192 
fxus63 karx 090944 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
344 am CST Friday Jan 9 2009 


Short term...through Sunday 
Winter Weather Advisory for snow accumulations today/tonight took 
the lion/S share of time this morning...with unfortunately a 
lowered confidence in how it will all play out. Little time spent 
elsewhere...but did try to focus in a little on Saturday 
night/Sunday light snow chances. 


Currently...snow has moved in with a vengeance this morning with 
band of moderate snow over much of southern Minnesota/northern Iowa/southwestern WI...a 
little on the faster side of the previous day/S model runs. 
Reports tough to come by thus far...but what has been received 
suggests 1-2 inches so far from Austin down through much of 
northeast Iowa into ovs/pdc areas as of 0930z. 09.08z Slater and 
kdmx wind profiler showing 30-40kt of flow at 850 mb in strong 
warm advection regime downstream of northern plains-to-cntl rockies 
shortwave trough driving the most intense portion of the snowband. 
Observations in Iowa on the southern fringe of the snowband have 
flirted with fzdz/sleet where ice bearing cloud has been thinning 
and elevated warm layer seen in RUC analysis. More convective 
looking elements upstream over eastern South Dakota immediately ahead of more 
concentrated pv advection/upper q/g forcing with steeper lapse 
rate aloft. 


As mentioned above...confidence has waned with 09.00z 
deterministic runs and associated sref probabilities making a 
break from going forecast especially this afternoon/evening. So 
the new forecast incorporates some of these movements without 
hopefully moving the forecast in the wrong direction. 


Of most certainty is the snowband covering the area this 
morning...which is forecast to continue to shift east/northeast 
over the area as strong warm advection maximizes low/middle level 
lift through about 15z. Most of the advisory area will see a good 
1-4 inches from this...a little more in spots...such as northeast 
Iowa into southwest Wisconsin. However...this thermodynamic 
forcing will be progressive...and while earlier models from 
Thursday had shown developing frontogenesis/deformation lingering 
accumulating snow over the area into the late morning and 
afternoon...the bulk of the hi res WRF runs...including 
spc/NSSL...and locally run GFS-based arw-WRF...now join the 09.00z 
and 09.06z NAM-WRF in showing a rather dry period for the 
afternoon hours...with little accumulation as warm advection snow 
shifts east...and deformation band slowly organizes over Iowa into 
northern Illinois. A shift was made to this drier scenario...with 
little accumulation noted in new forecast after 18z today...but 
did keep very light snow/light accums less than an inch/ going in the 
area of weak frontogenesis aloft for the afternoon hours. Don/T 
even have the confidence to say that ice-bearing cloud won't also 
leave...bringing a period of freezing drizzle...that may very well 
happen...as forecast soundings are wishy-washy in how much 
saturation remains later this morning/this afternoon as steadier 
snow ends. 


Late this afternoon/tonight...bulk of guidance seems to side with 
09.00z GFS/Gem that deformation snowband will develop over 
Iowa/northern Illinois...and 09.00z NAM-WRF likely a tinge too far 
south /keeping entire area dry this evening/. This in face of new 
09.00z European model (ecmwf) that brings good accumulations to northestern Iowa/southwestern wis. 
This evening. Think this solution...however...too far north...so 
play a middle of the Road solution that keeps continuity with 
previous forecast...which keeps snow going over southern forecast area 
this evening...but shows more gradient in snowfall with heaviest 
accumulations likely south of the area...but still another 1-2 
inches over northestern Iowa/far southwestern wisc as frontogenetic band pulls 
southeast by midnight. 


The bottom line...most areas probably will see a little less snow 
than previously thought...probably by an inch or two...especially 
southeast Minnesota into central Wisconsin. Given degree of 
agreement on the northern fringe of the snow band...pulled Clark County 
wisc from the advisory with only 1-2 inches expected in the far 
south. Most other areas expect a good 2-4"...with more across 
northeast Iowa and far southwestern wisc thanks to the proximity of 
deformation snows tonight. Also moved expiration times up to 
10.00z on southeastern Minnesota/cntl WI and portions of northestern Iowa 
advisory...though in reality...expect bulk of this accumulating 
snow to be done by noon...but confidence is low enough that seems 
prudent to keep this going through afternoon in case models 
double-back on afternoon snows. 


Rest of forecast largely unchanged...though shortwave energy 
Saturday night/Sunday was strong enough given forcing/quantitative precipitation forecast 
probabilities via sref to insert small snow chances in these periods. 


Long term...Sunday night through Thursday 
period will begin active with shortwave energy dropping into the 
evolving eastern Continental U.S. Longwave trough Sunday night/Monday. Snow 
chances increased a little in the new forecast...and may need to 
go higher yet depending on where this energy tracks especially 
into Monday. Even stronger wave Tuesday afternoon/night will not 
only bring snow accumulation potential...but will open the already 
well discussed Arctic flood gates. This period could be one of 
dangerous winter weather...with powdery snow...strong winds...low 
wind chills and plummeting temperatures. Began moving toward 
higher snow chances Tuesday/Tuesday night...and will address the winter 
hazards in severe weather potential statement. 


For Tuesday night-Thursday and beyond...all eyes on bitterly cold Arctic 
airmass. No reason...given the 09.00z deterministic 
GFS/ECMWF...and analogs to previous Arctic outbreaks...to believe 
this won't be one of the colder periods we/ve seen in a while. 
Massive 1045mb /ECMWF/ to 1055mb /GFS/ surface high with true 
Arctic airmass builds straight south through heart of the Continental U.S. 
Wed/thur. There/S detail differences...like which day will be the 
coldest et cetera...but the bottom line is that -20c to -25c 
temperatures at 925mb will pour into the region all the way 
through next weekend. When considering normalizing the surface 
pressures/850mb temperatures/500mb heights to climatology...most 
fields bottoming out a good 1.5 to 2.0 sigma below the 30 year 
climatology...which work by grumm and others have shown to be 
suggestive that local records may be broken...either minimum or 
record low maximum temperatures. Without getting lost in the many 
details to be worked out...like amount of fresh snow to fall ahead 
of this Arctic outbreak...when winds will decouple...which nights 
will be clear...the screaming message is one of bitterly cold 
air...low wind chills...and potential other hazards depending on 
how much snow/wind accompany the Arctic front. 


&& 


Aviation... 
aviation concerns center on reduced ceiling and visibility with 
snow. Looking like the lowest conditions and heaviest snowfall will 
be during the morning hours. Best forcing for precipitation prior 
to 18z today...with weakening lifting mechanisms later in the day. 
Deepest moisture also moves away by afternoon...with soundings 
supporting at least the possibility for freezing drizzle if indeed 
the heavier snows depart. This possibility will have to be watched 
later today. 


By evening the trend is for the taf sites to improve into the MVFR 
category with decreasing low level moisture. This will be addressed 
in greater detail for the 12z taf issuance. 


&& 


Arx watches/warnings/advisories... 
WI...Winter Weather Advisory until 6 PM CST this afternoon for wiz032- 
wiz033-wiz034-wiz041-wiz042-wiz043-wiz044-wiz053. 


Winter Weather Advisory until 6 am CST Saturday for wiz054- 
wiz055-wiz061. 


Minnesota...Winter Weather Advisory until 6 PM CST this afternoon for mnz079- 
mnz086-mnz087-mnz088-mnz094-mnz095-mnz096. 


Iowa...Winter Weather Advisory until 6 PM CST this afternoon for iaz008- 
iaz009-iaz010-iaz011-iaz018-iaz019. 


Winter Weather Advisory until 6 am CST Saturday for iaz029- 
iaz030. 


&& 


$$ 


Short/long term...binau 
aviation...mw 

National Weather Service Glossary of Abbreviations

Non-Expanded Version (with abbreviations)