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Area forecast discussion
National Weather Service Birmingham Alabama
1215 PM CST Tue Dec 18 2018

Update...
midday update and 18z aviation.

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Short term...

Model guidance was slightly fast on the clouds returning from the
west this afternoon. The lack of clouds, decent mixing and dry
atmosphere will lead to warmer temperatures this afternoon.
Therefore, have lowered the cloud cover and increased highs today.

75

Previous short-term discussion:
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...
18z taf discussion.

High pressure and little in the way of cloud cover anticipated
this afternoon. Winds will be 5kts or less out of the west
northwest. Light/calm winds overnight with only a few mid or high
clouds. No restrictions on flight expected on Wednesday as winds
swing around to the east southeast at 5kts.

75

&&

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 36 57 45 54 40 / 0 10 80 90 80
Anniston 40 58 46 56 41 / 0 10 80 90 80
Birmingham 40 59 48 57 41 / 0 10 80 80 80
Tuscaloosa 40 61 48 59 41 / 0 20 80 80 80
Calera 39 58 47 57 40 / 0 20 80 80 80
Auburn 42 57 48 57 42 / 0 10 80 90 80
Montgomery 41 60 49 61 43 / 0 20 90 80 70
Troy 42 60 50 62 43 / 0 10 90 80 70

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