The EPAC flow pattern near CA is ramping up to the wettest mode since Dec 2012, for at least NORCAL…finally! As has been advertised for the past week the forecast charts show heavy accumulations of precip produced by a Hawaiian Atmospheric River water hose aiming at NORCAL this weekend bringing QPF’s upwards of 8 to 10 inches of liquid equivalent for the wet spots in the higher terrain of the North Coast and Northern Sierra accompanied with high Freezing Levels. While a portion of this precip is likely to soak into the ground, much of it will also result in runoff into the mainstem rivers. In the case of the NW coastal region, the heavy rainfall should help somewhat to recharge the Klamath, Mad, Eel, Mattole, Russian and Napa Rivers from their recent record-setting low streamflows. In the case of the NORCAL the excessive runoff will be captured by the downstream reservoirs, such as Trinity, Shasta, Oroville and Folsom. Charts show the river basins feeding into the Feather, Yuba and American River …and Lake Tahoe, will benefit the most from this storm event, with lesser known impacts upon the Cosumnes, Stanislaus and Tuolumne River basins as QPF’s begin to rapidly drop off in those areas. If QPF estimates validate, then the impacts upon the river basins south of the Merced and into the San Joaquin and Kings River basins are highly suspicious and problematic due to the forecasted rapid drop in QPF’s. Those watersheds could receive an adequate soaking (favoring the San Joaquin), but runoff in this parched region of the Sierra will be far less than those neighboring watershed to the north. The extreme Southern Sierra, including the Kaweah, Tule, White and Kern River basins unfortunately may only see sparse amts of precip which have been depicted on the forecast charts for the past several days. This could all change if the Atmospheric River decides to sag further south instead of being stalled for such a long time over the Northern Sierra. The main 75 to 100 mile wide core of the Atmospheric River is of special interest to follow, due to its potential to produce flood conditions, and wherever that region ends up being could end up with a potentially dangerous situation. Right now that region appears to be on a line from near the Santa Rosa region E-NE toward the Feather, Yuba and American River basins.
A tight isohyetal (lines of equal rainfall) gradient over CENCAL, shows the moderate to heavy rains suggested over the Delta and into the Tuolumne and Stanislaus River basins with up to 2 inches, rapidly diminishes toward the south, with portions of Kern Co depicted with receiving only >0.1 inch or zero. This storm series is good news for an ailing NORCAL, and a prayer answered for at least parched NORCAL! If the QPF’s verify for the Northern Sierra with the QPF of 8-10 inches during the next 7 Days as is depicted on the HPC QPF chart below ↓, that would bring the monthly average for February close to normal/average…for the month of February. This does not mean an end to the Drought however, because not only was precip absent during December and January, the Long Range GFS cuts off most of the moisture during the remainder of the 16 Day Outlook for NORCAL, and after this weekend completely dries out most of CEN/SOCAL thru Feb 23rd, into the third week of February. For more precise QPF amts please refer to the QPF charts illustrated below ↓. We count our blessings and are thankful that this warm/wet flow pattern has finally knocked down the RRR High Pressure ridge and will put a substantial amt of water into Trinity, Shasta, Oroville and Folsom reservoirs. The main event of Sat/Sun is powered by an Atmospheric River initially full of 1.25 in PWAT Atmospheric Moisture which flows into CA on a SW flow …without the support of 500mb dynamics, and only producing precip due to strong ‘upslope glide’ isentropic lift along the moist SW flow pattern. This pattern is most difficult for charts to depict correctly due to the lack of the more typical vort dynamic lift that is absent in the upper 500mb charts. The lower 700mb layer of the atmosphere will be the primary driving force advecting the warm/moist subtropical moisture into CA. The western facing slopes of the Coast Range and Sierra will ramp up orographic lift to enhance heavy rainfall into the higher elevations. Snow levels will be high. The core of the Atmospheric River with 1.5 inch PWAT’s is moving onshore into the North Bay region, aiming at the northern Sierra producing copious rains which gradually sag southward into CENCAL on Sunday, then rapidly drop into SOCAL on Sunday night with diminishing rainfall to the north. Another Atmospheric River surge develops into far NORCAL again next Wednesday and Thursday producing another dose of rainfall for portions of NORCAL.
Flash Flood Warning issued for Sonoma County including the cities of Rohnert Park and Petaluma due to heavy rains, with some stream gages indicating localized flooding is occurring.
FORECAST SUMMARIES
Days 1-7;
Sat, Feb 8th Much of NORCAL being drenched by heavy rainfall produced by an Atmospheric River that stretches back to Hawaii bringing 1.2 inch PWAT’s inland over portions of NORCAL. The synoptic pattern remains similar with weak High Pressure SW of SOCAL and the Atmospheric River (AR) continuing to feed a steady flow of subtropical moisture into NOR/CENCAL with PWAT’s jumping up to 1.5 inches this evening. This will increase precip efficiencies over the Sierra and Coast Range where orographically enhanced precip will generate copious amts of rainfall, with high snowlevels. Overnight the moist flow remains nearly stationary with precip focused mostly into NORCAL. Note the 700mb chart below ↓ depicts a strong well-defined Atmospheric River reaching into CA, without the dynamic support of the GOA Low Pressure which is quite far away. Instead, this precip event will be primarily produced by upslope glide and enhanced by orographic uplift near western facing slopes. While QPF’s over NORCAL are assuredly going to be very high, by contrast the QPF’s over CENCAL are forecast to be much less and rapidly diminishing the farther south we go.
Sun, Feb 9th The charts indicate the Atmospheric River (AR) supply of moisture feeding into CA is chopped off west of CA and the remainder of the moist flow moves into NORCAL with continued heavy rainfall. Rainfall amts rapidly taper off to the south over CENCAL. Overnight the rains over NORCAL diminish as the event comes to an end. High Pressure offshore rebuilds west of CA. Chance Thunderstorms late afternoon/evening, Sunday for Fresno north.
Mon, Feb 10th The rebuilding RRR High Pressure ridge offshore temporarily shunts the moist flow northward into the PACNW, while CA dries out. Overnight the RRR High Pressure ridge once again weakens as another Atmospheric River from Hawaii re-connects to the SW flow pattern, producing heavy rains into Vancouver, BC and the WA coastline, with further development of the AR to the west of NORCAL.
Tue, Feb 11th As the High Pressure continues to weaken over NORCAL, the redeveloping AR begins to advect warm rains back into extreme NW CA. The longwave trough pattern over the NE PAC maintains the long fetched SW flow pattern carrying the AR into CA. Overnight the moist flow enhances and spreads over most of NORCAL down to the North Bay region to Tahoe with light to moderate rains, while skies over CENCAL are partly cloudy.
Wed, Feb 12th The next long-fetched Atmospheric River (AR) stretches back to the Dateline, with warm rains flowing into southern Canada down to Monterey Bay to YNP with cloudy skies thru the rest of CENCAL. Overnight High Pressure offshore SOCAL rebuilds which shunts the moisture feed northward into the CA/OR border.
Thu, Feb 13th High Pressure west of SOCAL allows the long-fetched AR to continue to bring mod/heavy rainfall into NORCAL from Cape Mendocino northward with fair skies over CENCAL.
Fri, Feb 14th Valentine’s Day shows the remnant left-over’s of the AR slides thru NORCAL and High Pressure remains over CEN/SOCAL. Overnight increased rains reach into far NOCAL as the large GOA Low Pressure swings a trough toward the
Sat, Feb 15th Continued heavy rainfall occurs in the far NW corner of NORCAL which spread down to the Golden Gate overnight, with fair skies over CEN/SOCAL.
Days 8-16, Long Range into Fantasyland Outlook; Day 8 begins;
Sun, Feb 16th shows the storm track shifting northward into the PACNW with fair skies continuing over CENCAL. Overnight a disturbance approaches the PACNW and extreme NW NORCAL.
Mon, Feb 17th shows the disturbance brings continued rains into the PACNW and spreads into extreme NW NORCAL. Fair skies over CEN/SOCAL.
Tue, Feb 18th shows a continual zonal –type moist flow into the PACNW while CEN/SOCAL remain fair.
Wed, Feb 19th shows more of the same with additional rains into the PACNW, while the rains shift out of the far NW Coastal region of NORCAL.
Thu, Feb 20th shows the RRR High Pressure ridge reforming along the West Coast with a dry NW flow for CA.
Fri, Feb 21st shows the RRR High Pressure ridge moves closer to the West Coast with a north flow pattern over NORCAL and NW flow pattern over CEN/SOCAL, with fair skies and cooler temps
Sat, Feb 22nd shows the RRR High Pressure ridge moving onshore over the West Coast, with fair skies.
Sun, Feb 23rd shows the RRR High Pressure ridge strengthening over the West Coast, with a warming trend.
Special Notations;
Forecast guidance and Satellite Imagery confirms what appears to be the ‘biggest storm event since Dec 2012’ reaching into NORCAL during the weekend, supplied by a narrow but super wet Atmospheric River (AR) which is riding over the top of the High Pressure ridge and beginning to flow into NORCAL and portions of CENCAL, with extremely heavy warm air advection type rains bringing much need precip which during the next 7 Day period should bring most NORCAL precip Stations close to normal/average for the month of February. Projected rainfall rates may run as high as 0.3 inch per hour or higher during the peak of the storm which could bring some localized small streams and creeks to flood levels, but mainstem rivers should remain below flood stage. With heavy rains in short duration time-spans also brings a real concern of debris and mudslides to the burn areas, especially into the RIM FIRE area of the Tuolumne River basin. The weekend deluge for NORCAL is great news, but does not erase the Drought deficit for the season, due to the lack of rainfall during Dec/Jan. Having one month of normal rainfall for NORCAL will not end the Drought but it increasingly looks like it will put a dent into it. The projected QPF if verified would bring the Season to Date Totals for some NORCAL Stations along the NW coast from a current 27% to 29% levels up to as high as 64% to 75% of normal for the Season to Date. This tremendous bump is occurring from just one weekend storm! That gives a testimonial to the potential effectiveness of what an Atmospheric River is capable of accomplishing.
On the other hand, the situation for CENCAL remains more tenuous and uncertain, since charts are consistently indicating that the primary focus of the heavy rains for the most part trend to the northern portions of CENCAL. It would a tremendous help to CENCAL if the activity over NORCAL during the next 2 days could spread further south about 100-150miles and drench the Southern Sierra, but the current forecast charts do not depict that happening. But even if the AR wavers or oscillates by only 50miles further to the south, the estimated QPF’s over CENCAL could roughly double, but that is wishful thinking. Nonetheless we are grateful for whatever rainfall we are gifted, even though it is warm with high snowlevels …at least it is putting water into the basins.
PRECIPITATION FORECAST
Day 1 – 5 QPF:
At 12Z (9am) Saturday thru the period Wednesday night (Thursday @4am) shows;
Sat shows up to 5 inches in the wet spots of the Feather River basin, surrounded with up to 4 inches from near Lassen southward to the I-80 corridor, and also along the northern Mendocino Co coastline and into the South Fork of the Eel River basin, with up to 3 inches from Cape Mendocino south to the Russian River basin and from the Feather River basin southward to the Stanislaus River basin, with up to 2 inches from Cape Mendocino region southward along the coast to Marin Co, and surrounding the Shasta River basin, and from Mt Lassen southward to the Tuolumne River basin, with up to 1 inch covering most of NORCAL, down to the Santa Cruz mts, and into the northern portion of the San Joaquin River basin, with up to 0.7 inch from Watsonville to Modesto to the Kings River basin, with up to 0.5 inch from the Santa Lucia’s to Merced to the Kaweah and upper Kern River basin, with 0.2 inch from San Simeon to Firebaugh to Hanford to the White River basin, and with >0.1 inch from near Malibu north to Lost Hills to near Shafter to the Kern Cyn Powerhouse. Most of Kern Co and SOCAL remain dry.
Sun shows lighter amts of rainfall over NORCAL, with up to 2 inches from the Feather River basin southward to the Stanislaus River basin, with up to 1 inch near Ben Lomond and Santa Cruz, and from Mt Lassen southward to include the northern San Joaquin River basin, with up to 0.7 inch along most of the NORCAL coastal region, the Shasta River basin and from the Northern Sierra SE to include most of the Kings River basin, with up to 0.5 inch covering most of NORCAL, down to the Santa Lucia’s to Merced to the Tule River basin, and with 0.2 inch from near San Simeon to Firebaugh to near Hanford to near Porterville, down to near Riverkern, with >0.1 inch from near Arroyo Grande to Kettleman City to near Shafter to near Bakersfield. Most of Kern Co and ALL of SOCAL remains dry.
Mon shows >0.1 inch of lingering showers remnant from the previous day into the far NW Coast, the northern Sierra from near Mt Lassen southward to near the Kern Cyn Powerhouse, across portions of the Tehachapi Mts and into the upper Mojave region and Mono/Inyo Co, and then down thru the higher terrain of SOCAL
Tue shows CA dry, except for the far NW Coastal region with up to 1 inch into Del Norte Co diminishing to 0.1 inch near Ukiah to Red Bluff
Wed shows CEN/SOCAL dry, with NORCAL wet to very wet, showing up to 3-4 inches of precip into the Klamath and Smith River basins with up to 1 inch into the Lassen on North Fork of the Feather River basin, diminishing down to 0.1 inch as far as SFO to Bridgeport.
5DAY ACCUMULATION CHART:
16-Day QPF estimates (from the 12Z GFS) for;
Days 1-7 shows NORCAL receiving heavy accumulations of precip with target focal points showing of up to 14.1 inches along the NW coastal region, with up to 11.4 inches into the Feather River basin, with up to 7-10 inches along much of the NW coast from the Russian River basin northward to the OR border, and also in the northern Sierra into the Feather, Yuba, and American River basins. Rainfall amts diminish rapidly to the south over the CENCAL region, with amts ranging up to 4 inches into the Southern Sierra into the Tuolumne River basin diminishing to 3 inches in to the Merced River basin, diminishing to 0.3 inch for Isabella. Freezing Levels rise from near 8K Friday up to 9.5K during the weekend storm and up to 12K by Wednesday.
Days 8-16 shows light amts of precip up to 1 inch for the NW coastal region of CA with rapidly diminishing amts to the south, with CEN/SOCAL remaining dry.
Wednesday’s comparison QPF charts;
Friday’s 12Z 9am GFS 384hr, 16Day QPF Chart, depicting an Atmospheric River (AR) stretching across the PAC and reaching the NORCAL coast, dropping up to 11-12 inches locally in the wetter favored areas of the NW coastal region and into portions of the Feather River basin, surrounded with a very large section of NORCAL receiving up to 5-10 inches of precip. Amts rapidly diminish to the south over CENCAL, with amts of nearly 2 inches for the northern SJV diminishing down to >0.1 inch for Kern Co. A slight oscillation or wavering of the AR could bring more precip further south. SOCAL is depicted as remaining DRY.
SATELLITE
Water Vapor 4KM Image of the West Coast Friday, February 7th, 2014, shows the development of an Atmospheric River (AR) taking aim at NORCAL originating from near the Hawaiian Islands. The AR will spread heavy rainfall across NORCAL and portions of CENCAL over the weekend.
Visible Satellite Image of the CA Friday, February 7th, 2014, shows subtropical cloudiness reaching into NOR/CENCAL, with upslope cloudiness along the Eastside SJV into the Sierra foothills. SOCAL remains mostly clear.
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Comments and suggestions always welcome.
Atmospherics Group International
Dan Gudgel Steve Johnson
559-696-9697 559-433-7316