470 
fxus63 kgid 231135 
afdgid 


Area forecast discussion 
National Weather Service Hastings NE 
535 am CST Monday Nov 23 2009 


Aviation...12z taf. The main issue will be fog this morning...and 
then rain chances later this afternoon into tonight. Observations 
from across south central Nebraska this morning show several spots 
with visibilities less than 1/2sm in fog. Currently kgri is closer 
to 2sm...but think that there is still the potential for lower 
visibilities over the next few hours due to nearly saturated 
conditions in the lower parts of the atmosphere. Will still drop 
visibilities to around 1/2sm at kgri for a few hours in a tempo 
group...but go with a prevailing visibility around 2sm. Forecast 
soundings suggest there will be slow improvement at kgri this 
morning..and will get rid of most of the visibility restrictions 
toward 16z. The low clouds will likely prevail longer...though 
ceilings should also gradually improve back to MVFR levels this 
afternoon. A storm system moving across the region will then bring 
a chance of rain to kgri starting late this afternoon...which will 
continue into tonight. Instead of carrying probability or tempo groups 
for the rain...decided to just carry rain in a prevailing group 
late this afternoon since several different forecast models show 
the rain beginning around that time. There is also some chance 
that snow could mix with the rain late tonight...and put that in 
towards 08z. Winds will also be on the increase later tonight with 
gusts of 25 to 30 kts possible. 


&& 


Previous discussion... /issued 446 am CST Monday Nov 23 2009/ 


Discussion...main concerns are dense fog this morning...and then 
precipitation type and amounts tonight. 


Upper trough was making its way across the northern/central rockies 
overnight per water vapor imagery. Upper air observation from last evening 
and RUC analysis data show a 110 knots upper jet on the backside of the 
trough that will continue to dig it to the southeast 
today...eventually developing a closed circulation over northwest 
Kansas/far southwest NE by this evening. Surface low was deepening over 
southeast Colorado/northeast nm ahead of the upper system...with surface 
cold front extending from the western NE Panhandle southwest through 
north central and western Colorado. Weak flow field was in place ahead of 
these features across the Central Plains...combining with moist low 
levels to produce widespread dense fog across the County Warning Area overnight. 
Regional radars have been showing some weak echoes over western 
NE...but surface observation indicating this light precipitation is struggling to 
reach the surface thus far. 


Will continue with the dense fog advisory for the entire County Warning Area as 
visibilities at all but one automated observing site in the County Warning Area are 
currently at 1/2sm or less. MOS guidance and model low-level relative humidity 
fields show this will continue into the middle to late morning...so 
expiration time of 17z still looks reasonable. Once the fog lifts 
late this morning...attention turns toward precipitation chances 
with upper trough and attendant cold front that will push through 
the area from west to east today. The morning looks mainly 
dry...although some rain may begin to sneak into the far west toward 
the noon hour. Precipitation chances then increase from west to east this 
afternoon as forcing increases ahead of upper low closing off over 
northwest Kansas/far southwest NE by 00z Tuesday. Strong q-vector 
convergence along with frontogenetic forcing from Pacific cold front 
support pretty good rain chances this afternoon and have increased 
probability of precipitation...keeping the highest values in the west. Thermal profiles 
remain warm enough through the afternoon to keep precipitation as rain. 


The main challenge with this system remains the changeover from rain 
to snow tonight and resultant snowfall amounts. Significant cold 
air is lacking with this system of Pacific origin...as confirmed 
by 850 mb 00z observation from last evening. However there does appear to be 
enough cold air to support a mix with and eventual changeover to 
snow from west to east this evening/overnight...and some dynamic 
cooling will supplement this somewhat as well. Near surface temperatures 
also remain fairly warm however...which will hamper snowfall 
accumulations. Surface wetbulbs are only forecast to fall into 
lower 30s in the western County Warning Area tonight...while staying in the middle to 
upper 30s in the east. Hence even though profiles aloft will 
support a changeover to snow...determining how much of the snow will 
accumulate remains a challenge. Based on the relatively warm near 
surface temperatures and the fact that the colder air lags the stronger 
middle-level forcing and resultant higher quantitative precipitation forecast...think that 
accumulations will remain fairly light. Much of the precipitation across 
the central and especially eastern County Warning Area will likely fall as rain or 
a rain/snow mix. Have kept the highest accumulations in the 
northwest County Warning Area...where 1-2 inches are possible northwest of a 
Fullerton/Grand Island/Phillipsburg line in the deformation band 
precipitation...with near an inch possible along this same 
line...tapering off to little to nothing in the southeast. 
Northwest winds will also pick up tonight on the backside of the 
system...reaching speeds of 20-30 miles per hour with gusts around 35 
miles per hour...which may combine with the precipitation to produce some reduced 
visibilities. 


The deformation/wraparound rain/snow may linger in the eastern 
County Warning Area into Tuesday morning...but the system is pretty quick to pull 
out into IA/MO...bringing an end to the precipitation by midday. Only 
anticipate some light snow accumulations if any /half inch or 
less/ in the east Tuesday morning. Breezy northwest winds will 
keep it on the cool side on Tuesday...although close to 
seasonal...with highs in the 40s. Another upper wave dives 
southeast from the northern plains through the upper Midwest/middle 
MS valley on Wednesday...sending a reinforcing cold front through 
the area. Not expecting any precipitation with this as a dry lower 10kft 
or so of the troposphere will likely keep anything limited to 
virga...at best a stray sprinkle or flurry. However it will keep 
temperatures around 40f northeast to the upper 40s southwest for highs. 


Dry conditions and seasonal temperatures are expected on Thanksgiving under 
a northwest flow aloft...with temperatures then moderating late week 
into the first part of the weekend as the western Continental U.S. Ridge 
propagates east. 


&& 


Gid watches/warnings/advisories... 
NE...dense fog advisory until 11 am CST this morning for nez039>041- 
046>049-060>064-072>077-082>087. 


Kansas...dense fog advisory until 11 am CST this morning for ksz005>007- 
017>019. 


&& 


$$ 

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