845 
fxus63 kmpx 231201 
afdmpx 


Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota 
601 am CST sun Nov 23 2008 


Discussion... 


The forecast challenge centers on the snow possibilities 
tonight...mainly in south central and southeast Minnesota. 


Early morning GOES satellite channels and observation indicating low 
clouds gradually advecting neward...with back-edge likely to clear 
south ctrl Minnesota if not much of the Minnesota portion of the forecast area 
through the morning. High clouds beginning to stream in from the 
west originating from next system. Will be a window for some sun 
in southern and possibly eastern communities into the early afternoon. This 
should allow for warmer and positive 925-850mb temperatures to be mixed 
down...with highs today still forecasted to be in the lower 40s 
across southern Minnesota into western WI. These highs and others across the area 
today will be slightly warmer than normal and overall the warmest 
day since the 14th. 


Interest turns to system coming out of Wyoming later this afternoon. This is 
another Pacific/Alberta-clipper like system in west-northwest flow. The 06z 
NAM has trended further south than the 00z NAM...and more in line 
with the short range forecast ensemble and 00z GFS. Compared to 
the Friday night system which dropped widespread light snow across 
the area...this one is not as deep in the 500 mb heights and temperatures or 
overall pv anomaly. However...this has much better moisture to 
work with and will be in the deepening phase as it passes just 
south of Minnesota this evening. This system will phase over the Great Lakes 
on Monday with a stronger one diving in from the northwest. But as it begins 
to feel the effects of that upstream system it will deepen this 
evening...placing southern Minnesota in deformation zone of this system. This 
correlates well with a left exit region of a predicted 95 knots jet. 
Greatest pressure advection on the isentropic surfaces is focused 
over south ctrl Minnesota this evening where moistening occurs fairly rapidly 
according to cross sections. Mixing ratios on the surfaces of 
upglide are 2 to 2.5 g/kg with precipitable waters near 150 
percent of normal. These values are greater than the Friday night 
system. Dendritic growth layer is fairly deep /50-70mb/ and Omega 
is found within that layer. Profile below this quickly warms 
however and some partial to complete melting is expected at 
first...especially given boundary layer temperatures in the upper 30s 
early this evening across southern Minnesota. Frontogenesis for focus snow bands 
develops as early as this afternoon but correlates with the 
aforementioned forcing and moistening mainly during the evening in southern 
Minnesota. So potential for brief moderate snow does therefore exist. 
Brief though is the key...as system is on the move fairly quickly 
east. A band of one to two inches appears likely to set-up 
somewhere in south ctrl Minnesota...with a broader area of up to one inch 
across the remainder of southern and eastern Minnesota and into western WI. 


Merging systems in the Great Lakes will offer back-side cold air 
advection by Monday afternoon with clouds and some isolated snow showers and 
flurries. 800 mb temperatures not as low as in recent dumps of cold air. So 
temperatures to drop to slightly below normal for Monday into Tuesday and then 
back to slightly above into the Holiday. For the Holiday travelers 
on Wednesday and into Thanksgiving...quiet weather looks to confidently 
prevail in this region. Friday the pattern becomes more 
unsettled...with the western lakes surrounded by systems of decent 
magnitude. All of them though seem to be blocking the 
flow...likely preventing any major precipitation on Friday and keeping temperatures 
from climbing or dropping much into next weekend. 


&& 


Aviation... /12z taf issuance/ 
the forecast challenge is the timing of clearing and influx of more 
stratus and low clouds over the taf sites during the period. The 
western and southern edge of the low clouds has moved into parts of 
western Minnesota and west central Wisconsin near Keau. An upper level 
disturbance over the Dakotas...will bring some light precipitation over the 
area later today and tonight. After midnight...low clouds should 
clear out temporarily before next batch of low clouds press south in 
the wake of a cold front that is forecast to plunge south on Monday. 
IFR to MVFR flying conds prevailing over most of the taf sites this 
morning...then becoming MVFR to VFR flying conds later tonight. 
Check tafs for more detail. 




&& 


Mpx watches/warnings/advisories... 
Minnesota...none. 
WI...none. 
&& 


$$ 


Mtf/jvm