Wednesday, November 5, 2014

Cooler Weather on the Way with Chances for Snow?

Public weather briefing

Rain showers will continue for the overnight hours. Turning cooler Thursday morning as winds pick up from the north and the precipitation could end as some flurries or flurries/drizzle. Staying cloudy most of the day and winds will ease as the day wears on. Next chance of precipitation comes Friday night as a cold front passes through area late in the evening with only a moderate chance of light showers. Another cold arctic airmass plunges in behind the frontal passage on strong north winds and below normal temperatures Saturday and Sunday. Highs running around 8 degrees below normal for the weekend. Attention then turns to possible storm system/frontal boundary for Monday into Veterans Day.  


Tonight. Showers likely, mainly before 3 am. Lows around 37. South winds around 10 mph shifting to the northwest late. Chance of precipitation 80%. Rainfall amounts less than a tenth of an inch. 

Thursday. Chance of rain and snow in the morning, then cloudy. High around 41. Breezy, with a north wind 15-20 mph, with gusts to 30, mainly before noon. Chance of precipitation 50%. Rainfall amounts less then a tenth of an inch. Little or no snow accumulation expected.  

Thursday night. Clearing. Lows around 26. Light northwest wind. 

Friday. Sunny in the morning, then turning partly sunny in the afternoon. Highs around 43. Light winds becoming south in the afternoon.  

Saturday. A slight chance of showers early, becoming mostly sunny in the afternoon. Highs in the lower 40s. Breezy early. North winds 15-20 mph. Chance of rain 20%. Rainfall amounts less than a tenth of an inch.

Outlook for Sunday through Veterans Day. Sunday mostly sunny, slight chance of rain at nigh. Highs in the lower 40s and lows in the lower to mid 30s. Monday chance of rain and snow. Highs in the lower to mid 40s. Veterans day mostly cloudy. Cooler highs in the low 30s. [CONFIDENCE: LOW TO MEDIUM]

Forecast Discussion

Overnight: Rain showers will overspread the area from the southwest with the approaching cyclone from the west within an area of moderate upward vertical motion in association with warm air advection at 850 mb. 850 mb temperatures hovering between 2 and 4 degrees C will drop to below 0 by 12Z changing an light rain or drizzle to flurries with little or no accumulation excepted. 

Thursday thru Friday Night: As the surface cyclone mentioned above passes to the east, it will deepen slightly producing a strong pressure gradient between it and the surface anticyclone building into the central plains. Strong cold air advection will cause highs Thursday to be cooler then today. Only low 40s excepted with 925mb temps only around -2 C. Dryer air an subsistence behind the shortwave will clear things out Thursday night into Friday. Clouds will increase Friday afternoon with southerly winds and warm air advection ahead of the next shortwave diving in from the northwest. NAM and GFS are in very good agreement in bringing a cold front though the area after midnight Friday night. There is positive vorticity advection with neutral temperature advection seen on both NAM and, to a lesser extent, the GFS, at 06Z Saturday, so chance of rain will be in the forecast late Friday night.

Saturday and beyond: Negative vorticity advection occurs Saturday as low levels dry out on breezy northwest winds and cold air advection producing downward vertical motion and an end to any precipitation. 850 mb temps drop to -8C on the NAM and -6 on the GFS. A blend will be perferred for highs on Saturday. Sunday looks to be sunny but cool with slightly warmer temperatures at 925mb climb to around 2C so will lean highs around 40. Monday is interesting because the GFS and the ECMWF diverge with the evolution of a winter event. 18Z run of the GFS has a baroclinic zone setting up just south of the area and a weak low pressure moves northeast along this boundary and produces an area of light snow with the 5400 m thickness line right overhead. The 12Z ECMWF run however, shows a well organized surface cyclone moving in from the central plains. ECMWF is a little warmer than the previous day's run so low confidence in this system. Will lean toward ECMWF because of the run to run consistently. After that, strong cold air advection ensues after departing surface cyclone or baroclinic zone depending on model. Coldest air of the season to be seen after Monday system as 850mb temps drop to -14 to -16 on GFS and -12 on ECMWF. Highs on Veterans day may struggle to reach 30 with some wind to put even more of a chill in the air. Stay tuned.     

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