Colder Weather Coming - Possible Thunderstorms

One of the most active storm periods this Water Year continues during the next 72hr Forecast period coming on the heels of a very productive warm/wet storm system which produced impressively high Storm Totals for portions of NORCAL with Storm Total Accumulations ranging up to 7 inches into the Feather River basin and up to 6 inches into the Shasta River basin. This has produced large increases of inflow into both important State reservoirs with Shasta Lake adding approximately 83,000 AF thus far and Oroville adding approximately 70,000 AF. Peak inflow into both reservoirs are illustrated below. The good news is that outflows have been held back, so increased storage is occurring. The next inbound storm is very cold, with a-32°C cold core approaching the OR coastline Monday, with snowlevels plummeting down to near 3.5K near YNP and near 4.5k into the Tehachapi Mts as cold air advects into the region tonight. Charts have recently reduced the impact of this storm for CENCAL, but still show cold air advection follows frontal passage with a -30°C isotherm digging into NORCAL with rapidly lowering snow levels. The timing schedule for this storm is not phased with post frontal heating to allow for post frontal Thunderstorms to be in optimal condition for most of the SJV, however Merced Co northward has a chance tonight and Tuesday. The potential for Thunderstorms on Tuesday should be monitored closely for the North Valley, due to the extreme cold nature of this disturbance, which could easily trigger Thunderstorms. Some of the Thunderstorms could reach strong levels in the far North Valley. Yet another storm system reaches into NORCAL on Fri, Apr 4^th, with light amts of precip. The GFS has shifted into a drier mode since the last forecast with a great deal of flip/flopping into the latter portion of the 16 Day forecast period, therefore low confidence.

The weekend storm produced a peak inflow into Shasta of 32,000cfs

The weekend storm produced a peak inflow into Oroville of 31,000cfs

NORTHERN SIERRA 8-Station Precip Index; after the weekend storm here’s an update that shows an increase of 3+ inches since last week, which pushes the 10 inches recorded for the month of March to 145% of average. This amt should increase by Tuesday, April 1^st, which could make March the wettest month in terms of precip accumulation as well as percent of monthly average. Currently the 145% of Month Average already makes March the wettest month by percent of average. By the end of Monday the Accumulated Precip Total for March maysurpass the 10.7 inches recorded in February, which would make March the wettest month of the Water Year.

San Joaquin 5-Station Precip Index; after the weekend storm here’s an update shows that shows an increase of 0.8 inch since last week, which raises the 4.7 inches for the month of March to 76% of average! This amt could increase by Tuesday, April 1^st.

CA Warning, Watches and Advisories

WINTER STORM WARNING; posted for the NORCAL Mts above 3,500 ft and the Sierra southward to YNP, from Monday afternoon thru Wednesday morning for periods of heavy low snowfall with of up to 2 feet at the highest elevations, accompanied by strong winds of 65mph or higher. Snow accumulations of 5-10 inches could occur to low elevations. The snow level will be near 2,000ft in the north and 3,000ft in the south by Tuesday. This will impact the Sierra from Lassen southward to the Kern Co border with possible road closures.

Winter Wx Advisory; posted for the foothills of the Sierra from east of Redding, east of Chico, and the Sierra from YNP southward to Kern Co, for snowfall of up to 4-7 inches for the Southern Sierra, and up to 11 inches above 7K, accompanied with gusty winds up to 55mph and up to 70mph near the Crest.

Wind Advisory; regarding high winds in the SJV, especially along the Westside for Monday afternoon and evening for strong winds gusting up to 25 to 35mph as high as 50mph near the coastal passes (Pacheco/Cottonwood, Twisselman)

Hazardous Wx; Thunderstorms possible Tuesday afternoon and evening, April 1^st for the Great Valley from Redding to Merced, capable of producing strong winds, frequent lightning, hail, urban and small stream flooding and possibly a funnel cloud or tornado…or possibly SEVERE WX. QPF’s in the SJV range from near 1 inch near the Delta up to 1 to 2 inches of precip for the Sierra foothills.

Special Notation; The GFS forecast charts continue a very interesting and important theme into the next 7 Day period with a very active storm period..which according to the past several. This storm series is producing the most significant precip event for NOR/CENCAL this Water Year. This is especially true with regards to NORCAL, with impressively high precip occurred over the weekend from a warm/wet storm, with impressive increases into Lake Shasta and Lake Oroville. Another storm reaches into CA on Monday/Tuesday which is much colder than the preceding weekend storm. The extended models into the long-range and Fantasyland realm are uncertain with low confidence especially with regards to timing and strength details of suggested storms within the latter half of the 16 Day period, however the good news is that the pattern is progressive and not locked into a High Pressure regime.

FORECAST SUMMARIES

Days 1-7;

Mon, Mar 31^st Weak High Pressure ridging between storms may produce patchy morning fog possible in the Great Valley due to the abundance of surface moisture as well as calm winds. Forecast charts show a strong late season winter cold front moving into CA and then into CENCAL during the afternoon, with a -32°C cold core storm center approaching the CA/OR border during the day. There is some discrepancy on the arrival timing of precip into CENCAL and the Sierra and recent model trends have been reducing the strength of the incoming system in general. As the upper Jet Stream punches into the Central Coast at 120kts by noon according to the NAM/WRF model, with increasing high winds for the Sierra Crest region and rapidly deteriorating conditions near the Crest region which spreads to lower elevations as the day progresses. It is possible that inclement wx could develop earlier over the Sierra with orographic lift enhancement due to the strong Jet Stream placement overhead of the Southern Sierra. By late afternoon the upper Jet Stream ramps up to 130kts over Kings/Tulare Co. Precip charts however do not show rains spreading inland until later in the afternoon and evening, then continuing overnight. During the evening strong vort dynamics accompany the powerful cold front as it spreads into CENCAL with cold air advection transporting a -30°C isotherm over Monterey to YNP during the evening. This would place the northern portions of the SJV into the left front quadrant of a very powerful 130kt upper Jet Stream (Blue chart below), an optimum position for Thunderstorm development north of the Jet. SPC has placed large portions of the Great Valley into a Categorical Risk for Thunderstorms as illustrated in the graphic chart below with the highest chances from Merced Co northward. Overnight the NAM/WRF keeps the front stalled out over CENCAL with moderate precip, especially in the orographically favored Sierra. Precip chart (green),upper level Jet Stream chart (blue) and the SPC Thunderstorm Outlook last.

Tue, Apr 1^st The Penn State Historic and Record website places this storm event with a -2 Standard Deviation Anomaly for temps, which may produce one or two records for low temps, but widespread records are not expected. The cold core Low Pressure circulation moves near Pt Arenas with rainshowers and Thunderstorms moving throughout the interior valleys of NOR/CENCAL and into the Sierra as well as low elevation snows for any mountainous terrain over NOR/CENCAL. As the trough axis moves onshore the upper level Jet Stream is placed over CENCAL at 95kts from the SW, supplying ample orographic lift to the Southern Sierra with a showery regime in the lowlands. 700mb charts show another significant precip band moving onshore over CENCAL during the evening with embedded Thunderstorms possible especially for the North Valley region. Overnight the cold core storm center drops SE into the SJV with additional rainshowers and snow into the Sierra continuing. Precip chart (green), upper level Jet Stream chart (blue) and the SPC Thunderstorm Outlook last.

Wed, Apr 2^nd A weak High Pressure ridge noses into CA by afternoon after morning showers diminish. A slow clearing trend from west to east occurs with lingering shower activity gradually ending over the Sierra. Overnight a new cold front and trough approach CA from the West, however the parent Low Pressure center is located much further away than the preceding storm which produces a much weaker cold front west of CA.

Thu, Apr 3^rd As the weakening trough approaches CA, it slows down. By evening it manages to reach onshore and spread rainfall into portions of NORCAL. Overnight the trough sluggishly moves east with light rainshowers spreading further inland across NOR/CENCAL.

Fri, Apr 4^th The trough axis slowly departs CA and is replaced by High Pressure which produces a cool/dry NNW flow pattern. Overnight High Pressure continues to build into CA as the storm track is shunted northward into the PACNW, producing heavy warm rains into WA and the Olympics which are supplied by a rich subtropicalAtmospheric River (AR) originating from near Hawaii.

Sat, Apr 5^th While High Pressure produces an offshore flow pattern over CEN/SOCAL, the heavy warm AR rains continue to intensify over WA/OR. Overnight the High Pressure continues to push further east into all of NV and western UT, while the non-stop heavy AR warm rains continue into WA State. Expect to see flooding in the western portions of WA by this time.

Sun, Apr 6^th The West Coast High Pressure builds northward into OR and WA which deflects the incoming warm AR rains to track further north into Vancouver and southern BC, Canada. While the WCONUS is covered by High Pressure, the upwind trough at 150W is slowly migrating east which is sharpening the ridge axis.

Days 8-16, Long Range into Fantasyland Outlook; Day 8 begins;

Mon, Apr 7^th shows the High Pressure ridge axis migrates east into the Great Basin as the trough pattern approaches 140W. The subtropical AR warm rains travel NNE into central BC, Canada as the southern portion of the AR begins to approach 130W. Fair skies prevail over CENCAL.

Tue, Apr 8^th shows the AR decays and rapidly diminishes intensity, just as it begins to reach the far NW coastal region of CA. Meanwhile over CEN/SOCAL a warm SSW flow pattern develops in advance of the trough. Overnight the trough axis passes thru 130W with a small moisture field approaching the CENCAL coastline as the trough appears to pinch off. Partly cloudy skies move over CA.

Wed, Apr 9^th shows the formation of a cut-off Low Pressure upper level devil, located west of Pt Conception, with a southerly flow pattern increasing over CEN/SOCAL. Overnight the Low Pressure ‘devil’ approaches Pt Conception, with a SE flow pattern developing over CENCAL with partly cloudy skies.

Thu, Apr 10^th shows the upper level ‘devil’ travels across SOCAL with shower activity possible up to and including the Tehachapi Mts with clouds for the rest of CENCAL. As the Low Pressure center moves eastward the winds over CENCAL veer as backwash over the top of the circulation, ultimately becoming NW as the Low Pressure moves into the Vegas region.

Fri, Apr 11^th shows the Low Pressure closed circulation opens up into a trough configuration and travels east into the Great Basin and Rockies while a NW flow pattern brings cool/dry air into CA with a return of fair skies.

Sat, Apr 12^th shows a narrow High Pressure ridge axis centered at 140W with a minor shortwave trough pushing thru the WCONUS with a few clouds, but at this time the charts do not depict much precip reaching into CA, with only a few showers possible along the Central Coast. However, further north charts show cold air diving southward into the PACNW.

Sun, Apr 13^th shows the cold air in the PACNW rapidly dives southward into the existing trough configuration over CA, which rapidly energizes the trough and increases rainshowers over NORCAL and portions of CENCAL with cooler temps. Overnight the trough kicks eastward into NV leaving clear skies with rapid radiational cooling for the SJV producing cold overnight minimums into the mid 30°’s. A more zonal-type flow pattern develops over the PAC basin, flowing along 40N and undercutting the High Pressure ridge which is pushing northward into Alaska.

Mon, Apr 14^th shows an undercutting zonal-type flow pattern across most of the PAC basin. The zonal flow is weak however, with several weak disturbances traveling eastward within the main flow with most of the energy focused upon the PACNW. Extremely cold air is observed rapidly digging southward into the Bering Sea, past the Aleutians into the GOA. Meanwhile CA remains fair and mild.

Tue, Apr 15^th shows the cold ARCTIC air from the Bering Sea has rapidly developed into a very large Low Pressure disturbance in the GOA with a zonal-type flow pattern reaching along 40N into CA. This could be the beginning of a significant storm period for CA, as High Pressure is nowhere in sight and the moist westerlies are aiming toward NORCAL.

NEW CA RECORDS

Station New Record Old Record__

Medford, OR 0.82 inch 0.45 inch set in 1925

Modesto 0.51 inch 0.45 inch set in 1982

Merced 0.48 inch 0.01 inch set in 2005

Madera 0.32 inch 0.03 inch set in 2005

Hanford 0.14 inch 0.01 inch set in 2010

San Diego 61° Ties 61° set in 1978

Elsinore 55° Ties 55° set in 1950 High Min

Riverside 56° Ties 56° set in 1903 High Min

Idyllwild 43° 41° set in 2013 High Min

PRECIPITATION FORECAST

Day 1 – 5 QPF:

At 12Z (9am) Sunday thru the period Thursday night (Friday @4am);

7 Day QPF Chart; the 0Z Sunday evening WPC 7-Day QPF showing up to 3 inches of rainfall into the North Coastal region near the Russian River, with amts diminishing to near 2 inches for most of the North Coast and into the North Bay region, with a precip focal point of 1.8 inches into the Southern Sierra near the Tuolumne River basin. Most of NORCAL is painted with 1.25inches or more, while CENCAL receives up to 1.5 inch in the South Bay region diminishing to about 0.5 inch near Vandenberg, with the SJV receiving about 1 inch near Stockton to 0.2 inch into the Southern SJV and the Southern Sierra receive about 1.5 inches in the northern regions diminishing to about 0.75 inch into the lower Kern River region. SOCAL receives up to 0.9 inch along the Coastal region which rapidly diminishes to >0.1 inch into portions of the lower Mojave Desert region and zero for the Imperial Valley and lower Colorado River Valley.

16-Day QPF estimates (from the 0Z GFS model) for;

Days 1-7; shows 3 inches of rainfall into the North Coastal region near the Russian River, with amts diminishing to near 2 inches for most of the North Coast and into the North Bay region, with a precip focal point of 1.8 inches into the Southern Sierra near the Tuolumne River basin. Most of NORCAL is painted with 1.25 inches or more, while CENCAL receives up to 1.5 inch in the South Bay region diminishing to about 0.5 inch near Vandenberg, with the SJV receiving about 1 inch near Stockton to 0.2 inch into the Southern SJV and the Southern Sierra receive about 1.5 inches in the northern regions diminishing to about 0.75 inch into the lower Kern River region. SOCAL receives up to 0.9 inch along the Coastal region which rapidly diminishes to>0.1 inch into portions of the lower Mojave Desert region and zero for the Imperial Valley and lower Colorado River Valley.

Days 8-16; depicts several storm systems to travel thru CA, with light amts of 0.2 inch or less reaching thru NORCAL into portions of CENCAL and with up to 0.25 inch progged for coastal SOCAL. The Week #2 time frame is highly uncertain and with a low confidence forecast.

GFS 384hr, 16Day QPF Chart shows up to 4 inches of precip falls into the wet spots of the Northern Coastal region with up to 2-3 inches over portions of Humboldt and Mendocino Co extending NE toward the Shasta basin. With up to 1-1.5 inches of precip for the Northern Sierra and along the Coast down to Big Sur, with up to 0.5 inch down to Vandenberg along the coast and down to the Kaweah River basin with up to 0.5 to 0.75 inch for most of the SJV and the Santa Barbara coastal region. With about 0.25 inch for the coastal region of SOCAL diminishing to less than >0.1 inch into the lower Mojave Desert and zero for the lower desert region.

EVAPOTRANSPIRATION

Weekly Evapotranspiration rates for the local 7 Counties shows up to 0.88 inch near Merced up to 1.09 inches into Tulare Lake during the next 7 Days. The Sierra Crest evaporation loss is 0.49 inch in the upper Tuolumne River basin up to 0.74 inch near Isabella.

SATELLITE

Water Vapor 4KM Image of the West Coast Sunday, March 30^th, 2014, shows the next inbound storm system rapidly developing west of the OR coastline, dropping SE toward CA. The next storm is very cold.

Visible Satellite Image of the CA Sunday, March 30^th, 2014, shows mostly clear skies over CENCAL with a few clouds over the Sierra and South Valley regions and along the Westside SJV where a few instability cumulus have temporally formed.