start of May. Frosts: about average number of frosts (minimal or unlikely) in last week of Apr, and in May. After the 10th, May will be getting hot, with intermittent hot spells in foothill terrain above the marine layer. Per the usual trend, warmest nighttime temperatures will tend to be in those areas above the marine influence area...within the marine inversion that overlies the top of the marine fog/stratus layer.
LONG TERM OUTLOOK — May 25-June 30
- Northern California: Near average precipitation, highest precipitation anomaly will be at the North Coast Mendocino Co north from coastal drizzle and light rains from weak end-of-season cold fronts in the first week or so of May. Note that we dry out quickly after about 10 May. At that time, temperatures turn to well above average Sacramento Valley, and well above average in central to southern Sierras (warm nights on slopes with inversion conditions). Frosts: Frost conditions are not an issue after about 10 May. Trending to above average nighttime temperatures most areasafter the first week of May. Watch for frosts in the Mendocino Co valleys (viticulture areas) and possibly southern Lake Co during the first few days of May, before 10 May, even when clearing and drying occurs overnights. For valleys and mountains of Northern California, turning hotter and drier than average during late spring and first month of summer (June).
- Southern California: Near average precipitation with near average temperature. We expect generally below average number of frosts/freezes in valleys of SOCAL outside the marine influence zone, especially north part, i.e. valleys of San Luis Obispo, Santa Barbara, Ventura Counties during the first week to 10 days of May followed by dry and hot conditions starting to develop in mid to late May. Please note that well above normal temperatures are suggested in the NCEP/CPC guidance for June. Therefore, we suggest heat wave conditions in the Sierra Nevada foothills and west slope areas from El Dorado Co south. This is important because with dry conditions in May and June, this may set
the stage for an early start to fire season. For the valleys of Southern California: Turning warmer and drier than average during the late spring and the first month of early summer (June). We note from the NCEP monthly guidance included next page that significant hot spells are a possibility in June for inland SOCAL citrus and avocado regions. There is risk of highs reaching to near 110 in the Ramona-Temecula areas and a little over 100 in Fallbrook during mid to late June.
Figures: Alan Fox, Text: Alan Fox, Fox Weather, LLC.
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