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fxus63 keax 082343 
afdeax 


Area forecast discussion 
National Weather Service Kansas City/Pleasant Hill MO 
542 PM CST Thursday Jan 8 2009 


..updated aviation discussion... 


Discussion... 


Short term (tonight through Saturday night)... 


Surface ridge and band of warm air advection cloudiness in northern Missouri...are 
both progressing to the east this afternoon. The surface ridge 
should be east of the forecast area by early evening with return 
flow setting up overnight. Thus expecting a non diurnal trend to the 
temperatures overnight...especially in western areas where winds 
should increase some late tonight as the pressure gradient increases 
between the departing surface high and the pressure falls taking 
place over the plains in response to a short wave moving through The 
Rockies. 


This short wave will have a decidedly positive tilt as it moves east 
of the northern rockies by Friday afternoon...with its southern 
portion still extending back across the southern rockies even by 
Friday evening. In the middle level flow expect an 850 mb low associated 
with the northern portion of this trough...to be moving through 
southern Iowa on Friday...gradually weakening as it moves east. The 
flow ahead of this low as well as the positively tilted 700 mb trough 
should remain mainly from the southwest with the better moisture 
ahead of the front expected to be along the I-44 corridor. Expect 
much of the forecast area to be dry slotted during the day on 
Friday...and the best chance of precipitation will come in the Post 
frontal phase...when middle level frontogenetic forcing moves across 
northern Missouri. Am preferring the WRF solution which shows the 
better forcing and moisture in the 750-700mb level during the late 
evening to early morning hours across north central Missouri. Will 
have chance probability of precipitation at this point to cover this possibility...since 
position and strength of this forcing is not consistently 
represented by all the operational models. The 15z sref does show 
support for this idea...with the better forcing aimed toward north 
central and northeast Missouri where the entrance region of the 
northern jet will reside during the evening hours. 


Regarding temperatures on Friday...will continue the trend of the 
slower WRF/European model (ecmwf) timing of the surface front. WRF shows 10c 850 mb 
temperatures into southern areas of forecast area and given central 
Oklahoma where 850 mb temperatures today were prognosticated around 10c...have 
reached the lower 60s...will go above the warm met guidance and put 
some lower 60s in southern areas. 


Will leave some flurries in eastern areas Saturday morning as the 
700 to 500mb troughing moves through and layers below 800 mb remain 
saturated. Will be cold air advecting through the day...so will go 
with the colder met numbers...which are supported by the sref. Mav 
guidance looks way too warm. 


Pc 


Medium range (sunday through thursday)... 


For late in the weekend and next work week the Middle-Range models are 
beginning to all point in the same direction...and that direction 
indicates that it is going to be cold as a classic positive pna 
/Pacific North Atlantic/ pattern is setting up. This will result in 
an amplified pattern across the Continental U.S. Which will allow for a strong 
meridional flow to develop across the central Continental U.S. Next week. 
Notable effects for the lower Missouri River valley will be clipper 
systems as the northwest flow steepens across the region...and 
increasingly cold temperatures as more Arctic air is allowed to 
spill south from northern Canada. The European model (ecmwf) has been advertising 
this for a little while...but the GFS and ensemble members are also 
coming onboard with this idea. 


Sunday...before the big chill...we will see temperatures rise to a 
bit above normal. Surface and boundary layer winds will back to the 
southwest and west...respectively...in advance of a shortwave that 
will move through Monday allowing a bit of a warmup. 


By Monday...first of several shortwaves will move southeast through 
the plains states as the east Pacific Ridge begins building. GFS and 
European model (ecmwf) are still having timing and amplitude issues with this wave. 
However...with moisture spilling over the ridge with the 
shortwave...there is enough agreement in the general mass fields of 
of the models to warrant adding some chance probability of precipitation in north central 
and northeast Missouri for Monday. This would be the 
climatologically preferred area for accumulating snow with this type 
of system. Kept in some silent slight chance probability of precipitation for snow going 
through Monday night with the northwest flow and model timing 
issues...but any flakes that fly after the main clipper shortwave 
moves through will probably be only flurries. 


By Tuesday night...another shortwave will be beginning to round the 
base of the trough that will be prevailing across the eastern half 
of the Continental U.S. At that time. Available moisture could be a little more 
sketchy with this second wave...so have only gone with a silent 
slight chance pop for now. 


Otherwise...took an axe to temperatures from Tuesday through the 
end of the going forecast...and chopped them down well below normal. 
European model (ecmwf) and GFS advertise a 1050mb+ surface high sliding south into 
the Central Plains from Canada as the flow GOES meridional. As an 
example...lowered high temperatures into the teens and twenties for 
next thursdays highs...but if there is any snow pack around...or to 
our north...temperatures will ultimately verify a lot lower. 


Cutter 






&& 


Aviation... 


Strengthening warm advection regime aloft will further intensify 
later this evening as weak pressure falls allow boundary layer flow 
to remain backed and decouple. This will promote the development of 
a 40-45 knot low level jet between roughly 06 and 14z...and this has 
been introduced into the 00z tafs. 


Otherwise southeast winds of 7 to 10 knots will gradually veer toward the 
south after daybreak...and then toward the west and eventually 
northwest by tomorrow afternoon as a weak surface low dropping 
through Oklahoma drags a SW-NE oriented cold front across the area. 


Other than the winds...tafs are otherwise straightforward with VFR 
conditions prevailing as moisture remains meager in northwest upper 
flow regime. 


Bookbinder 


&& 


Eax watches/warnings/advisories... 
MO...none. 
Kansas...none. 


$$