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Following a brief lull in storm activity on Wednesday the next storm is rapidly approaching the West Coast and will soon begin to spread unsettled wx across the State.  Clouds are already increasing over NORCAL as the cold front sags into southern OR.  The storm has slowed down, with its peak intensity to reach into CENCAL late Friday into Friday evening with moderate to possibly briefly heavy precip.  Thunderstorms are less likely due to the late arrival schedule but nonetheless are still possible even during frontal passage.  A new twist to the forecast is the additional precip that may occur on Saturday from an overrunning warm air advection Atmospheric River (AR) spreading across NORCAL during Saturday and into CENCAL overnight Saturday.  An overrunning warm air rain event is rare on a WNW flow pattern.  The forecast for next week’s brief Heat Wave indicates that widespread 90°’s are likely especially on Wed.  Some may occur as early as Tue at the warm spots.  The cutoff of the Heat Wave is uncertain due to major model differences which point in very different directions.  Some charts continue the heat into Thu/Fri, while other charts show cooling initiating on Thu and Fri.  Week #2 forecasts are also highly questionable and therefore issued with a low confidence level.   

 

Hazardous Wx Outlookfor CENCAL regarding gusty winds up to 55mph near the mountain passes and canyons for portions of Kern Co, with a wintery mix of snowfall down to 4,500ft over the Tehachapi Mts and down to 4,000ft in the Southern Sierra accompanied with winds gusting up to 70mph, and with chances of Thunderstorms on Friday throughout CENCAL, with winds up to 35mph throughout the central and southern SJV.

 

Winter Weather Advisory; for the Sierra above 6,000ft, with snow accumulations up to 12 inches or more around YNP to Mammoth Mt, with up to 6-9 inches or more as far south as Sequoia NP.  High winds will accompany this storm.

 

FORECAST SUMMARIES

 

Days 1-7;                                                          

       

Thu, Apr 24th               The zonal-type flow pattern across the PAC basin is buckling Thursday, as High Pressure builds near 160W sends colder GOA air to the south in a deepening large broad trough that is digging southward at 130W with winds aloft over CA beginning to veer to the SW.  This storm has been on the GFS charts for the past 16 Days in one form or another and now it’s finally showtime.  Chalk this one up for the GFS Fantasyland.  The nose of a 100kt Jet Stream at the base of the trough points at CENCAL, as a tightening pressure gradient develops over NORCAL with increasing onshore winds.  A narrow frontal boundary brings light rains as far south as the North Bay region by afternoon with clouds spreading rapidly southward thru all of NORCAL and into CENCAL.  Initially the prefrontal warm sector carries a moisture content of up to 1.10 PWAT thru NORCAL with descent rainfall efficiencies, however as the front sags southward it weakens and dries out, with PWAT’s of only 0.75 inch by the time it penetrates into CENCAL, thus lower rainfall rates.  Overnight as the trough axis continues to deepen and approach the CA coastline, the cold front dips southward into CENCAL running from near the Central Coast to Tahoe with rainshowers spreading thru the SJV and into the Southern Sierra, with clouds extending inland as far south as Santa Barbara into the Tehachapi Mts and into the entire Southern Sierra.  Afternoon Convective charts show a portion of the Southern Sierra is painted with Supercell Parameters into the Kings River basin and with Hail Parameters showing up over the SAC Valley. Orographics kick into gear producing precip into the Southern Sierra reaching as far south as Sequoia NP overnight, initially with fairly high snowlevels.

                                    MAX                                                     MIN                                                     PRECIP   

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Fri, Apr 25th                 A very interesting day is in store for CA as a late season winter type storm plows onshore.  The latest GFS charts have shown a slight slowdown in the forward speed of this storm as compared to previous models which means a later frontal passage thru CENCAL.  This places the threat of Thunderstorms out of phase with diurnal heating during the afternoon, therefore the threat of Thunderstorms is either reduced or delayed until late in the day or evening hours, especially for the South Valley where considerable cloudiness may exist from the cold front, thus hampering instability.  While the cold front has now passed thru NORCAL, it is still trudging thru the Tehachapi region of CENCAL at modest pace producing moderate to perhaps occasionally heavy rainshowers in the lowlands and moderate snowfall into the Sierra aided by the perfect alignment of the SW flow aloft hitting the Sierra range at a perpendicular angle.  As the trough axis moves onshore it is accompanied with the upper Jet Stream at 100kts aligned from the SW which advances southward into SOCAL.  Imbedded Thunderstorms within the frontal boundary itself are not out of the question for the SJV… unlikely but possible.  Currently the latest Convective charts show the western Merced Co region with the earliest Convective Parameters by 21Z.  This area is a favored region for SJV Thunderstorm development, in a post frontal environment.  3 hrs later the Convective Parameters show high readings over a larger region of the Westside from Fresno Co northward to the Delta with Supercell Parameters popping up near Chowchilla/Madera and Hail Parameters painted from the Delta southward to Fresno Co.  3 hrs later, in the early evening, the Convective Parameters show high readings from Madera Co southward into Kings Co coinciding with Hail Parameters.  Convection is conditional with surface heating which can only occur with sunshine, therefore if extensive frontal band cloudiness exists then Convective Potential is lowered considerably. As the -30°C cold core moves onshore over NORCAL by mid afternoon, freezing levels begin to plummet in the Northern Sierra as the cold air aloft is arriving later than previously scheduled.  By 06Z the trough axis is passing thru CENCAL with strong upper level support producing mod/heavy rain over the SJV from Fresno Co northward with lighter amts further south.  Top-of-the-scale vort dynamics spread across the Tehachapi Mts during the evening with possible strong winds and heavy precip.  An additional update on Friday’s CONVECTIVE POTENTIAL is warranted tomorrow.  Current QPF show 1+ inch possible from the higher elevations of the Feather River basin south into the Tuolumne River basin southward into the San Joaquin River basin, with diminishing amts further south. It also should be noted that a long-fetched Atmospheric River (AR) is accompanying a high speed Jet Stream at 160kts that is racing across the PAC basin from the WPAC, with the moisture supply originating near 20N at 140E in the subtropics.  This AR is loaded with high octane PWAT’s of up to 1.75 inches with the leading edge approaching 40N at 130W a few hundred miles west of Cape Mendocino containing a core PWAT value of 1.5 inches.  It is interesting to note that the GFS shows a bomb storm with impressive rapid intensification occurring in the WPAC located at 28N 155W on Friday night.  The 24hr intensification goes from a 1016mb pressure down to a 990mb pressure! Is this a brief Tropical Cyclone?  Not sure, but the important role that it plays for the West Coast is that it is a major contributing feature to the stable conveyor belt of subtropical moisture reaching across the PAC basin flowing into the Atmospheric River (AR)

MAX                                                     MIN                                                     PRECIP   

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Sat, Apr 26th                As the trough shifts eastward into the Rockies and pressures rebound over CA slightly the flow pattern switches to NW while the aforementioned Atmospheric River (AR) brings renewed warm-type rains into NORCAL which gradually sag southward thru NORCAL during the day and into CENCAL overnight, with rising snowlevels.  This is a new development with a new storm reaching into CENCAL which was not even showing possible just 24hrs ago. WOW, surprise, thanks for the 72hr notice, we’ll accept the rainfall anytime… Although this storm was on the charts it was not pumped up with an AR, nor was it progged to reach into CENCAL, but only skirt the far northern portion of NORCAL, what a difference 24hrs can make on the charts!  Cold air left in the wake of Friday’s storm system lays in place over the lowlands with cool, below average temps, as warmer air aloft spreads over CA.  Since this next surprise system is drying out rapidly as it reaches into CENCAL due to the lack of upper air support, the Sierra from the San Joaquin River basin northward are gifted with the most precip, with rising snowlevels.  Without dynamic support the lowlands are less inclined to see much precip from this warm air advection type overrunning event, yet the Sierra may benefit from orographic lift producing showers even though the flow pattern is WNW.

MAX                                                     MIN                                                     PRECIP

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Sun, Apr 27th               Broad High Pressure at 130W rebuilds northward, forcing the Jet Stream and storm track to shift northward into the PAC, carrying the 140kt Jet and the accompanying Atmospheric River (AR)with it, leaving CA with diminishing Sierra showers and clearing skies with warming temps.  Overnight 576dm High Pressure moves northward over CENCAL running from Monterey SE to near Blythe as an upcoming Heat Wave initiates its first strong hold on CA.

MAX                                                     MIN                                                    PRECIP

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Mon, Apr 28th              Models are recently trending with much stronger High Pressure building northward over CA which produces a brief heat wave Tue/Wed/Thu!  Currently the 0Z GFS shows 582dm heights spread onshore at Pt Conception on Monday and then spread northward over most all of CEN/SOCAL overnight.  This produces a rapid warm up over all CA with the storm track displaced far north into southern BC, Canada with warm type showers still flowing into OR/WA.  SJV temps jump by an average of about +10°.  Overnight the flow pattern over SOCAL becomes offshore with Santa Ana conditions possible.

                                    MAX                                                     MIN   

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Tue, Apr 29th               The entire State of CA is basking beneath 582dm High Pressure with a 587dm circulation centered SW of San Diego.  The warm up continues with fair skies producing 90°’s in the hot spots of the SJV.  The 582dm heights spread northward to the WA/OR border along the Columbia River overnight.  Warm temps overnight range from the upper 50°’s and in the 60°’s in the SJV, with light winds.

                                    MAX                                                     MIN                                                  

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Wed, Apr 30th              The 582dm High Pressure ridge is in retreat out of OR during the morning and dropping southward to the Central Coast by afternoon.  This is too late to prevent SJV temps from gaining a degree or two from Tuesday, producing widespread 90°’s in the SJV.  Overnight an onshore flow develops which brings cooler marine air thru the gaps and passes of the Coast Range, with lower temps inland.

                                    MAX                                                     MIN                                                  

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Special Notation;   In the nearer term forecasts the Friday storm system and the Heat Wave next week continue to evolve on forecast charts.  In the long range charts into Week #2 there is much uncertainty due to the lack of model consistency between runs as well as no consistency between models…period!  They are bouncing all over the place.  The explanation is that the atmosphere is in a state of flux due to the change of seasons, so we’ll just have to wait and see if models settle down so we can have a better look into the future.  

 

Note; The first 100° days in Fresno and Bakersfield occurred on April 23rd and 24th in 1910.

                                   

Days 8-16, Long Range into Fantasyland OutlookDay 8 begins;

 

Thu, May 1st shows lots of disagreement with regards to the evolution of the High Pressure retracting southward into SOCAL, with some models faster than others with regards to the cool-down from Wednesday’s high temps.   For now, we’ll suggest the max temps to be near persistence from the Wed max.  This will be refined when additional models begin to agree.  An approaching cut-off Low Pressure offshore west of the Central Coast may provide for a southerly flow pattern ahead of its arrival, which could extend the warm temps into Friday!

 

Fri, May 2nd shows much disagreement between models with some holding the Low Pressure circulation together as it nears the CENCAL coastline, while others wash it out into a weak trough configuration which brings high cloudiness into NORCAL.   

 

Sat, May 3rd shows a weak trough axis situated offshore near the CA coastline with onshore flow providing a marine influence to the inland valleys, with partly cloudy skies for NORCAL.

 

Sun, May 4th shows no strong upper level features over the PAC basin or NA.  Highly unusual to see such a docile atmosphere over such a large region this time of year.  This appears more like the summer doldrums.   Charts show a weak upper level disturbance over the PACNW with a westerly flow pattern over CA.

 

Mon, May 5th shows the mild and quiet conditions continuing over CA with the dry westerly flow pattern.  Persistence rules over CA.

 

Tue, May 6th shows no changes from the previous day with dry/mild conditions over CA, with a few high clouds possible over NORCAL.

 

Wed, May 7th shows a split flow possibly developing over the West Coast as High Pressure builds over western Canada up into the Arctic, with the onshore westerly flow veering slightly to the NW, producing persistent conditions into the Great Valley and CENCAL.  Overnight cold air from the Aleutians forms a Low Pressure storm center west of Cape Mendocino with a 100kt Jet Stream wrapping around the base of the Low Pressure west of CA aiming at SFO.

 

Thu, May 8th shows a large 540dm Low Pressure circulation which contains a -30°C cold core and is encompassed with high speed vorticity around the Low.  As this system slowly approaches CA winds aloft veer to the SW as temps aloft begin to fall when the -20° isotherm reaches Cape Mendocino during the afternoon.  700mb charts show light rains spreading across NORCAL from the Golden Gate northward into the Shasta River basin, as winds veer more southerly and the cold front aligns more N to S.  Clouds increase from San Simeon northward to Tahoe, while most of the interior of CENCAL remains clear.  Overnight the cold front spreads eastward with rains finally reaching into the northern Sierra and extending due southward into the northern SJV with showers possible down to near Fresno.

 

Fri, May 9th shows the 525dm Low Pressure center remains nearly stationary while pressures over CA drop.  The sphere of influence of the Low Pressure continues to spread over NOR/CENCAL as the -26°C isotherm reaches into Del Norte and Humboldt Co, and the -20°C isotherm passes over FAT.  The upper 300mb Jet Stream on a SW alignment moves onshore over CENCAL, with increased clouds spread over CENCAL.  Charts show rainfall is limited to the North Coast with this system while it rotates offshore.  This feature may possibly become locked into position near its current location as it becomes tucked beneath a giant High Pressure ridge over eastern Alaska and western Canada.  What an interesting way for the GFS to leave us hanging. 

 

                     

 

NEW  CA  RECORDS

 

Station                         New Record                Old Record__

Newport Beach              60°       Tied                  60° set in 1997

Riverside                       57°       Tied                  57° set in 1997

El Cajon                        59°       Tied                  59° set in 1997

Big Bear                        44°                               43° set in 1999

 

 

 

PRECIPITATION FORECAST

 

Day 1 – 5 QPF:

At 12Z (9am) Thursday thru the period day Monday night (Tuesday @5am);

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CENCAL/SJV close up QPF for Friday;

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7 Day QPF Chart; shows a precip focal point of up to 1.9 inches for the NW coastal region and the Smith River basin, with 1.7 inches for the Northern and Southern Sierra with orographically enhanced precip increasing from the Westside SJV toward the Eastside SJV, and then increasing further into the Sierra. 

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

Days 1-7; shows up to nearly 2 inches for the NW coastal region with up to 1.7 inches for the Sierra wet spots near YNP and the Feather River basin, with the Great Valley receiving up to 0.75 inch near the Eastside foothills diminishing to about 0.25 inch along the Westside.  SOCAL shows up to 0.5 inches into the LA basin while the lower deserts remain dry.

Days 8-16; shows very little precip, even for NORCAL which shows only >0.1 inch for the wet spots.

 

GFS 384hr, 16Day QPF Chart shows up to 1.75 inch for the wet spots of coastal NORCAL, with up to 1 inch of precip from the Bay region into the Sierra, with diminishing amts further south into CENCAL, with light amts reaching into SOCAL.  Since the bulk of precip occurs during Days 1-7, suggest using the WPC 7 Day chart above for a more accurate detailed look at the precip quantities.

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EVAPOTRANSPIRATION

 

Weekly Evapotranspiration rates for the local 7 Counties shows up to 1.24 inch near Merced, up to 1.45 inch in SW Fresno Co and up to 1.82 inch near Ridgecrest during the next 7 Days.  The Sierra Crest evaporation loss ranges from 0.77 inch in the upper Tuolumne River basin up to 1.41 inch near Isabella.

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SATELLITE

 

Water Vapor 4KM Image of the West Coast Monday, April 23rd, 2014, shows the outline of High Pressure over CA, with the storm track placed into the PACNW where a Low Pressure trough is digging southward toward CA.  This system will bring stormy wx to CA as early as Thu night for NORCAL and then Fri into CENCAL.

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Visible Satellite Image of the CA Wednesday, April 23rd, 2014, shows abundantly clear skies over all of CEN/SOCAL, with increasing cloudiness over NORCAL as a storm begins to approach CA on Thursday from the NW.

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Color Image of CENCAL Wednesday, April 23, 2014;

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If you have any questions please call or text at any time of day or night. 

Comments and suggestions always welcome.

 

Atmospherics Group International

Dan Gudgel              Steve Johnson

559-696-9697                 559-433-7316