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FXUS64 KBMX 212321

Area Forecast Discussion
National Weather Service Birmingham AL
521 PM CST Thu Nov 21 2019

For 00Z Aviation.


/Updated at 0321 PM CST Thu Nov 21 2019/
Through Tonight.

Upper level flow across the area today has remained zonal ahead of
an approaching trough and surface front. In response to a weak
impulse within that zonal flow, light rain showers are moving 
eastward across northern MS and AL. Stronger returns are making it
to ground, overcoming the still relatively dry air below 700mb. PW
values are now around 1 inch across far northwest Central Alabama,
and drop to 0.6 inches near and south of the I-20 corridor. 

For the remainder of this evening, light showers extend back to
the Mississippi River and expect this activity to continue across
northern areas through early evening. The weak impulse will move
east of the area and rain activity will pull north of Central
Alabama, closer to better upper level dynamics ahead of the
approaching front. Expect cloud cover and low level moisture to 
continue to increase overnight. This will keep overnight lows in 
the mid 40s east to low 50s west.


Friday through Saturday.

/Updated at 0321 PM CST Thu Nov 21 2019/

No wholesale or major changes were made to the extended forecast, 
though some PoP adjustments were for the weekend and mid next week 
to become more aligned with trends in recent guidance prognostics. 
In particular, PoPs were advanced slightly more progressive for the 
weekend system. This remains a widespread chance of rain for the 
forecast area with totals generally ~0.50" or less in the southeast, 
to as high as ~1.75" in the northwest through Saturday night. A 
slight chance of embedded thunderstorms was maintained Friday 
evening through Saturday afternoon as MUCAPE <500 J/kg is possible 
along the frontal boundary and primary area of rain as it swings 
through Central Alabama. Rain is expected to quickly advance out of 
the area with dry conditions expected by early Sunday morning, 
though cooler air is on its heels. The next period of rainy & 
unsettled weather is forecast for the Tue/Wed timeframe. 
Inconsistencies with how this system evolves has yielded low 
forecast confidence, thus PoPs were adjusted slightly more 
conservative with this forecast update. These changes aside, the 
previous long-term discussion is a good analysis for the forecast 
across Central Alabama in the coming days including the varying 
scenarios for next week.


/Updated at 0353 AM CST Thu Nov 21 2019/

A broad area of moist deep layer southwesterly flow will be present 
along and ahead of the front on Friday, with several embedded 
shortwaves. PWATs will increase to around 1.5 to 1.6 inches 
Friday/Friday night. Moist isentropic lift will result in several 
waves of showers moving through Friday/Friday evening across the 
northwest half of the area as the column saturates with the front 
also providing some forcing. This will also result in a tricky high 
temperature forecast with increased cloud cover. Southerly flow and 
rain-free conditions will result in highs continuing to be above 
guidance in the south, while highs across the north will be 
dependent on how quickly the rain moves in, and a secondary warm 
front may set up in between. An additional area of showers will set 
up along the cold front which moves in late Friday night and 
Saturday. An isolated embedded thunderstorm is possible late Friday 
night in the west with some weak, primarily elevated instability. 
Dew points only in the 60-62F range and lack of stronger theta-e 
advection, combined with expected cloudcover and precipitation 
should prevent any strong or severe surface-based activity late 
Friday night. 

Dew points may creep up to around 65F on Saturday in our far 
southern counties, which may allow some weak surface-based 
instability to develop, though the consensus of guidance keeps CAPE 
values still only around 200 J/kg or less. Forecast soundings also 
suggest hodographs will straighten as low-level flow veers with time 
even as instability begins to increase, and profiles remain 
saturated. If an overlap between enough instability and curved 
hodographs could develop, then there would be a non-zero threat of a 
brief tornado on Saturday, but this seems unlikely at this time, 
though some gusty winds will be possible. Rainfall amounts look to 
be around 0.5 to 1.5 inches, with locally higher amounts in the far 
northwest, so with a lack of saturated conditions any minor flooding 
should be limited to poor drainage areas. Conditions mainly dry out 
Saturday night, but can't rule out some drizzle across the far north 
as some wrap-around low-level moisture rotates through. 

Sunday through Thursday.

Rain-free and cooler conditions are expected Sunday, as a few 
moisture-starved waves move through in northwest flow aloft. 
Southerly winds on Monday will signal a warming trend. Meanwhile a 
strong North Pacific jet will carve out broad troughing from the 
northwestern to the north-central CONUS. Models have been trending 
less amplified with a shortwave and strong mid-level speed max 
rounding the base of this trough Tuesday/Tuesday night, but vary 
regarding how much less amplified. There is still ensemble support 
for a surface low lifting northeast from the Southern Plains to the 
Great Lakes, with high rain chances and possible breezy conditions 
for Central Alabama with the cold front passage Tuesday night. 
Moisture return and instability within the narrow warm sector is 
expected to be somewhat limited, and heights remain neutral/rising 
with time. Therefore any potential severe threat remains very 
marginal at this time. Strengthening ridging over the Gulf of Mexico 
is expected through the end of the period downstream of West Coast 
troughing. A very early look at the Thanksgiving forecast suggests a 
warm front lifting northward across West Alabama while a CAD wedge 
tries to build in from the east, suggesting varied temperatures 
across the area and some light rain chances especially the further 
north you go, but details remain murky at this time.



00Z TAF Discussion.

Sct light showers will continue north of I-20 thru 03z, but not 
likely have any significant impact on TAF sites as vsbys should 
stay above 5 miles. Mostly mid to high level clouds will continue 
across central Alabama thru 09z. Isentropic lift will increase 
across north Alabama between 09z and 12z, and cigs 2500-3000 feet 
agl will likely develop along and north of I-20. Showers will 
increase across north Alabama after 18z, but coverage uncertain 
for northern TAF sites, and handled with VCSH. MVFR cigs likely to
remain across northern TAF sites thru the end of the period.




Most of the area will be dry tonight, with only scattered light
rain showers across the far north. The chance for wetting rains 
returns increase Friday and lasts through Saturday thanks to a 
frontal system that will move across Central Alabama. Minimum 
relative humidity values will be elevated through the weekend.
Drier conditions return by Sunday through early next week. 


Gadsden     46  67  59  65  37 /  30  80  90  90  10 
Anniston    48  70  59  66  40 /  20  60  80  90  10 
Birmingham  51  69  60  65  39 /  20  70  90  90   0 
Tuscaloosa  51  71  61  65  38 /  20  70  90  80   0 
Calera      49  71  60  65  39 /  10  50  80  90  10 
Auburn      48  69  58  67  43 /   0  10  50  80  20 
Montgomery  49  75  60  70  43 /   0  20  60  90  10 
Troy        48  74  60  72  44 /   0  10  30  80  20 



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