What pushed the Cen­tral Coast ‘storm door’ back open?

The Tribune (SLO) (Sunday) - - Local - BY JOHN LIND­SEY

The East­ern Pa­cific High was an­chored off the Cal­i­for­nia coast­line and com­bined with tran­si­tory high-pres­sure sys­tems over the Great Basin — the area be­tween the Sierra Ne­vada range to the west and the Rocky Moun­tains to the east — cre­ated per­sis­tent Santa Lu­cia north- east­erly (off­shore) winds. These winds brought in a cold air mass that cre­ated frosty morn­ings and bone-dry weather af­ter Christ­mas to the Cen­tral Coast.

Then a sig­nif­i­cant change oc­curred in the first part of 2019; the East­ern Pa­cific High weak­ened and shifted, al­low­ing the up­per-level winds (jet stream) and the storms it car­ries and nour­ishes to travel at high speeds across the Pa­cific from west to east to­ward the Golden State. Cur­rently, these up­per­level winds are about 150 mph at 30,000 feet. With tail­winds like this, a pas­sen­ger jet fly­ing from Ja­pan to Cal­i­for­nia would see ground speeds (how fast the air­craft is mov­ing over land or ocean) faster than the speed of sound.

Many of the weather mod­els are pre­dict­ing a cu­mu­la­tive rain­fall to­tal rang­ing be­tween 4 and 8 inches from Satur­day through next Mon­day (Jan. 14) for most of the Cen­tral Coast — with rain or show­ers oc­cur­ring just about ev­ery day. Not only will we re­ceive sig­nif­i­cant rain but heavy snow in the Sierra Ne­vada along with pe­ri­ods of gale-force southerly winds and high sea swells along the coast­line.

So why did the East­ern Pa­cific High weaken and open the storm door to an im­pres­sive se­ries of storms that will march across the Pa­cific into Cal­i­for­nia through mid-Jan­uary, if not longer?

At this time, no­body knows for sure, but it could be the fol­low­ing oceano­graphic, and at­mo­spheric, phe­nom­ena. Here is why.

The first pos­si­ble cause is El Niño (warmer than av­er­age sea sur­face tem­per­a­tures in east­ern Equa­to­rial Pa­cific). These warmer wa­ters in the east­ern Pa­cific pro­duce a more con­sid­er­able amount of evap­o­ra­tion. As this wa­ter va­por as­cends into the at­mos­phere, it of­ten con­denses into thun­der­storms and re­leases tremen­dous amounts of la­tent heat, which fur­ther de­creases the at­mo­spheric pres­sure. This area of low pres­sure, in turn, changes the path of the south­ern branch of the po­lar jet stream, pulling it far­ther south­ward to­ward the Cen­tral Coast.

How­ever, de­spite the for­tunetelling from the warmer-thanaver­age sea sur­face tem­per­a­tures (SST) in the cen­tral equa­to­rial re­gion of the Pa­cific Ocean, “the pat­terns of con­vec­tion and winds are mostly near av­er­age over the trop­i­cal Pa­cific,” ac­cord­ing to NOAA’s Cli­mate Pre­dic­tion Cen­ter.

In other words, the at­mos­phere is not yet re­sponded to the warmer-than-av­er­age tem­per­a­tures such as trade winds weak­en­ing. Their most cur­rent re­port dated Dec. 31 states that “El Niño is ex­pected to form and con­tinue through the North­ern Hemi­sphere win­ter 2018-19.

An­other con­ceiv­able rea­son is the Mad­den-Ju­lian Os­cil­la­tion (MJO).

Un­like El Niño and La Niña, both of which are a stand­ing pat­tern (in other words, they stay fixed in the same ge­o­graphic area) the MJO is a large trav­el­ing pat­tern of in­creased rain- fall and thun­der­storm ac­tiv­ity that propagates east­ward at ap­prox­i­mately 8 to 18 mph across the trop­i­cal parts of the In­dian and Pa­cific oceans.

In 1971, Roland Mad­den and Paul Ju­lian stum­bled upon the pat­tern when an­a­lyz­ing wind anom­alies in the trop­i­cal Pa­cific. How­ever, lit­tle at­ten­tion was paid to the os­cil­la­tion un­til the strong 1982-83 El Niño event, which led re­searchers to be­lieve that the pat­tern may have en­hanced the amount of rain in Cal­i­for­nia. The MJO is also called the “30- to 60-day os­cil­la­tion” and the “30- to 60-day wave.” The lat­est MJO fore­cast from the Cli­mate Pre­dic­tion Cen­ter in Sil­ver Spring, Mary­land, in­di­cates an os­cil­la­tion is present in the western Pa­cific but its fore­cast re­mains un­cer­tain.

There is grow­ing ev­i­dence that what hap­pens in the trop­ics can in­flu­ence our weather along the Cen­tral Coast. Mod­els sug-


gest the os­cil­la­tion will travel east­ward. His­tor­i­cally, es­pe­cially dur­ing El Niño phases, as the Mad­den-Ju­lian Os­cil­la­tion moves across the Pa­cific Ocean, a split in the po­lar jet stream can de­velop. The south­ern branch of the po­lar jet stream can ex­tend far out over the Pa­cific to­ward the coast of Cal­i­for­nia. That river of air in the up­per-at­mos­phere can steer moist, sub­trop­i­cal air to­ward Cal­i­for­nia and can bring sev­eral days of rain.

An­other cause for aboveav­er­age rain is an­other great oceano­graphic cy­cle that can or­ches­trate changes in our weather: a longer last­ing cy­cle called the Pa­cific Decadal Os­cil­la­tion or PDO.

While the ENSO phase typ­i­cally lasts from eight to 13 months, the PDO al­ter­nates be­tween a warm phase ( pos­i­tive) and a cooler (neg­a­tive) phase that can last a few years to decades.

Un­like El Niño, which fo­cuses on SST in the cen­tral equa­to­rial re­gion of the Pa­cific Ocean, the PDO is clas­si­fied by seawa­ter tem­per­a­tures through­out the north­ern Pa­cific Ocean. Ac­cord­ing to Josh Wil­lis, oceanog­ra­pher and cli­mate sci­en­tist at NASA’s Jet Propul­sion Lab­o­ra­tory in Pasadena, the PDO shifted to the pos­i­tive phase. His­tor­i­cally, the pos­i­tive phase of the PDO typ­i­cally en­hances the ef­fects of El Niño events.

Last, but per­haps not least, is the Arc­tic vor­tex that could split into three sec­tions cre­at­ing a block­ing high in the Gulf of Alaska that may con­tinue to help steer storms to Cal­i­for­nia and the po­ten­tial for frigid weather along the East Coast.

All of these or just one or two of these cou­pled con­di­tions be­tween the ocean and at­mos­phere may help to pro­duce above-av­er­age rain­fall for Cal­i­for­nia this year. How­ever, when try­ing to pre­dict that far into the fu­ture, there are no guar­an­tees.

John Lind­sey’s col­umn is spe­cial to The Tri­bune. He is PG&E’s Di­ablo Canyon marine me­te­o­rol­o­gist and a me­dia re­la­tions rep­re­sen­ta­tive. Email him at [email protected] or fol­low him on Twit­ter: @PGE_John.

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