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fxus63 kdtx 212017 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
317 PM EST Sat Nov 21 2009 


Short term...tonight 


With surface dew points still hovering in the low to middle 40s and 
skies mostly cleared out...fog appears to be a good bet tonight as 
anti-cyclonic flow builds into lower Michigan. Easterly flow off 
Lake Erie will help contribute some additional moisture across the 
south third of the County Warning Area. Even without this moisture...relatively high 
crossover temperatures should be sufficient for fog with mins 
falling into the upper 30s. A quick redevelopment of the low stratus 
is about the only fly in the ointment...but the only significant 
inversion looks to be the surface based one and leaning toward a fog 
scenario...which could become dense. 


&& 


Long term...Sunday through next Saturday 


Middle level ridging is forecast to build across the eastern Great 
Lakes on Sunday...allowing the high now centered over Ohio to build 
into northern New England. The surface high and associated upper 
ridge is then forecast to slide off the coast of New England on 
Monday...with surface ridging persisting across the eastern Great 
Lakes. 


With no appreciable advection of low level dry air into the region 
and given the limited mixing potential this time of year...dense fog 
will be the main weather concern over the next several days. Given 
the warm ambient airmass...once fog is able to mix out Sun 
afternoon...temperatures should achieve the lower 50s. Although the low 
level gradient is forecast to increase Sun night into Tuesday...the low 
level flow will remain off Lake Erie. This will supply additional 
low level moisture into the already moisture rich boundary layer. 
While the tightening gradient raises some concerns that low stratus 
will be favored over fog...model soundings and wind speeds below 
950mb suggest enough boundary layer decoupling to allow a chance of 
fog redevelopment both Sun night and Monday night. Weak shallow cold 
air advection on Monday and persistent middle level subsidence will 
strengthen the inversion. This may make it difficult for morning 
fog/stratus to completely mix out during the afternoon. Thus...have 
kept mondays maximum temperatures mainly in the upper 40s. 


Attention then turns to the weather system expected to impact the 
state by middle week. The 12z model suite continue to show middle level 
trough amplification over the central rockies on Monday...evolving 
into a cut off circulation over the Central Plains by Tuesday. Both 
GFS and NAM have trended slower with this system /more toward that 
of the ECMWF/. Satellite representation of this wave /now off the 
coast of Washington state/ appears stronger than what the GFS 
initialized. These factors seems to support the slower European model (ecmwf). With 
the wave still off the coast...model confidence is still not high. 
In light of recent European model (ecmwf) trends however...the best chance of rain 
associated with the warm conveyor belt will occur between Tuesday night 
and Wednesday. 


As the system lifts across Michigan /roughly in the Wednesday time period/ 
a secondary northern stream wave is forecast to drop into the 
northern plains/upper Mississippi Valley from Canada. There remains 
a high degree of uncertainty with respect to timing and amplitude of 
this wave and whether there will be any phasing with the wave 
preceding it. In light of recent model trends...a chance of 
precipitation /albeit light/ continues to look warranted Thursday 
through Friday. The magnitude of cold air advection with the 
northern stream system has not been shown to be as strong over the 
last couple of model cycles /the coldest air now does not look to 
arrive until Thursday night or Friday/. Since even the faster GFS 
solution is now indicating that the cold air advection will not be 
strong enough to offset the depth of above freezing air in the 
boundary layer...have removed all mention of snow on Thanksgiving 
day and just carried a mix Thursday night into Friday. 


&& 


Marine... 


High pressure will remain in place over the central Great Lakes 
through tomorrow...resulting in very light winds. There will be a 
gradual increase in southeast winds Monday into Tuesday as the high 
retreats east away from the area. The pattern becomes active again 
middle/late next week as a strong shortwave and associated jet streak 
dig into the upper Midwest and lead to the development of a large 
upper level low over/near the region. This time frame will bring 
unsettle weather and higher wind and waves. 


&S 


&& 


Previous discussion...issued 1201 PM EST Sat Nov 21 2009 


Aviation... 


Remnant low level moisture with ill defined low level inversion will 
make for a difficult taf forecast...as the debate will be low 
stratus vs fog as calm/light winds promote good radiational cooling 
tonight. Slow drying of the column this afternoon should allow for 
partial clearing...but boundary layer cooling when the sun sets 
will allow for either MVFR vsby's or ceilings quickly 
redeveloping...with either IFR vsby's or ceilings expected 
overnight with at least a strong surface based inversion setting 
up as general subsidence will be the rule with anti-cyclonic flow 
building in. Southeast flow off Lake Erie will make dtw especially 
susceptible for possible dense fog or low stratus tomorrow 
morning. As typical for this time of year...fog and stratus will 
likely take all morning to burn off/dissipate with the light winds 
and limited mixing. 


&& 


DTX watches/warnings/advisories... 
Michigan...none. 
Lake Huron...none. 
Lake St Clair...none. 
Michigan waters of Lake Erie...none. 


&& 


$$ 


Short term...sf 
long term....SC 
marine.......sf 
aviation.....Sf 




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