Weather

424 
fxus63 kfgf 070213 
afdfgf 


Area forecast discussion 
National Weather Service Grand Forks ND 
856 PM CDT Sat Sep 6 2008 


Discussion... 
we will make some minor adjustments to tonight/S forecast...mainly to 
tidy up probability of precipitation and sky now that observation make it more obvious that much 
of the convection is diurnally-induced and thus will not survive 
the night. The main exception will be with the batch of -shra now 
entering southeast ND...which should last longer /despite its convective 
origins/ given moderate 500-300 hpa q-vector convergence per the 
18z NAM/GFS. Our chance probability of precipitation will follow this upward motion into 
northwest Minnesota 05-09z. Modifying surface temperatures on the 00z bis/abr radiosonde observations 
suggest that once we fall into the middle 50s f /as most sites are 
now/ the buoyancy will be too marginal for thunder so this will 
also be dropped from the database. Further upstream...satellite 
shows a cellular nature to most clouds which suggests at least 
partial clearing shifting southeast into eastern ND just as the 00z RUC 
shows. This will be tempered some by low/middle level cold air advection...which drops 
850-hpa temperatures down to +2 c in the northwest forecast area by 12z. Not an ideal 
radiational cooling situation...but even this morning Langdon was 
down to 38 f...so we will lower temperatures about 3 f there. The 18z NAM 
actually drops that area to 35 f. 


One other issue may again be patchy fog or at least lower 
ceilings...especially where it rained today...and in northwest Minnesota where most 
12/18z models and the 00z RUC all develop IFR ceilings 09-12z. We will 
not add any fog at this point but it is something to keep an eye 
on. 


Aviation... 
scattered -shra will move east at 20kts into northwest Minnesota through 06z... 
though it is questionable how long they will last now that the sun 
has set. Marginal MVFR ceilings/visible could accompany this activity. The 
low confidence part of the aviation forecast is 08-13z when IFR or 
lower ceilings /or visible/ are possible. This happened the past two 
mornings and model guidance suggests it will again...at least at 
ktvf/kbji. This is hinted at in the present taf set...but until 
confidence increases the forecast will remain somewhat optimistic. 


&& 


Previous discussion... /issued 339 PM CDT Sat Sep 6 2008/ 


Short term... 
main forecast challenge is precipitation. When looking at the latest 
surface analysis...an area of low pressure is centered in NE mb with 
a weak cold front that extends east mb to along a Rox to dtl line. When 
comparing the models...the models remain in pretty good agreement 
for the short term...so ended up using model compromise for the 
forecast. Blended the GFS and NAM for probability of precipitation as they differ on what 
part of the County warning forecast area will be most affected by tonight's trough passage. As 
far as temperatures...with adjmavbc still doing pretty well...used it as 
a starting point and then adjusted from there. 


Kmvx currently shows two areas of convection...one in the east and 
another in the west. East precipitation is ahead and along of surface front and west 
precipitation is associated with push of next wave as seen on WV in 
association with the 500mb trough. 500mb trough is expected move through 
the North Plains tonight and sun...so kept precipitation in the forecast. Models 
do keep showalters at or below zero over parts of the area until 06z 
tonight. With occasional lightning found in storms that have developed... 
kept thunder mention in for this evening and transitioned to rain showers 
after 06z. 


For Sun night into Tuesday...surface high pressure dominates the 
weather...so kept the forecast dry. There is a another 500mb 
trough that moves through on Monday night...but with minimal upward vertical velocity at 
700mb...850mb q-vector divergence indicated and also stability 
indices above zero...believe that will only see clouds from this 
trough passage at this time. 


Long term (tue through sat)... 
medium-range models continue to differ with strength and timing 
of next Pacific short wave. 00 UTC Canadian and 12 UTC European model (ecmwf) 
solutions briefly cut-off weak 500 hpa low over the central 
rockies Thursday morning before ejecting primary energy eastward...whereas 
GFS is stronger and much more progressive. Current forecast 
primarily follows HPC guidance...which favored faster GFS. Model 
consensus shows best chance for measurable precipitation Wednesday and 
Thursday out ahead of primary middle-level wave...although this time 
frame may be pushed later or extended if slower Canadian/European model (ecmwf) 
solutions become reality. Either way...temperatures will continue 
to trend below seasonal normals through next week. 


&& 


Fgf watches/warnings/advisories... 
ND...none. 
Minnesota...none. 
&& 


$$ 


Schultz/ng/Rogers