Wednesday, October 29, 2014

A Frighteningly Frigid Friday Around the Corner!

Public weather briefing

The current weather over Madison and southern Wisconsin is quite fair with mostly clear skies and seasonal temperatures.  Clear skies are expected to persist overnight with clouds increasing Thursday morning and becoming overcast by the afternoon.  Temperatures will remain pleasant during the day with a high near 51 and moderate westerly winds shifting northerly.  A temperature decrease is expected to begin late Thursday as an anticyclone to our west and a weak cyclone to our east begin funneling cold Canadian air directly into the heart of the mid-west.  This will also produce strong northerly winds in the coming days and a 20% chance of precipitation, mostly in the form of rain with a few flurries possible, Thursday night.  This blast of cold air will linger into Halloween Friday with a high temperature of 39 and a low near 25, well below average.  Wind chill will also become a factor Friday with consistent winds around 25mph during the day and 10mph overnight making it feel more like 29 during the day and 15 during the night.  Clearer skies will also return Friday and persist through the weekend.  Temperatures will gradually warm over the weekend, reaching the lower 50's by Monday, as a shift from northerly to southwesterly winds begins bringing warm air to the region.  A chance for precipitation also presents itself late Monday afternoon as a surface cyclone centered over northern Minnesota with a frontal zone that extends to Texas begins to pass through southern Wisconsin.  


Tonight. Skies becoming mostly cloudy after midnight with a low temperature near 34.  Westerly winds around 6mph

Thursday. Continued overcast conditions with a high of 51.  Westerly winds around 8mph with a shift to northwesterly later in the afternoon. 

Thursday night. Mostly cloudy with clearing skies after midnight, a low near 32.  Northwesterly winds increasing to near 18mph by the early morning hours.  20% chance of precipitation with trace accumulation.  

Friday. Mostly sunny and much cooler temperatures with a high around 39.  Strong northerly winds up to 25mph.  Clear skies overnight with a low near 25 and weaker winds at 10mph 

Saturday. Clear throughout the day with a high in the lower 40's.  Slightly breezy with 10mph northerly winds tapering off to light and variable.  Low in the upper 20's overnight. 

Outlook for Sunday through Tuesday.  Mostly clear skies will persist through Sunday with westerly/southwesterly winds around 12mph with a high in the upper 40's and a low in the upper 30's.  Skies will become cloudier as the day progresses Monday with continued westerly/southwesterly winds around 12mph.  A high in the low 50's and a low in the low 40's is expected with a 40% chance of showers late Monday afternoon. Tuesday will be cooler once again as a band of cold air moves in through the area with moderate westerly winds blowing in at around 15mph, high in the lower 40's and low in the upper 20's.[CONFIDENCE: MEDIUM]

Forecast Discussion 

Today's weather was primarily dominated by an anticyclone centered over southern Illinois, clearing the skies and bringing westerly winds at a moderate 5 mph. A strong shortwave ridge-trough couplet moving through the area at 500mb will be the primary influence for our weather over the next few days.  As the trough axis drops into northern Wisconsin from Ontario it will advect a strong area of positive curvature vorticity that is projected to pass over eastern Wisconsin in the early morning hours Thursday. This will bringing us our greatest chance for precipitation by forcing upward vertical motion according to the QG omega equation over the four day period. The GFS model shows oscillating percentages of relative humidity (between 70% and 40%) in the southern Wisconsin area with more humidity to the west than the east, while the NAM model shows a more solid area of consistent relative humidity (around 60%) in the same area. This supports the possibility for cloud formation and precipitation, particularly in the southeastern portion of the state. However, the advection of cold air from the north will work against the vorticity advection by causing subsidence, hence providing some uncertainty as to whether or not to expect precipitation. Consideration of Q-Vectors and their convergence between the 900mb and 700mb levels indicates the likely hood of upward vertical motion over the eastern part of the state at this time. This leaves a good chance for precipitation Thursday night in that area. Forecasing up to .175in of accumulated precipitation by 12z Friday, NAM model seems to agree with this projection. The GFS model, however, shows trace amounts of precip anywhere in the state. The uncertainty from these factors is why our forecast confidence is medium. In any case, with the 5400m thickness line from the 500:1000mb layer extending well south of Wisconsin it is clear that any precipitation will likely include snow showers.

This ridge-trough couplet will continue to advect cold air into the Midwest well into Friday with northerly wind speeds blasting in at a consistent 23 mph. With the trough axis well downstream from us, skies will clear as the cold-air advection brings subsidence into the forecasting region. As the ridge axis passes through late Friday night and into Saturday morning we will see a decrease in the pressure-gradient force and thus weaker northerly winds tapering off to around 10 mph, finally minimizing to light and variable winds as the center of the ridge enters the area mid-day Saturday. Skies will remain clear with a slight warm-up Sunday as the ridge passes through and advects southwesterly wind at around 10 mph. This wind will persist into Monday, where the NAM model shows an upper-level trough moving into the region. This will cause divergence aloft due to curvature acceleration downstream of the trough axis. Positive vorticity advection into the northwestern part of the state supports possible vertical motion in that area, bringing clouds and possible precipitation later in the day. Finally, Tuesday will see another cooldown as a band of cold air extending south from a Canadian cyclone passes through the area with westerly winds blowing in at around 15 mph.

Craig Oswald
Cody Kuchinski

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