FXUS64 KBMX 162352

Area Forecast Discussion
National Weather Service Birmingham AL
552 PM CST Sat Feb 16 2019

For 00Z Aviation.


Through Tonight.

Surface cold front has shifted just south of the forecast area
this afternoon, with sunny skies now across the south. Post
frontal stratus has lingered near and north of the I-20 corridor.
This has caused quite a range in temperatures today, with low 40s
in the far northwest to mid 70s in the far southeast. For the rest
of this afternoon and early evening, expect rain free conditions,
but the clouds will remain across the north with near steady
temps north of I-20.

Low level winds will become southerly this evening in response to
a weak surface low strengthening to our west, and the front to our
south will begin to move northward as a warm front. After 9-10pm,
temperatures should gradually increase overnight. Weak isentropic
lift will bring a return of clouds to the entire area, and
isolated showers across the north before midnight. Deep layer
southwesterly flow increases as a upper level trough in the
northern Plains. Rain coverage increases to our north and west,
where a new cold front develops, and spreads into northern
portions of Central Alabama before sunrise. With increasing shear
and weak elevated cape, cannot rule out a few sustained updrafts
and thunderstorms in the northwest, but strong or severe storms
are not expected.


Sunday through Friday.

Strong/moist southwest flow between a positively tilted trough
over the southwestern CONUS and a strong 594 decameter subtropical
ridge centered near the Bahamas will be the dominant weather
feature for the extended period. As a front oscillates over the
area, multiple rounds of rainfall are expected with flooding
concerns by the middle of the week.

Sunday/Sunday night:
An upper low over the Siouxland region of the Plains will be in
the process of shearing into an open wave on Sunday, while a
strong (185kts at 250mb and 115 kts at 500mb) southwesterly mid
and upper-level jet streak will extend from Texas to the Ohio
Valley. At the surface, a warm front will be lifting northward
through Central Alabama as a weak surface low moves northeastward
into Kentucky and Tennessee, while a cold front moves eastward
into Mississippi and West Alabama. A weak wedge will build into
north Georgia in response to elongated high pressure over southern
Canada. Southwest surface winds should help erode the wedge over
all but our far northeastern areas. Moist isentropic lift and
associated showers will be ongoing in our northern counties in the
morning. Strengthening isentropic lift and frontogenetical
forcing along with increasing moisture will result in a band of
light to moderate rain developing across the northern two-thirds
of the area by afternoon. Despite warm temperatures across our
southern counties, little to no surface-based instability will be
present. Additionally, 500mb height rises will be present,
indicative of synoptic-scale subsidence. Therefore, severe storms
are not expected, and there is only a small chance of a rumble of
thunder with very weak elevated instability. Rain will continue
into the evening hours, ending from northwest to southeast later
in the night as the cold front pushes southeastward. Rainfall
totals of up to an inch are possible.

Monday/Monday night:
Monday now looks to be a dry day outside of a few lingering
morning showers in the far southern counties. Cool northerly post-
frontal winds will be present with highs at or just below seasonal
averages, as a 1040 mb high pressure builds into the Northern
Plains. Southerly flow re-develops aloft after midnight Monday
night, resulting in warm air advection and moist isentropic lift
aloft over a cooler and drier air mass at the surface. Meanwhile
easterly flow will develop at the surface as high pressure moves
eastward to New England and a weak surface low begins to develop
near the Texas Gulf Coast. Light rain will develop after midnight
Monday night as the column begins to saturated.

Tuesday through Thursday:
Another shortwave trough will eject
out over the Southern Plains from the southwestern CONUS trough
Tuesday/Tuesday night. A wave of low pressure will lift
northeastward into North Mississippi, preceded by an inverted
trough. Southerly flow aloft will continue to strengthen as
moisture increases. The strong isentropic lift will result in
widespread moderate rainfall especially across the northern half
of the area during the day on Tuesday. The surface low will try to
pull the warm front northward from the coast, but will be
inhibited at least initially given the developing CAD wedge, and
evaporative cooling effects of rain falling north of the warm
front. High temperatures were lowered Tuesday given the easterly
flow and evaporative cooling effects. Rainfall during the day does
not look to be heavy enough to cause major flooding issues outside
of poor drainage areas, but will cause soils to begin to saturate.

Main forecast challenge will be Tuesday night into Wednesday.
Drier mid-level air will temporarily move into most of the area
except for the northwest/far west Tuesday night, decreasing
rainfall rates over all but the northwest/far west. In the
northwest/far west, PWATs and instability will be increasing. A
band of heavy rainfall will set up in the vicinity of the surface
low/surface trough track. Some model trends keep this just to the
northwest of the forecast area Tuesday night, but it`s still
possible it could shift back to the southeast, especially if the
effective warm front is not able to make it as far north due to
rain falling north of it. Will keep in a low confidence flooding
mention Tuesday night, mainly for the northwest.

The cold front will slowly move into the western part of the state
on Wednesday, with PWATs increasing to around 1.7 inches. Models
indicate some differences with the speed of the front and where it
will stall Wednesday night. This will be key to the flooding
potential, with an increased flooding threat the further west it
stalls. Also, some surface-based instability will develop,
resulting in a potential marginal threat of severe weather.
However, limiting factors will be uncertainty of where the warm
front will be and if the atmosphere can destabilize given all the
rainfall. Also, upper-level support will be very limited with only
some weak glancing 500mb height falls. Therefore, confidence is
too low to mention any severe threat in the HWO at this time given
the dependence on mesoscale details. This does appear to be the
main period to focus on for flooding potential, so will bump up
the confidence slightly in the HWO for Wednesday. Limiting factors
are the lack of uni-directional flow which will limit cell
training except along the boundary, and lack of confidence in the
position of the boundary. However the high PWATs are certainly
something to watch. The flooding threat will probably linger into
Thursday as the front stalls near the area.

Sunday through Thursday rainfall totals from WPC have come down
slightly, with 4-6 inches now forecast along and north of I-20.
This is in line with the ensemble means. The higher values
depicted by the deterministic models still remain possible, as
this will depend on where any heavier bands set up. Areal flooding
and river flooding look to be a good bet, with GEFS ensemble based
river forecasts indicating the greatest risk of flooding being in
the Tombigbee basin. The flash flooding threat remains more
unclear, which will be closely monitored over the next couple



00Z TAF Discussion.

A cold front has stalled out across southeast Alabama. A colder
air mass over north Alabama has produced a stronger low level
inversion, and stratus has remained entrenched along and north of
I-20. The southern edge of the stratus has not showed much erosion
lately, and it may take awhile for the low clouds to erode. North
boundary layer winds will veer overnight and become southeast by
09z. This will help erode lower clouds, probably in the 03z to 06z
time frame. Showers will spread into north Alabama by 06z, but
probably remain north of I-20 thru 09z. Showers will increase
along the I-20 corridor by 09z and spread southward. Included VCSH
at northern TAF sites initially, becoming prevailing showers
between 15z and 17z. Moist southerly flow will increase during the
day Sunday, and cigs will likely fall below 1000 feet agl across
north Alabama between 15z and 18z, with MVFR cigs at TOI and MGM.
Elevated storms possible after 15z, but coverage too small for
inclusion in TAFs.




A brief period of rain free conditions this evening, but rain
chances return tonight as a warm front lifts northward through the
area, and a cold front slides eastward Sunday. Another break
between weather systems is expected Monday, before a prolonged
period of rainfall from Tuesday through Friday. Very wet
conditions are expected, especially across the northern half of
the area. There are no fire weather concerns at this time.


Gadsden     45  65  45  57  40 /  50  80  80  10  50
Anniston    50  67  48  60  43 /  50  80  80  10  50
Birmingham  48  68  46  59  43 /  50  80  80  10  50
Tuscaloosa  51  69  46  60  44 /  40  80  70  10  50
Calera      49  69  48  60  44 /  40  80  80  10  50
Auburn      55  71  53  64  49 /  20  50  80  20  40
Montgomery  59  76  54  65  50 /  20  40  80  20  40
Troy        59  77  56  66  52 /  20  30  60  30  30





NWS BMX Office Area Forecast Discussion