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Area forecast discussion 
National Weather Service Aberdeen South Dakota 
1011 am CDT Friday Jul 18 2008 


Update... 
all bets are off with forecast package today as widespread 
convective debris moves from western counties to the east. This 
will make for a very difficult maximum temperature forecast adjustment 
with going highs in the 80s. Across eastern counties...mid/high convective 
debris means Reliance on any breaks in late am/PM hours. Any sun hereafter 
will cause rapid jump in temperature...so will hold off on any 
changes to highs in eastern County Warning Area. For western counties...same issue as 
well defined clearing line lacking with the current convective 
activity...supported by general upslope flow extending into 
western South Dakota from surface high in southeast ND. Next issue is pop chances. 
Have updated for current convection...and will keep afternoon 
forecast for slight chance in west...however latest NAM overdoing 
drying as Morning Sun evaporated dew/fog and eastern County Warning Area dew 
points have jump 3 to 5 degrees. Will be tough mixing this out 
with cloud cover...and although this has little bearing on the 
near term minus additional mugginess...the main issue is that 
forecast for tonights nocturnal convective activity hinged on 
larger dew point depressions in the NE County Warning Area. Therefore will likely 
increase pop coverage overnight with expected complex. 


&& 
Previous discussion... 
short term...today through 00z Monday 
main forecast challenge this morning deals with precipitation chances through 
the period. 


Msas is currently showing a weak surface convergence boundary draped 
along the northern tier counties...with some isolated -tsra developing 
along this feature. This boundary is very subtle...but is defined 
by dewpoints in th 50s north of boundary...and dewpoints in the 60s south 
of this feature. Models are having difficult time defining such a 
subtle feature. Models do show next in series of waves moving into 
northwestern South Dakota in quick zonal flow. Currently convection is firing across 
southeastern Montana...and with boundary in place across northern 
counties...have included some small probability of precipitation for the northwest/western County Warning Area this 
morning and afternoon. With respect to precipitation chances for later 
today/tonight...big unknown in this equation is role dry low level 
air will play. The GFS shows surface/925mb trajectories off of dry 
high dropping into Minnesota during the day today...with GFS/NAM also 
showing relative humidity minima at 850 mb across western Minnesota/far eastern South Dakota through 12z 
Saturday. RUC shows 7h Theta-E downglide taking place during the 
day acros the east in conjunction with high settling in...so dry 
forecast across the east looks to be a good bet. GFS is showing that 
warm air advection forcing...seen in the 850 mb-7h q-vector fields...wanes fairly 
quickly during the morning hours..with a reset taking place by 00z 
Saturday as next upper wave approaches. Anticipate that mesoscale convective system will 
develop later tonight west of South Dakota...then advect east late tonight 
into Saturday morning. Subsidence/downglide behind system should 
lead to rapid drying/ending of precipitation from west to east during the 
day Saturday as complex moves into Minnesota. Feeding into system tonight 
will be 850 mb winds from 25 to 35 kts...so if complex can overcome 
dry air intrusion down low...should be able to survive the night 
as it moves east. 


Another dry period should set up Saturday night into Sunday 
morning for most of the County Warning Area as high pressure remains in control. 
By Sunday afternoon...the GFS develops another surface low in central 
Nebraska with inverted trough nosing NE through eastern County Warning Area. 7h temperatures are 
close to capping temperatures across southwestern County Warning Area. The northestern County Warning Area though has 7h 
temperatures below 10 degrees celcius...so thunderstorms and rain should be easier to 
develop there than in the SW. 


Temperature wise...weak cold air advection across the east today should keep temperatures below 
normal for this time of year...with warming temperatures expected across 
the west. A warming trend is then anticipated through Sunday with 
slowly rebounding heights/thickness values. 




Long term...Sunday night through Thursday 
the timing of a short wave at the beginning of the period is the 
main challenge in the extended. The period opens with an upper 
level ridge in the west and trough in the east. This leave the 
northern plains under northwest flow aloft. The GFS shows a closed 
700 mb low over northern Wisconsin...with a weak middle level trough back 
across the Dakotas. Boundary layer dew points are shown to be in 
the upper 60s to lower 70s over much of South Dakota. Low level jet of 35 
kts points to western Iowa...which would leave at least some quantitative precipitation forecast 
in the eastern County Warning Area. This wave moves into the Midwest with the 
upper level ridge rebounding in the western Dakotas. The region 
dries out behind this wave with boundary layer dew points 
struggling into the lower 50s...plus a close 700 mb high is shown to 
rule the northern plains. High pressure slides east...allowing for 
moisture to return up the plains states. Not real confident with 
precipitation chances for Tuesday evening with the GFS showing a delay in 
moisture return. The European model (ecmwf) however is more progressive in the 
extended...and is trying to bring a strong surface low into the 
northern High Plains with plenty of available moisture. Made only 
minor changes to the weather/pop grids with this big of disagreement 
among models. 


&& 


Aviation... 
broken-overcast conditions will be found over all but the far eastern County Warning Area 
today. The lowest clouds...between 1000-2000 feet...will be found 
across the southwestern County Warning Area...affecting kpir. The rest of the area 
will see VFR ceilings. The cloud bases in the southwest are expected 
to lift and become VFR by 18z. Visibilities will be unrestricted all day. A 
weak upper level disturbance will bring a chance of thunderstorm 
activity this evening...reaching kpir and kmbg after 0z. 
Thunderstorm chances spread eastward during the overnight hours. 


&& 


Abr watches/warnings/advisories... 
South Dakota...none. 
Minnesota...none. 
&& 


$$ 
Update...Connelly 
short term...Hintz 
long term...South Dakota 
aviation...Parkin 


Weather.Gov/Aberdeen 

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