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afdbmx 


Area forecast discussion 
National Weather Service Birmingham Alabama 
545 am CST Tue Dec 18 2018 


Update... 
for 12z aviation. 


&& 


Short term... 


Today and tonight. 


Going to start out cloudy and foggy across the west and central. 
Areas in the far west will stay cloudy through 9 or 10 am and then 
warm up into the the upper 50s and low 60s again this afternoon as 
surface high pressure continues to build into the area. As the high 
pressure slides east, we will see some high clouds develop and slide 
across the area tonight. This should be enough to limit the fog 
potential, at least for Alabama. The best chances look to be to our 
west. Look for the lows to be in the mid 30s north to low 40s 
south. 


16 


Long term... 


Wednesday through Friday. 


A dynamic upper-level trough will dig southeastward across the 
central Continental U.S. Wednesday morning with a neutrally-tilted axis 
paralleling the Mississippi River valley by Thursday afternoon. 
Downstream synoptic forcing/ascent will be in abundance across 
central Alabama overnight Wednesday and into Thursday morning. By 
then, warm air & moisture advection maximizes within an intensifying 
low-level jet responding to cyclogenesis to our west and upper-level 
jet streak support & diffluent flow aloft. Initially, rain showers 
are expected to develop within isentropic flow and encompass the 
area as early as Wednesday evening across the southwest then spread 
northeastward gradually increasing in intensity and coverage towards 
Thursday morning. 


Have placed highest pops (70-90%) from 00z Thursday through 12z 
Friday as the low pressure strengthens & continues eastward 
across our area and pws approach ~1.2", particularly across the 
south. During this time, mid/upper-level dynamics continue a 
rather impressive southward trend as the trough undergoes 
amplification. 500 mb height anomalies may approach ~40 decameters 
below average when the trough passes over the area Friday morning, 
indicative of very cold temperatures aloft and a lowered 
tropopause. However, pops & quantitative precipitation forecast continue a downward trend by then, 
though cloud cover & light rain showers along a deformation axis 
are expected to stick around Friday afternoon as the upper-level 
low moves into Georgia. A tight pressure gradient and cold-air 
advection on the western side of the surface low will provide 
gusty northwesterly winds & chilly conditions Friday afternoon as 
well. Overall, forecast rainfall totals are expected to be in the 
0.7-1.1" range across central Alabama. 


Saturday through Tuesday. 


A stable Continental airmass and surface high pressure will make its 
way into the southeastern Continental U.S. Behind the mid-latitude cyclone 
exiting our area as we head into the weekend. Upper-level split flow 
makes its return as well as we await the next significant rossby wave 
in the long-term. For now, a brief opportunity exists for isolated 
light rain showers early next week as a shortwave passes to our 
north Monday, but moisture will be limited and lift likely stays out 
of our area so have kept pops <20% for now. Thereafter, more quiet 
and mild weather remains in place as we look ahead towards 
Christmas. Generally expecting highs to rebound to the 50s/60s 
Sunday-Tuesday with corresponding overnight lows in the 30s/40s. 


40/sizemore 




&& 


Aviation... 
12z taf discussion. 


The main widespread fog is just west of all taf sites, but river 
fog has and will continue to limit vis at tcl and mgm through at 
least 14z maybe 15z. Anb and asn has been up and down all night, 
so continued the chance of IFR through 14 to 15z. After this time 
the sun will shine and VFR conditions are expected. 


16 


&& 


Fire weather... 


Areas of dense fog have developed overnight, particularly in 
areas west of I-65 and along river valleys across the area. This 
is expected to persist through 9 am this morning. Thereafter, dry 
conditions will persist across the area through Wednesday morning 
before rain chances increase Wednesday night across the 
southwest. Relative humidity values will remain above critical values and there 
are no fire weather concerns at this time. 


&& 


Preliminary point temps/pops... 
Gadsden 58 36 57 45 54 / 0 0 10 80 90 
Anniston 60 40 58 46 56 / 0 0 10 80 90 
Birmingham 59 40 59 48 57 / 0 0 10 80 80 
Tuscaloosa 61 40 61 48 59 / 0 0 20 80 80 
Calera 60 39 58 47 57 / 0 0 20 80 80 
Auburn 63 42 57 48 57 / 0 0 10 80 90 
Montgomery 63 41 60 49 61 / 0 0 20 90 80 
Troy 64 42 60 50 62 / 0 0 10 90 80 


&& 


Bmx watches/warnings/advisories/... 
dense fog advisory until 10 am CST this morning for the 
following counties: Autauga...Bibb...Chilton...Coosa...Dallas... 
Elmore...Fayette...Greene...Hale...Jefferson...Lamar...Lowndes... 
Marengo...Marion...Montgomery...Perry...Pickens...Shelby...St. 



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