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Area forecast discussion...updated 
National Weather Service Fort Worth Texas 
1020 am CST Monday Nov 23 2009 


Update... 
the dense fog advisory will be allowed to expire at 11 am. As of 
16z... visibilities were still 1/4 mile or less across much of 
north central North Texas from Bowie...to Granbury...to 
Lancaster...to Sherman. The fog should continue to disperse 
during the next hour so we will let the advisory expire at 11 am. 
Otherwise...have adjusted the hourly trends but made no 
significant change to the previous forecast. 


58 


&& 


Aviation... 
517 am 
dense fog has formed over much of North Texas...except in the 
metroplex where the heat island effect is slowing down the radiative 
cooling. Still expect patchy dense fog even in the city centers by 
sunrise...as evidenced by the fog beginning to affect city web cams 
we are watching. Because this radiation fog should remain fairly 
shallow...expect it to burn off near middle morning...with rapid 
improvement between 9 and 10 am. Cold front will shift winds 
northwest just before midnight...but precipitation chances are very low and 
kept the vcsh for the few sprinkles that may fall. 84 




&& 




Previous update... 
update... 
we extended the dense fog advisory for the north central and 
northeastern parts of North Texas until 11 am. Visibilities are 
slowly improving and the remaining dense fog should disperse 
between 10 am and 11 am. 


58 


&& 




Previous discussion... /issued 517 am CST Monday Nov 23 2009/ 
433 am 
today through Tuesday...09z water vapor imagery indicates a strong 
shortwave trough moving southeast through Colorado early this morning. 
This is the upper level disturbance that will move east across Kansas 
and OK during the day today...sending a cold front south through 
North Texas during the overnight hours. Shallow moisture remains in 
place through the forecast area this morning...where clear skies 
and light winds have once again allowed areas of dense fog to 
develop. Winds are expected to pick up out of the south today 
ahead of the upper level disturbance pushing through the 
plains...which should bring some meager return of Gulf moisture 
through the area ahead of the cold front for tonight. With only 
around 12 hours of good moisture transport expected...thinking is 
that Gulf moisture will remain relatively shallow over much of 
North Texas. Upper level potential vorticity analysis indicates that 
the bulk of the large scale forcing for ascent associated with 
this upper level disturbance will remain north of the Red 
River...as has been advertised the past couple of days of model 
runs. With relatively shallow moisture in place and without much 
in the way of large scale forcing for ascent...think that rain 
shower activity will tend to remain along and just behind the cold 
front with only isolated to scattered coverage along it as it 
pushes southeast tonight. With this in mind...maintained the 20 to 
30 percent chance of precipitation for tonight from previous 
forecast. Think that rain shower activity will develop as the 
front moves through...but coverage is expected to be limited at 
this time. Behind the front...models depict a significant drop in 
low level moisture as air currently over the Texas/OK panhandles moves 
into North Texas. This airmass will probably be modified slightly as 
it moves our way...so we may not see the lower 20s dew point 
values that are currently in place...but dew points in the upper 
20s to lower 30s look likely at this time. 


Tuesday night through Friday...with drier air in place...chances 
of overnight fog/stratus that we have been seeing the past couple 
of days should be small. With the drier air and expected clear 
skies we will have a greater chance to see many locations approach 
or dip below the freezing mark during this period...with rural and 
low lying areas being most likely to see near freezing temperatures for 
lows. Urban areas will most likely not see freezing temperatures during 
the period because the airmass is not cold enough to combat the 
heat island effect...but something to keep an eye on if daily 
highs do not climb as high as expected behind the cold front. As 
the initial strong shortwave trough kicks off to the northeast 
towards the Great Lakes...another powerful trough is forecast to 
dive south through the plains Wednesday night and Thursday bringing a 
reinforcing shot of cool...dry air into the forecast area. This 
secondary front is only expected to keep highs a few degrees lower 
than earlier in the week as there will be no time in between 
systems for any return of moisture. On Friday...upper level 
ridging looks to set up over the Great Plains as another strong 
shortwave trough moves onshore the western Continental U.S.. this should 
bring a return of southerly flow to North Texas by Friday afternoon 
bringing a gradual warm up headed into the weekend. 


Saturday through Monday...models are coming into better agreement 
that we will have some sort of strong trough to the west of Texas 
promoting southwest flow aloft over much of the state. Models 
continue to differ on when exactly this trough may move northeast 
over the state providing good forcing for ascent and 
precipitation. Regardless...southwest flow aloft signals a wetter 
than average pattern for the forecast area...so have gone ahead 
with slight chance probability of precipitation for the entire area Sunday through Monday. 
It seems more likely that North Texas will see another cold front 
push through either Sunday night or into early next week...but 
ahead of that...some good moisture return looks likely at this 
time. Could see some elevated thunderstorm activity across the area by 
Sunday afternoon...and then would expect the best precipitation 
chances to occur along and behind any cold front that moves 
through the region. Too much uncertainty to pin down a frontal 
passage this far out...so chose to highlight some chance of 
precipitation for the period in general instead of trying to place 
the front/better precipitation chances. 


Cavanaugh 






&& 




Preliminary point temps/pops... 
Dallas-ft. Worth, Texas 71 50 62 39 64 / 5 20 5 0 0 
Waco, Texas 73 52 65 39 64 / 5 20 10 0 0 
Paris, Texas 69 47 61 38 61 / 5 30 20 0 0 
Denton, Texas 71 43 61 35 64 / 10 20 5 0 0 
McKinney, Texas 70 45 62 36 63 / 5 20 10 0 0 
Dallas, Texas 71 51 62 41 64 / 5 20 5 0 0 
Terrell, Texas 71 52 64 39 63 / 5 30 10 0 0 
Corsicana, Texas 72 52 65 39 64 / 5 30 20 0 0 
Temple, Texas 73 53 65 39 64 / 5 20 10 0 0 


&& 


Forward watches/warnings/advisories... 
dense fog advisory until 11 am CST this morning for txz091>095- 
102>105-117>120-131>134-144-145-159. 




&& 


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