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Area forecast discussion 
National Weather Service La Crosse WI 
303 PM CDT Wednesday Mar 17 2010 


Short term...tonight through Saturday night 


Forecast concerns this package...fog threat tonight...precipitation 
chances Friday into Sat...temperatures through the period. 


18z data analysis had a broad area of high pressure over the central 
Continental U.S....centered over Kansas/eastern Nebraska. Low pressure was located over 
southern saskat in response to the shortwave energy over southern 
BC/alb...with strongest pressure falls from Lake Winnipeg to southwest 
ont. Visible imagery/surface observation show plenty of low level moisture/strato-cu 
trapped under subsidence in the high across the low MO valley into 
IA/MN. These clouds and some fog slow to erode/dissipate. Drier low 
level airmass remains over eastern WI...with surface dew points once 
again falling into the middle teens-middle 20s and relative humidity values falling to 20 
to 30 percent. Early afternoon temperatures again in the upper 50s to middle 
60s in these sunny/dry areas and in the 40s under the low clouds to 
the west/southwest of the area. 


17.12z models look to have initialized without large errors and 
offer rather similar solutions for tonight into Sat night. NAM now 
the more robust/further north with precipitation Friday into Sat...with GFS 
now more in line with rather unable-wavering European model (ecmwf)/can-Gem solutions. 
Dprog/dt of 500mb heights at 17.12z showed model runs of 16.12z and 
15.12z rather well across noam/eastern Pacific...with slight edge to 
European model (ecmwf) on run-to-run consistency over eastern Pacific and GFS over 
eastern noam. Through 36hrs...models converging toward a common 
solution of their earlier runs as the shortwave energy digs into 
the Great Basin/northern rockies...with better consistency to 
Gem/ECMWF. For 36-60hrs trend is a bit faster with the portion of 
the energy moving through the central/southern rockies and energy 
moving east along the US/can border. European model (ecmwf) with the better 
consistency and other models trending toward it. For 60-84hrs trend 
is faster with the middle level trough axis pushing east across the 
region and more of a positive tilt trough/less closed 500mb low 
moving through the central Continental U.S.. European model (ecmwf) again with the better run-to- 
run consistency. European model (ecmwf) does remain a stronger/slower solution with 
the southern portion of the trough for Sat night but less so than 
its 17.00z run. Check of observation vs. Model data at 18z showed models 
good with the surface mass field across the central Continental U.S....but again 
struggling with the mixed/falling dew points in the dry airmass over 
eastern WI. Per WV imagery...NAM/GFS/ECMWF/Gem all appear quite 
reasonable with the shortwave details across noam/eastern Pacific. With 
no favorite based on 18z data...European model (ecmwf) with the better run-to-run 
consistency and other models trending toward its solution through most 
of the short-term period...favored a compromise solution weighted 
toward the European model (ecmwf). Short term forecast confidence average to good this 
cycle. 


In the short term...saskat low will track to southwest ont tonight 
as the British Columbia/alb shortwave energy moves east. Pressure gradient over 
the area will tighten some tonight with more of a general west wind 
tonight. This boundary layer wind should be enough to limit the 
dense fog...but with dew points still in the upper 30s-low 40s over 
the west 1/2 to 2/3 of the forecast area...some fog...mostly 1 to 4 mile 
visibilities...still expected to develop for late tonight/Thursday morning and 
added this to grids. With increasing winds/mixing Thursday...expect low 
clouds/fog to lift quickly Thursday morning. Model continue to spread 
some high level moisture/weak upper level divergence/cirrus across 
the area Thursday...that seen in infrared imagery over western Dakotas/central 
rockies this afternoon. 925mb temperatures warm into the +6c to +10c range 
Thursday...ahead of the surface-700mb trough/wind shift for late Thursday/Thursday 
night. With some sunshine through the cirrus...Thursday could be the warmest 
day of the week but highs will depend on how much mixing occurs. 
925-700mb cold advection spreads across the area for Thursday night with 
a tightening thermal gradient over the area. 925-850mb temperatures fall to 
near/below 0c by 00z Sat...but for the far southeast end of the 
forecast area. 850mb moisture transport/Theta-E convergence and 850- 
500mb fn convergence increase into the tight thermal ribbon for Friday/ 
Friday night. Again NAM furthest north with the deeper forcing/lift... 
with other models clipping mainly the southeast third of the forecast 
area with the deeper moisture/lift. Previous grid set had rain/snow 
chances for Friday/Friday night well trended...based on more consistent 
European model (ecmwf)/Gem solutions. With GFS trending south with the precipitation band... 
did lower some rain/snow chances north of an Austin-Winona-Black 
River Falls line Friday afternoon/night. With the trend of the model 
soundings toward more cooling just above the boundary layer to 925mb 
Friday...did speed up the change-over from -ra to -sn into the late Friday 
afternoon and Friday evening hours. Also added falling temperatures Friday 
afternoon as the -ra spreads into the area and low level cold 
advection continues. However...not a lot of confidence in the change 
over time...or snow amounts over the southeast end of the forecast area 
Friday night into Sat. Much of any snowfall accumulation will depend on 
snowfall rates due to the now thawed and warming ground. Potential 
exists for 3 to 6 inches in Grant County and nearby areas...tapering 
quickly to little if any accumulation north of a kaum-klse-kvok 
line. With the more progressive system and deformation/fn band 
translating east of the area...carried highest snow chances Sat in 
the morning then trended to a dry forecast after midnight Sat night. 


Generally favored a blend of guidance lows for tonight/Thursday night. 
With the 925mb thermal ridge over the area Thursday and at least some 
filtered sunshine...raised highs about a category above guidance. 
Favored a blend of guidance highs Friday...but these tricky based on 
when/where the -ra would move into the forecast area the earliest. With 
plenty of clouds expected Friday night...carried lows about a category 
above the numerical guidance. Generally favored a blend of GFS 
mex/ensemble MOS and HPC guidance highs/lows for Sat/Sat night. 


Long term...Sunday through Wednesday 


17.00z medium range models in reasonable agreement with the large 
scale features sun...but differ on some timing/strength details. 
Overall trend is a bit slower/stronger with the southern stream 
portion of the moving through the Lower/Middle MS valley...the northern 
stream trough dropping into the region and toward a tighter 
consensus with the next trough moving toward the West Coast. By Monday 
trend is more progressive with the longwave trough into the eastern 
Continental U.S. And stronger with the trough onto the West Coast. For Tuesday/Wednesday 
trend remains stronger/slower with the troughing into the western 
Continental U.S. Tuesday then central Continental U.S. Wednesday...with better between model 
consistency. With reasonably consistency among GFS/ECMWF/ensemble 
means through the period...long-term forecast confidence average to good 
this cycle. With a stronger southern stream low/trough in the 
Lower/Middle MS valley Sunday...less northern/southern stream phasing 
looks to occur...with bulk of moisture/lift with the southern 
stream system east of the area sun. 17.12z GFS/Gem trend more 
robust with moisture/lift as the northern stream shortwave drops 
into the region sun...and added a small -shsn chance to the 
northeast end of the forecast area for sun. Cooler...drier high pressure 
and a seasonably cold airmass with 850mb temperatures in the -10c to -16c 
range spreads across the area by 12z Monday. Northwest gradient winds 
to keep Sun night lows from falling too far. Coldest airmass is 
short-lived with the strong troughing moving onto the West Coast and 
keeping the pattern progressive. Heights rise with shortwave ridging 
over the region already by Tuesday and 850mb temperatures warming to near/above 
0c. With the shortwave ridging and slowing of the next approaching 
trough...removed most of the -ra chance from Tuesday. Moisture/lift 
ahead of the low/trough as it ejects into the Central Plains spreads 
across the region Tuesday night/Wed. Lower level/boundary layer temperatures 
warm enough for bulk of the 20 to 40 percent precipitation chances Tuesday 
night/Wednesday to be rain...but for the late night/early morning hours. 
Generally favored a blend of GFS mex/ensemble MOS and HPC guidance 
for highs/lows through the period...but for Tuesday night with increasing 
clouds and low level warm advection and raised lows this period 
toward warmer of guidance values. 


&& 


Aviation...tonight and Thursday 


Earlier fog has dissipated across the region leaving VFR clear 
conditions at the taf sites. Light northwest winds will persist 
into the evening...but will start backing to the west and southwest 
tonight as a ridge of high pressure drops to the southeast of the 
region. The main forecast concern is with fog potential again 
tonight into Thursday morning as the setup looks decent for at least 
some reduced visibilities. The low level winds look like they will 
be stronger...which may help to keep the visibility up. For 
now...have started a trend down with MVFR visible developing later this 
evening and persisting into the morning Thursday. Conditions should 
rapidly improve Thursday morning as the winds increase to between 10 
to 15 knots out of the southwest. 


Expect VFR conditions into the early evening hours before some MVFR 
fog is expected to develop. 


&& 




Arx watches/warnings/advisories... 
WI...none. 
Minnesota...none. 
Iowa...none. 


&& 


$$ 


Short/long term...rrs 
aviation..........halbach 

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