Af­ter a wild warmup, colder air re­turn­ing on Satur­day

Richmond Times-Dispatch - - METRO - John Boyer jboyer@times­dis­patch.com Check Richmond.com/weather for John Boyer’s videos and up­dates as the fore­cast evolves. Con­tact him at JBoyer@ times­dis­patch.com or (804) 649-6209, and fol­low him on Twit­ter, @boy­er­weather.

Richmond didn’t just thaw out af­ter the snow­storm; the ther­mome­ter soared from zero to 70 de­grees in just 79 hours.

There isn’t an of­fi­cial record for Richmond’s fastest warmup, but that was cer­tainly among the area’s big­gest tem­per­a­ture rises in such a short time.

Thurs­day’s high of 70 fell a de­gree short of ty­ing the daily record for Richmond. Jan. 12, 2005, brought a high of 71.

It should be no sur­prise that the pen­du­lum is about to swing back to colder weather this week­end. For­tu­nately it won’t swing all the way to more record, bit­ter cold.

The mag­ni­tude of this week’s warmup was un­usual, but it ac­tu­ally makes sense in the con­text of the larger weather pat­tern. A strong up­per-level ridge or trough is usu­ally re­quired for a tem­per­a­ture ex­treme. When Vir­ginia is colder than nor­mal, the op­po­site ex­treme can of­ten be found on the other side of the coun­try. The west-to-east move­ment of our weather cre­ates the fa­mil­iar back-and-forth tem­per­a­ture swings. The pat­tern over the past sev­eral days was more am­pli­fied than usual.

On Sun­day and Mon­day, the jet stream gen­er­ally took a curvy path from the north­ern Rock­ies down to the South­east U.S. and back up to the Cana­dian Mar­itimes. In other words, a deep up­per trough was cross­ing right over Vir­ginia.

By it­self, the pat­tern prob­a­bly wouldn’t have led to record lows; but the snow-cov­ered ground helped push the tem­per­a­ture at or below zero around Richmond on Mon­day morn­ing.

That big dip in the jet stream weak­ened and mi­grated out into the north­ern At­lantic Ocean. By Thurs­day, the jet stream flowed from south­west to north­east over the coun­try, from Cal­i­for­nia to the Great Lakes.

Vir­ginia and the East Coast are now un­der the in­flu­ence of an up­per-level ridge that will per­sist into the week­end and next week.

Richmond will ac­tu­ally cool down over the week­end de­spite the pres­ence of the ridge aloft. A north­east wind will force a shal­low layer of chilly air into the re­gion — the fa­mil­iar “wedge.” It will be no­tice­ably colder, but not dan­ger­ously cold.

An­other taste of spring­like warmth is a good pos­si­bil­ity next week, but it won’t ap­pear un­til sur­face winds blow in from the south­west on Tues­day and Wed­nes­day.

Win­try mix may re­turn soon

The in­tru­sion of cold air on Satur­day might be enough for light win­try pre­cip­i­ta­tion in some parts of Vir­ginia, but don’t ex­pect a re­peat of last Satur­day’s snow­storm.

Cold air will slide be­neath a layer of warmer air aloft, which could mean sleet pel­lets or freez­ing driz­zle in North­ern Vir­ginia. The chance may get as far south as Richmond, but the lack of an or­ga­nized sys­tem will keep any amounts on the very light side.

In Richmond, ex­pect an over­cast sky with a nar­row range of tem­per­a­tures in the mid-to-up­per 30s through­out the day.

Even light amounts of freez­ing driz­zle can cause messy roads. It’s a good idea to stay aware of con­di­tions if trav­el­ing to­ward Wash­ing­ton D.C., Staunton or Har­rison­burg on Satur­day.

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