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Scientific Forecaster Discussion
Area forecast discussion National Weather Service Detroit/Pontiac Michigan 647 am EDT Thursday Oct 2 2014 Aviation... //discussion... The light southeast winds have kept a very low stratus deck at ptk/fnt/mbs early this morning. This stratus does not extend very far west of the terminals suggesting that daytime heating will erode these clouds over the next couple of hours. A band of showers now moving into the west side of the state may decrease in coverage slightly has they head east today. Confidence is still high enough to include a period of showers in the terminals. Aside from the shallow moisture this morning...the low levels are expected to remain dry and well mixed enough this afternoon and evening to keep ceilings VFR. For dtw...the rate at which visibilities improve this morning carries some uncertainty as there has been some rapid lifting of the fog just southwest of metropolitan. So while this type of fog usually takes quite some time to lift...recent observations and an increasing low level wind field suggests visibility improvements may occur by 13z. //Dtw threshold threats... * low confidence on timing of visibility above a half mile this morning. * Low confidence on thunderstorms impacting the terminal tonight. && Previous discussion...issued 335 am EDT Thursday Oct 2 2014 Short term...today and tonight The interval of clear skies last evening provided enough boundary layer cooling to support the development of a nocturnal inversion. Southeast winds winds atop this inversion have sustained enough moisture advection from the east to lead to both low clouds and fog again this morning. Some thick cirrus which has moved in from the west has thus far proven to have little effect in preventing fog development. Based on observational trends...some of the fog will be dense. A strengthening southerly flow atop the inversion and the onset of daytime heating will erode the fog during the middle-late morning. A low level anticyclone extending across the eastern Great Lakes will continue its departure to the east today as a progressive long wave trough moves over the plains. The 00z upper air analysis showed a large region of deep layer moisture extending across the MS valley into the western Great Lakes ahead of this system. The showers now advancing across Lake Michigan are occurring within an 850-700mb Theta-E ridge...which will slowly make its way across the forecast area late this morning through the afternoon. This will support a chance of showers and a fair amount of cloud cover across the forecast area today. Increasing southerly flow will boost 925mb temperatures into the upper teens. The NAM and regional Canadian suggest the warm front now extending across the MS/Ohio valleys will lift toward the Michigan/Ohio border by late afternoon. Temperatures will be well in the 70s south of this boundary. To the north /over southeast mi/...forecast highs will remain optimistic /upper 60s-low 70s/ given the degree of low level warming expected. A strengthening upper jet core ahead of a rapid amplification in the long wave trough over the northern plains tonight will lead to a deepening surface low. By 12z Friday...the low is forecast to be centered over Lake Superior with the trailing cold front across Lake Michigan. Strengthening low level southerly flow /45-50 knots at 925-850mb/ will send another surge of low level moisture into Southeast Michigan. While surface based instability is expected to remain SW of the area today...this southerly flow will transport some weak elevated instability into the forecast area tonight. Lift will also be enhanced by a remnant middle level short wave impulse approaching from the southwest late. The result will be increasing chances of showers and a few thunderstorm during the night. The advection of low 60 surface dewpoints into the forecast area tonight and strong low level wind fields will keep min temperatures fairly mild /low-middle 60s/. Long term... Powerful upper level low/trough over western Canada diving southeast this morning...and will be merging/phasing with upper wave/energy exiting The Four Corners region...leading to massive upper level low/trough encompassing eastern North America this weekend...and into early next week. Temperatures will average well below normal (10 to 15 degrees). Before we get there...will have to contend with the strong cold front barreling through Southeast Michigan during Friday with deepening surface low riding along the front. Excellent large scale ascent and plenty of moisture (850 mb dew points of 10-12 c) will assure showers (near 100 percent)...with modest instability and strong dynamics/vertical forcing likely enough to generated isolated-scattered thunderstorms as well. With strong low level jet of 50 knots...there is some concern some of these strong winds will translate to the ground with any vigorous convection. However...12z NAM/GFS both suggesting convective available potential energy predominately under 500 j/kg. The upper level trough axis GOES negative Friday evening...and although the surface front will have cleared the area...enough wrap around moisture/deformation...coupled with the Lake Michigan enhancement to support widespread showers...as core of the cold airmass (-12 c at 700 mb) rotates through the Ohio Valley...with southwest flow leading to the good cold advection into lower Michigan. Good cyclonic flow on Saturday with residual cold air (850 mb temperatures -2 to -3 c) will continue to support scattered-numerous showers...with highs topping around 50 degrees...ultimately dependent on the areal coverage of showers to knock down the temperature. Northwest winds Saturday night into Sunday will continue to supply/maintain sufficiently cold airmass (-2 to -4 c at 850 mb) to produce Lake Michigan showers...which should survive the eastward to support scattered activity. Modest late day warning (925 mb temperatures of 5 c per 00z euro) expected to bolster maxes into the lower 50s most locations...with middle 50s toward Ohio border. Marine... Southeast winds will steadily increase today and tonight as strengthening low pressure tracks through the western Great Lakes. Latest model guidance points to stronger southerly flow over open waters of Lake Huron...solidly around 30 knots...with the potential for gale force winds even with the arrival of warmer air which will maintain a higher degree of stability near the surface. Have thus issued a gale watch for the central portion of Lake Huron...where wind speeds look to be maximized. Isolated to scattered thunderstorms are also expected as widespread showers move through during this time. Any thunderstorm could produce stronger winds. A seasonably strong cold airmass will overspread the Great Lakes region over the weekend...with westerly winds of 25 to 30 knots expected on Saturday...with the potential for gale force winds toward the Canadian side of Lake Huron due to the deeper boundary layer mixing. && DTX watches/warnings/advisories... Michigan...none. Lake Huron... Small Craft Advisory...nearshore waters from Port Austin to Port Sanilac...from 1 am Friday to 8 PM Friday. Small Craft Advisory...lhz421-lhz443...from 8 am Friday to 8 PM Friday. Gale watch...lhz362-lhz363-lhz462...from 6 am Friday to 5 PM Friday. Lake St Clair...none. Michigan waters of Lake Erie...none. && $$ Aviation.....SC short term...SC long term....sf marine.......sf You can obtain your latest National Weather Service forecasts online at www.Weather.Gov/Detroit (all lower case).
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