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Area forecast discussion 
National Weather Service Topeka Kansas 
1146 PM CDT Wednesday Aug 27 2008 


Update... 


Updated for 06 UTC aviation. 


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Discussion... 


See previous discussion. 


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Aviation... 


VFR expected for through the taf period. Convective complex in 
northeast Nebraska will not affect taf sites through 12z. A cold 
front will move through the taf sites and shift the winds from the 
southwest to the northwest in the 19z-22z time period. Also enough 
instability and forcing will be present for scattered 
thunderstorms and have added thunderstorms in the vicinity to the tafs. Visibilities and ceilings 
could lower to MVFR category in the 20z Thursday to 02z Friday 
period. 


&& 


Previous discussion... /issued 336 PM CDT Wednesday Aug 27 2008/ 


Short term...tonight and Thursday 


Initial forecast concerns are convective chances for the remainder 
of this afternoon. An outflow boundary has stalled out north of 
Interstate 70 in north central Kansas...but continues to slide south 
across portions of east central Kansas. Good daytime heating along with 
better than expected low level moisture has destabilized the 
atmosphere across much of the County Warning Area with Storm Prediction Center mesoscale analysis 
indicating ml cape values of 2500 to 3000 j/kg along a narrow axis 
extending from Clay Center southeast towards Topeka and Lawrence 
where ml cin values had eroded to near zero. The main area of 
concern over the next couple of hours looks to be from Topeka 
southeast to the MO border including Lawrence and Ottawa where at 
least some meager low level convergence resides associated with the 
now fairly weak outflow boundary. A weak warm air advection pattern 
remains over the County Warning Area...which may also provide some weak isentropic 
lift for low level ascent this afternoon...however no large scale 
synoptic forcing mechanisms exist in the upstream flow to the 
County Warning Area...so think that any thunderstorm activity that occurs would likely 
remain isolated in nature. Deep layer shear values continue to look 
weak over the County Warning Area with h500 flow of 5-10 kts aloft...would think 
that despite strong ml cape values thunderstorms would have a difficult time 
organizing. Any isolated thunderstorm that develops may become strong 
enough to drop some dime to nickle size hail...but the primary 
threat of any severe weather would seem to be damaging outflow winds 
associated with collapsing thunderstorm cores. 


Tonight...water vapor imagery and h500 profilers indicated the 
presence of a weak shortwave trough over the Western Plains. As 
this feature translates east tonight there is some possibility that 
this wave would provide enough to set off some elevated convection 
along and north of Interstate 70 with the help of increasing low 
level jet. Kept probability of precipitation in the 20 percent range as confidence in thunderstorm 
development given weak forcing is low. Lows tonight are likely to 
remain warm...with temperatures only dipping into the upper 60s to 
near 70 degrees. 


Thursday...12z h250 analysis indicated a strong jet streak entering 
the Pacific northwest digging into a broad trough over the 
northwestern Continental U.S.. the associated shortwave through is expected to 
move through the northern plains Thursday morning sending a cold 
front currently stalled out from northwestern Iowa southwest into 
central NE through the plains and into the County Warning Area during the early 
afternoon hours. Ahead of the cold front models indicate an h850 
thermal ridge while mostly sunny skies provide efficient low level 
mixing to the southeast. With the combination of good daytime 
heating and the thermal ridge...should see temperatures climb into 
the middle 90s for parts of northeast and much of east central Kansas. 
Sufficient low level moisture looks to combine with good daytime 
heating to destabilize the atmosphere ahead of the cold front. 
Forecast soundings indicate around 3000 j/kg of ml cape weakly 
capped by 21z. With the cold front expected to move into the 
southeastern half of the County Warning Area during this period of peak 
heating/destabilization think convergence and ascent associated with 
the cold front will be sufficient to break any weak cap...and 
scattered thunderstorms will be possible along and just behind the cold 
front. If thermodynamics are well handled by the models could see 
some severe weather potential with ml convective available potential energy near 3000 j/kg and increased 
deep layer shear with strong shortwave trough moving through the 
northern plains. At this time marginally severe hail and damaging 
outflow winds look to be the most likely severe weather threats 
Thursday afternoon. Thunderstorm and severe weather chances look best southeast 
of a Hiawatha to Abilene line Thursday. 


Cavanaugh 


Long term...Thursday night through Wednesday 


Strong upper level shortwave will continue to move into the Great Lakes on 
Thursday night with the trailing cold front making good progress through the 
forecast area. Convection which develops in the Thursday time frame will continue in a 
scattered nature alng and slightly behind the cold front through the evening hours. 
Synoptic scale support for convection is generally weak /although 
not unfavorable/ but what looks to be fairly strong frontogenesis 
alng the fnt shld allow for sufficient mesoscale forcing for 
sustained convection through the evening. Deep layer shear is generally 
weak...so pulse storms is most likely the Mode of convection for 
our County Warning Area. Current forecast seemed to handle the scattered nature of the 
expected convection well and very little changes were made here. 


Air behind the cold front looks pretty cool and dry. Surface hi pressure 
will settle into the region on Friday and should remain in the 
area through sun. This should yield clear and cool nights and a 
dry period. Going forecast looked pretty good...but did lower 
overnight lows in a few places. Current forecast is on the lower 
end of the guidance envelope and is closer to the operational GFS 
MOS at this time. Models likely under doing the low level drying given the 
upstream dewpoints in the 30s this afternoon...and thus temperatures might be a 
little cooler than going forecast...and an additional trend 
downward may be needed later. 


There are then sig differences in the extended. General synoptic 
pattern favors troughing moving into the Pacific northwest and a downstream 
ridge over the eastern US. The surface hi shld be moving off to the east 
during the Tuesday/Wednesday time frame as a new cold front moves through the northern 
plains as the trough develops west. Moisture will start to return to 
the area on Tuesday and temperatures will increase as upper level heights 
increase Tuesday and Wednesday. This might favor some kind of nocturnal 
precipitation as the moisture returns...but uncertainty in the quality of 
the moisture and timing precludes mentioning at this time. Otherwise it 
appears the cold front will not reach the County Warning Area until the end of the 
period or slightly after due to the blocking ridge over the east. 
Thus will continue with a dry forecast. 


Boustead 


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Top watches/warnings/advisories...none. 


&& 


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