063 
fxus63 klbf 210906 
afdlbf 


Area forecast discussion 
National Weather Service North Platte NE 
306 am CST Sat Nov 21 2009 


Synopsis... 
long wave trough crashing onshore across the West Coast with well 
defined warm air advection zone out ahead of it and developing middle and upper level 
clouds over the central rockies into the northern plains. Lowering 
pressures in response to elvolving southwesterly flow further west allowing 
decent low level jet for the time of year and associated mixing across the sandhills 
keeping temperatures up this morning. 


&& 


Short term...(today through tuesday) 
brief transitory ridge exists today being replaced by developing 
Lee trough as lead short wave progresses quickly across southern Canada. 
Southwesterly flow as upper high exists allows eml to overspread the area 
this morning and although mixing may be limited initially due to 
cloud cover...areas east of a line from Oga to vtn will benefit 
from mechanical mixing to allow temperatures to reach u50s to possibly 
l60s. As short wave continues across southern Canada tonight...weak 
front will progress through the County Warning Area overnight. Initially...middle 
level fgen will allow a coupled circulation to keep front moving 
eastward...but as upper support outruns surface feature frontolysis and 
eventually low level frontal fracture occurs just southeast of County Warning Area. Dry 
middle and lower layers will inhibit preicp reaching the ground and 
have left forecast dry Sat night and early Sunday. Low level moisture 
poised to return northward Sunday morning...but should be deplaced east of 
advancing front and therefore east of County Warning Area and have removed mention of 
fog in zones. This will have to be monitored if front weakens 
early. 


Next...more impressive...short wave begins to dig across the central 
rockies Sunday. Lee trough in advance of this will allow winds to 
shift to srly again and will begin the transport of moisture nwwd. 
Sunday will be cooler behind the front. The moisture will have a 
hard time initially moving too far west and dry middle levels will inhibit 
any precipitation development through Sunday night. Low levels will 
saturate Sunday night and br and stratus will likely be the result. 
Low temperatures have been adjusted slightly upward to account for increase 
of ll moisture and cloud cover. Though decent sunshine for 
Sunday...southeasterly winds will be recycling cooler Post frontal airmass. 


Significant diffs in models by early Monday. 00z 21 Nov run of the 
European model (ecmwf) would have the most impact for the County Warning Area...but this model has 
drifted northward with the upper low compared to its 12z 20 Nov 
counterpart. At the same time...the GFS has trended slightly slower 
and southward...but would leave western County Warning Area mainly dry and precipitation for northestern County Warning Area 
would come during the day Monday. Even if the European model (ecmwf) verifies...a look at 
moisture transport vectors on isentropic sfcs would suggest wcb 
would not have a chance to work into the system until it reaches eastern 
nebr. There does appear to be a small window of opportunity though 
for wcb/ccb interaction and associated fgen in favored northwest quadrant of 
developing upper low...over northestern County Warning Area beginning Monday night and 
continuing Tuesday. In addition to and as a result of timing diffs between 
models...precipitation type is at issue as well. Warm low levels as 
suggested by bufr soundings would indicate little chance for 
significant accumulating snows until Tuesday...and only if European model (ecmwf) 
verifies. GFS does bring more cold air southward than European model (ecmwf) on Monday 
and therefore snowfall would start sooner...but would end sooner 
as well. Too many diffs at this point to work out details unfortunately. 


Long term...(wednesday through friday) 
earlier discussed diffs magnify later in the week leading to a wide 
envelope of plausible solutions. However end result by the end of 
the week would be to trend toward increasing heights aloft and a dry 
forecast. No changes made beyond Monday/Tuesday time frame for now...but early 
indications for Thanksgiving are mainly dry...though trending more 
active toward the weekend. 


&& 


Aviation... 
a surface trough of low pressure will deepen and track 
east today across western and north central Nebraska. Strong 
southerly winds of 15 to 25 kts will develop east of this 
feature...while westerly winds of 10 to 20 kts will be common west 
of the surface trough. Winds are expected to diminish this evening with 
westerly winds of 5 to 15 kts expected. Middle to high level cloudiness 
will be on the increase this afternoon and tonight east of an 
approaching upper level trough of low pressure. Ceilings at or above 15000 
feet above ground level are likely at both the kvtn and klbf terminals through 12z 
Sunday. 


&& 


Lbf watches/warnings/advisories...none. 
&& 


$$ 


Short term/extended...jws 
aviation...clb