Monday, September 29, 2014

It's Going to be a Troughy Week

Public weather briefing

After a near perfect and mild weekend, Monday started off continuing this trend as the morning temperatures were again above average and there were mostly sunny skies. Temperatures for this afternoon are expected to stay well above normal. Clouds are expected to move in this afternoon that may bring a slight chance of rain for this evening followed by a much cooler Tuesday with mostly sunny skies. chances of rain and possibly thunderstorms increase into Wednesday through early Friday. A much cooler weekend is in store with highs only in the 50s on Saturday and highs in the low 60s on Sunday.


Tonight. Slight chance of showers early, mainly before 7pm then cloudy and cooler with lows around 44. Northeast winds 5-10mph.

Tuesday. Mostly Sunny and cooler with a highs around 62. East winds around 5mph.

Tuesday night. Partly cloudy. Lows around 46 with light southeast winds.

Wednesday. Mostly cloudy. A 30% chance of showers after 1pm. Highs around 70 and southeast winds 5-10mph. 

Thursday. Showers and thunderstorms likely. Southwest winds 5-10mph. Highs around 74. Chance of rain 60%.

Outlook for Friday through Sunday. Showers ending early Friday turning cooler and breezy Friday with highs in the low 60s. Sunny Saturday with highs in the mid 50s. Partly cloudy Sunday with highs in the low 60s. [CONFIDENCE: MEDIUM-HIGH]

Forecast Discussion

Shortwave trough with associated vorticity maximum swinging southeast through the central Great Lakes region will drag a cold front and a weak 850 mb front through the region early in the period. Weak to moderate 850-700 mb negative omega associated with right jet entrance region of upper jet streak will result in at least a chance of SHRA as well. Very little instability is seen so no mention of thunder. Omega wanes quickly after 00Z and drying at 850mb will bring an end to any showers. Expect temperatures to drop to the mid 50s by mid evening. Low stratus deck should overtake the area. Weak high pressure on Tuesday will bring a sunny but cooler day as 850 mb temperatures drop to around 8 degrees Celsius. Cool east winds will result in temperatures in the low to mid 60s. Low level WAA quickly occurs in the wake of departing high as longwave trough approaches from the west.

Wednesday through Thursday night there is upper flow that becomes southwest as upper ridge moves off to the east during the day Wednesday. Upper jet increases over Minnesota and puts southern Wisconsin in an area of upper divergence resulting in upward vertical motion after 21Z on Wednesday. GFS quite a bit weaker with upward motion and vorticity advection. Small chances of rain seem reasonable with the bulk to our Northwest. Meanwhile upper jet strengthens to the west Wednesday night through Thursday night which results in upward vertical motion with embedded vorticity maximums through the period. Will put likely wording for rain/storms after midnight Wednesday night through at least early Thursday. NAM dries things out Thursday while GFS keeps precipitation in area through early Friday morning.

Early rain on Friday will give  way to cooler and dryer conditions with 1000-500 mb thickness dropping to about 5420 meters. Much cooler weather is forecast for Saturday under mostly sunny skies. An embedded vorticity maximum may bring light showers overnight Saturday into Sunday.

Ryan Tvedt
Ross Braatz

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